FINAL DRAFT UGANDA 
STANDARD  
  FDUS 854-2
First Edition
2009-mm-dd
 
Reference numbe r
FDUS 854-2: 2009
  
  © UNBS 2009 
  
Thermal solar systems and components – Solar collectors – 
Part 2: Test methods  
 FDUS 854-2: 2009 
ii © UNBS 2009 – All rights reserved
 Compliance with this standard does not, of itself confer immunity from legal obligations 
A Uganda Standard does not purport to include all necessary provisions of a contract. Users are 
responsible for its correct application  
© UNBS  2009 
All rights reserved. Unless otherwise sp ecified, no part of this publication may be reproduced or utilised in any 
form or by any means, electronic or mechanical, in cluding photocopying and microfilm, without prior written 
permission from UNBS. 
Requests for permission to reproduce this document should be addressed to 
The Executive Director 
Uganda National Bureau of Standards 
P.O. Box 6329 
Kampala  
Uganda 
Tel: 256 41 4505 995 
Fax: 256 41 4286 123 
E-mail: unbs@infocom.co.ug  
Website: www.unbs.go.ug   FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  iii
 Contents Page 
1 Scope.......................................................................................................................... ............................ 1 
2   Normative references.......................................................................................................... ................. 1 
3   Terms and de finitions ......................................................................................................... ................. 1 
4   Symbols and uni ts............................................................................................................. ................... 2 
5    Reliability testing of liquid heating collectors............................................................................. .....4 
5.1    General ...................................................................................................................... ........................... 4 
5.2    Internal pressure tests for ab sorbers........................................................................................ ........ 5 
5.3    High-temperatur e resistan ce test ............................................................................................. ......... 7 
5.4   Exposur e test................................................................................................................. ....................... 9 
5.5    External ther mal shock test .................................................................................................. ........... 10 
5.6    Internal ther mal shock test.................................................................................................. ............. 11 
5.7    Rain pene tration test........................................................................................................ ................. 12 
5.8    Freeze resi stance test....................................................................................................... ................ 13 
5.9    Mechanical  load test ......................................................................................................... ................ 14 
5.10    Impact resistan ce test ( optional) ............................................................................................ ......... 16 
5.11    Final in spectio n ............................................................................................................. .................... 18 
5.12    Test report.................................................................................................................. ........................ 18 
6    Thermal performance testing of liquid heati ng collectors............................................................ 18 
6.1    Glazed solar collectors under steady st ate conditions (inc luding pressure drop).................... 18 
6.2    Unglazed solar collector s under steady state conditions  (including pressure drop)................ 38 
6.3    Glazed and unglazed solar coll ectors under quasi-dyn amic conditions .................................... 48 
Annex A (normative)Schematics fo r durability and re liability tests ........................................................... 59 
Annex B (normative) Durability and reliability test report sheets ............................................................. .68 
Annex C (normative) Stagnation temper ature of liquid h eating collectors................................................ 84 
Annex D (normative) Performance test re port for glazed so lar collectors ................................................ 85 
Annex E. (normative) Performance test re port for unglazed solar collectors ........................................... 90 
Annex F  (normative) Modelling of the coefficients c 1 to c 6 of  the collector model of 6.3...................... 924 
Annex G (normative) Measurement of  effective th ermal capacity .............................................................. 95 
Annex H (informative)   Comparison of the collector model of 6.1 to the coll ector model 6.3 ............... 958 
Annex 1 (informative) Prope rties of water............................................................................................ .......100 
Annex J (informative) Performance test report summary for quasi dynamic test method .................. 1031 
Annex K (informative)  General guidelines for the assessment of uncertainty in solar collector 
efficiency testin g ............................................................................................................. .................. 103 
Annex L (informative) Determination of  presure drop acr oss a collector................................................ 107 
Bibiliography ............................................................................................................................... .................. 1079 
 FDUS 854-2: 2009 
iv © UNBS 2009 – All rights reserved
 Foreword 
Uganda National Bureau of Standards (UNBS) is a pa rastatal under the Ministry  of Tourism, Trade and 
Industry established by the Act of Parliament of 198 3, of the Laws of Uganda.  UNBS is mandated to co-
ordinate the elaboration of standards and is  
(a) a member of International Organisation for Standardisation (ISO) and  
(b) a contact point for the WHO/FAO Codex A limentarius Commission on Food Standards, and  
(c) the National Enquiry Point on TBT/SPS Agr eements of the World Trade Organisation (WTO). 
The work of preparing Uganda Standards is carried  out through Technical Committees. A Technical 
Committee is established to deliberate on standards in a gi ven field or area and consists of representatives of 
consumers, traders, academician s, manufacturers, governme nt and other stakeholders. 
Draft Uganda Standards adopted by the Technical Committ ee are widely circulated to stakeholders and the 
general public for comments. The committee reviews the comments before recommending the draft standards 
for approval and declaration as Uganda Standards by the National Standards Council.  
Committee membership 
The following organisations were represented on Subcommittee on Solar Energy, UNBS TC 13/SC 3, in the 
preparation of this standard: 
• Renewable Energy Department, Minist ry of Energy and Mineral Development 
• Makerere University 
• Incafex Solar Systems 
• Battery Masters (U) Limited 
• Equator-sun (U) Limited 
• Ultratec (U) Limited 
• Sonnerkraft Solar Systems 
• Solar Masters 
• Uganda National Plumbers Association 
• Uganda Institute of Professional Engineers 
• Uganda National Bureau of Standards 
 
 FINAL DRAFT UGANDA STANDARD FDUS 854-2: 2009
 
© UNBS 2009– All rights reserved  1
 Thermal solar systems and components – Solar collectors – 
Part 2: Test methods 
1 Scope 
This Uganda Standard specifies test methods for validat ing the durability, reliability and safety requirements 
for liquid heating collectors as specified in DUS 854-1. Th is standard also includes three test methods for the 
thermal performance characterization for liquid heating collectors. 
It is not applicable to those collectors in which the therma l storage unit is an integral part of the collector to 
such an extent that the collection pr ocess cannot be separated from the storage process for the purpose of 
making measurements of these two processes. It is basically applicable to tracking concentrating colle ctors, thermal performance testing as given in 6.3 
(quasi dynamic testing) is also applicable to most co ncentrating collector designs, from stationary non-imaging 
concentrators as CPCs to high concentrating tracking designs. Parts of the so lar radiation measurement 
should be adjusted in case of a tracking collector and in case a pyrheliometer is used to measure beam 
radiation. 
Collectors that are custom built (built -in; e.g. roof integrated collectors  that do not compose of factory made 
modules and are assembled directly on the place of in stallation) cannot be tested in their actual form for 
durability, reliability and thermal performance according to this standard. Instead, a module with the same 
structure as the ready collector may be tested. The module gross area should be at least 2 m
2. The test is 
valid only for larger collectors than the tested module. 
2   Normative references 
The following referenced documents are indispensable fo r the application of this Uganda Standard. For dated 
references, only the edition cited applies. For undated references, the latest edition of the referenced 
document (including any amendments) applies. 
ISO 15928-1, Houses — Description of performance — Part 1: Structural safety 
ISO 15928-2, Houses — Description of performance — Part 2: Struct ural serviceability 
US 854-1, Thermal solar systems and components ―Solar collectors ― Part 1: General requirements 
ISO 9488, Solar energy ― Vocabulary 
ISO 9060, Solar energy ― Specification and classification of inst ruments for measuring hemispherical solar 
and direct solar radiation 
3   Terms and definitions 
For the purposes of this Uganda Standard, the terms and definitions given in ISO 9488 apply. FDUS 854-2: 2009 
2 © UNBS 2009 – All rights reserved
 4   Symbols and units 
a1   heat loss coefficient at ( Tm - Ta)  =  0              W m2K-1 
a2   temperature dependence of the heat loss coefficient         W m-2K-2 
AA   a b s o r b e r  a r e a  o f  c o l l e c t o r                m2 
Aa   aperture area of collector               m2 
AG   g r o s s  a r e a  o f  c o l l e c t o r                m2 
AM   o p t i c a l  a i r  m a s s                  m2 
bu   collector efficiency coefficient (wind dependence)          m-1 s 
b0   constant for the calculation of the incident angle modifier 
b1   heat loss coefficient at (Tm – T a)  =  0              Wm-2K-1 
b2   c o l l e c t o r  e f f i c i e n c y  c o e f f i c i e n t               W sm-3K-1 
c1   heat loss coefficient at ( Tm – T a) =  0              Wm-2K-1 
c2   temperature dependence of the heat loss coefficient         W m-2K-2 
c3   wind speed dependence of the heat loss coefficient         Jm-3K-1 
c4   sky temperature dependence of the heat loss coefficient        Wm-2K-1 
c5   e f f e c t i v e  t h e r m a l  c a p a c i t y                J m-2K-1 
c6   wind dependence in the zero loss efficiency           sm-1 
cf   specific heat capacity of heat transfer fluid           Jkg-1K-1 
C   effective thermal capacity of collector            JK-1 
D   d a t e                    Y Y M M D D  
EL   longwave irradiance ( λ > 3μm )               W m-2 
Eβ   longwave irradiance on an inclined surface outdoors         Wm-2 
Es   longwave irradiance                Wm-2 
F  radiation view factor 
F'   collector efficiency factor 
G   h e m i s p h e r i c a l  s o l a r  i r r a d i a n c e               W m-2 
G*   global hemispherical solar irradiance           Wm-2 
G"   n e t  i r r a d i a n c e                  W m-2 
Gb   direct solar irradiance (beam irradiance)            Wm-2 
Gd   d i f f u s e  s o l a r  i r r a d i a n c e                W m-2 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  3
 LT   l o c a l  t i m e                   h  
Kθ   incidence angle modifier 
Kθb   incidence angle modifier for direct radiation 
Kθd   incidence angle modifier for diffuse radiation 
m   thermally active mass of the collector            kg 
m   mass flowrate of heat transfer fluid             kgs-1 
Q   useful power extracted from collector            W 
QL   p o w e r  l o s s  o f  c o l l e c t o r                W  
SF   safety factor 
T   t i m e                    s  
ta   ambient or surrounding air temperature            °C 
tdp   atmospheric dew point temperature             °C 
te   c o l l e c t o r  o u t l e t  ( e x i t )  t e m p e r a t u r e              ° C  
tin   c o l l e c t o r  i n l e t  t e m p e r a t u r e                ° C  
tm   mean temperature of heat transfer fluid            °C 
ts   atmospheric or sky temperature             °C 
tstg   stagnation temperature               °C 
T   absolute temperature                      K 
Ta   ambient or surrounding air temperature            °C 
T*m  reduced temperature difference ( = (tm–ta) I G * )            m2KW1 
Ts   atmospheric or equivalent sky r adiation temperature         K 
U   measured overall heat loss coefficient of collector, with reference to T*m     Wm-2K"1 
UL   overall heat loss coefficient of a collector with uniform absorber temperature    Wm K 
u   surrounding air speed                ms-1 
Vf   fluid capacity of the collector              m3 
Δp   pressure difference between fluid inlet and outlet          Pa 
Δt   time interval s 
ΔT   temperature difference between fluid outlet and inlet (te– tin)        K 
α   solar absorptance 
β   tilt angle of a plane with respect to horizontal           degrees FDUS 854-2: 2009 
4 © UNBS 2009 – All rights reserved
 γ   azimuth angle                  degrees 
ε   hemispherical emittance 
ω   s o l a r  h o u r  a n g l e                   d e g r e e s  
θ   angle of incidence                 degrees 
Φ   latitude                    degrees 
λ  wavelength μm 
η  collector efficiency, with reference to T*m 
ηo   zero-loss collector efficiency ( η at T*m = 0), reference to T*m 
σ   Stefan-Boltzmann constant               Wm-2K-4 
ρ   density of heat transfer fluid               kgm-3 
τc  c o l l e c t o r  t i m e  c o n s t a n t                 s  
τ   transmittance 
(τα)e  effective transmittance-absorptance product 
(τα)ed  effective transmittance-absorptance product for diffuse solar irradiance 
(τα)en  effective transmittance–absorptance product for direct solar radiation at normal incidence 
(τα)eθ  effective transmittance-absorptance product fo r direct solar radiation at angle of incidence θ 
NOTE 1  In the field of solar energy the symbol G is used to  denote solar irradiance, rather than the generic symbol E for irra diance. 
NOTE 2  C is often denoted ( mC)e in basic literature (see also Annex F) 
NOTE 3  For more information about thermal performance coefficients (parameters) c1 to c6, see Annex F. 
5    Reliability testing of  liquid heating collectors 
5.1    General 
The details regarding the number of collectors and se quences used to carry out the qualifications tests 
detailed in the list below (Table 1) shall be given in the report. 
For some qualification tests, a part of the collector ma y have to be tampered with in some way, for example a 
hole may have to be drilled in the back of the collector to attach a temperature sensor to the absorber. In 
these cases care should be taken to ensure that any  damage caused does not affect the results of 
subsequent qualification tests, for example by allowing water to enter into a previously rain tight collector. 
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  5
 Table 1 – Test list 
Subclause Test  
5.2 Internal pressure 
5.3 High-temperature resistance a.b 
5.4 Exposure b 
5.5 External thermal shock c 
5.6 Internal thermal shock 
5.7 Rain penetration d 
5.8 Freeze resistance e 
5.9 Mechanical load 
5.10 Impact resistance (optional test) 
6.1 – 6.2 – 6.3 Thermal performance 
a For organic absorbers, the high-temperature resistance test shall be performed first in order to determine 
the collector stagnation temperature ne eded for the internal pressure test. 
b The high temperature and ex posure test shall be carried out on the same collector 
c The external and internal thermal shock tests may be combined with the exposure test or the high-
temperature resistance test. 
d The rain penetration test shall be carried out only for glazed collectors. 
e The freeze resistance test shall be carried out onl y for collectors claimed to be freeze resistant. 
f The thermal performance test shall be carried out onl y a collector that had not been used for other tests. 
 
NOTE Regarding the durability and reliability of elastic mate rials it is recommended to refer to ISO 9808 and ISO 9553. 
5.2    Internal pressure tests for absorbers 
5.2.1  Inorganic absorbers 
5.2.1.1  Objective 
The absorber shall be pressure-tested to assess the ext ent to which it can withst and the pressures which it 
might meet in service. 
5.2.1.2  Apparatus and procedure 
The apparatus, shown in Figure A.1, consists of a hydraul ic pressure source (electrical pump or hand pump), 
a safety valve, an air-bleed valve and a pressure g auge with a standard uncertainty better than 5 %. The air 
bleed valve shall be used to empty the absorber of air before pressurization. The inorganic absorber shall be 
filled with water at room temperature and pressurized to the test pressure for the test period (see 5.2.1.3.2). 
This pressure shall be maintained while the absorber is inspected for swelling, distortion or ruptures. 
5.2.1.3  Test conditions 
5.2.1.3.1 Temperature 
Inorganic absorbers shall be pressure -tested (see 5.2.1.3.2) at ambient te mperature within the range 5 °C to 
30 °C. 
5.2.1.3.2 Pressure 
The test pressure shall be 1.5 times the maximum collec tor operating pressure specified by the manufacturer. 
The test pressure shall be maintained for 15 min. 
5.2.1.4  Results FDUS 854-2: 2009 
6 © UNBS 2009 – All rights reserved
 The collector shall be inspected for leakage, swelling an d distortion. The results of this inspection shall be 
reported together with the values of pressure and temperature used and the duration of the test. 
5.2.2  Absorbers made of organic materials (plastics or elastomers) 
5.2.2.1  Objective 
The absorber shall be pressure-tested (see 5.2.1.3.2) to assess the extent to which it can withstand the 
pressures which it might meet in service while operati ng at elevated temperature. The tests shall be carried 
out at elevated temperatures, beca use the pressure resistance of an organic absorber may be adversely 
affected as its temperature is increased. One of the methods described in 5.2.2.2.2 through 5.2.2.2.4 may be 
chosen. 
5.2.2.2  Apparatus and procedure 
5.2.2.2.1 General 
The apparatus consists of either a hydraulic or a pneumatic pressure source, and a means of heating the 
absorber to the required test temperature. 
The characteristics of a solar irradiance simulator shall be the same as those of the simulator used for 
efficiency testing of liquid heating solar collectors. 
A temperature sensor shall be attached to the absorber to monitor its temperature during the test. The sensor 
shall be positioned at two-thirds of the absorber height and half the absorber width. It shall be fixed firmly in a 
position to ensure good thermal contact with the absorber. The sensor shall be shielded from solar radiation. The test conditions specified in 5.2.2.3 shall be main tained for at least 30 min prior to test and for the full 
duration of the test. The pressure in the absorber shall be  raised in stages as specified in  5.2.2.3, and the absorber shall be 
inspected for swelling, distortion or rupture after each increase in pressure. The pressure shall be maintained 
while the absorber is being inspected. For safety reasons, the collector shall be encased in a transparent box to protec t personnel in the event of 
explosive failure during this test. 
One of the methods described in 5.2. 2.2.2 through 5.2.2.2.4 may be chosen. 
5.2.2.2.2 Organic absorbers for use in unglazed collectors (test temperature < 90 °C) Where the maximum test temperature is below 90 °C, absorbers may be submerged in a heated water bath 
and pressure-tested. The pressurized fl uid supply to the absorber shall be fitted with a safety valve, air-bleed 
valve (if required) and pressure gauge having a standard uncertainty better than 5 %. The apparatus is shown 
in Figure A.2. 
5.2.2.2.3 Organic absorbers for use with o il-based fluids (test temperature > 90 °C) 
When the test temperature exceeds 90 °C, the absorber  may be connected to a hot oil circuit. The absorber 
and hot oil circuit are then pressurized. The hot oil circui t shall be fitted with a safety valve, air-bleed valve and 
pressure gauge having a standard uncertainty better than 5 %. 
The absorber may be heated by any of the following methods: 
a)  connecting a heater in the oil circuit (see Figure A.3); 
b)  heating the whole collector in a sola r irradiance simulator (see Figure A.4); 
c)  heating the whole collector  outdoors under natural solar irradiance (see Figure A.4). FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  7
 Safety measures should be taken to protect personnel from  hot oil in the event of explosive failure during this 
test. 
5.2.2.2.4 Organic absorbers — High temperature pneumatic pressure test 
The absorber may be pressure-tested using compressed ai r, when heated by either of the following methods: 
a)  heating the whole collector in a sola r irradiance simulator (see Figure A.5); 
b)  heating the whole collector  outdoors under natural solar irradiance (see Figure A.5). 
The compressed air supply to the absorber shall be fitted with a safety valve and a pressure gauge having a 
standard uncertainty better than 5 %. 
5.2.2.3 Test conditions 5.2.2.3.1 Temperature 
For absorbers made of organic materials, the test te mperature shall be the maximum temperature which the 
absorber will reach under stagnation conditions. 
The reference conditions given in Table 2 shall be used. 
The calculations employed to determine the test temperature are included in Annex C and shall either: 
-  use measured collector performance characteristics, or 
-  extrapolate from average values, measured in the hi gh-temperature resistance test (see 5.3.3), of the 
global solar irradiance (natural or simulated) on t he collector plane, the surrounding air temperature and 
the absorber temperature. 
Table 2 – Climate reference conditions to determine test temperatures for internal pressure 
test of organic absorbers 
Climate parameter Value for all climate classes 
Global solar irradiance on collector plane, G in W/m2 1000 
Surrounding air temperature, t a in oC 30 
 
5.2.2.3.2 Pressure 
The test pressure shall be 1.5 times the maximum collec tor operating pressure specified by the manufacturer. 
For absorbers made of organic material s, the pressure shall be raised to t he test pressure in equal stages of 
20 kPa (approximately) and maintained at each intermediat e pressure for 5 min. The test pressure shall then 
be maintained for a least 1 h. 
5.2.2.4  Results 
The collector shall be inspected for leakage, swelling and distortion. The results of the inspection shall be 
reported. 
Full details of the test procedure used, including th e temperature, intermediate pressures and test periods 
used, shall be reported with the test results. 
5.3    High-temperature resistance test 
5.3.1  Objective FDUS 854-2: 2009 
8 © UNBS 2009 – All rights reserved
 This test is intended to assess rapidly whether a collect or can withstand high irradiance levels without failures 
such as glass breakage, collapse of plastic cover, melt ing of plastic absorber, or significant deposits on the 
collector cover from outgassing of collector material. 
5.3.2  Apparatus and procedure 
The collector shall be tested outdoors, or in a solar i rradiance simulator. A schema for testing is shown in 
Figure A.6. 
The characteristics of the solar irra diance simulator to be used for the hi gh-temperature resistance test shall 
be those of the solar irradiance simulator used for efficiency testing of liquid heating solar collectors. 
The collector shall be mounted outdoors or in a solar simulato r, and shall not be filled with fluid. All of the fluid 
pipes except for one shall be sealed to prevent  cooling by natural circulation of air. 
A temperature sensor shall be attached to the absorber to monitor its temperature during the test. The sensor 
shall be positioned at two-thirds of the absorber height and half the absorber width. It shall be fixed firmly in a 
position to ensure good thermal contact with the absorber. The sensor shall be shielded from solar radiation. 
NOTE 1  When testing collectors, such as  evacuated tubular collectors, for which it is not appropriate to measure the 
stagnation temperature at the absorber, the te mperature sensor should be placed at a suitable location in the collector, 
and this location should be clearly described with the test results. 
NOTE 2  In some cases, such as evacuated collectors, it ma y be difficult to attach a thermocouple to the absorber. In 
such cases, instead of attaching a thermocouple to the absorber, the testing laboratory may partially fill the absorber with 
a special fluid, seal the absorber and measure the temperat ure of the fluid directly or measure the pressure in the 
absorber. The relationship between the internal pressure in the absorber and its temperature should be known from the 
standard vapour pressure/temperature relationship for the fluid. 
NOTE 3  It is recommended to perform the determinati on of the stagnation temperature together with the high-
temperature resistance test. 
The test shall be performed for a minimum of 1 h after steady-state conditions hav e been established, and the 
collector shall be subsequently inspected fo r signs of damage as specified in 5.3.4. 
5.3.3 Test conditions 
The set of reference conditions given in Table 3 or conditions resulting in the same collector temperature 
according to Equation C.1 shall be used for all climate classes. 
Table 3 – Climate reference conditions for high-temperature resistance test 
Climate parameter Value for all climate classes 
Global solar irradiance on collector plane, G in W/m2 >1000 
Surrounding air temperature, t a in oC 20-40 
Surrounding air speed in m/s <1 
 
 5.3.4  Results 
The collector shall be inspected for degradat ion, shrinkage, outgassing and distortion . 
The results of the inspection shall be recorded together with the average values of solar irradiance (natural or 
simulated) on the collector plane, surrounding air tem perature and speed, and absorber temperature (and the 
pressure of the special fluid in the absorber, if  that method is used) recorded during the test. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  9
 5.4   Exposure test 
5.4.1     Objective 
The exposure test provides a low-cost reliability test sequence, indicating (or simulating) operating conditions 
which are likely to occur during real service and also a llows the collector to "settle", such that subsequent 
qualification tests are more likely to give repeatable results. 
5.4.2  Apparatus and procedure 
The collector shall be mounted outdoors (see Figure A.7), but not filled with fluid. All except one of the fluid 
pipes shall be sealed to prevent cooling by natural ci rculation of air. One shall be left open to permit free 
expansion of air in the absorber. 
The air temperature shall be recorded to a standard uncer tainty of 1 K and the global irradiance on the plane 
of the collector recorded using a pyranometer of class I or better in accordance with ISO 9060 . Irradiation and 
mean air temperature values shall be recorded at least every 5 min and rainfall shall be recorded daily. The 
collector shall be exposed until the test conditions have been met. 
At the end of the exposure, a visual inspection shall be made for signs of damage as specified in 5.4.4. 
5.4.3  Test conditions 
The set of reference conditions given in Table 4 shall be used. 
The collector shall be exposed until at least 30 days (w hich need not be consecut ive) have passed with the 
minimum irradiation H shown in Table 4. The irradiation is determined by recording irradiance measurements 
using a pyranometer. The collector shall also be exposed for at least 30 h to  the minimum irradiance level G given in Table 4, as 
recorded by a pyranometer, when the surrounding air temp erature is greater than the value shown in Table 4 
or conditions resulting in the same  collector temperature according to  Equation C.1. These hours shall be 
made up of periods of at least 30 min. 
NOTE In regions where these conditions cannot be met duri ng certain periods of the year , the 30-h exposure to high 
irradiance levels (Table 4) can be conducted in a solar irradiance simulator having characteristics identical to those of a 
simulator used for efficiency testing of liquid heating solar collectors. The 30-h exposure test should be conducted after the 
collector has completed at least 10 days, but no more than 15 days, of the exposure to t he minimum irradiation level 
(Table 4). 
If the external and internal thermal shock tests are co mbined with the exposure test, the first external and 
internal shocks shall be caused during the first 10 of the 30 h defined above, and t he second during the last 
10 of the 30 h. 
Table 4 – Climate reference conditions for exposure test as well as for external and internal 
thermal shock tests 
Climate parameter Value for all climate classes 
Global solar irradiance on collector plane, G in W/m2 850 
Global daily irradiation on collector plane, H in MJ/m2 14 
Surrounding air temperature, ta in oC 10 
NOTE     Values given are minimum values for testing. 
 
 
 FDUS 854-2: 2009 
10 © UNBS 2009 – All rights reserved
 5.4.4  Results 
The collector shall be inspected for damage or degradat ion. The results of the inspection shall be reported 
together with a record of the climat ic conditions during the test, including daily irradiation, surrounding air 
temperature and rain. 
5.5    External thermal shock test 
5.5.1  Objective 
Collectors may from time to time be exposed to sudde n rainstorms on hot sunny days, causing a severe 
external thermal shock. This test is intended to assess  the capability of a collector to withstand such thermal 
shocks without a failure. 
5.5.2  Apparatus and procedure 
The collector shall be mounted either outdoors or in a solar irradiance simulator, but shall not be filled with 
fluid. All except one of the fluid pipes shall be sealed to prevent cooling by natural circulation of air. One shall 
be left open to permit free expansion of air in the absorber (see Figure A.8). 
A temperature sensor may be optionally attached to the absorber to monitor its temperature during the test. 
The sensor shall be positioned at two-thirds of the absorber height and half the absorber width. It shall be 
fixed firmly in a position to ensure good thermal contac t with the absorber. The sensor shall be shielded from 
solar radiation. 
NOTE 1 When testing collectors, such as evacuated tubular co llectors, for which it is not appropriate to measure the 
stagnation temperature at the absorber, the te mperature sensor should be placed at a suitable location in the collector, 
and this location should be clearly described with the test results. 
NOTE 2 In some cases, such as evacuated collectors, it may be difficult to attach a thermocouple to th e absorber. In such 
cases, instead of attaching a thermocouple  to the absorber, the testin g laboratory may partially fill the absorber with a 
special fluid, seal the absorber and measure the pressure in the absorber. The relationship between the internal pressure 
in the absorber and its temperature should be known from the standard vapour pressure/temperature relationship for the 
fluid. 
An array of water jets shall be arranged to prov ide a uniform spray of water over the collector. 
The collector shall be maintained under a high level of solar irradiance for a period of 1 h before the water 
spray is turned on. It is then cooled by the water spray for 15 min before being inspected. 
The collector shall be subjected to two external thermal shocks. 
5.5.3  Test conditions 
The set of reference conditions given in Table 4 sha ll be used. The specified operating conditions shall be: 
-  solar (or simulated solar) irradiance G greater than the value shown in Table 4. 
-  surrounding air temperature ta greater than the value shown in Tabl e 4. Or conditions resulting in the 
same collector temperature according to Equation C.1 
The water spray shall have a temperature of less than 25  °C and a flow rate in the range 0.03 kg/s to 0.05 
kg/s per square meter of collector aperture. 
If the temperature of the water which first cools the colle ctor is likely to be greater than 25 °C (for example if 
the water has been sitting in a pipe in the sun for some  time), then the water shall be diverted until it has 
reached a temperature of less than 25 °C before being directed over the collector. 
5.5.4  Results FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  11
 The collector shall be inspected for any cracking, distortion, condensation, water penetration or loss of 
vacuum. The results of the inspection shall be reported. The measured values of solar irradiance, surrounding 
air temperature, absorber temperature (if measured), water temperature and water flow rate shall also be 
reported. 
5.6    Internal thermal shock test 
5.6.1  Objective 
Collectors may from time to time be exposed to a sudden intake of cold heat transfer fluid on hot sunny days, 
causing a severe internal thermal shock, for example,  after a period of shutdown, when the installation is 
brought back into operation while the collector is at its stagnation temperature. This test is intended to the 
capability of a collector to withstand such thermal shocks without failure. 
5.6.2  Apparatus and procedure 
The collector shall be mounted either outdoors or in a so lar irradiance simulator (see Figure A.9), but shall not 
be filled with fluid. One of its fluid pipes shall be connec ted via a shutoff valve to the heat transfer fluid source 
and the other shall be left open initially to permit the free ex pansion of air in the absorber and also to permit 
the heat transfer fluid to leave the absorber (and be colle cted). If the collector has more than two fluid pipes, 
the remaining openings shall be sealed in a way that ensur es the designed flow pattern within the collector. 
A temperature sensor may be optionally attached to the absorber to monitor its temperature during the test. 
The sensor shall be positioned at two-thirds of the absorber height and half the absorber width. It shall be 
fixed firmly in a position to ensure good thermal contac t with the absorber. The sensor shall be shielded from 
solar radiation. 
NOTE 1  When testing collectors, such as  evacuated tubular collectors, for which it is not appropriate to measure the 
stagnation temperature at the absorber, the te mperature sensor should be placed at a suitable location in the collector, 
and this location should be clearly described with the test results. 
NOTE 2  In some cases, such as evacuated collectors, it ma y be difficult to attach a thermocouple to the absorber. In 
such cases, instead of attaching a thermocouple to the absorber, the testing laboratory may partially fill the absorber with 
a special fluid, seal the absorber a nd measure the pressure in the absorber. The relationship between the internal 
pressure in the absorber and its temperature should be known from the standard vapour pressure/temperature 
relationship for the fluid. 
The collector shall be maintained under a high level of sola r irradiance for a period of 1 h before it is cooled by 
supplying it with heat transfer fluid for at least 5 min or until the absorber temperature drops below 50 °C. 
The collector shall be subjected to two internal thermal shocks. 
5.6.3  Test conditions 
The set of reference conditions given in Table 4 shall be used. 
The specified operating conditions shall be: 
-  solar (or simulated solar) irradiance G greater than the value shown in Table 4; 
-  surrounding air temperature ta greater than the value shown in Table 4. 
or conditions resulting in the same collector temperature according to Equation C.1 
The heat transfer fluid shall have a temperature of less  than 25 °C. The recommended fluid flow rate is atleast 
0.02 kg/s per square meter of collector aperture (unless otherwise specified by the manufacturer). 
5.6.4  Results FDUS 854-2: 2009 
12 © UNBS 2009 – All rights reserved
 The collector shall be inspected for any cracking, distor tion, deformation, water penetration or loss of vacuum. 
The results of the inspection shall be reported. The m easured values of solar irradiance, surrounding air 
temperature, absorber temperature (if measured), heat transfer fluid temperature and heat transfer fluid flow 
rate shall also be reported. 
5.7    Rain penetration test 
5.7.1  Objective 
This test is applicable only for glazed collectors and is  intended to assess the extent to which glazed collectors 
are substantially resistant to rain penetration. They shall normally not permit the entry of either freefalling rain 
or driving rain. Collectors may have ventilation holes and drain holes, but these shall not permit the entry of 
drifting rain. 
5.7.2  Apparatus and procedure 5.7.2.1  General 
The collector shall have its fluid inlet and outlet pipes  sealed (unless hot water is circulated through the 
absorber, see 5.7.2.2), as shown in Figure A.10, and be placed in a test rig at the shallowest angle to the 
horizontal recommended by the manufacturer. If this angle is not specified, then t he collector shall be placed 
at a tilt of 30° to the horizontal. Collectors designed to be integrated into a roof structure shall be mounted in a 
simulated roof and have their underside protected. Ot her collectors shall be m ounted in a conventional 
manner on an open frame or a simulated roof. 
The collector shall be sprayed on exposed sides, using spray nozzles or showers. 
5.7.2.2  Detection of ingress of water The collector shall be mounted and sprayed as explained above while the absorber in the collector is kept 
warm (minimum 50 °C). This can be done either by circulating hot water at about 50 °C through the absorber 
or by exposing the collector to solar radiation. The pe netration of water into the collector shall be determined 
by inspection (looking for water droplets, condensation on the cover glass or other visible signs) and by one of 
the following methods: 
a)  by weighing the collector (standard uncertainty better than 5 g/m
2 collector area); or 
b)  by means of humidity measurement (standard uncertainty better than 5 %) or 
c)  by means of measuring the condensation level. 
The heating up of the collector should be started before t he spraying of the water in order to ensure that the 
collector box is dry before testing. 
In cases of collectors having wood in the backs (or ot her special cases), the laboratory shall take all 
necessary measures during the conducti on of the test so that the final resu lt will not be influenced or altered 
by the special construction of the collector. 
5.7.3  Test conditions 
The collector shall be sprayed with water at a temperature lower than 30 °C with a flow rate of more than 0.05 
kg/s per square meter of sprayed area. The duration of the test shall be 4 h. 
5.7.3.1  Weighing method 
If the weighing method is chosen, the collector shall be put on the scale before the start of the test on three 
consecutive occasions. The weights record ed shall not vary by more than ± 5 g/m2 collector area. 
5.7.3.2  Humidity measurement method FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  13
 When measuring the penetration of water into the co llector by means of humidity measurement, an absolute 
humidity sensor is placed in the air gap between absorber and glazing. Collector and sensor are connected to 
a hot fluid loop for at least five hours before the rain  is switched on in order to stabilize. When testing 
outdoors, in order to minimize disturbances of t he measurement, the collector shall be shaded during the 
raining. Ingress of water might also be detected at a later stage, during the "Final Inspection" (5.11). 
5.7.3.3  Condensation level method 
If the condensation level method is chosen, the penetration of water is determined by measuring the 
condensation level on the cover glass and by measuring the water that comes out of the collector when 
tipping it. 
The heating of the collector shall be started at least 30 min before the spraying of water and shall continue 
until it can be ensured that the collector box is dry befor e testing. This shall be done by circulating hot water 
(or other fluid) above 50 °C through the absorber before but also during the complete test. The water will 
thereafter condense on the inside of the glazing, which is being cooled by cold water on the outside. After 2 h 
an intermediate inspection of condensation on the cover glass shall be done in order to facilitate the reporting of the places where water penetrates. 
After finishing the spraying the inspection of condensat ion should be done after a short time for ventilating, in 
order to distinguish collectors with good ventilation qua lifications that are without accumulation of humidity 
inside the collector. However, the inspection should be done within one minute after finishing the spraying 
before the collector will make  any temperature changes. 
5.7.4  Results 
The collector shall be inspected for water penetration. The results of the inspection, i. e. the extension of water 
penetration and the places where water penetrated shall be reported. 
5.8    Freeze resistance test 
5.8.1  Objective 
This test is intended to assess the extent to whic h water heating collectors which are claimed to be freeze 
resistant can withstand freezing, and fr eeze/thaw cycling. This test is not  intended for use with collectors for 
which it is clearly stated in the installation manual th at they may only be used with an antifreeze fluid. Two test 
procedures are recommended: 
-  one for collectors which are claimed to be freeze-resistant when filled with water, and 
-  one for collectors which are claimed to resist freezing after being drained. 
5.8.2  Apparatus and procedure 
5.8.2.1  Freeze-resistant collectors 
For collectors which are claimed to be able to withstand freezing, the collector shall be mounted in a cold 
chamber (see Figure A.11). The collector shall be fitted correctly, shut completely and inclined at the 
shallowest angle to the horizontal recommended by the manufacturer. If no angle is specified by the 
manufacturer, then the collector shall be inclined at an angle of 30° to the horizont al. Unglazed collectors shall 
be tested in a horizontal position unless this is excluded by the manufacturer. Next, the collector shall be filled 
with water at the operating pressure. 
The cold-chamber temperature shall be cycled, and at t he end of each cycle the collector shall be refilled with 
water at operating pressure. 
The temperature of the water shall be monitored throughout the test. 
5.8.2.2  Collectors with drain-down protection FDUS 854-2: 2009 
14 © UNBS 2009 – All rights reserved
 For collectors which employ a drain-down system to prot ect them from freezing damage, the collector shall be 
mounted in a cold chamber (see Figure A.11). The collector shall be then inclined at the shallowest angle to 
the horizontal recommended by the manufacturer. If no angle is specified by the manufacturer, then the 
collector shall be inclined at an angle of 30° to t he horizontal. Unglazed collectors shall be tested in a 
horizontal position unless this is excluded by the manufac turer. The collector shall be next filled with water, 
kept at operating pressure for 10 min and then drain ed using the device installed by the manufacturer. 
If, 5 min after the start of the drain process, the amount  of drained water from t he collector corresponds to 
approximately 95 % of the total water which filled that colle ctor, then in that case, no test in cold chamber is 
required. 
The cold-chamber temperature shall be cycled. The temperature shall be measured insi de the absorber close to the inlet. 
After the last cycle the collector shall be refilled with water at operating pressure. 
5.8.3  Test conditions 
The contents of the absorber shall be maintained at (-20 ± 2) °C for at least 30 min during the freezing part of 
the cycle, and are raised to above 10 °C  during the thawing part of the cycl e. The duration of the thawing part 
of the cycle shall be at least 30 min. 
The collector shall be subjected to three freeze-thaw cycles. 
5.8.4  Results 
The number of cycles carried out sh all be reported. The collector shall be  inspected for leakage, breakage, 
distortion and deformation. These shall be reported toget her with the absorber temperatures reached during 
the cycles and the times spent  by the collector at the test temperatur es. The tilt angle used for the test shall 
also be reported. 
5.9    Mechanical load test 
5.9.1  Positive pressure test of the collector 
5.9.1.1  Objective This test is intended to assess the extent to which th e transparent cover of the collector and the collector box 
are able to resist the positive pressure load due to the effect of wind and snow. 
5.9.1.2  Apparatus and procedure 
The collector shall be placed horizontally on an even gr ound. On the collector a foil shall be laid and on the 
collector frame a wooden or metallic frame shall be placed, high enough to contain the required amount of 
gravel or similar material (see Figure A.12). 
The gravel, preferably type 2-32 mm, shall be weighed in portions and distributed in the frame so that 
everywhere the same load is creat ed (pay attention to the bending of the glass), until the desired height is 
reached. 
The test can also be carried out installing the collector  in accordance with 5.9.2.2 and loading the cover using 
suction cups, gravel or other suitable means (e.g. water). As a further alternative, the necessary load may be cr eated by applying an air pressure on the collector cover. 
The load may also be created by applying a negative pressure on the collector cover. In this case, apparatus 
in accordance to ISO 6612 can be used. However this  method cannot be applied on all collector types. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  15
 5.9.1.3  Test conditions 
The test pressure shall be increased at maximum steps of 250 Pa until a failure occurs or up to the value 
specified by the manufacturer. The test pressure shall be at least 1000 Pa. A failure can be the destruction of 
the cover and also the permanent deformati on of the collector box or the fixings. 
NOTE  A permanent deformation should be assigned to a load  value, while it is completely relieved after every load 
increment of 250 Pa and the distortion is measured compared to the beginning of  the test sequence. The value of an 
inadmissible permanent deformation amounts to max. 0.5 %. (Example: 10 mm distortions at 2 m length of collector 
5.9.1.4  Results 
The pressure at which any failure of the collector cover or the box or fixings occurs shall be reported together 
with details of the failure. If no failure occurs, then the maximum pressure which the collector sustained shall 
be reported. 
The maximum positive pressure is the pressure reache d before occurring a failure. The permissible positive 
pressure is the maximum pressure divided by the safety factor SF+ = 1.5: 
Fpem+ = F max+ / SF+ with SF+ = 1.5 
NOTE  When the test is done with an on-roof mounting system the test results are also valid for the roof integrated mounting sy stem. 
5.9.2  Negative pressure test of the collector 
5.9.2.1  Objective 
This test is intended to assess the extent to which t he fixings between the collector cover and collector box 
are able to resist uplift forces caused by the wind. 
For the design of the static of  the mounting system the requireme nts of ISO 15928 have to be applied. 
5.9.2.2  Apparatus and procedure 
The collector shall be installed horizontally on a stiff fr ame by means of its mounting fixtures. The frame which 
secures the cover to the collector box shall not be restricted in any way. 
A lifting force which is equivalent to the specified negative pressure load shall be applied evenly over the 
cover. The load shall be increased in steps up to the fi nal test pressure. If the cover has not been loosened at 
the final pressure, then the pressure may be stepped up until failure occurs. The time between each pressure 
step shall be the time needed for the pressure to stabilize. 
Either of two alternative methods may be used to apply pressure to the cover: 
-  Method (a): The load may be applied to the collector  cover by means of a uniformly distributed set of 
suction cups (see Figure A.13). 
-  Method (b): For collectors which have an almost ai rtight collector box, the following procedure may be 
used to create a negative pressure on the cover (see Figure A.14). Two holes are made through the 
collector box into the air gap between the collector cover and absorber, and an air source and pressure 
gauge are connected to the collector air gap through these holes. A negative pressure on the cover is 
created by pressurizing the collector box. For safe ty reasons the collector shall be encased in a 
transparent box to protect personnel in the event of failure during this test. 
During the test, the collector shall be visually inspec ted and any deformations of the cover and its fixings 
reported. The collector shall be examined at the end of the test to see if  there are any permanent 
deformations. 
5.9.2.3  Test conditions FDUS 854-2: 2009 
16 © UNBS 2009 – All rights reserved
 The test pressure shall be increased in steps of 250 Pa until a failure occurs or up the value specified by the 
manufacturer. The test pressure shall be at least 1000 Pa. A failure can be the destruction of the cover and 
also the permanent deformation of the collector box or the fixings. 
NOTE  A permanent deformation should be assigned to a load  value, while it is completely relieved after every load 
increment of 250 Pa and the distortion is measured compared to the beginning of  the test sequence. The value of an 
inadmissible permanent deformation amounts to max. 0.5 %.  (Example: 10 mm distortions at 2 m length of collector 
frame). 
5.9.2.4  Results 
The pressure at which any failure of the collector cover or the box or fixings occurs shall be reported together 
with details of the failure. If no failure occurs, then the maximum pressure which the collector sustained shall 
be reported. 
The maximum negative pressure is the pressure reache d before occurring a failure. The permissible negative 
pressure is the maximum pressure divided by the safety factor SF- = 2: 
Fpem- = F max- / SF- with SF- = 2 
5.10    Impact resistance test (optional) 
5.10.1  Objective 
This test is intended to assess the extent to whic h a collector can withstand the effects of heavy impacts 
caused by hailstones. 
5.10.2  Apparatus and procedure 
5.10.2.1 General The testing of the solar collector to determine its impact  resistance can be done by one of two methods, i.e. by 
using steel balls or ice balls. 
5.10.2.2 Method 1 
The collector shall be mounted either vertically or ho rizontally on a support (see Figure A.15). The support 
may be stiff enough so that there is negligible di stortion or deflection at the time of impact. 
Steel balls shall be used to simulate a heavy impact. If the collector is mounted horizontally then the steel 
balls are dropped vertically, or if it is mounted vertically then the impacts are directed horizontally by means of 
a pendulum. In both cases, the height of the fall is the vertical distance between the point of release and the 
horizontal plane containing the point of impact. 
The point of impact shall be no more than 5 cm from t he edge of the collector cover, and no more than 10 cm 
from the corner of the coll ector cover, but it shall be moved by seve ral millimeters each time the steel ball is 
dropped. A steel ball shall be dropped onto the collector 10 times from the first test height, and then 10 times 
from the second test height, etc. until the maximum test  height is reached (as specified by the manufacturer). 
The test is to be stopped when the collector sustains  some damage or when the collector has survived the 
impact of steel balls at the maximum test height. 
NOTE  This method does not correspond to the physical effe ct of hailstones as the deformation energy absorbed by the 
ice particles is not being considered. 
5.10.2.3 Method 2 
The apparatus consists of the following equipment: 
a)  Moulds of suitable material for casting spher ical ice balls of the required diameter (25 mm). FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  17
 b)  A freezer, controlled at -10 °C ± 5 °C. 
c)  A storage container for storing the ice balls at a temperature of -4 °C ± 2 °C. 
d)  A launcher capable of propelling an ice ball at the specified velocity (as specified by the manufacturer), 
within ± 5 %, so as to hit the collector within the sp ecified impact location. The path of the ice ball from the 
launcher to the collector may be horizontal,  vertical or at any intermediate angle. 
e)  A rigid frame for supporting the collector, with t he impact surface normal to the path of the projected ice 
ball; the support shall be stiff enough so that there is negligible distortion or deflection at the time of 
impact. 
f)  A balance for determining the mass of an ice ball to a standard uncertainty of ± 2 %. 
g)  An instrument for measuring the velocity of the ice ball to a standard uncertainty of ± 2 ms-1. The velocity 
sensor shall be no more than 1 m from the surface of the collector. 
As an example, Figure A.16 shows in schematic form a suitable apparatus comprising a horizontal pneumatic 
launcher, a vertical collector support and a velocity me ter which measures electronically the time it takes the 
ice ball to traverse the distance between two light beams. 
The testing procedure shall be the following: a)  Using the moulds and the freezer, make sufficient  ice balls of the required size for the test, including 
some for the preliminary adjustment of the launcher. 
b)  Examine each one for cracks, size and mass. An  acceptable ball shall meet the following criteria: 
-  no cracks visible to the unaided eye; 
- diameter within ±5 % of the ball (25 mm); 
-  mass within ±5 % of the ball (25 mm). 
c)  Place the balls in the storage container and leave them there for at least 1 h before use. 
d)  Ensure that all surfaces of the launcher like ly to be in contact with the ice balls are near room 
temperature. 
e)  Fire a number of trial shots at a simulated target in  accordance with step g) be low and adjust the launcher 
until the velocity of the ice ball, as measured with the velocity sensor in the prescribed position, is within 
±5 % of the required hailstone test velocity. 
f)  Install the collector at room temperature in t he prescribed mount, with the impact surface normal to the 
path of the ice ball. 
g)  Take an ice ball from the storage container and plac e it in the launcher. Take aim at the impact location 
and fire. The time between the removal of the ice ball from the container and impact on the collector shall 
not exceed 60 s. 
The point of impact shall be no more than 5 cm from t he edge of the collector cover, and no more than 10 cm 
from the corner of the coll ector cover, but it shall be moved by se veral millimeters each time the ice ball is 
launched. 
An ice ball shall be launched onto the collector 10 times; the test shall be stopped when the collector sustains 
some damage or when the collector has survived the impact of 10 ice balls. 5.10.3  Test conditions FDUS 854-2: 2009 
18 © UNBS 2009 – All rights reserved
 If the test is conducted according to method 1, t he steel ball shall have a mass of 150 g ± 10 g and the 
following series of test heights shall be used: 0.4 m, 0.6 m, 0.8 m, 1.0 m, 1.2 m, 1. 4 m, 1.6 m, 1.8 m and 2.0 m 
If the test is conducted according to method 2, the ice ball shall have a diameter of 25 mm ± 5 %, a mass of 
7.53 g ± 5 % and its velocity shall be 23 m/s ± 5 %. 
5.10.4  Results 
The collector shall be inspected for damage. The results of the inspection shall be r eported, together with the 
height from which the steel ball was dropped (if method 1 is used) and the number of impacts which caused 
the damage. 
NOTE  As test method 2 is closer to rea lity, this method (5.10.2.3) is preferable . 
5.11    Final inspection 
When the full tests have been completed, the collector  shall be dismantled and inspected. All abnormalities 
hall be reported and accompanied by a photograph. 
5.12    Test report 
The format sheets given in Annex B shall be completed for each test, together with the introductory format 
sheet (B.1) reporting a summary of main results, including the test methods. 
6    Thermal performance testi ng of liquid heating collectors 
The thermal performance of glazed solar collectors shall be tested according to either 6.1 or 6.3. 
6.1    Glazed solar collectors under steady state conditions (including pressure drop) 
6.1.1  Collector mounting and location 
6.1.1.1  General 
The way in which a collector is mounted will influence t he results of thermal performa nce tests. Collectors to 
be tested shall therefore be mounted in accordance with 6. 1.1.2 to 6.1.1.8. Full-size collector modules shall be 
tested, because the edge losses of small collectors may significantly reduce t heir overall performance. 
6.1.1.2  Collector mounting frame 
The collector mounting frame shall in no way obstruct t he aperture of the collector, and shall not significantly 
affect the back or side insulation. Unless otherwise spec ified (for example, when the collector is part of an 
integrated roof array), an open mounting structure shall be  used which allows air to circulate freely around the 
front and back of the collector. The collector shall be m ounted such that the lower edge is not less than 0.5 m 
above the local ground surface. Currents of  warm air, such as those which rise  up the walls of a building, shall 
not be allowed to pass over the collector. Where collector s are tested on the roof of a building, they shall be 
located at least 2 m away from the roof edge. 
6.1.1.3  Tilt angle 
The collector shall be tested at tilt angles such that th e incidence angle modifier for the collector varies by no 
more than ±2 % from its value at normal incidence. Fo r single glazed flat plate co llectors, this condition will 
usually be satisfied if the angle of incide nce of direct solar radiation at the collector aperture is less than 20°. 
NOTE  For many collectors, the influenc e of tilt angle is small, but it can be an important variable for specialized 
collectors such as those incorporating heat pipes. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  19
 6.1.1.4  Collector orientation outdoors 
The collector may be mounted outdoors in a fixed position facing the equator, but this will result in the time 
available for testing being restricted by the acceptance range of incide nce angles. A more versatile approach 
is to move the collector to follow the sun in  azimuth, using manual or automatic tracking. 
6.1.1.5  Shading from direct solar irradiance 
The location of the test stand shall be such that no shadow is cast on the collector during the test. 
6.1.1.6  Diffuse and re flected solar irradiance 
For the purposes of analysis of outdoor test results, so lar irradiance not coming directly from the sun's disc is 
assumed to come isotropically from the hem ispherical field of view of the collector. 
In order to minimize the errors resulting from this app roximation, the collector sha ll be located where there will 
be no significant solar radiation reflected onto it from su rrounding buildings or surfaces during the tests, and 
where there will be no significant obstructions in the field of view. With some collector types, such as 
evacuated tubular collectors, it may be equally important to minimize reflections on both the back and the front 
fields of view. 
Not more than 5 % of the collector's field of view sha ll be obstructed, and it is particularly important to avoid 
buildings or large obstructions subtending an angle of greater than approximately 15° to the horizontal in front 
of the collectors. The reflectance of most rough surfaces  such as grass, weathered concrete or chippings is 
usually low enough so no problem is caused during collector testing. Surfaces to be avoided in the collector's field of view include large expanses of glass, metal or water. In most 
solar simulators the simulated beam approximates direct solar irradiance only. In order to simplify the 
measurement of simulated irradiance, it is necessary to  minimize reflected irradiance. This can be achieved 
by painting all surfaces in the test chamber with a dark (low reflectance) paint. 6.1.1.7  Thermal irradiance 
The performance of some collectors is particularly sensitive to the levels of thermal irradiance. The 
temperature of surfaces adjacent to the collector shall be as close as possible to that of the ambient air in 
order to minimize the influence of thermal radiation. Fo r example, the outdoor field of view of the collector 
shall not include chimneys, cooling towers or hot exhausts . For indoor and simulator testing, the collector shall 
be shielded from hot surfaces such as radiators, air-conditioning ducts and machinery, and from cold surfaces 
such as windows and external walls. Shielding is im portant both in front of and behind the collector. 
6.1.1.8  Airspeed The performance of many collectors is  sensitive to air speeds. In order to maximize the reproducibility of 
results, collectors shall be mounted such that air ca n freely pass over the aperture, back and sides of the 
collector. The mean air speed, parallel to the collecto r aperture, shall be between the limits specified in 
6.1.4.3. Where necessary, artificial wind generators shall be used to achieve these air speeds. Collectors 
designed for integration into a roof may have their backs protected from the wind; if so, this shall be reported 
with the test results. 
6.1.2    Instrumentation 6.1.2.1    Solar radiation measurement 6.1.2.1.1 Pyranometer 
6.1.2.1.1.1 General A class I or better, as specified in ISO 9060, pyranom eter shall be used to measure the global short-wave 
radiation from both the sun and the sky. The recommended practice for use given in ISO/TR 9901 should be FDUS 854-2: 2009 
20 © UNBS 2009 – All rights reserved
 observed. Before each test the pyranometer should be checked for dust, soiling etc. on the outer dome and it 
should be cleaned if necessary. 
A class I or better pyranometer equipped with a shading ring or alternatively a pyrheliometer together with a 
pyranometer shall be used to measure the diffuse short-wave radiation. 
6.1.2.1.1.2 Precautions for effects of temperature gradient 
The pyranometer used during the test(s) shall be placed in  a typical test position and allowed to equilibrate for 
at least 30 min before data-taking commences. 
6.1.2.1.1.3 Precautions for effect s of humidity and moisture 
The pyranometer shall be provided with a means of pr eventing accumulation of moisture that may condense 
on surfaces within the instrument and affect its reading. An instrument with a desiccator that can be inspected 
is required. The condition of the desiccato r shall be observed on a regular basis. 
6.1.2.1.1.5 Precautions for infrared radi ation effects on pyranometer accuracy 
Pyranometers used to measure the irradi ance of the solar irradiance simulator shall be mounted in such a way 
as to minimize the effects on its readings of  the infrared radiation of wavelength above 3 μm from the 
simulator light source. 6.1.2.1.1.5 Mounting of pyranometers outdoors 
The pyranometer shall be mounted such that  its sensor is coplanar, within a tolerance of ±1° with the plane of 
the collector aperture. It shall not cast a shadow onto the collector aperture at any ti me during the test period. 
The pyranometer shall be mounted so as to receive the same levels of direct, diffuse and reflected solar 
radiation as are received by the collector. 
NOTE  For angles of incidence of 50°, a deviation of ±1° l eads to an error of 2 % when measuring the solar irradiance. 
For outdoor testing, the pyranometer shall be mount ed at the midheight of the collector. The body of the 
pyranometer and the emerging leads of the connector shall be shielded to minimize solar heating of the 
electrical connections. Care shall also be taken to mi nimize energy reflected and reradiated from the solar 
collector onto the pyranometer. 
6.1.2.1.1.6 Use of pyranometers in  solar irradiance simulators 
Pyranometers may be used to measure the distribution of  simulated solar irradiance over the collector 
aperture and the variation in simulated irradiance with time  (see 6.1.5.6). Alternatively, other types of radiation 
detector may be used, provided that they have been eval uated and calibrated for simulated solar radiation in 
question. 
The pyranometer(s) shall be mo unted such that its sensor(s) is (are) coplanar, with a tolerance of ±1 ° with the 
plane of the collector aperture. The pyranometer(s) shall not cast a shadow onto the collector aperture at any 
time during the test period. The pyranometer(s) shall be mounted so as to receive the same levels of indirect, 
diffuse and reflected solar radiati on as are received by the collector. 
The method and equipment used to measure any variation in simulated irradiance with time during the test 
period shall be well proven to give the required accuracy. 
NOTE  E.g. for the most commonly used type of solar simulator, using an electrical arc in a sealed beam parabolic 
reflector as the light source, the mounting of a pyranometer in a typical test position at the midheight of the collector as 
described for outdoor testing, will not be adequate. Especially , when the lamp array is powered from unstabilized mains 
power supply and from 3 different phases. For solar simulators of this kind not equipped with stabilized power supply, 
integration of power supply voltage dur ing each test period is recommended. Solar simulators equipped with stabilized 
power supply, capable of stabilizing the power supply voltage wi thin ±0.5 %, normally will need no integration of simulated 
irradiance during (each) test period. For most types of solar simulators, the rela tionship between the spatially integrated FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  21
 value of the simulated solar irradiance and the integrated power supply voltage during each test period, can be found. In 
all cases, sufficient knowledge of burn in and life time  characteristics of the lamp type used, is assumed. 
6.1.2.1.2 Measurement of the angle of incidence of direct solar radiation 
A simple device for measuring the angle of incidence of direct solar radiation can be produced by mounting a 
pointer normal to a flat plate on which graduated concentri c rings are marked. The length of the shadow cast 
by the pointer may be measured using the concentric ring s and used to determine the angle of incidence. The 
device shall be positioned in the collector plane and to one side of the collector. 
6.1.2.2  Thermal radiation measurement 6.1.2.2.1 Measurement of thermal irradiance outdoors 
The variations of thermal irradianc e outdoors are not normally taken into  account for collector testing. 
However, a pyrgeometer may be mounted in the plane of  the collector aperture and to one side at midheight, 
to determine the thermal irradian ce at the collector aperture. 
6.1.2.2.2 Determination of thermal irradiance indoors and in solar simulators 
6.1.2.2.2.1 Measurement 
The thermal irradiance may be measured using a py rgeometer as indicated in 6.1.2.2.1 for outdoor 
measurements. Pyrgeometer shall be well ventilated in order to minimize the influence of solar or simulated 
solar irradiance. For indoor testing, the thermal irradi ance shall be determined to a standard uncertainty of 
10Wm-2 
6A.2.2.2.2 Calculation 
Provided that all sources and sinks of t hermal radiation in the field of view  of the collector can be identified, 
the thermal irradiance at the collector aperture may be calculated using temperature measurements, surface 
emittance measurements and radiation view factors. The thermal irradiance incident on a collector surface 
(designated 1), from a hotter surface (designated 2) is given by 
    
σε2F12T24                  ( 1 )  
Or, more usefully, the additional thermal irradiance (com pared with that which woul d be present if surface 2 
had been a perfect black body at ambient temperature) is given by 
    σε2F12 (T24-Ta4)                 ( 2 )  
Radiation view factors are given in te xtbooks on radiation heat transfer. The thermal irradiance at the collector 
aperture may also be calculated from a series of measurements made for small solid angles in the field of 
view. Such measurements can be made using a pyrheliome ter with and without a glass filter to identify the 
thermal component of the global irradiance. 
6.1.2.3  Temperature measurements 
6.1.2.3.1 General 
Three temperature measurements are r equired for solar collector testing. These are the fluid temperature at 
the collector inlet, the fluid temperature at the collector  outlet, and the ambient air temperature. The required 
accuracy and the environment for these measurem ents differ, and hence the sensor for temperature 
measurement and associated equipment may be different. 
6.1.2.3.2 Measurement of heat tr ansfer fluid inlet temperature (tin) 
6.1.2.3.2.1 Required accuracy FDUS 854-2: 2009 
22 © UNBS 2009 – All rights reserved
 The temperature of the heat transfer fluid at the collecto r inlet shall be measured to a standard uncertainty of 
0.1 K, but in order to check that the temperature is not drifting with time, a very much better resolution of the 
temperature ‘signal to ±0.02 K is required. 
6.1.2.3.2.2 Mounting of sensors 
The sensor for temperature measurem ent shall be mounted at no more than 200 mm from the collector inlet, 
and insulation shall be placed around the pipework both upstream and downstream of the sensor. If it is 
necessary to position the sensor more than 200 mm aw ay from the collector, then a test shall be made to 
verify that the measurement of fluid temperature is not a ffected. To ensure mixing of the fluid at the position of 
temperature measurement, a bend in the pipework, an orifice or a fluid-mixing device shall be placed 
upstream of the sensor, and the sensor probe shall point  upstream and in a pipe where the flow is rising (to 
prevent air from being trapped near the sensor), as shown in Figure 1. 
DIAGRAM 
 
 
  
 
  
 
  
 
Figure 1 – Recommended transducer positions for measu ring the heat transfer fluid inlet and outlet 
temperatures 
6.1.2.3.3 Determination of heat tran sfer fluid temperature difference ( Δ T) 
The difference between the collector outlet and inlet temperatures ( ΔT) shall be determined to a standard 
uncertainty of <0.05 K. Standard uncertainties approach ing 0.02 K can be achieved with modern well-matched 
and calibrated transducers, and hence it is possible to measure heat transfer fluid temperature differences of 
1 K or 2 K with a reasonable accuracy. Delta-T sensors shall be calibrated in the relevant temperature range. 
6.1.2.3.4 Measurement of surrounding air temperature ( t
a) 
6.1.2.3.4.1 Required accuracy 
The ambient or surrounding air temperature shall be measured to a standard uncertainty of 0.5 K. 
6.1.2 .3.4.2 Mounting of sensors 
For outdoor measurements the sensor shall be shaded from  direct and reflected solar radiation by means of a 
white-painted, well-ventilated shelter, preferably with forced ventilation. T he shelter itself shall be shaded and FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  23
 placed at the mid height of the collector but at least 1 m above the local ground surface to ensure that it is 
removed from the influence of ground heating. 
The shelter shall be positioned to one side of the collector and not more than 10 m from it. If air is forced over 
the collector by a wind generator, the air temperature shall be measured in the outlet of the wind generator 
and checks made to ensure that this temperature does no t deviate from the ambient air temperature by more 
than ±1 K. 
6.1.2.4  Measurement of collector liquid flowrate 
Mass flowrates may be measured directly or, alternativ ely, if the density is known, they may be determined 
from measurements of volumetric flowrate and temperature. The standard uncertainty of the liquid flowrate 
measurement shall be within ±1 % of the measured value, in mass per unit time. The flow meter shall be calibrated over the range of fluid flowrates and temp eratures to be used during collector testing. 
NOTE  The temperature of the fluid in volumetric flow mete rs should be known with sufficient accuracy to ensure that 
mass flowrates can be determined to within the limit s specified by the manufacturer. 
6.1.2.5  Measurement of air speed 
6.1.2.5.1 General 
The heat losses from a collector increase with increasi ng air speed over the collector, but the influence of air 
speed direction is not well understood. Measurements of air speed direction are therefore not used for 
collector testing. 
The relationship between the meteorological wind speed and the air speed over the collector depends on the 
location of the test facility, so meteorological wind s peed is not a useful parameter for collector testing. By 
using the air speed measured over the collector, it is pos sible to define clearly the conditions in which the tests 
were performed. 
6.1.2.5.2 Required accuracy 
The speed of the surrounding air ov er the front surface of the collect or shall be measured to a standard 
uncertainty of 0.5 ms-1 for both indoor and outdoor testing. Under outdoor conditions the surrounding air 
speed is seldom constant, and gusting frequently oc curs. The measurement of an average air speed is 
therefore required during the test period. This may be obtained either by an arithm etic average of sampled 
values or by a time integration over the test period. 
6.1.2.5.3 Mounting of sensors for the measurement of air velocity over the collector 
During indoor testing, the air speed may vary from one end of the collector to the other. A series of air speed 
measurements shall therefore be taken, at a distance of 10 mm to 50 mm in front of the collector aperture, at 
equally spaced positions over the collector area. 
An average value shall then be determined. Air speed measurements indoors in stable conditions shall be 
made before and after performance test points to avoid obscuring the collector aperture. 
When testing outdoors in locations where the mean wind speed is below 2 ms-1, an artificial wind generator 
shall be used, and anemometer measurements shall be fitt ed for the continuous measurement of air velocity. 
This anemometer shall be mounted on a board so that there is continuous surface pointing towards the wind 
generator from the collector edge to 0.3 m behind the anemometer. The uniformity of air speed in the field of 
collector aperture shall be checked. The wind speed shall be measured while adjusting the wind generator, 
using a hand held anemometer in a height of 10 mm to 50 mm above the collector aperture plane. 
In windy locations, wind speed measurement shall be ma de near to the collector at the midheight of the 
collector. The sensor shall not be shielded from the wi nd and it shall not cast a shadow on the collector during 
the test periods. FDUS 854-2: 2009 
24 © UNBS 2009 – All rights reserved
 NOTE  The recorded value of the wind speed is diffe rent to the air speed above the collector aperture. 
6.1.2.6  Elapsed time 
Elapsed time shall be measured to a standard uncertainty of 0.2 %. 
6.1.2.7  Instrumentation/data recorders 
In no case shall the smallest scale division of the inst rument or instrument system  exceed twice the specified 
standard uncertainty. For example, if the specified standard uncertainty is 0.1 K, the smallest scale division 
shall not exceed 0.2 °C. Digital techniques and electroni c integrators shall have a standard uncertainty equal 
to or better than 1.0 % of the measured value. 
Analog and digital recorders shall have an error equal to  or better than 0.5 % of the full-scale reading and 
have a time constant of 1s or less. The peak signal indication shall be between 50 % and 100 % of full scale. 
The input impedance of recorders shall be greater t han 1000 times the impedance of  the sensors or 10 M Ω 
whichever is higher. 
6.1.2.8  Collector area 
The collector area (absorber, gross or aperture) shall be measured to a standard uncertainty of 0.3 %. Area 
measurements shall take place at a collector temperature of (20 ± 10) °C and under operating pressure if the 
absorber is made of organic material. 
6.1.2.9  Collector fluid capacity 
The fluid capacity of the collector shall be measured to a standard uncertainty of no more than 10 %. 
Measurements may be made either by weighing the collect or when empty and again when filled with fluid, or 
by filling and emptying the colle ctor to determine the mass of fluid whic h it will contain. Th e temperature of the 
fluid shall be kept within ±2 K of the ambient temperatur e. As an alternative, a det ermination by calculation, 
which is based on the geometrical circumstances, can be given. 
6.1.3  Test installation 
6.1.3.1  General consideration 
Examples of test configurations for testing solar coll ectors employing liquid as the heat transfer fluid are 
shown in Figures 2 and 3. These are schem atic only, and are not drawn to scale. 
 
DIAGRAM  
 
  
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  25
  
 
 
 
 
 
  
 
  
 
 
 
  
 
  
Figure 2 – Example of a closed test loop 
 
DIAGRAM 
  
 
 
  
 
 FDUS 854-2: 2009 
26 © UNBS 2009 – All rights reserved
  
 
 
 
 
 
  
 
 
Figure 3 – Example of an open test loop 
6.1.3.2  Heat transfer fluid 
The heat transfer fluid used for collector testing may be water or a fluid recommended by the collector 
manufacturer. The specific heat capaci ty and density of the fluid used shall be known to within ±1% over the 
range of fluid temperatures used during the test s. These values are given for water in Annex I. 
Some fluids may need to be changed periodically to ensure that their properties remain well defined. 
The mass or volume flowrate of the heat transfer fluid s hall be the same throughout the test sequence used to 
determine the thermal efficiency curve, time const ant and incident angle modifiers for a given collector. 
6.1.3.3  Pipework and fittings 
The piping used in the collector loop shall be resistant  to corrosion and (for testing of glazed collectors) 
suitable for operation at temperatures up to 95 °C. If non-aqueous fluids are used, then compatibility with 
system materials shall be confirmed. Pipe lengths shall generally be kept short. In particular, the length of 
piping between the outlet of the fluid te mperature regulator and the inlet to the collector shall be minimized, to 
reduce the effects of the environment on the inlet temperature of the fluid. 
This section of pipe shall be insulated to ensur e a rate of heat loss of less than 0.2 WK
-1, and shall be 
protected by a reflective weatherproof coating. Pi pework between the temperature sensing points and the 
collector (inlet and outlet) shall be protected with in sulation and reflective (for outdoor measurements also 
weatherproof) covers to beyond the positions of t he temperature sensors, such that the calculated 
temperature gain or loss along either pipe portion does not exceed ±0.01 K under test conditions. Flowmixing 
devices such as pipe bends are required immediat ely upstream of temperature sensors (see 6.1.2.3). 
NOTE 1  A short length of transparent tube should be installed in the fluid l oop so that air bubbles and any other 
contaminants will be observed if present. The transparent tube should be placed clos e to the collector inlet but should not 
influence the fluid inlet temperature control or temperature measurements. 
A variable area flowmeter is convenient for this purpos e, as it simultaneously gives an independent visual 
indication of the flowrate. An air separator and air v ent should be placed at the outlet of the collector, and 
other points in the system where air can accumulate. 
NOTE 2  Filters should be placed upstream of the flow measuring device and the pump, in accordance with normal 
practice (a nominal filter size of 200 μm is usually adequate). 
6.1.3.4  Pump and flow control devices FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  27
 The fluid pump shall be located in the collector test l oop in such a position that the heat from it which is 
dissipated in the fluid does not affect either the cont rol of the collector inlet temperature or the measurements 
of the fluid temperature rise through the collector. With some types of pump, a simple bypass loop and 
manually controlled needle valve may provide adequate fl ow control. Where necessary, an appropriate flow 
control device may be added to stabilize the mass flowrate. 
The pump and flow controller shall be capable of maintaining the mass or volume flowrate through the 
collector stable to within 1 % despite temperature va riations, at any inlet temperature chosen within the 
operating range. 
6.1.3.5  Temperature regulation of the heat transfer fluid 
The collector test loop shall be capable of maintain ing a constant collector inlet temperature at any 
temperature level chosen within the operating range. Si nce the rate of energy colle ction in the collector is 
deduced by measuring instantaneous values of the fluid inlet and outlet temperatures, it follows that small 
variations in inlet temperature could lead to errors in  the rates of energy collection deduced. It is particularly 
important to avoid any drift in the collector inlet temperature. 
NOTE 1  Test loops may contain two stages of fluid inlet tem perature control, as shown in Figures 2 and 3. The primary 
temperature controller should be placed upstream of the flowmeter and flow contro ller. A secondary temperature regulator 
should be used to adjust the fluid temperature just before the collector inlet. This secondary regulator should normally not 
be used to adjust the fluid temperature by more than ±2 K. 
NOTE 2  A secondary temperature controll er close to the collector inlet is usef ul, provided that it does not introduce 
problems with measuring the “mean cup" inlet temperature. On  the other hand, bypasses should also remain allowed as a 
means to reduce the influence of heat loss and to improve t he control characteristics of t he heating circuits. A constant 
and high flowrate through the heaters will allow PID controlling with fast I and D action, while any flowrate through the 
collector can be chosen. 
6.1.4  Outdoor steady-state performance test 
6.1.4.1  Test installation 
The collector shall be mounted in accordance with the sp ecifications given in 6.1.1, and coupled to a test loop 
as described in 6.1.3. The heat transfer fluid shall fl ow from the bottom to the top of the collector, or as 
recommended by the manufacturer. 
6.1.4.2 Preconditioning of the collector 
The collector shall be visually inspected and any dam age recorded. The collector aperture cover shall be 
thoroughly cleaned. If moisture is formed on the collec tor components, then the heat transfer fluid shall be 
circulated at approximately 80 °C for as long as is nece ssary to dry out the insulation and collector enclosure. 
If this form of preconditioning is carried out, then it shall be reported with the test results. The collector 
pipework shall be vented of trapped air by means of an air valve or by circulating the fluid at a high flowrate, 
as necessary. The fluid shall be inspected for entrained ai r or particles, by means of the transparent tube built 
into the fluid loop pipework. Any contaminants shall be removed. The empty collector shall be exposed to 
irradiation for 5 h at the level of more than 700 Wm-2. 
6.1.4.3  Test conditions At the time of the test, the hemispherical solar irradiance  at the plane of the collector aperture shall be greater 
than 700 Wm
-2. 
NOTE1  lf the manufacturer has limitati ons on operation with respect to maximum irradiance but not less than 800 Wm-2, 
this can be requested with the test. That maximum value should be clearly reported . 
The angle of incidence of direct solar radiation at the collector aperture shall be in the range in which the 
incident angle modifier for the collector varies by no more than ±2 % from its value at normal incidence. For FDUS 854-2: 2009 
28 © UNBS 2009 – All rights reserved
 single glazed flat plate collectors, this condition will usually  be satisfied if the angle of incidence of direct solar 
radiation at the collector aperture is less than 20°. 
However, much lower angles may be required for particular designs. In order to characterize collector 
performance at other angles, an incident angle modifier may be determined (see 6.1.7). 
Where diffuse solar irradiance is less than 30 %, its influence may be neglected. The collector shall not be 
tested at diffuse irradiance levels of greater than 30 %. 
The average value of air speed parallel to the collector aper ture, taking into account spatial variations over the 
collector and temporal variations during the test period, shall be 3 ms ± 1 ms-1. 
Unless otherwise specified, the fluid flow rate shall be set at approximately 0.02 kgs-1 per square meter of 
collector aperture area. It shall be held stable to within ±1 % of the set value during each test period, and shall 
not vary by more than ±10 % of the set value from one test period to another. 
The testing at other flowrates may be accommodated by adhering to manufacturer's specification. In some 
collectors the recommended fluid flowrate may be clos e to the transition region between laminar and turbulent 
flow. This may cause instability of the internal heat tr ansfer coefficient and hence variations in measurements 
of collector efficiency. In order to characterize such a collector in a reproducible way, it may be necessary to 
use a higher flowrate, but this shall be clearly stated with the test results. 
NOTE 2  In the transition regime, the flowrate should first be set high (turbulent) and then reduced to the setpoint value. 
This will prevent transition from laminar to turbulent during the measurements. Measurements of fluid temperature 
difference of less than 1 K shall not be included in the test resu lts because of the associated prob lems of instrument error. 
6.1.4.4  Test procedure The collector shall be tested over its operating temper ature range under clear sky conditions in order to 
determine its performance characteristic. Data points, which satisfy the requirements given below, shall be 
obtained for at least four fluid inlet temperatures spac ed evenly over the operati ng temperature range of the 
collector. If possible, one inlet temperature shall be se lected such that the mean fluid temperature in the 
collector lies within ±3 K of the ambient air temperat ure, in order to obtain an accurate determination of η
o. if 
water is the heat transfer fluid, the maximum temperat ure shall be at least around 80 °C. A maximum value of 
Tm* of at least 0.09 is recommended if test conditions permit. 
 At least four independent data points shall be obtained for ea ch fluid inlet temperature, to give a total of 16 
data points. If test conditions permit, an equal number of  data points shall be taken before and after solar 
noon for each fluid inlet temperature. The latter is not r equired if the collectors are moved to follow the sun in 
azimuth and altitude using automatic tracking. 
During a test, measurements shall be made as specified in 6.1.4.5. These may then be used to identify test 
periods from which satisfacto ry data points can be derived. 
6.1.4.5  Measurements 
The following data shall be measured: 
- gross collector area A
G, the absorber area AA and the aperture area Aa; 
-  fluid capacity 
-  hemispherical solar irradiance at the collector aperture 
-  diffuse solar irradiance at the collector aperture (only outdoors) 
-  angle of incidence of direct solar radiation (alter natively, this angle may be determined by calculation) FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  29
 -  air speed parallel to the collector aperture 
-  surrounding air temperature 
-  temperature of the heat transfer fluid at the collector inlet 
-  temperature of the heat transfer fluid at the colle ctor outlet flowrate of the heat transfer fluid 
6.1.4.6  Test period (steady-state) 
The test period for a steady state dat a point shall include a pre-conditioning period of at least 4 times the time 
constant of the collector (if known), or not less than 15 min (if time constant is not known), with the correct 
fluid temperature at the inlet, followed by a steady stat e measurement period of at least 4 times the time 
constant of the collector (if known), or not le ss than 10 min (if time constant is not known). 
A collector is considered to have been operating in steady-state conditions over a given measurement period 
if none of the experimental parameters deviate from thei r mean values over the measurement period by more 
than the limits given in Table 5. To establish that a steady state exists, average va lues of each parameter 
taken over successive periods of 30 s shall be comp ared with the mean value over the measurement period 
Table 5 – Permitted deviation of measured parameters during a measurement period 
Parameter                                                            Permitted deviation from the mean value 
(Global ) Test solar irradiance                                        ± 50Wm-2 
Surrounding air temperature (indoor)                             ±  1K 
Surrounding air temperature (outdoor)                           ± 1.5K 
Fluid mass flowrate                                                         ± 1% Fluid temperature at the collector inlet                            ± 0.1K 
 
6.1.4.7  Presentation of results 
The measurements shall be collated to produce a set of  data points which meet the required test conditions 
(see 6.1.4.3), including those for steady-state operati on. These shall be presented using the data format 
sheets given in Annex D. 
6.1.4.8  Computation of collector output 
6.1.4.8.1 General    
The actual useful power extracted, .
Q, is calculated from: 
TcmQ fΔ=. .
                ( 3 )   
A value of cf corresponding to the mean fluid temperature shall be used. If m& is obtained from volumetric 
flowrate measurement, then the density shall be determined for the temperature of the fluid in the flowmeter. 
6.1.4.8.2 Solar energy intercepted by the collector 
Provided that the angle of incidence is less than 20°, the use of an incident angle modifier, as described in 
6.1.7, is not required for singl e glazed flat plate collectors. 
The solar energy intercepted is A.G where the area is AA when referred to the absorber area of the collector 
and Aa when referred to the aperture area of the collector. Introducing the collector efficiency, the actual useful 
power extracted, .
Q , can also be written as FDUS 854-2: 2009 
30 © UNBS 2009 – All rights reserved
 ηG QΛ=.
                 ( 4 )  
6.1.4.8.3 Reduced temperature difference 
When the mean temperature of the heat transfer fluid fm is used, where 
 2Ttt in mΔ+=                  (5) 
the reduced temperature difference is calculated as: 
GttTa m
m−=*                  ( 6 )  
6.1.4.8.4 Modeling of instantaneous efficiency 
6.1.4.8.4.1 General 
The instantaneous efficiency η shall be calculated by statistical curv e fitting, using the least squares method, 
to obtain an instantaneous efficiency curve of the form 
() 2* * 2 1 0 m m TGa Ta−−=ηη               (7) 
Normally a second-order curve shall be used which can be achieved by least sq uares regression. A second-
order fit shall not be used if the value deduced for a2 is negative. The test conditions shall be recorded on the 
data format sheets given in Annex D. 
Where necessary, tables of measurements of the collector performance are admitted. 
6.1.4.8.4.2 Instantaneous efficiency based on absorber area / aperture area 
With reference to the reduced temperature difference T*m the equations for instantaneous efficiency are: 
2
2 1 0 ⎟
⎠⎞⎜
⎝⎛−−−−=GttGaGttaa m a mηη             (8) 
6.1.4.8.4.3 Conversion of thermal performance test characteristics 
To convert the thermal performance test characteri stics the following basic conversions shall be used: 
   
ΑΑΑΑ=a
a0 0ηη                  ( 9 )  
   
ΑΑ=AAa aa
a1 1                  ( 1 0 )  
   
ΑΑΑΑ=a
aa a 2 2                  ( 1 1 )  
6.1.4.8.5 Collector output Using Equation 4 and 8 the collector output per module can be written as: FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  31
    () ()⎟
⎠⎞⎜
⎝⎛ −−−−Α=GttaGtta G Qa m a m
2 1 0.
..η            ( 4 . 1 )  
Where the area is AA when referred to the absorber area of the collector and Aa when referred to the aperture 
area of the collector. 
The collector output per module shall be presented graphically as a function of the temperature difference 
between mean fluid and ambient temperature ( tm – ta) using G = 1000W/m2. The product AGηo shall be 
referred to as Wpeak. 
6.1.5  Steady-state efficiency test using a solar irradiance simulator 
6.1.5.1  General The performance of most collectors is better in direct so lar radiation than in diffuse and at present there is little 
experience with diffuse solar simulation. This test me thod is therefore designed for use only in simulators 
where a near-normal incidence beam of simulated so lar radiation can be directed at the collector. 
In practice it is difficult to produce a uniform beam of  simulated solar radiation and a mean irradiance level has 
therefore to be measured over the collector aperture 
6.1.5.2  The solar irradiance simulator for steady-state efficiency testing 
A simulator for steady-state efficiency testing shall have the following characteristics: 
The lamps shall be capable of producing a mean irradian ce over the collector aperture of at least 700 Wm
-2. 
Values in the range 300 Wm to 1000 Wm-2 may also be used for specialized test s, provided that the accuracy 
requirements given in Table 5 can be achieved and the irradiance values are noted in the test report. 
At any time the irradiance at a point on the collector aper ture shall not differ from the mean irradiance over the 
aperture by more than ±15 %. The spectral distribution of the simulated solar radiation shall be approximately 
equivalent to that of the solar sp ectrum at optical air mass 1.5. 
Where collectors contain spectrally selective absorber s or covers, a check shall be made to establish the 
effect of the difference in spectrum on the ( τα) product for the collector. If the effective values of ( τα) under the 
simulator and under the optical air mass 1.5 solar r adiation spectrum differ by more than ±1 %, then a; 
correction shall be applied to the test results. 
   ()()()()
()∫∫
=m
mm
m
d Gd G
Effectiveμ
μμ
μ
λλλλλαλτ
τα3
3.03
3.0           (12) 
Measurement of the solar simulator's spectral qualitie s shall be in the plane of the collector over the 
wavelength range of 0.3 μm to 3 μm and shall be determined in band widths of 0.1 μm or smaller. 
For certain lamp types, i.e. metal halide designs, it is recommended that the initial spectral determination be 
performed after the lamps have completed their burn-in pe riod. The amount of infrared thermal energy at the 
collector plane shall be suitably measured (measu rements in the wavelength range above about 2.5 μm if 
possible, but starting not beyond 4 μm) and reported (see 6.1.2.2). 
The thermal irradiance at the collector shall not exceed that of a blackbody cavity at ambient air temperature 
by more than 5 % of global irradiance. 
The collimation of the simulator shall be such that the a ngles of incidence of at least 80 % of the simulated 
solar irradiance lie in the range in which the incident angle modifier of the collector varies by no more than ±2 FDUS 854-2: 2009 
32 © UNBS 2009 – All rights reserved
 % from its value at normal incidence. For typical flat plat e collectors, this condition usually will be satisfied if at 
least 80 % of the simulated solar radiation received at any point on the collector under test shall have 
emanated from a region of the solar irradiance simula tor contained within a subtended angle of 60° or less 
when viewed from any point. 
NOTE 1 Additional requirements concerning collimation apply to  measurement of the incident an gle modifier (see 6.1.7.2). 
The method used for measuring the irradiance during the test period shall produce values of mean irradiance 
which agree with those determined by spatial integration to within ±1%. 
NOTE 2 The spectral distribution of the lamps (indoors) and of the sky (outdoors) can and do lead to very wide 
discrepancies in spectrally selective absorbers or covers. 
6.1.5.3  Test installation 
Collector mounting and location requirement s outlined in 6.1.1 shall be followed. 
A wind generator shall be used with a solar simulator to produce an air flow in accordance with 6.1.1.8. 
6.1.5.4  Preconditioning of the collector 
The procedure outlined in 6. 1.4.2 shall be followed. 
6.1.5.5  Test procedure 
The collector shall be tested over its operating temper ature range in approximately the same way as specified 
for outdoor testing (see 6.1.4.4). 
However, eight test points shall be adequate for testing in solar simulators provided that at least four different 
inlet temperatures are used, and adequate time is allowed for temperatures to stabilize. One inlet temperature 
should lie within ±3 K of the ambient air temperature,  if possible. During a test, measurements shall be made 
as specified in 6.1.5.6. These may then be used to ident ify test periods from which satisfactory data points can 
be derived. 
6.1.5.6 Measurements during tests in solar irradiance simulators 
6.1.5.6.1 General Measurements shall be made as specified in 6.1.4. 
6.1.5.6.2 Measurement of simulated solar irradiance 
NOTE  Simulated solar irradiance usually varies spatially over the collector aperture  as well as varying with time during a 
test. It is therefore essential to employ a procedure for integrating the irradiance over  the collector aperture. Time 
variations in irradiance are usually caused  by fluctuations in the electricity s upply and changes in lamp output with 
temperature and running time. Some lamps take more than 30 min to reach a stable working condition when warming up 
from cold. 
Pyranometers may be used to measure the irradiance of simulated solar radiation in accordance with 6.1.2.1. 
Alternatively, other types of radiation detector may be  used, provided they have been calibrated for simulated 
solar radiation. Details of the instruments and the met hods used to calibrate them shall be reported with the 
test results. The distribution of i rradiance over the collector aperture shall be measured using a grid of 
maximum spacing 150 mm, and the spatial mean deduced by simple averaging. 
6.1.5.6.3 Measurement of thermal irradiance in simulators 
The thermal irradiance in a solar simulator is likely to  be higher than that which typi cally occurs outdoors. It 
shall therefore be measured to ensure that it does not exceed the limit given in 6.1.5.8. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  33
 The mean thermal irradiance in the collector test  plane shall be determined whenever changes are made in 
the simulator which could affect the thermal irradiance, and at least annually. The mean thermal irradiance in 
the collector test plane and the date when it was last measured shall be reported with collector test results. 
6.1.5.6.4 Ambient air temperature in simulators 
The ambient air temperature ta in simulators shall be measured, taking the mean of several values if 
necessary sensors shall be shielded in order to minimize  radiation exchange. The air temperature in the outlet 
of the wind generator shall be used for t he calculations of collector performance. 
6.1.5.7  Test period 
The test period may be determined in the same way as for outdoor steady-state testing. 
The more stable environment of an indoor test facility may allow steady-state conditions to be maintained 
more easily than outdoors, but adequate time shall still be allowed to ensure proper steady-state operation of 
the collector as specified in 6.1.4.6. 
6.1.5.8  Test conditions 
The test conditions described in 6.1.4.3 for outdoor testing shall be observed with the following additions: 
-  thermal irradiance in the plane of the collector apertur e shall not exceed that fr om a blackbody cavity at 
ambient air temperature by more than 5 % of the global irradiance. 
-  air issuing from the wind generator shall not differ in  temperature from ambient air by more than ±1 K. 
6.1.5.9  Computation and presentation of results The analysis presented in 6.1.4.8 for outdoor testing is also applicable to solar simulator tests, and the results 
shall be presented on the format sheets shown in Annex D. 
6.1.6  Determination of the effective thermal capacity and the time constant of a collector 
6.1.6.1  General 
The effective thermal capacity and the time constant of  a collector are important parameters, which determine 
its transient performance. 
A collector can usually be considered as a combinati on of masses, each at a different temperature. When a 
collector is operating, each collector component responds differently to a c hange in operating conditions, so it 
is useful to consider an effective ther mal capacity for the whole collector.  
Unfortunately, the effective thermal capacity depends on the operating conditions and is not a collector 
parameter with a unique value. Several different test  methods have been used to measure or calculate the 
effective thermal capacity of colle ctors and it has been show n that similar results can be obtained by using 
quite different methods. Just as there is no unique va lue of effective thermal capacity, there is no unique 
overall time constant for a collector. For most collect ors, the dominant influence on the response time is the 
fluid transit time, and hence the first-order response va ries with the fluid flowrate. Other collector components 
respond with different times to give an effective overall time constant, which depends on the operating 
conditions. 
6.1.6.2  Determination of effective thermal capacity 
The effective thermal capa city of the collector C (expressed as Joules per Kelvin) is calculated as the sum, for 
each constituent element of the collector (glass, absorber,  liquid contained, insulation), of the product of its 
mass m: (expressed in kilograms), its specific heat c
i (expressed as joules per kilogram Kelvin) and a 
weighting factor pi; FDUS 854-2: 2009 
34 © UNBS 2009 – All rights reserved
    ∑=
iiiicmp C                  (13) 
The weighting factor pi (between 0 and 1) allows for the fact that  certain elements are only partially involved in 
collector thermal inertia. The values of pi are given in Table 6. 
Table 6 — Values of weighting factors 
Elements pi 
Absorber Insulation 1 
Heat transfer liquid 0.5 External glazing 0.01× a1 
Second glazing 0.2× a1 
Third glazing 0.35× a1 1 
0.5 
0.01xa
1 
0.2xa 1 
0.35xa 1 
 
For drain-back and drai n-down systems, the capacity should be reported for the collec tor while it is filled with 
water and while it is empty. 
Effective thermal capacity may also be measured by  applying the procedures described in Annex G. 
6.1.6.3  Test procedure for collector time constant (optional) 
Testing shall be performed either outdoors or in a sola r irradiance simulator. In either case, the solar 
irradiance on the plane of the collect or aperture shall be greater than 700 Wm-2. The heat transfer fluid shall 
be circulated through the collector at the same flowrate as  that used during collector thermal efficiency tests. 
The aperture of the collector shall be shielded from the solar radiation by means of a solar-reflecting cover, 
and the temperature of the heat transfer fluid at the collector inlet shall be set approximately equal to the 
ambient air temperature. When a steady state has been reached, the cover shall be removed and 
measurements continued until steady-state conditions have been achieved ag ain. For the purpose of this test, 
a steady-state condition is assumed to exist when the out let temperature of the fluid varies by less than *0.05 
K per minute. The following quantities shall be measured in accordance with 6.1.2: 
-  Collector fluid inlet temperature ( tin); 
- Collector fluid outlet temperature ( te); 
-  Surrounding air temperature ( ta); 
6.1.6.4  Calculation of collector time constant (optional) 
The difference between the temperature of the fluid at  the collector outlet and that of the surrounding air (te - 
te) shall be plotted against time, beginning with the initial steady-state condition ( te - ta)0 and continuing until 
the second steady state has been ac hieved at a higher temperature ( te - ta)2 (see Figure 4). 
DIAGRAM pg 43 
 
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  35
  
 
 
 
 
 
Figure 4 – Collector time constant 
The time constant τc of the collector is defined as the elapsed time between the removal of the cover and the 
point where the collector outlet  temperature rises to 63.2 % of the total increase from ( te - ta)0 to (te - ta)2. If the 
response time of the temperature sensors is significant  when compared with that measured for the collector, 
then it shall be taken into account in calculating the test results. 
6.1.7 Collector incidence angle modifier 
6.1.7.1  General 
The effective transmittance-absorbance product ( τα)e can be replaced by the value at normal incidence 
provided that another factor ca lled the incidence angle modifier, Kθ, is introduced in Equation (14). 
   ()2
2 1 ' ⎟
⎠⎞⎜
⎝⎛−−−− =GttGaGtta KFa m a m
enταη θ          ( 1 4 )  
Hence: 
   ()()en eKτα τα θ=                ( 1 5 )  
Figure 5 shows the variation of Kθ with angle of incidence for two solar collectors. 
For those collectors (e.g. evacuated tube collectors a nd CPC collectors) for which the incidence angle effects 
are not symmetrical with direction of incidence, it is necessary to meas ure the incident angle effects from 
more than one direction to fully characterize the incident angle modifier. 
The complex individual incident angle modifier can be es timated by considering it to be the product of the 
separate incident angle modifiers, ΚθL and ΚθΤ for two perpendicular symmetry planes (Equation 15.1). 
K θ = ΚθL •ΚθT              ( 1 5 . 1 )  
The longitudinal plane (index L) runs parallel to the optical axis of the collect or, and the transversal plane 
(index T) is perpendicular to the optical axis. The angles θL and θT are the projections of the incidence angle 
θ onto the longitudinal and transversal planes, respectively. 
For the correlation between θ, θ L and θ T, the following equation holds: 
tan2θ = tan2θL + tan2θT             ( 1 5 . 2 )  
DIAGRAM Pg 44 
 
 FDUS 854-2: 2009 
36 © UNBS 2009 – All rights reserved
  
 
 
 
 
 
  
 
  
Figure 5 – Typical incidence angle modifiers K  θ  
The significance of the incidence angle modifier to the test procedures outlined in this standard is that the 
thermal efficiency values are determined for the collector at or near normal incidence conditions. Therefore, 
the y intercept η of the efficiency curve is equal to F’(τα)en, for a flat plate collector. A separate measurement 
shall be conducted to determine the value of Kθ so that the performance of  the collector can be predicted 
under a wide range of conditions and/or time of day using Equation (14). 
6.1.7.2  Solar irradiance simulator for the measurement of incidence angle modifiers 
For the measurement of the incidence angle modifier, only solar irradiance simulators with the following 
collimation specificat ion shall be used. 
The collimation shall be such that at least 90% of the simulated solar irra diance at any point on the collector 
under test has emanated from a region of the solar irr adiance simulator contained within a subtended angle of 
20° or less when viewed from the point. 6.1.7.3  Test procedures 
6.1.7.3.1 General 
The testing of the solar collector to determine it s incidence angle modifier may be done by one of two 
methods. However, during each test peri od, the orientation of the collector shall be such that the collector is 
maintained within ±2.5° of the angle of incidence for wh ich the test is being conducted. The solar irradiance on 
the plane of the aperture s hall be greater than 300 W/m
2. 
While measuring the incident angle modifier in one pl ane of an optical unsymmetrical collector the incident 
angle within the other plane should be kept to a value where the incident angle modifier does not differ more 
than 2% from the one at normal incidence. 
NOTE 1  Care should be taken that the measurement of the inci dent angle modifier is not affected by inappropriate tilt angles. 
NOTE 2  For angles of incidence of 50°, a deviation of ±1° l eads to an error of 2 % when measuring the solar irradiance. 
6.1.7.3.2 Method 1 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  37
 This method is applicable for testing indoors using a solar simulator with the characteristics specified in 
6.1.5.2, or outdoors using a movable test rack (altazimuth collector moun t) so that the orientation of the 
collector can be arbitrarily adjusted with respect to  the direction of the incident solar radiation. 
The collector shall be orientated so that the test angle of  incidence between it and the direct solar radiation for 
the test condition is 50°. For conventio nal flat plate collectors, this angle will be sufficient. For some collectors 
with unusual optical performance characte ristics, or if it is required for sy stem simulation, angles of 20°, 40°, 
60° and others may be necessary. 
The mean temperature of the heat transfer fluid shall be co ntrolled as closely as possible (preferably within ±1 
K) to the ambient air temperature. The efficiency va lue shall be determined in accordance with 6.1.4.4. 
6.1.7.3.3 Method 2 
This method is applicable for testing outdoors using a stat ionary test rack on which the collector orientation 
cannot be arbitrarily adjusted with respect to direction for incident solar radiation (except for adjustments in 
tilt). The mean temperature of the heat transfer fluid shall be co ntrolled, if possible, to within ±1 K of the ambient 
air temperature. The efficiency value shall be determined in  such a way that one value of efficiency is taken 
before solar noon and a second value after solar noon. The average incidence angle between the collector 
and the solar beam for both data points is the same. The e fficiency of the collector for the specific incidence 
angle shall be considered equal to the average of the two values. 
The efficiency value shall be determined in general in accordance with the method described in 6.1.4.4. As 
with Method 1, data shall be collected for angle of incide nce of 50°. For some collectors with unusual optical 
performance characteristics or if it is required for syst em simulation, angles of 20°, 40°, 60° and others may be 
necessary. 
NOTE  More experience is required to confirm whether this me thod is applicable to special geomet ries, such as tubular collector s. 
6.1.7.4  Calculation of collect or incidence angle modifier 
Regardless of which experimental method in 6.1.7.3. is used, values for the thermal efficiency of the collector 
shall be determined for each value of angle of incidence.  For conventional flat plate collectors, only one angle 
of incidence is needed which is 50° (It is noted that a rating standard using this test method may require that 
Kθ be measured for a different set of angles of incidence). The mean fluid temperature is held very close to 
the ambient air temperature so that 
(tm– ta) ≈ 0. The relationship between Kθ and the efficiency is: 
   () en FKταη
θ'=                 ( 1 6 )  
Since F’(τα)en will have already been obtained as the y-axis inte rcept of the efficiency curve, values of kb can 
be computed for the different angles of incidence (see  6.1.7.3.). If the mean fluid temperature cannot be 
controlled to equal the ambient air temperature within ±1 K, each value of Kθ shall be computed as: 
   () ena m a m
FGttGaGtta
Kταη
θ'2 1 ⎟
⎠⎞⎜
⎝⎛−+−+
=             ( 1 7 )  
Due to more exact results Equat ion (17) should be used generally. 
Alternatively, each data point may be plotted on the sa me graph with the efficiency curve determined in 
accordance with 6.1.4 or 6.1.5, and a curve drawn through each point pa rallel to the efficiency curve and 
made to intersect the y-axis. The values of the y interc ept are the efficiency values that would have resulted FDUS 854-2: 2009 
38 © UNBS 2009 – All rights reserved
 had the mean fluid temperature been controlled to equal ambient air temperature. Therefore, these values 
may be used in conjunction with Equation (17) to compute the different values of Kθ. 
6.1.8  Determination of the pressure drop across a collector 
If determination of pressure drop across a collector is required, this should be done according to Annex L. 
6.2    Unglazed solar collectors under steady state conditions (including pressure drop) 
6.2.1  Collector mounting and location 
6.2.1.1  General 
Specification given in 6.1.1.1 applies. 
6.2.1.2  Mounting 
The collector shall be mounted in the manner specified by the manufacturer. 
The collector mounting frame shall in no way obstruct t he aperture of the collector, and shall not significantly 
affect the back or side insulation, unless otherwise spec ified (for example, when the collector is part of an 
integrated roof array). 
Collectors designed to be mounted direct ly on standard roofing material may be mounted over a simulated 
roof section. In case of roof int egrated collectors, a model consisting of a small scale collector placed on an 
artificial roof should be prepared for the purpose of the tests. If mounting instructions are not specified, the coll ector shall be mounted on an insulated backing with a 
quotient of the materials thermal cond uctivity to its thickness of 1 Wm
-2 K-1 ± 0.3 Wm-2 K-1 and the upper 
surface painted matt white and ventilated at the back. 
NOTE  Example material suited for the insulated backing is 30 mm of polystyrene foam. 
The collector shall be mounted such that the lower ed ge is not less than 0.5 m above the local ground surface. 
Collector arrays constructed from pipe or strip compo nents shall be mounted with the pipes (or strips) spaced 
10 mm or one diameter (width of strip) apart, whichever is  the smaller. If a different pipe or strip spacing is 
specified in the manufacturers' installation instructio ns then the recommended spacing shall be used. If the 
collector is delivered with mounting spacers or any devi ce fixing the spacing of the pipes (or strips) then the 
collector shall be tested as delivered and its geom etry shall be reported in the test report. 
Currents of warm air, such as those which rise up the walls of a building, shall not be allowed to pass over the 
collector. Where collectors are tested on the roof of a bu ilding, they shall be located at least 2 m away from 
the roof edge. 
The performance of some forms of unglazed solar collect ors is a function of module size. If the collector is 
supplied in fixed units of area greater than 1 m2 then a sufficient number of modules shall be linked together to 
give a test system aperture of at least 3 m2. If the collector is supplied in the form of strips the minimum built-
up module area shall be 3 m2 (gross area). 
6.2.1.3  Tilt angle 
The collector shall be tested at tilt angles so t hat the incidence angle with direct solar radiation θ is less than 
30°, or at angles of tilt so that t he incidence angle modifier varies by less than ±2 % from normal incidence. 
Before deciding on a tilt angle it may be necessary to check the incidence an gle modifier at two angles prior to 
commencing the tests. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  39
 NOTE  For most unglazed collectors, the influence of tilt angle and radiation incidence angle on collector efficiency is 
small and unglazed collectors are commonly installed at low incl inations. However care should be taken to avoid air locks 
at low inclinations. 
6.2.1.4  Collector orientation outdoors 
The collector orientation shall conform to 6.1.1.4. 
6.2.1.5  Shading from direct solar irradiance 
The collector shall be shaded from direct solar irradiance in accordance with 6.1.1.5. 
6.2.1.6  Diffuse and re flected solar irradiance 
The collector shall diffuse and reflect solar radiation in accordance with 6.1.1.6. 
6.2.1.7  Thermal irradiance 
The performance of some collectors is particularly sensitive to the levels of thermal irradiance. The 
temperature of surfaces adjacent to the collector shall be as close as possible to that of the ambient air in 
order to minimize the influence of thermal radiation. Fo r example, the outdoor field of view of the collector 
shall not include chimneys, cooling towers or hot exhausts . For indoor and simulator testing, the collector shall 
be shielded from hot surfaces such as radiators, air-conditioning ducts and machinery, and from cold surfaces 
such as windows and external walls. Shielding is im portant both in front of and behind the collector. 
The major difference between indoor and outdoor testing of unglazed collectors is the long wave thermal 
irradiance. The relative long wave radiation in a simulator shall not be higher than ±50 Wm-2 (typically — 100 
Wm-2 for outdoor conditions). 
6.2.1.8  Airspeed 
The performance of unglazed collectors is sens itive to air speed adjacent to the collector. 
In order to maximize the reproducibility of results, ungl azed collectors shall be mount ed so that air can freely 
pass over the front side of the colle ctor, and exposed back and sides of the collector. Collectors designed for 
integration into a roof may have their backs protected fr om the wind, but this shall be reported with the test 
results. 
The average surrounding air speed at a distance of 100 mm above and parallel to the collector aperture shall 
cover the range 0 ms-1 to 3.5ms-1 subject to the tolerance specified in Table 7. If these conditions cannot be 
achieved under natural conditions then an artificial wind generator shall be used. If a wind generator is used 
the turbulence level shall be in the range of 20% to 40% to simulate natural wind conditions. The turbulence 
level shall be checked at the leading edge of the colle ctor 100 mm above the collector surface. The turbulence 
level shall be monitored using a linearized hot wire anem ometer with a frequency response of at least 100 Hz. 
If the absorber is not mounted directly on a roof or a sheet of backing material, the air speed shall be 
controlled and monitored on the front and back of the absorber. 
6.2.2  Instrumentation 
6.2.2.1  Solar radiation measurement 
Specification given in 6.1.2.1 applies. 
6.2.2.2  Thermal radiation measurement 
6.2.2.2.1 Measurement of long wave irradiance 
A pyrgeometer mounted in the plane of the collector s hall be used to measure global hemispherical long wave 
radiation. FDUS 854-2: 2009 
40 © UNBS 2009 – All rights reserved
 6.2.2.2.2 Precaution for effects of temperature gradient 
The pyrgeometer used during the tests shall be placed in  the same plane as the collector absorber and 
allowed to equilibrate for at least 30 min before measuring. 
6.2.2.2.3 Precautions for effect s of humidity and moisture 
The pyrgeometer shall be provided with a means of pr eventing accumulation of moisture that may condense 
on surfaces within the instrument and affect its reading. An instrument with a desiccator that can be inspected 
is required. The condition of the desiccator shall be observed prior to and following each daily measurement 
sequence. 
6.2.2.2.4 Precautions for effect of short wave heating 
The influence of short wave solar heating effects should be minimized. 
6.2.2.3  Temperature measurements 
Temperatures shall be measured in accordance with 6.1.2.3. 
6.2.2.4  Measurement of collector liquid flowrate 
Liquid flowrates shall be measured in accordance with 6.1.2.4. 
6.2.2.5  Measurement of air speed 
6.2.2.5.1 General 
Air speed shall be measured in accordance with 6.1.2.5.1. 
6.2.2.5.2 Required accuracy of air speed 
The speed of the surrounding air ov er the front surface of the collect or shall be measured to a standard 
uncertainty of 0.25 ms-1. Under outdoor conditions the surrounding ai r speed is seldom constant and gusting 
frequently occurs. The measurement of an average air spe ed is therefore required during the test period. This 
may be obtained either by an arithmetic average of sampl ed values or by a time integration over the test 
period. 
NOTE  It should be taken into consideration that anemometers have starting limits which lie between 0.5 ms-1 and 1 ms-
1. Therefore, considerable errors may occur for air velocities less than 1 ms-1. 
6.2.2.5.3 Mounting of sensors 
The wind speed shall be measured while adjusting the wind generator, using a hand held anemometer at a 
height of 10 mm to 50 mm above the collector aperture  plane. A permanently installed anemometer shall be 
placed at one edge of the collector in order to superv ise the operation of the wind generator. This anemometer 
shall be mounted on a board so that there is a cont inuous surface pointing towards the wind generator from 
the collector edge to 0.3 m behind the anemometer. 
NOTE  The recorded value of the wind speed is  not the air speed above the collector aperture. 
A series of measurements shall be taken at nine equally  spaced positions over the collector area. An average 
value shall then be determined. For a collector that does not have back insulation or is not mounted on a 
simulated roof surface, the air speed shall be meas ured over the front and back surfaces. The average air 
speed on the front and back surface shall be used in the data correlation. 
During the test the air speed shall be monitored at a c onvenient point that has been calibrated relative to the 
mean air speed over the collector. The anemometer shall not cast a shadow on the collector during the tests. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  41
 6.2.2.6  Pressure measurements 
The inlet pressure to the collector and the pressure dr op across the collector shall be measured with a device 
having an error of 5 % of the measured value or ±10 Pa. If the collector is suppli ed in modules the pressure 
drop shall be specified per module. Fo r strip absorbers the pressure shall be  specified per metre run of strip. 
6.2.2.7  Elapsed time 
Elapsed time shall be measured in accordance with 6.1.2.6. 
6.2.2.8  Instrumentation/data recorders 
Instrumentation and data recorder s shall conform to 6.1.2.7. 
6.2.2.9  Collector area The collector area shall conform to 6.1.2.8. 
6.2.2.10 Collector fluid capacity 
The collector fluid capacity shall conform to 6.1.2.9. 
6.2.3  Test installation 
The test installation shall be in accordance with 6.1.3. 
6.2.4  Outdoor steady state efficiency test 
6.2.4.1  Test installation 
The test installation shall conform to 6.1.4.1. 
6.2.4.2  Preconditioning of the collector 
The collector shall be preconditioned in accordance with 6.1.4.2. 
6.2.4.3  Test conditions 
At the time of the test, the net irradiance at the plane of the collector apertur e shall be greater than 650 Wm
-2. 
NOTE  If the manufacturer has limitations  on operation with respect to maximum irradiance but not less than 800 Wm, 
this can be requested with the test. That maxi mum value should be clearly stated in the report. 
The angle of incidence of direct solar radiation at the collector aperture shall be in the range in which the 
incidence angle modifier for the collector varies by no mo re than ±2 % from its value at normal incidence. An 
angle of incidence modifier shall be calculated so that  collector performance can be characterized at other 
angles. 
The average value of the surrounding air speed, taking in to account spatial variatio ns over the collector and 
temporal variations during the test period,  shall lie in the range defined in 6.1.5.5. 
Unless otherwise specified, the fluid flowrate shall be set at approximately 0.04 kg/s per square metre of 
collector gross area. It shall be held stable to within ±1  % of the set value during each test period, and shall 
not vary by more than ±10 % of the set value from one test period to another. The testing at a lower flowrate may be accommodated by adhering to manufacturer’s specification. 
In some collectors the recommended fluid flowrate may be close to the transition region between laminar and 
turbulent flow. This may ca use instability of the internal heat trans fer coefficient and he nce variations in FDUS 854-2: 2009 
42 © UNBS 2009 – All rights reserved
 measurements of collector efficiency. In order to characte rize such a collector in a reproducible way, it may be 
necessary to use a higher flowrate, but this s hall be clearly stated with the test results. 
Measurements of fluid temperature difference of less t han 1 K shall not be included in the test results because 
of the associated problems of instrument accuracy. 
6.2.4.4  Test procedure 
The collector shall be tested over its operating temper ature range under clear sky conditions in order to 
determine its efficiency characteristic. Data points  which satisfy the requirements given below shall be 
obtained according to Table 7. 
One inlet temperature shall be selected so that the mean fluid temperature in the collector lies within ± 3 K of 
the ambient air temperature, in order to obtain an accurate determination of η0. 
NOTE 1  Under consideration of the condition that the fluid inlet temper ature shall be higher than the dew point 
temperature of the surrounding air, the inle t temperature should be chosen such that Tm = Ta ± 3 K applies. However, the 
inlet temperature should in no case be lower than the dew point temperature. 
Table 7 – Range of thermal performance test conditions 
Test point Net irradiance 
Wm-2 Tm (mean temperature) 
K Air Speed parallel to Collector 
ms-1 
1 > 650 Tm = Ta ± 3K < 1 
2 > 650 Tm = Ta ± 3K 1.5 ± 0.5 
3 > 650 Tm = Ta ± 3K 3 ± 0.5 
4 > 650 Tm = Ta  + 0.5 (Δtmax) ± 3K < 1 
5 > 650 Tm = Ta + 0.5 (Δtmax) ± 3K 1.5 ±  0.5 
6 > 650 Tm = Ta + 0.5 (Δtmax) ± 3K 3 ± 0.5 
7 > 650 Tm = Ta + 0.5 Δtmax± 3K < 1 
8 > 650 Tm = Ta + 0.5 Δtmax± 3K 1.5 ± 0.5 
9 > 650 Tm = Ta + Δtmax ± 3K 3 ± 0.5 
 
Δtmax is the expected maximum temperature difference between absorber mean temperature and ambient 
temperature in real operation. 
NOTE 2  For typical swimming pool applications this Δtmax will usually be limited to about 10 K or less. 
At least two independent data points shall be obtained for each fluid inlet temperature. If test conditions 
permit, an equal number of data points shall be taken before and after solar noon for each fluid inlet 
temperature. The latter is not requir ed if the collectors are moved to follow the sun in azimuth and altitude 
using automatic tracking. 
During a test, measurements shall be made as specified in 6.2.4.5. These may then be used to identify test 
periods from which satisfacto ry data points can be derived. 
6.2.4.5  Measurements 
The following data shall be measured: 
-  collector gross area AG and absorber area AA, measured at operating pressure conditions; 
-  fluid capacity; FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  43
 -  global solar irradiance at the collector area; 
-  diffuse solar irradiance at the collector area (only outdoors); 
-  long wave thermal irradiance in the collector plane (or dew point temperature tdp); 
- surrounding air speed; 
- surrounding air temperature; 
-  temperature of the heat transfe r fluid at the collector inlet; 
-  temperature of the heat transfe r fluid at the collector outlet; 
-  flowrate of the heat transfer fluid. 
6.2.4.6 Test period (steady state) 
Specification given in 6.1.4.6 applie s, taking into account Table 8. 
Table 8 – Permitted deviation of measured parameters during a measurement period 
Parameter Symbol Deviation from the mean value
Total short wave solar irradiance G ±50 Wm-2 
Long wave thermal irradiance E L ±20 Wm-2 
Surrounding air temperature ta ±1 K 
Fluid mass rate .
m ±1 % 
Collector fluid inlet temperature tin ±0.1 K 
Surrounding air speed u ±0.5 ms-1 
 
6.2.4.7  Presentation of results 
Specification given in 6.1.4.7 applies. 
6.2.4.8  Computation of collector output. 
6.2.4.8.1 General 
Specifications given in 6.1.4.8.1 apply, with the following additions: 
The test results shall be used to compute efficiency η from the following equation: 
''GQ
Λ=η               ( 1 8 )  
G” is the net irradiance determined by the equation 
                                            G ” = G + (ε/α) (EL - σΤa 4)                  ( 1 9 )  
the value ( ε/σ) shall be taken 0.85 unless the manuf acturer can supply a measured value. 
EL is the measured longwave irradiance in the collector plane. FDUS 854-2: 2009 
44 © UNBS 2009 – All rights reserved
 .
Q is the useful power output, calculated from:   () in ef ttcmQ −=. .
            ( 2 0 )  
A value of cf appropriate to the mean fluid temperature shall be used. 
If the fluid mass flowrate m is obtained from volumetric flowrate measurements, then the density shall be 
determined for the fluid at the temperature in the flowmeter. 
The test data shall be correlated by curve fitting usin g the least squares method to obtain an efficiency 
function of the form 
   () ( )()
''1 2 1 0Gttubb uba m
u−−−−=ηη             ( 2 1 )  
 ηo, bu, b1 and b2 are coefficients to be determined by curve fitting. 
If instrumentation is not available for measuring long wave irradiance EL, the following clear sky long wave 
model may be used to determine sky emittance  εs from measured dew point temperature tdp. 
2
10073.010056.0 711.0 ⎟
⎠⎞⎜
⎝⎛+ +=dp dp
st tε          ( 2 2 )  
where the dew point temperature tdp shall be measured to a standard uncertainty of 0.5 K. 
The long wave irradiance is calculated by the expression: 
   as sT4σε=Ε                  ( 2 3 )  
If the collector is inclined there will be thermal radiation exchange with both the sky and ground. 
The relative long wave irradiance Eβ on a collector inclined at an angle β is given by: 
   2cos1
2cos1 4 4 βσεβσεβ−++=Εagas T T           (24) 
The ground temperature will have little influence on long wa ve radiation on a collector inclined at less than 
since the view factor between a collector and the ground is only 0.15 for β = 45° 
In this case, Equation (24) can be written as: 
   2cos14βσεβ+=ΕΤas                ( 2 5 )  
Thus, in Equation (19) the longwave irradiance EL in the collector plane is equal to Eβ when the collector is 
located outdoors 
NOTE 1 Positive EL values are a downward oriented irradiance onto a surface with a temperature of 0 K. 
NOTE 2 When calculating Es Equation 23 should be used. 
6.2.4.8.2  Solar energy intercepted by the collector 
The solar energy intercepted is AG" and so in this case FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  45
   ''.
AGQ=η                  ( 2 6 )  
6.2.4.8.3 Reduced temperature difference 
Specification given in 6.1. 4.8.3 applies, by replacing G by G". 
6.2.4.8.4 Modeling of instantaneous efficiency 
6.2.4.8.4.1 General Specification given in 6.1.4.8.4. 1 applies, by replacing G by G". 
6.2.4.8.4.2 Instantaneous efficiency 
Specification given in 6.1.4. 8.4.2 applies, by replacing G by G". 
6.2.4.8.5 Graphical presentation of the collector output 
Specification given in 6.1. 4.8.5 applies, by replacing G by G" 
6.2.5  Steady-state efficiency test using a solar irradiance simulator 
6.2.5.1  General 
Specification given in 6.1.5.1 applies. 
6.2.5.2  The solar irradiance simulator for steady-state efficiency testing 
Specification given in 6.1.5.2 applies. 
6.2.5.3  Test installation 
Specification given in 6.1.5.3 applies. 
6.2.5.4  Preconditioning of the collector 
Specification given in 6.1.5.4 applies. 
6.2.5.5  Test procedure 
The collector shall be tested over its operating temp erature range in order to determine its efficiency 
characteristic. One inlet temperature shall be selected such that the mean fluid te mperature in the collector 
lies within ±3 K of the ambient air temperature, in order to obtain an accurate determination of η 0 
The range of thermal performance test conditions is specified in Table 7. 
At least two independent data points shall be obtained for each fluid inlet temperature. During a test, 
measurements shall be made as  specified in 6.2.4.5. 
6.2.5.6 Measurements during tests in solar irradiance simulators. 
Measurements shall be in accordance with 6.1.5.6. 
6.2.5.7 Test period 
The test period shall conform to 6.1.5.7. FDUS 854-2: 2009 
46 © UNBS 2009 – All rights reserved
 6.2.5.8 Test conditions 
Test conditions shall be in accordance with 6.1.5.8. 
6.2.5.9 Computation and presentation of results Results shall conform to 6.1.5.9. 
6.2.6  Determination of the effective thermal capacity and the time constant of a collector 
6.2.6.1 General 
The effective thermal capacity and the time constant of  a collector are important parameters which determine 
its transient performance. 
A collector can usually be considered as a combinati on of masses, each at a different temperature. When a 
collector is operating, each collector component responds differently to a c hange in operating conditions, so it 
is useful to consider an effective t hermal capacity for the whole collector. 
Unfortunately, the effective thermal capacity depends on the operating conditions and is not a collector 
parameter with a unique value. Several different te st methods have been used to measure the effective 
thermal capacity of collectors and it has been shown that similar results can be obtained by using quite 
different methods. 
Just as there is no unique value of effective thermal ca pacity, there is no unique overall time constant for a 
collector. For most collectors, the dominant influence on t he response time is the fluid flowrate. Other collector 
components respond with different times to give an e ffective overall time constant which depends on the 
operating conditions. 
6.2.6.2 Determination of effective thermal capacity 
The effective thermal capacity of the collector C (expressed as joules per Kelvin) shall be calculated as the 
sum, for each constituent element of the collector (abs orber, heat removal fluid), of the product of its mass mi 
of collector (expressed in kilograms), times its specific heat ci (expressed as joules per kilogram Kelvin) 
   ∑=
iiicm C                   ( 2 7 )  
Effective thermal capacity may also be measured by  applying the procedures described in Annex G. 
6.2.6.3  Test procedure for collector time constant 
The test procedure shall be in  accordance with 6.1.6.3. 
6.2.6.4  Calculation of collector time constant 
The time constant shall be calcul ated in accordance with 6.1.6.4. 
6.2.7  Incidence angle modifier (optional) 
6.2.7.1 General 
For solar beam incidence which is not near normal, the efficiency η 0 in Equation (21) may be replaced by 
Κθη 0 where Kθ is the incidence angle modifier. 
    () ( )''1 2 1 0Gttubb ub Ka m
u−+−−=ηη θ              ( 2 8 )  FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  47
 Figure 6 shows the typical variation of Kθ with incidence angle for an unglazed solar collector. 
For those collectors for which the incidence angle effect s are not symmetrical with direction of incidence 
specifications given in 6.1.7.1 apply. 
DIAGRAM   
 
 
  
 
  
 
  
 
Figure 6 – Typical incidence angle modifier 
The significance of the incidence angle modifier to t he test procedures outlined in this clause is that the 
thermal efficiency values are determined for the collector at or near normal incidence conditions. Therefore, 
the y intercept η of the efficiency curve is equal to η 0. 
A separate measurement shall be conducted to determine the value of Kθ so that the performance of the 
collector can be predicted under a wide range of co nditions and/or time of day using Equation (28). 
6.2.7.2 Solar irradiance simulator for the measurement of incidence angle modifiers 
Specification given in 6.1.7.2 applies. 
6.2.7.3 Test procedures 
Specification given in 6.1.7.3 applies. 
6.2.7.4 Calculation of collector incidence angle modifier 
Regardless of which experimental method in 6.2.7.3 is us ed, values for the thermal efficiency of the collector 
shall be determined for each value of angle of incidence. 
For unglazed collectors, only one angle of incidence is needed which is 50°. 
NOTE  A rating standard using this test method may require that Kθ be measured for a different set of angles of 
incidence. 
The mean temperature of the fluid shall be held very  close to the ambient air temperature so that ( tm - ta) ≈ 0 
The relationship between Kθ and the efficiency is: FDUS 854-2: 2009 
48 © UNBS 2009 – All rights reserved
  
   ()
0ηθη
θ=K                   ( 2 9 )  
Since η 0 will have already been obtained as the y-axis in tercept of the efficiency curve, values of Kθ can be 
computed for the different angles of incidence (see 6.2. 7.3). If the mean fluid temperature cannot be controlled 
to equal the ambient air temperature within ±1 K, the value of Kθ shall be computed as: 
   () ( )
() ubGttubb
K
ua m
−⎟
⎠⎞⎜
⎝⎛−++
=1''
02 1
ηθη
θ              ( 3 0 )  
Due to more exact results Equation (17) should be used generally. Alternatively, each data point may be 
plotted on the same graph with the efficiency curve determ ined in accordance with 6. 2.4 or 6.2.5, and a curve 
drawn through each point parallel to the effi ciency curve and made to intersect the y-axis. 
The values of the y intercept are the efficiency va lues that would have resulted had the mean fluid 
temperature been controlled to equal ambient air temp erature. Therefore, these values may be used in 
conjunction with Equation (28) to compute the different values of Kθ. 
6.2.8  Determination of the pressure drop across a collector 
If determination of pressure drop across a collector is required, this should be done according to Annex L. 
6.3    Glazed and unglazed solar collectors under quasi-dynamic conditions 
6.3.1  Collector mounting and location 
6.3.1.1  General 
Collectors shall be located and mounted in accordance with 6.1.1.1. 
6.3.1.2  Collector mounting 
Glazed collectors shall conform to 6.1.1.2 and un glazed collectors shall conform to 6.2.1.2. 
6.3.1.3  Tilt angle 
Glazed collectors shall conform to 6.1.1.3 and un glazed collectors shall conform to 6.2.1.3. 
6.3.1.4 Collector orientation outdoors 
The collector shall be mounted outdoors in a fixed position facing the equator, within ± 5°. 
NOTE  The azimuthal deviation of collector (or pyranomet er) from due south, should be taken into account when 
calculating the angle of incidence of so lar radiation onto the collector aperture . Larger deviations from south may be 
acceptable, but will lead to a nonsymmetrical angular distributio n of beam radiation in Figure 8 (see 6.3.4.6.2). This may 
lead to slightly biased incidence angle dependence of the colle ctor. The actual incidence angle should be calculated with a 
standard uncertainty better than ±1°. 
In case of concentrating collectors, the tracking device of  the manufacturer should be used if possible. In case 
of non imaging stationary collectors as  CPCs, they should be mounted so that the beam radiation from the 
sun falls within the angular acceptance range of the design. 
6.3.1.5  Shading from direct solar irradiance FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  49
 Shading shall be in accordance with 6.1.1.5. 
6.3.1.6  Diffuse and re flected solar irradiance 
Specification given in 6.1.1.6 applies. Last section of 6.1.1.6, text about solar simulator tests is not applicable. 
6.3.1.7  Thermal irradiance 
The performance of some collectors is particularly sensitive to the levels of thermal irradiance. The 
temperature of surfaces adjacent to the collector shall be as close as possible to that of the ambient air in 
order to minimize the influence of thermal radiation. Fo r example, the outdoor field of view of the collector 
shall not include chimneys, cooling towers or hot exhausts . Shielding is important both in front of and behind 
the collector. 
6.3.1.8  Surrounding air speed 
Glazed collectors shall conform to 6.1.1.8 and un glazed collectors shall conform to 6.2.1.8. 
6.3.2  Instrumentation 
6.3.2.1  Solar radiation measurement 
Measurements shall co nform to 6.1.2.1. 
6.3.2.1.1 Pyranometer 
Pyranometers shall conform to 6.1.2.1. 1 with the following exception: Subc lause 6.1.2.1.1.5 is not applicable. 
6.3.2.2  Thermal radiation measurement Measurements shall be in accordance with 6.2.2.2. 
6.3.2.3  Temperature measurements 
Measurements shall co nform to 6.1.2.3. 
6.3.2.4  Measurement of collector liquid flowrate 
Measurements shall be in accordance with 6.1.2.4. 
6.3.2.5  Measurement of air speed 6.3.2.5.1 General 
Measurements shall be in accordance with 6.1.2.5.1 
6.3.2.5.2 Required accuracy 
The speed of the surrounding air ov er the front surface of the collect or shall be measured to a standard 
uncertainty of 0.5 ms
-1 (glazed collectors) and 0.25 ms-1 (unglazed collectors). Under outdoor conditions the 
surrounding air speed is seldom constant, and gusting fr equently occurs. The measurement of an average air 
speed is therefore required during the test period. This  may be obtained by time integration over the test 
period. 
6.3.2.5.3 Mounting of sensors 
When testing outdoors in locations where the mean air speed lies below 2 ms-1 an artificial wind generator 
shall be used, and anemometer measurements shall be fitt ed for continuous measurement of air velocity. This 
anemometer shall be mounted on a board so that t here is continuous surface pointing towards the wind FDUS 854-2: 2009 
50 © UNBS 2009 – All rights reserved
 generator from the collector edge to 0.3 m behind the anemometer. The uniformity of air speed in the field of 
collector aperture shall be checked as the air speed ma y vary from one end of the collector to the other. A 
series of air speed measurement shall therefore be tak en, at a distance of 100 mm in front of the collector 
aperture and equally spaced positions over the collector area. An average value shall then be determined and 
related to the continuous measurement. 
In windy locations, the wind speed measurement shall be made near to the collector at the mid height of the 
collector. The sensor shall not be shielded from the wi nd and it shall not cast a shadow on the collector during 
test periods. 
6.3.2.6 Pressure measurements 
Measurements shall be in accordance with 6.2.2.6. 
6.3.2.7 Elapsed time 
Time shall be measured in accordance with 6.1.2.6. 
6.3.2.8 Instrumentation/data recorders 
Instruments shall conform to 6.1.2.7. 
6.3.2.9 Collector area 
The collector area shall conform to 6.1.2.8. 
6.3.2.10 Collector fluid capacity 
Fluid capacity shall conform to 6.1.2.9. 
6.3.3  Test installation 
The installation shall be tested in accordance with 6.1.3. 
6.3.4  Outdoor efficiency test 
6.3.4.1  Test installation 
The installation shall be tested in accordance with 6.1.4.1. 
6.3.4.2  Preconditioning of the collector 
The collector shall be preconditioned in accordance with 6.1.4.2. 
6.3.4.3  Test conditions 
In the following, test requirements are given for what can be called a quasi-dynamic test method (QDT). To 
facilitate the understanding and accept ance of this kind of approach, t he recommended test sequence and 
other test requirements given here are closely connect ed to those widely accepted for steady-state testing of 
solar thermal collectors, as outlined in 6.1 and 6.2. Ba sically the demand on suitable test data are the same 
for both types of approaches, and hence the recommended test sequence will allow also for conventional 
steady-state parameter identificat ion, by obtaining and cutting out those measurement data sequences 
corresponding to steady-state requirements. The test method and recommended test sequence, will combine 
and allow for the evaluation of the ef fective thermal capacitance, inci dence angle modifier — (IAM), wind 
speed — as well as sky temperature — dependence of the collector efficiency. In the collector parameter 
identification phase the collector model  of actual useful power is used. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  51
 The average value of the surrounding air speed, taking in to account spatial variatio ns over the collector and 
temporal variations during the test  period, shall be greater than 1 ms-1 and less than 4 ms-1. Wind generators 
may be used if necessary to achieve sufficient wind speeds. 
Unless otherwise specified, the fluid flow rate shall be set at approximately 0.02 kgs-1 per square meter of 
collector reference area (A). It shall be held stable to wi thin ±1 % of the set value during each test period, and 
shall not vary by more than ±10 % of the set value fr om one test period to another. Testing at other flowrates 
can be accommodated by adhering to  manufacturer's specification. 
In some collectors the recommended fluid flowrate may be close to the transition region between laminar and 
turbulent flow. This may ca use instability of the internal heat trans fer coefficient and he nce variations in 
measurements of collector efficiency. In order to characte rize such a collector in a reproducible way, it may be 
necessary to use a higher flowrate, but this s hall be clearly stated with the test results. 
Measurements of fluid temperature diff erence of less than 1.0 K shall not be included in the test results, 
because of the associated probl ems of instrument accuracy. 
NOTE  As the quasi-dynamic method is based on the minimization  of the error in the power ou tput of the collector (not 
efficiency as in stationary testing described in 6.1 and 6.2) t he relative error of low fluid temperature differences will no 
longer cause a problem. 
Therefore the limitation to 1.0 K temperature difference can be removed in a later revision of the standard and 
more data from each test day can be used. 
6.3.4.4  Test procedure 
The collector shall be tested over its operating temper ature range under outdoor conditions in order to 
determine its efficiency characteristics. Data points which satisfy the requirements given below shall be 
obtained for at least 4 fluid inlet temperatures spac ed evenly over the operating temperature range of the 
collector. If possible, one inlet temperature shall be selected such that the mean fluid temperature in the collector lies 
within ±3 K of the ambient air temperature at around solar noon, in order to obtain an accurate determination 
The inlet temperature shall be kept above the dew point, so that condensation of water on the absorber is 
avoided, which otherwise would result in erroneous test results. Weather conditions shall be as described in 
6.3.4.6, sequence type 1 and 2. 
The second and third inlet temperature shall be selected so  that the mean fluid temperature in the collector is 
evenly spaced between the lowest and highest operating range of the collector, as measured at about solar 
noon. For unglazed collectors only 3 fluid inlet temperat ures are requested. The second shall then be chosen 
in to be close to the middle of the operating range of t he collector. Weather conditions shall be as described in 
6.3.4.6, sequence type 3. Depending on type of collector, the highest fluid inlet temperature shall be chosen as specified in 6.1.4.4 and 
6.2.4.4. 
The change in inlet temperature should be done after each test sequence has been completed. Data recorded 
during this "step-change" period shall not be included in  the test data. The inlet temperature shall be kept 
stable within ±1 K during each test sequence. 
NOTE 1  If comparison with steady-state parameters is to  be done, at least 4 data poi nts with the required duration 
should be obtained for each fluid inlet temperature. If te st conditions permit, an equal number of data points should be 
taken before and after solar noon for each fluid inlet temperature. 
NOTE 2 As the collector model used here, more accurately describes the collector perfo rmance, the importance of 4 
measuring points as well as independent data points within th ese is reduced or eliminated. In a later revision of this 
method, only 3 measuring points should be considered. The more  complete characterization of the collector also leads to 
fewer restrictions on the collector designs and a wider r ange of collectors will be covered by this test method. FDUS 854-2: 2009 
52 © UNBS 2009 – All rights reserved
 During a test, measurements shall be made as specified in 6.3.4.5. These may then be used to identify test 
periods from which satisfactory test data can be derived. 
6.3.4.5  Measurements and data acquisition 6.3.4.5.1 Measurements 
The following measurements shall be carried out: 
- aperture area A
a, the absorber area AA and the gross collector area AG; 
- fluid capacity; 
- global solar irradiance at the collector aperture; 
- diffuse solar irradiance at the collector aperture; 
- incident longwave radiation at the collector aperture; 
- angle of incidence of direct solar radiation or may be determined by calculation; 
- azimuth and tilt angle of the collector aper ture (standard uncertainty better than ±1°); 
- surrounding air speed; 
- surrounding air temperature; 
- temperature of the heat transfer fluid at the collector inlet; 
- temperature of the heat transfe r fluid at the collector outlet; 
- flowrate of the heat transfer fluid. 
6.3.4.5.2 Data acquisition requirements 
Sampling rate: 1s to 6 s 
Averaging interval: 5 min to 10 min 
Each data line (record) shall contain a unique time label  (standard uncertainty better than ±1 min), giving the 
possibility to calculate the angle of incidence of solar r adiation onto the collector for each such data line (time 
period). See also under 6.3.1.4. The following on line calculations should be perfo rmed and included in the measurement database: 
 - useful output power of the collector or Q 
  -  time derivative of t
m in the collector, i.e. dtm/dt as (tm new - tm old) / (sampling interval for tjn and te) 
The calculation of time derivative dtm/dt should be performed on-line as it has a big impact on the final results. 
Sampling rate and averaging interval as for measured values. 
NOTE  If the measurement system allows for on line calcul ation of collector model output with expected collector 
parameters, this will be a very useful tool to detect any measurement error or problem. If not and in general, it is 
recommended to plot a diagram of measured output  versus modelled output after each test day. 
6.3.4.6  Test period FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  53
 6.3.4.6.1 General 
The recommended test sequence consists of 4-5 days. Th e number of actual days will, as for all outdoor 
collector testing, be dependent on the actual weather condit ions on the test site. The data record shall contain 
data equivalent to all important normal operating co nditions (enough variability and dynamic range), to give 
decoupled collector parameters. This is done similar to the stationary method (see 6.1 and 6.2), by varying the 
inlet temperature to the collector within its design ra nge. If sufficient data has been recorded after 4-5 days, 
this data shall be evaluated for each test day, follo wing the guidelines outlined below in 6.3.4.6.2. 
6.3.4.6.2 Description of test sequences 
The minimum length of a test sequences according to the requirements of 6.3.4.3 shall be 3 h. The test 
sequence under η 0 -conditions as specified in 6.3.4.4 should be conducted under mostly clear sky conditions. 
It shall include values of the incident angle from lar ger than 60° down to values where the difference of the 
incident angle modifier of the beam irradiance differs not  more than 2 % from the value at normal incidence. 
One test sequence shall be conducted under partly cl oudy conditions, including broken cloud as well as clear 
sky conditions. This can be a test sequence under elevated operating temperature or under η 0 –conditions 
specified in 6.3.4.4. 
The relative order of the different test  sequences is not critical, but may be  adjusted to the actual weather on 
the test site. 
6.3.4.6.3 Optional test: Dependence of tilt angle 
If dependence of tilt angle shall be evalua ted, one extra test day shall be added. 
During this day, the collector shall be tested at t he other requested tilt angle  and at the high operating 
temperature (Day type 4). This additional test dat abase may be evaluated with extended MLR, together with 
and at the same time as all other collector parameters. 
NOTE  Extended MLR, see note 2 under 6.3.4.8.1. 
6.3.4.6.4 Evaluation of test data 
In the following, guidelines for evaluating the suitability of the data recorded are outlined. It is reminded that 
when evaluating the suitability of test data the following criteria should be satisfied: 
-  Tout – Tin > 1 K 
-  Tin stable within ± 1K 
-  flowrate stable within ± 1% of the set value during test day or test sequence and 10 % from one sequence to       
another 
During evaluation of the test data a pre-conditioning period of  at least 4 times the time constant of the collector 
(if known), or not less than 15 min (if time constant is not  known), with the correct fluid temperature at the inlet 
shall be skipped to ensure that the initial state of the co llectors fades away and does not influence the result of 
the parameter identification. It is also noted that outliners that cannot be explained shall not be excluded from the data set. 
For clearness reasons, most requirements are given in  the form of idealized diagrams, showing important 
relationships between different test conditions, includi ng the dynamic ranges that shall be in the data to§; 
achieve reliable and de-coupled collecto r parameters. These diagrams sha ll be plotted for the evaluation of 
the goodness of the test data used for parameter ident ification and shall be included in the test report. FDUS 854-2: 2009 
54 © UNBS 2009 – All rights reserved
 Figure 7 shows tm – ta versus G* to check if sufficient data has been taken under η0-conditions and at higher 
inlet temperatures. This  data will give all necessary info rmation for the identification of F'(τα)en and the 
collector heat losses. 
 
Figure 7 – tm - ta versus G* 
Figure 8 and 9 show if the data includes  enough data at high and low incident angle of the beam irradiance to 
identify Kθb(θ) and if enough data at high diffuse radiation levels was taken to identify Kθd. 
 
Figure 8 – G b versus θi 
NOTE  Measurement data with higher Gb-values (upper curve), will give Kθb(θ). The lower values will give Kθd. 
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  55
 
 
 
Figure 9 – G d versus G* 
If wind speed dependence of the collector was considered  Figure 10 shall be included. Figure 10 shows the 
ideal distribution of the relati onship of wind speed versus G* 
The wind speeds as described in 6.3.4.3 should be considered . 
 
 
Figure 10 – Wind speed versus G* 
6.3.4.7  Presentation of test results 
The test results shall be presented in a report usin g the data format sheets, gi ven in Annex D and Annex E 
and with the text and content adjusted in accordance with what is given by 6.3 (see also under 6.3.4.8.4). The 
measurements shall be collated to produce a set of dat a points, which meet the requirements of the test 
conditions and provide sufficient information in the test  data. In addition to what is given by Annex D and E, 
the measurement data used for collector parameter identific ation shall be presented in four diagrams, 1 to 4 
as described under 6.3.4.6.4, Figure 7 to Figure 10. A diagram 5, showing measured collector output versus 
modelled output, shall also be included in the test report. Diagram 5 shall include all test data used for 
collector parameter identification in one diagram (see also 6.3.4.5.2, note). The incidence angle modifier 
(IAM), Kθb(θ)shall be presented in a diagram 6 as indicated in Figure 5 or 6. 
In addition to the collector performance coefficients as  requested by Annex D or E, the full set of quasi-
dynamic performance coefficients as identified by Equation (32), should be included in the test report. 
6.3.4.8  Parameter identification and co mputation of useful collector output FDUS 854-2: 2009 
56 © UNBS 2009 – All rights reserved
 6.3.4.8.1 Collector parame ter identification tool 
Multiple Linear Regression (MLR), is a non iterative very fast matrix method,  that is available in most standard 
programme packages with statistical functions, such as spreadsheets or more specialized statistical 
programmes like MINITAB or SISS. Linear, in this instance means that the model has to be written as a sum 
of terms with the parameters pn as a multiplier in front of the terms. 
For example: Yout = p0 + p 1f(x1, x2) + p 2g(x1, x3, x4) + p 3h(x2, x5) 
The sub models f(x..) g(x..) and h(x..) in each term can be highly non-linear. 
The MLR method allows for fully free selection of dat a from the test database, according to any test 
specification before applying the MLR parameter identification. 
This selection can be made afterwards from measurements during a few days. 
NOTE 1  To give an example, this  means e.g., that test data with G* > 700 Wm-2, dtm/dt < 0.002 Ks-1, u > 2 ms-1 and ta – 
ts > 10k could be selected for the MLR parameter identification,  if those test requirements  are considered. Even for an 
extensive database, the parameter identification will need only a few seconds of comput er time, making MLR very 
versatile also in development and research. 
NOTE 2  For several years, a special ca se of MLR (extended MLR) has also been tested out, which makes it possible to 
identify the same parameter in different subsets of the database. This  has made it possible to identify for example the zero 
loss efficiency angle by angle without the need to have an equation and also in two axis θL and θtKθb (θi ) is than 
generalized and replaced by Κθb (θL ,θt ) in Equation (32). The parameters  can still be identified with standard 
MLR software in the same run. This is very useful fo r special collectors as ETC, CPC or unglazed collectors 
with round separate absorber tubes that cannot be modelled with the standard IAM equations. The derived 
IAM results can be used directly in simulation prog rammes like TRNSYS, WATSUN or  MINSUN. Recently it 
was also found, that the heat loss factor can be identified in successive ranges of ΔT. This overcomes the 
problem of a slight correlation between the ΔT and ΔT2 terms. The heat loss coefficient can also be modelled 
in this way for collectors with special heat loss effects as heat pipe collectors or other special designs. 
Other non linear methods, provided that  they are minimizing the error in the output power of the collector as in 
the MLR method, can be used as parameter ident ification tool as beside the MLR method. 
6.3.4.8.2 The Collector Model for different collector 
This model is basically the same as the steady-state  model used in 6.1 and 6.2, but with some extra 
correction terms. Here the dependence of direct and diffu se radiation, wind speed, sky temperature, incidence 
angle effects and effective thermal capacitance are modelled. For more information see Annex H. 
Q/A = F '(τα)enKθb(θ)Gb + F'(τα)enKθdGd – c6 uG*- c1 (tm - ta) –c2 (tm - ta)2-c3 u(tm- ta) + c4 (EL - σTa4)- c5dtm/dt   (32) 
where the area is AA when referred to the absorber area of the collector and Aa when referred to the aperture 
area of the collector (see Annex J). 
NOTE  Kelvin degrees are used for convenienc e in the radiation term. At all other pl aces Celsius degrees are used (see Clause 4 .) 
6.3.4.8.3 Use of the collector model for different collector types 
The collector model as described in 6.3.4.8.2, should cover most collector designs available on the market, 
except ICS collectors. If the full collector model should be applied for a certain type of collector (or collector 
design) or not, will in general be given by the result of the parameter identification, but for all types of 
collectors, the use of F’(τα )en, Κθb(θ), Κθd and the coefficients c1, c2 and c5 are mandatory and they should be 
identified. 
NOTE 1  For sun tracking, high concentrating collectors the inclusion of Κθd may not always be significant and should 
therefore be determined by the T-ratio of the parameter identification as given below. Then Kθb (θ) = 1.0 and Κθd = 0 
should be used in Equation (32) and the par ameter identification should be repeated. FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  57
 If the coefficients c3, c4, and c6 shall be included in the collector model, will be determined by the T-ratio 
(parameter value / standard deviation of parameter val ue) of the parameter identification. The T-ratio should 
be greater than 2 for those parameters presented in the test results. If the T-ratio is less than 2, (enough 
variability in the input data is assumed), the coefficien t shall be set to zero and the parameter identification 
should be repeated with the adjusted collector model. 
For unglazed collectors, the use of t he full collector model is mandatory. 
NOTE 2  Empirically found, the full collector model including  capacitance, diffuse and incidence angle corrections is very 
accurate. If the model does not fit to the data, in most cases the problem will be f ound in the collector, the test installatio n 
or the measurements. 
6.3.4.8.2 Graphical presentation of test results 
To conform with the presentation of test results, when te sting in accordance with 6. 1 and 6.2, the test results 
shall be presented in the form of a power curve as a function of the temperature difference between mean 
fluid and ambient temperature ( tm – ta), which shall be calculated from the power function, Equation 32, using 
the value of G* = 1000 Wm-2 and a diffuse fraction of 15 %, i.e. Gd = 150 Wm-2. The parameter d tm/dt is set to 
zero and θi to 15° (d tm/dt = 0 and θi = 15) to adjust to stationary operating conditions around solar noon 
(Equation 32.1). If wind speed dependence of the heat loss es and the zero loss efficiency are used in the 
collector model for glazed collectors (c 3>0 and c 6>0) as outlined in 6.3.4.8. 3, the wind speed u = 3 ms-1 should 
be used in the equation. If sky temperature dependence of  the heat loss coefficient is used in the collector 
model (c 4>0), then ( ( EL −σTa 4) = -100 Wm-2 should be used in the equation. 
.
Q =(AG*)(F’(τα)enKθb(15)0.85+ F’(τα) Kθd0.15−c6 (3m/s)−c1 (tm −ta)−c2 (tm−ta)2 −c3(3m/s)( t m−ta)−c4(100W/m2)) 
                            (32.1) 
Graphical presentation of test results for unglazed collec tors should be made accordingly, but with reference 
to Annex E. 
The product .
Q = (AG*)(F'(τα)enKθb(15)0.85+ F'(τα)enKθd0.15) shall be referred to as Wpeak. 
NOTE  ( EL −σTa 4 ) has normally a negative value as the effective sk y radiation temperature is lower than the ambient 
air temperature. A net longwave irradiance of minus 100 W m-2 will correspond to about a clear sky condition when 
ta = 20 °C and ts = 0 °C. 
6.3.5  Determination of the effective thermal capacity 
6.3.5.1  General 
The effective thermal capacity ( C) and the time constant of a colle ctor are important parameters which 
determine its transient performance. A collector can usua lly be considered as a combination of masses, each 
at a different temperature. When a collector is operat ing, each collector component responds differently to a 
change in operating conditions, so it is useful to cons ider an effective thermal capacity for the whole collector. 
The determination of c 5 (see Equation 32) requires a large enough variation in dtm/dt. In the test, this can only 
be achieved by variations in irradiation level as the inlet temperature is fixed due to the requirement for 
compatibility with testing according to 6.1 and 6.2. 
6.3.5.2  Procedure 
The effective thermal capacitance, modelled as c 5 and equal to C/A, is a mandatory part of the collector 
model, Equation 32, and is identified simultaneously together with all other collector parameters. 
It is essential to have large enough variability in solar radi ation during the test so that the thermal capacitance 
effects will be significant. In all empirical data so far, partly cloudy conditions will induce enough variability in 
dtm/dt for the determination of c 5. dtm/dt should exceed ± 0.005 K/s during the partly cloudy day. If the unlikely FDUS 854-2: 2009 
58 © UNBS 2009 – All rights reserved
 case happens that this is not met during the test perio d, an extra test day, type 2, as described under 6.3.4.6.1 
with partly cloudy conditions, shall be added to the data used for identification. 
6.3.6  Collector incidence angle modifier 
The Collector incidence angle modifiers (IAM), modelled as K θb(θ) for direct radiation and as Kθd for diffuse 
radiation (see also 6.3.4.8.3, Note 1), are mandator y parts of the collector model, Equation 32. These are 
identified simultaneously together with  all other collector parameters. 
The basic modeling of lAM-dependence of flat plate collectors shall be done with the equation 
Kθb(θ) = 1− b0 ((1/cosθ1) −1 )                 ( 3 3 )  
as described in e.g. ASHRAE 93-77. 
For those collectors (e.g. evacuated tube collectors a nd CPC collectors) for which the incidence angle effects 
are not symmetrical with direction of incidence, it is necessary to meas ure the incident angle effects from 
more than one direction to fully characterize the incident angle modifier. 
The complex individual incident angle modifier can be es timated by considering it to be the product of the 
separate incident angle modifiers, KθL and KθT for two perpendicular symmetry planes (Equation 33.1). 
          K θb(θ) = KθL ·KθT                     ( 3 3 . 1 )  
The longitudinal plane (index L) runs parallel to the optical axis of the collect or, and the transversal plane 
(index T) is perpendicular to the optical axis. The angles θL and θT are the projections of the incidence angle θ 
onto the longitudinal and transversal planes, respectively. 
For the correlation between θ, θL and θT, the following equation holds: 
tan2θ = tan2 = tan2θL + tan2θT                   ( 3 3 . 2 )  
While measuring the incident angle modifier in one pl ane of an optical unsymmetrical collector the incident 
angle within the other plane should be kept to a value where the incident angle modifier does not differ more 
than 2 % from the one at normal incidence. For collectors with special lAM-depen dence, see also note under 6.3.4.8.1. 
K
θb shall be modelled as a collector constant. 
For general information, also refer to 6.1.7. 
 
 
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  59
 Annex A 
(normative)  
 
Schematics for durabilit y and reliability tests 
A.1 
 DIAGRAM  
 
 
 
 
 
  
 
Figure A.1 – Schematic for internal pr essure test of inorganic absorbers 
 
 
  
 
 
 
 
  FDUS 854-2: 2009 
60 © UNBS 2009 – All rights reserved
  
 
Figure A.2 – Schematic for internal pressure test of organic absorbers for use in unglazed collectors 
 
 
 
  
 
  
 
  
 
 Figure A.3 – Schematic for internal pressure test of or ganic absorbers for use with oil-based fluids (hot oil 
source)   
 
  
 
 
 
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  61
  
Figure A.4 – Schematic for internal pressure test of or ganic absorbers for use with oil-based fluids (test under 
solar irradiance)  
 
 
  
 
  
 
  
 
  
 
 Figure A.5 – Schematic for internal pressure test of or ganic absorbers (pneumatic test under solar irradiance) 
   
 
 
 
 
  
 FDUS 854-2: 2009 
62 © UNBS 2009 – All rights reserved
  
Figure A.6 – Schematic for high-temperature re sistance test (outdoor of in simulator) 
  
 
 
  
 
  
 
  
 
  
Figure A.7 -  Schematic for exposure test 
  
 
  
 
  
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  63
  
 
Figure A.8 – Schematic for external thermal shock test 
 
 
 
  
 
  
 
  
 
  
Figure A.9 – Schematic for internal thermal shock test 
  
 
  
 
  
 
  
 FDUS 854-2: 2009 
64 © UNBS 2009 – All rights reserved
  
 
 
Figure A.10 – Schematic for rain penetration test 
 
 
  
 
  
 
  
 
  
 
 Figure A.11 – Schematic for freeze resistance test 
 
  
 
  
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  65
  
 
Figure A.12 – Schematic for mechanical test (positive pressure on the collector cover) 
 
 
 
  
 
  
 
  
 
  
 
 Figure A.13 – Schematic for mechanical test (negative pressure on fixings between the cover and the collector 
box)   
 
 
 
 
  
 FDUS 854-2: 2009 
66 © UNBS 2009 – All rights reserved
  
 
 
Figure A.14 – Schematic for mechanical test (n egative pressure on collector mounting) 
 
 
  
 
  
 
  
 
  
Figure A.15 – Schematic for impact resistance test using steel balls 
 
 
 
  
 
  
 
  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  67
  
 
Figure A.16 – Schematic for impact resistance test using ice balls 
 
 FDUS 854-2: 2009 
68 © UNBS 2009 – All rights reserved
 Annex B 
(normative) 
 
Durability and reliability test report sheets 
Collector Identification 
Manufacturer: 
Brand name: Collector Type:    Unglazed/Glazed/Evacuated 
Year of Production: 
Serial No: 
Drawing Document No; 
 
 
B.1  Record of test sequence and summary of main results 
All significant damage to the collector, including rain penetration, should be summarized in Table B.1.  Full 
details should be given in the individual test result sheets. 
Table B.1 
Date Summary of main test results Test 
Start  End  
Internal pressure    
High-temperature resistance    
Exposure    
First     External thermal shock 
Second     
First     Internal thermal shock 
Second     
Rain penetration    
Freeze resistance    
Mechnaical load    
Thermal performance    
Impact resistance (optional)    
Final Inspection     
 
Remarks :……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………… 
 Collector reference No: ………………………………………………………………FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  69
  
 
B.2  Internal pressure test for inorganic absorbers 
NOTE  See B.3 for internal pressure test for absorbers made of organic materials. 
B.2.1 Technical details of collector 
B.2.1.1 Collector type: 
     Glazed 
     Unglazed 
B.2.1.2 Maximum collector operating pressure specified by manufacturer:      k P a  
B.2.2 Test conditions 
  Test temperature:………………………………………………………………………………….oC 
  Test pressure:………………………………………………………………………………………kPa 
  Test duration:……………………………………………………………………………………….min 
B.2.3 Test results 
Give details of any observed or measured leak age, swelling or distortion and any of the failures 
denoting “major failure”. Defied in 5.3.1 of US 854-1. 
 ………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………..  
 
  
 
  
 
 Collector reference No : ……………………………………………………………… FDUS 854-2: 2009 
70 © UNBS 2009 – All rights reserved
  
 
 
B.3  Internal pressure test for absorbers made of organic materials 
NOTE See B.2 for internal pressure test for absorbers made of inorganic materials. 
B.3.1 Technical details of collector  
B.3.1.1 Collector type: 
      Glazed 
      Unglazed 
B.3.1.2 Maximum collector operating pressure specified by manufacturer     k P a  
B.3.1.3 Calculated collector stagnation temperature:          oC 
Provide details of calculation, showing input data used (attach separate page if necessary) 
B.3.2 Test conditions 
B.3.2.1 Fluid used to pressurize absorber: 
      Oil 
      Air 
      Other (specify) 
B.3.2.2 Method used to heat absorber: 
      Water bath 
      Heater in fluid loop 
      Natural solar irradiance 
      Simulated solar irradiance 
B.3.2.3 Measured absorber test temperature:            oC 
B.3.2.4 Final test pressure:                k P a  
B.3.2.5 Duration of test at final test pressure:            m i n  
 
 
  Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  71
  
 
 
B.3.2.6 
Intermediate test pressures 
 
kPa Duration of test at each 
intermediate pressure 
min 
 
 
  
 B.3.2.7 For absorber tested under irradiance   Collector tilt angle (degrees from horizontal):………………………………………..…..
o 
  Average irradiance during test:…………………………………………….………..……W/m2 
  Average ambient temperature during test:……………………………………………….oC 
  Average wind speed during test:……………………………………………………..…….m/s 
B.3.3 Test results 
Details of any observed or measured leakage, sw elling or distortion and the test pressure at 
which it occurred and any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1. 
……………………………………………………………………………………………………………… 
……………………………………………………………………………………………………………… 
……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… 
……………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………  
   
 Collector reference No : ……………………………… FDUS 854-2: 2009 
72 © UNBS 2009 – All rights reserved
  
 
 
B.4  High-temperature resistance test 
B.4.1 Method used to heat collectors 
      Outdoor testing 
      In solar simulator 
B.4.2 Test conditions 
B.4.2.1 General  
  Collector tilt angle (degrees from horizontal):………………………………………………………o 
  Average irradiance during test:…………………………………………………………….……….W/m2 
  Average surrounding air temperature:………………………………………………………      ….oC 
  Average surrounding air speed:……………………………………………………..……………….m/s 
  Average absorber temperature:……………………………………………….. …. ………………….oC 
  Duration test:………………………………………………………………………………..…………..min B.4.2.2 Additional information required if an evacuated tubular collector was tested 
  The temperature of the collector was measured at the location shown below: 
B.4.2.3 Additional information required if th e absorber temperature was measured using a 
special fluid (as described in 5.3.2, note 2) 
The absorber was partially filled with ………………………..and the average pressure as………………Pa, 
which corresponds to the average temperature given in B.4.2. 
B.4.3 Test results 
Given details of any observed or measured degradation, distor tion, shrinkage or outgassing and 
any of the failures denoting “major fa ilure”, defined in 5.3.1 of US 854-1. 
……………………………………………………………………………………………………………… 
……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… 
……………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………
…………………………………………. 
 Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  73
  
 
B.5  Exposure test 
B.5.1 Test conditions 
  Collector tilt angle (degrees from horizontal); 
In Tables B.2 and B.3 full details should be given of  the climatic conditions for all days during the 
test, including: 
- daily global irradiation, H (MJ/m2); 
- periods when the global irradiance G and the surrounding air temperature t a have values 
greater than those specified in Table 4; 
- surrounding air temperature, ta (oC); 
- rain (mm). 
B.5.2 Test results 
Inspection should be conducted according to B.5.5. A full description and evaluation should be given of 
any problems or failures observed, including any of t he failures denoting “major failure”, defined in 5.3.1 
of US 854-1, together with  appropriate photographs. 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… 
 
  
 
  
 
 Collector reference No : ……………………………… FDUS 854-2: 2009 
74 © UNBS 2009 – All rights reserved
  
 
 
B.5.3 Climatic conditions for all days during the test 
Table B.2 
Date H 
MJ/m2 ta 
oC Rain 
mm Date H 
MJ/m2 Ta 
oC Rain 
mm 
 
  
 
  
 
        
  Total:……………………………..days in which H>……………………………….MJ/m2 
B.5.4 Time periods in which irradiance and surrounding air temperature have values 
greater than those specified in Table 4 
Table B.3 
Date G 
W/m2 ta 
oC Time periods 
min 
 
  
 
  
 
    
  Total:  
 Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  75
  
 
B.5.5 Inspection results 
  Evaluate each potential problem according to the following scale: 
  0 - No problem 
  1 -   Minor problem 
  2 -  Severe problem 
  *  Inspection to establish the condition was not possible 
 Collector component   Potential problem 
         Evaluation 
a)  Collector box/fasteners   Cracking/warping/corrosion/rain penetration 
         ………………………… 
b) Mountings/structure    Strength/safety 
         ………………………….. 
c) Seals/gaskets     Cracking/adhesion/elasticity 
         ………………………… 
d) Cover/reflector     Crac king/crazing/buckling/delamination 
         ……………………….. 
         Warping/outgassing 
e) Absorber coating     Cracking/crazing/blistering          ………………………... 
Absorber tubes and headers   Deformation/corrosion/leakage/loss of bonding 
         ……………………. Absorber mountings     Deformation/corrosion 
f) Insulation      Water retention/outgassing/degradation 
   
 
 Collector reference No : ……………………………… FDUS 854-2: 2009 
76 © UNBS 2009 – All rights reserved
  
 
B.6  External thermal shock test: 
B.61 Test conditions 
B.6.1.1 General 
  Test performed 
   Outdoor       In solar irradiance simulator 
Test combined with exposure test 
   Y e s         N o    Test combined with high-temperature resistance test: 
   Y e s         N o  
  Collector tilt angl e (degrees from horizontal):………………………………………………....
o 
  Average irradiance during test:…………………………………………………………………W/m2 
  Minimum irradiance during test:………………………………………………………………..W/m2 
  Average surrounding air temperature:………………………………………………………….oC 
  Minimum surrounding air temperature:…………………………………………………………oC 
  Period during which the required operating conditions were 
  Maintained prior to external thermal shock:……………………………………………………min 
  Flowrate of water spray:…………………………………………………………………………kg/(sxm2 
  Temperature of water spray:………………………………………………………………….…oC 
  Duration of water spray: ………………………………… ……………………………………....min 
  Absorber temperature immediatel y prior to water spray:…………………………………....oC 
B.6.1.2 Additional information required if an evacuated tubular collector was tested 
  The temperature of the collector was measured at the location shown below: 
B.6.1.3 Additional information required if the ab sorber temperature was measured using a special 
fluid (as described in 5.5.2, note 2 ) 
  The absorber was partially filled with … …………and the average pressure was      Pa, 
  which corresponds to the absorber temperature given in B.6.1.1. 
B.6.2 Test results 
Give details of any cracking. Distortion, condensation, water penetration or loss of vacuum found and 
any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1 when examining the collector 
after the test. 
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………… Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  77
  
 
B.7  Internal thermal shock test : 
B.7.1 Test conditions 
B.7.1.1 General 
  Test performed: 
   Outdoors     In solar irradiance simulator 
   Test combined with exposure test 
    Y e s        N o  
   Test combined with high-temperature resistance test: 
    Y e s        N o     Collector tilt angle (degrees  from horizontal):………………………………………………....
o 
   Average irradiance during test:…………………………………………………………………W/m2 
   Minimum irradiance during test:………………………………………………………………..W/m2 
   Average surrounding air temperature during test…………………………………………….oC 
   Minimum surrounding air temperature:…………………………………………………………oC 
Period during which the required operating conditions were maintained prior to external  
Thermal shock……………………………………………………………………………..………min 
   Flowrate of heat transfer fluid:……………………………………………………………….kg/(s-m2 )  
   Temperature of heat transfer fluid:………………………………………………..……….…oC 
   Duration of heat transfer fluid flow:…………………… ……………………………………....min 
   Absorber temperature immediately prior to heat transfer fluid flow:……………………...oC 
B.7.1.2 Additional information required if an evacuated tubular collector was tested 
  The temperature of the collector was measured at the location shown below: 
B.7.1.3 Additional information required if the ab sorber temperature was measured using a special 
fluid (as described in 5.6.2, note 2 ) 
The absorber was partially filled with … …………and the average pressure was………….Pa, which 
corresponds to the absorber te mperature given in B.7.1.1. 
B.7.2 Test results 
Give details of any cracking, distortion, deformati on, water penetration or loss of vacuum found and 
any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1 when examining the collector 
after the test. 
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………… Collector reference No : ……………………………… FDUS 854-2: 2009 
78 © UNBS 2009 – All rights reserved
  
B.8  Rain penetration test 
B.8.1 Test conditions 
B.8.1.1 Collector mounting 
Collector mounted on: 
   Open frame 
   Simulated roof 
Collector tilt angle (degrees from horizontal): 
B.8.1.2 Method used to keep absorber warm: 
   Hot water circulation 
   Exposure of collector to solar radiation 
B.8.1.3 Water spray 
 W a t e r  s p r a y  f l o w r a t e :                 k g / ( s - m2) 
D u r a t i o n  o f  w a t e r  s p r a y :                h  
B.8.2 Test results 
 Area with visible sign of water penetration (expressed as a percentage of aperture area):    % 
Give details of water penetration, reporting the pl aces where water penetrated and the time the sign of 
rain penetration took to vanish. 
……………………………………………………………………………………………………………………… 
…………………………………………………………………………………………………………………….. 
Give details of any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1 
……………………………………………………………………………………………………………………. 
……………………………………………………………………………………………………………………. 
…………………………………………………………………………………………………………………… 
………………………………………………………………………………………………………………….. 
   
 
 Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  79
  
 
B.9  Freeze resistance test 
B.9.1 Collector type 
   Freeze-resistant when filed with water 
   Drain-down 
B.9.2 Test conditions 
B.9.2.1 Tilt angle of collector during test (degrees fro horizontal): B.9.2.2 Details of freeze-thaw cycles 
No. of freeze-thaw 
cycles Freeze conditions  Thaw conditions  
 Test temperature 
oC Duration 
min Test temperature Duration 
min 
1 
2 
3     
For freeze-resistant collectors, this is the temperature of the contents of the collector, e.g. water, ice. 
For drain-down collectors, this is the temperatur e measured inside the absorber close to the inlet. 
 
B.9.2.3 Rate of chamber cooling: 
B.9.2.4 Rate of chamber heating: 
B.9.3 Test results 
Give details of leakages distortion or deformat ion and any of the failures denoting “major failure” 
defined in 5.3.1 of US 854-1. 
  ………………………………………………………………………………………………………………   ………………………………………………………………………………………………………………   ……………………………… ……………………………… ……………………………… ……………... 
  ……………………………… ……………………………… ……………………………… ……………... 
B.10 Mechanical load test 
B.10.1 Positive pressure test of the collector cover 
B.10.1.1 Method used to apply pressure: 
  Loading with gravel or similar material      Loading with water Collector reference No : ……………………………… FDUS 854-2: 2009 
80 © UNBS 2009 – All rights reserved
      S u c t i o n  c u p s  
     Pressurization of collector cover 
B.10.1.2 Test conditions 
Maximum pressure load: 
B.10.1.3 Test results 
Give details of any damage to the collector cover after the test, reporting the value of pressure load which 
caused the damage and any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1. 
…………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………… 
B.10.2 Negative pressure test of fixings between the cover and the collector box 
B.10.2.1 Method used to apply pressure : 
     Suction cups     Pressurization of collector box 
B.10.2.2 Test conditions 
   Maximum pressure load: 
B.10.2.3 Test results 
Give details of any damage to the collector cover or cover fixings after the test, reporting the value of 
pressure load which caused the damage and any of t he failures denoting “major failure”, defined in 5.3.1 
of US 854-1 
…………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………….. 
B.10.3 Negative pressure test of collector mountings 
B.10.3.1 Method used to apply pressure : 
     S u c t i o n  c u p s     A i r  b a g s  
B.10.3.2 Test conditions 
   M a x i m u m  p r e s s u r e  l o a d :                P a  
B.10.3.3 Test results 
Give details of any damage to the collector mounting fi xtures or fixing points after the test reporting the 
value of pressure load which caused the damage and any  of the failures denoting “major failure”, defined 
in 5.3.1 of US 854-1 Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  81
 …………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………… 
 
B.11 Impact resistance test using steel balls 
B.11.1 Test conditions 
  D i a m e t e r  o f  b a l l :                 m m  
  M a s s  o f  b a l l :                  g    
  Test performed using: 
   Vertical impact (dropping ball) 
   Horizontal impact (pendulum) 
B.11.2 Test procedure 
  Drop height 
m No. of drops 
  
  
  
  
  
B.11.3 Test results 
Give details of any damage to the collector and any of the failures denoting “major failure”, 
defined in 5.3.1 of US 854-1 ………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………… 
 
B.12 Impact resistance test using ice balls 
 B.12.1 Test conditions 
   Diameter of ball:………………………………………………………………………………………mm 
   Mass of ball:…………………………………………………………………………………………..g 
   Velocity of ball:………………………………………………………………………………………m/s 
 B.12.2 Test procedure 
   No. of impacts: 
 B.12.3 Test results 
Give details of any damage to the collector and any of the failures denoting “major failure”, 
defined in 5.3.1 of US 854-1 Collector reference No : ……………………………… FDUS 854-2: 2009 
82 © UNBS 2009 – All rights reserved
 ……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………… 
B.13 Final inspection results 
  Evaluate each potential problem according to the following scale: 
  0 - No problem 
  1 - Requirement apart from testing not fulfilled 
  2  - Requirements for testing not fulfilled 
  *  - Inspection to establish the condition was not possible 
Collector component    Potential problem        Evaluation 
a)  Collector box/fasteners   Cracking/warping/corrosion/rain penetration  …………… 
b) Mountings/structure    Strength/safety        ……………. 
c) Seals/gaskets     Cracking/adhesion/elasticity     ……………. 
d)  Cover/reflector     Cracking/cr azing/buckling/delamination/   ……………. 
         Warping/outgassing 
e) Absorber coating     Cracking/ crazing/blistering of bonding   ……………. 
 Absorber tubes and headers  Deformation/corrosion/leakage/loss of bonding ……………. 
 Absorber mountings    Deformation/corrosion       ……………. 
f)  Insulation      Water retention/outgassing/degradation   …………….. 
 
 
 
  
 
  
 
  
 Collector reference No : ……………………………… FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  83
  
 FDUS 854-2: 2009 
84 © UNBS 2009 – All rights reserved
 Annex C 
(normative)  
 
Stagnation temperature of liquid heating collectors 
C.1  General 
This annex provides a method for calcul ating the stagnation temperature of a collector, i.e. the temperature of 
the collector during periods of no useful heat removal fr om the collector with high solar radiation and ambient 
surrounding temperatures. 
The stagnation temperature should be deter mined for a selected solar irradiance Gs and a selected ambient 
temperature tas: 
The calculated stagnation temperature is used for determining the test temperature for:  
- internal pressure testing of collectors with organic absorbers (see 5.2.2); 
- high-temperature resistance test using a hot fluid loop (see 5.3). 
C.2   Determination of stagnation temperature 
The stagnation temperature tstg, for the selected values of solar irradiance G s and ambient temperature tas, is 
calculated by extrapolating from measured steady-state values of: 
-  solar irradiance Gm (natural or simulated) on the collector plane; 
-  surrounding air temperature tam; 
-  absorber temperature tsm 
while the collector is exposed to the available solar irradiance and ambient temp erature (outdoors, or in a 
solar irradiance simulator) under steady-state conditions wi thout heat extraction from the collector (stagnation 
conditions). 
The expression for determining the stagnation temperature for the selected parameters ( Gs and tas) is: 
   () am sm
ms
as stg ttGGt t −+=  
It is based on the approximation that ratio () m am sm Gtt /− remains constant under steady-state collector 
stagnation conditions. 
This approximation is acceptable only if the irradiance level (G m) used during the test is within 10% of the 
irradiance specified for the stagnation conditions (G s). FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  85
 Annex D 
(normative)  
 
Performance test report for glazed solar collectors 
D.1  General 
Collector reference No: 
Test performed by: 
Address: 
Date, Tel. Fax: 
D.2  Solar collector description 
Name of manufacturer: 
Brand Name:       Serial No: Collector Type: Glazed/Evacuated  Drawing Document No: 
Year of Production: 
F l o w  r a n g e :                 t o  k g / s  
O p e r a t i n g  p r e s s u r e :               k P a  
Stagnation temperature at 1000 W/m
2 
and 30 oC ambient temperature:           oC 
Collector mounting: 
Collector 
Type name: 
Flat plate/evacuated/subat mospheric: Gross area: 
Aperture area: Absorber area: 
Dimensions of collector unit 
Length:    mm       Absorber area:    m
2 
Width:    mm       Aperture area:   m2 
H e i g h t :     m m        G r o s s  a r e a :     m2 
Weight empty: 
Fluid content: Number of covers: 
Cover materials: FDUS 854-2: 2009 
86 © UNBS 2009 – All rights reserved
 Cover thickness: 
Cover solar transmittance: 
Absorber 
Material: 
Fin width: 
Fin thickness: 
Solar absorptance α; 
Hemispherical emittance ε: 
Surface treatment: 
Construction type: 
Number of risers: 
Riser diameter or dimensions: Distance between risers: 
Dimensions: 
Thermal insulation and casing  
Thermal insulation thickness: 
Insulation material: 
Casing material: Sealing material: Limitations: 
Maximum operation temperature: 
Maximum operation pressure:       other limitations: 
Photograph of the collector 
Comments on collector design Schematic diagram of collector mounting 
Schematic diagram of the test loop 
Heat transfer medium:       water/oil/other Specifications (additives etc): 
Alternative acceptance heat transfer fluids: 
  FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  87
 D.3  Test results 
Thermal performance has been tested based on test methods : 
  16.1 Outdoor-steady     6.1 Indoor – Steady    16.3 Outdoor-Quasi-dynamic 
  State Method     State Method       Method 
Outdoor             Indoor 
Latitude:           Mean solar irradiance: Longitude:          Type of the lamps 
Collector azimuth:         shading of longwave radiation:  yes    no Collector tilt: 
Orientation of absorber tubes during testing (horizontal or vertical): 
Peak Power (G=1000 W/m
2) per collector unit……………………………………………..W peak 
Power output per collector unit (W): 
Irradiance 
Tm-Ta in=2 K 400 W/m2 700 W/m2 1000 W/m2 
10    
30    
50    
NOTE The reported values are for normal incidence: 
 
 10000                      
                       
Power output                       
per 
collector 8000                      
unit (W)                       
                       
 6000                      
                       
                       
 4000                      
                       
                       
 2000                      
                       
                       
 0                     
  0  20  40  60  80  100 
(tm-ta) [K] 
Figure D.1 – Power output per collector unit (for G = 1000 W/m2) FDUS 854-2: 2009 
88 © UNBS 2009 – All rights reserved
 Instantaneous efficiency curve based on (absorbe r/aperture) area and mean temperature of heat 
transfer fluid. 
Reference area: Absorber area AA      Reference area: Aperture area Aa 
absorber area used for curve in m2 : aperture area used for curve in m2 : 
The instantaneous efficiency is defined by: 
  GAQ
ΑΛ=.
η                   ( D . 1 )  
  GAQ
aa.
=η                   ( D . 2 )  
Fluid flowrate used for the tests:               kgs-1 
G r o s s  c o l l e c t o r  a r e a :                   m2 
Second order fit to data: 
2
2 1 ⎟
⎠⎞⎜
⎝⎛−−⎟
⎠⎞⎜
⎝⎛−−= Α ΑΑΑGttGaGttaa m a m
oηη  
2
2 1 ⎟
⎠⎞⎜
⎝⎛−−⎟
⎠⎞⎜
⎝⎛−−=GttGaGttaa m
aa m
a oa aηη  
Based on Absorber Area  Std. Deviation Based on Aperture Area  Std. Deviation 
ηoA   ηoa   
a1A   a 1a   
a2A   a 2a   
In case of 6.3, test results according to Annex J should be attached to this document 
 
Time constant 
τc = s 
Effective thermal capacity 
C = JK-1 
Determination: 
Calculation: Indoors: 
Outdoors: FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  89
 Incident angle modifier 
Angle: K
ө: 
Observed failures 
Give details of any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1 
Delivery of sample: 
Start of test: End of test: 
Test Institute:…………………………………….   Date:………………………………… FDUS 854-2: 2009 
90 © UNBS 2009 – All rights reserved
 Annex E  
(normative) 
 
Performance test report for unglazed solar collectors 
E.1  General 
Collector reference No: 
Test performed by: Address: 
Date, Tel, Fax: 
E.2  Solar collector description 
Name of manufacturer: 
Brand Name:              Serial No: 
C o l l e c t o r  T y p e :              D r a w i n g  D o c u m e n t  N o :  
Year of Production: 
F l o w  r a n g e :               t o  k g / s  
Operating pressure:            kPa 
Stagnation temperature at 1000W/m2 
And 30 oC ambient temperature:         oC 
Collector mounting: 
Collector: 
Type name: Flat plate / evacuated / sub atmospheric: 
Gross area: Aperture area: 
Absorber area: 
Serial No:  Drawing Document No: 
Dimensions of collector unit 
Length:      mm      Absorber area:       m2 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  91
 W i d t h :        m m       A p e r t u r e  a r e a :        m2 
H e i g h t :        m m       G r o s s  A r e a :         m2  
Weight empty: 
Fluid content: 
Absorber: 
Material: 
Fin Width: Fin thickness: 
Solar absorptance α: 
Hemispherical emittance ε: 
 Surface treatment: 
Construction type: Number of risers: 
Riser diameter or dimensions: 
Distance between risers: Dimensions: 
Limitations: 
Maximum operation temperature: 
Maximum operation pressure at 45 
oC: 
Maximum operating pressure at maximum temperature of operation: 
Other limitations: 
Photograph of the collector 
Comments on the collector design 
Schematic diagram of collector mounting Instantaneous efficiency 
Heat transfer medium:          water / oil / other 
Specifications (additives etc): Alternative acceptance heat transfer fluids: FDUS 854-2: 2009 
92 © UNBS 2009 – All rights reserved
 E.3  Test results 
Thermal performance has been tested based on test methods: 
 16.1 Outdoor – Steady     6.1 Indoor – Steady   16.3 Outdoor-Quasi –dynamic 
  State Method       State Method      Method 
Outdoor:           Indoor: 
Latitude:             Mean solar irradiance: Longitude:            type of the lamps: 
Collector tilt:            shading of longwave radiation:  yes   no Collector azimuth: 
Peak Power (G = 1000 W/m
2) per collector unit:…………………………………………………………W peak 
Power Output per collector unit (W): 
Irradiance 
Tm-Ta = 2K 400 W/m2 700 W/m2 1000 W/m2 
u< 1 m/s    
u = 1.5 ± 0.5 m/s    
u = 3 ± 0.5 m/s    
 
The power output per collector unit shall be presented graphically according to Figure E.1 for the following 
wind conditions: u< 1 m/s, u = 1. 5 ± 0.5 m/s  and u = 3 ± 0.5 m/s 
 10000                      
                       
Power output                       
per 
collector 8000                      
unit (W)                       
                       
 6000                      
                       
                       
 4000                      
                       
                       
 2000                      
                       
                       
 0                      
  0  20  40  60  80  100 
(tm-ta) [K] 
Figure E.1 – Power output per collector unit  
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  93
 Instantaneous efficiency curve based on collector area and mean temperature of heat transfer fluid. 
The instantaneous efficiency is defined by:  ".
GQ
Α=η            ( E . 1 )  
Collector area used for curve:  m2 
Fluid flowrate used for tests:  kgs-1 
       () ( ) ⎟
⎠⎞⎜
⎝⎛−+−−="1 1 2 0Gttubb uba m
uηη        ( E . 2 )  
Based on Absorber Area  Based on Aperture Area  
ηoA  ηoa  
buA  b ua  
b1A  b 1a  
b2A  b 2a  
In case of 6.3, test results according to Annex J should be attached to this document 
Time constant 
τc = s 
Effective thermal capacity 
C = JK-1 
Determination: 
Calculation: 
Indoors: 
Outdoors: Incident angle modifier 
Angle: 
:θΚ 
Observed failures 
Give details of any of the failures denoting “major failure”, defined in 5.3.1 of US 854-1 
Delivery sample: 
Start of test: 
End of test: Test Institute:…………………………………   Date:…………………………………… FDUS 854-2: 2009 
94 © UNBS 2009 – All rights reserved
 Annex F 
(normative) 
 
Modelling of the coefficients c 1 to c 6 of the collector model of 6.3. 
f.i  
Coefficient c 1 is the Heat loss coefficient at ( tm - ta )  = 0          [ W m-2K-1] 
c1 is modelled as F’U0 
Coefficient c 2 is the Temperature dependence of the heat loss coefficient     [Wm-2K-2] 
C2 is equal to F’U1 
Coefficient c 3 is the Wind speed dependence of the heat loss coefficient      [Jm-3K-1] 
c3 is equal to F’Uu 
Coefficient c 4 is the Long wave irradiance dependence of the heat loss coefficient      [-] 
c4 is equal to F'ε 
NOTE  The modelling of the long wave irradiance dependence of the collector is made in the same principle way as 
described in the basic ISO 9806-3, 8.8, for test ing of unglazed collectors. The Net long wave irradiance, is defined as        
(EL −σT4
a ) and where EL is the measured long wave thermal irradiance in the collector plane. 
However, a purely mathematical difference exists between IS O 9806-3 and this standard that eliminates G" and the use of 
the ε/α- coefficient in the equation. Physically, the long wave radiation corrections are the same. In this standard, the 
correction factor for long wave radiation is treated as a separate  heat loss term and is not involved in an effective radiation  
term G" as in ISO 9806-3. The main reason for this is that the co llector equation is simplified by doing so, as this approach 
also takes into account incidence angle effects and diffuse radiation effects. In this case, α otherwise should be corrected 
for these effects. In ISO 9806-3, these fact ors are not taken into account and the equation is simplified by using G" and the 
ε/α-factor. 
Coefficient c 5 is the Effective thermal capacitance           [ J m-2K-1] 
c5 is equal to C/A (definition of C, see under 6.1.6.2 ). 
NOTE  c is often denoted ( mC)e in basic literature 
Coefficient c 6 is the Wind speed dependence in the zero loss efficiency      [sm-1] 
C6 is modelled as a collector constant 
Kθb(θ) is the Incidence angle modifier (IAM) for direct radiation         [-] 
The basic modelling of lAM-dependence is made with the equation 
Kθ(θ) = 1-b 0((1/cosθ1) - 1 )               ( H . 1 )  
as described in e.g. ASHRAE 93-77 
For collectors with special lAM-dep endence, see note 2 under 6.3.4.8.1. 
Κθd is the Incidence angle modifier for diffuse radiation 
K θd is modelled as a collector constant FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  95
 Annex G 
(normative) 
 
Measurement of effective thermal capacity 
G.1    Test installation 
The collector is mounted in accordance with the reco mmendations of 6.1.1 and coupled to a test loop for 
thermal capacity measurement. 
Effective thermal capacity measurem ents may be carried out indoors, where only heat loss is measured. They 
may also be made outdoors in steady-state clear sky c onditions, or in a solar irradiance simulator. 
G.2    Indoor test procedure 
G.2.1  General 
The heat fluid is circulated from the t op to the bottom of the collector with a constant inlet temperature, using a 
flowrate similar to that defined for collector efficien cy testing, until steady-state  conditions are reached. 
The inlet temperature of the fluid is raised rapidly by about 10 K, and measurements made continuously until 
steady-state conditions are achieved again. This process is performed four times and an arithmetic mean 
value of the effective thermal capacity calculated. 
G.2.2  Measurements 
The following quantities are measured: 
a)  heat transfer fluid mass flowrate; 
b)  temperature of the heat trans fer fluid at the collector inlet; 
c)  temperature of the heat trans fer fluid at the collector outlet; 
d)  surrounding air temperature. 
NOTE  When testing collectors having a low thermal capacity, the sampling fre quency selected for measuring the fluid 
temperatures may need to be greater than the usually used for collector efficiency testing, in order to adequately follow the 
transient behaviour of the collector. 
G.2.3  Calculation of the effective thermal capacity 
The transient behaviour of the colle ctor between the two indoo r steady states 1 and 2 is represented by the 
following equation: 
() a m fmttAU mCdtdtC −−ΔΤ−=               (G.1) 
where 
Δ T = (ta – ta)  ( n e g a t i v e )                 ( G . 2 )  
and tin and te are the heat transfer fluid temper atures at the collector inlet and outlet (exit), respectively, under 
the new flow direction of the heat transfer fluid. FDUS 854-2: 2009 
96 © UNBS 2009 – All rights reserved
 Integrating the equation over the peri od between the two steady-states gives 
() ( ) dtttdt mct tCt
tt
ta m mt m∫∫−ΔΤ−2
12
1.
1 2              ( G . 3 )  
Since 
2ΔΤ+=in mtt                  (G.4) 
we may express (tm-ta) as 
()2ΔΤ+−=− a m a m tt tt                ( G . 5 )  
Combining the above equations, and rearranging, gives the following equation for the collector thermal 
capacity. 
  ()
1 21 122
12
21
m mt
tt
ta mt
tf
t tdt dttt U dt mc
C−⎥
⎦⎤
⎢
⎣⎡ΔΤ+−Α−ΔΤ−
=∫∫∫
        ( G . 6 )  
G.2.4  Determination of effective thermal capacity from experimental data 
From the test results, ( tin - ta) and ΔT are plotted as a function of time. The areas under the curves, between 
the two steady states, are 
  () dtttt
ta in∫−2
1 and  ∫ΔΤ2
1t
tdt  
respectively. 
The heat transfer coefficient U of the collector may already have been determined during indoor collector heat 
loss measurement. However, AU may be obtained directly from two steady  states since in a steady state we 
have: 
0 = -mcf ΔΤ − ΑU( t m − ta )               (G.7) 
and hence 
a mf
ttmcAU−ΔΤ=                  ( G . 8 )  
AU is evaluated for both steady states, and arithmetic mean value taken. 
A value of the effective thermal capacity is determin ed by inserting these experimental values in Equation 
(G.6). FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  97
 G.3   Outdoor or solar irradiance simulator test procedure 
The fluid is circulated with a constant temperature, using a flowrate similar to that defined for collector 
efficiency testing, until steady-state conditions are reac hed. The aperture of the collector should be shielded 
from the solar radiation (natural or simula ted) by means of a solar-reflecting cover. 
The cover is removed and measurements are made cont inuously until steady-state conditions are achieved 
again. This process is performed four times and an ar ithmetic mean value of the effective thermal capacity 
deduced. 
The measurements indicated in G.2.2 are made. In additi on, the solar irradiance (natural or simulated) G is 
measured. 
The transient behavior of the collector between two steady  states 1 and 2 is repres ented by the following 
equation 
().
0 a m fmttU mcGdtdtC −Δ−Α=η              ( G . 9 )  
at where, as in G.2.3, 
 ΔΤ = (te − tin) (positive) 
Integrating the Equation (G.9) over the period between the two steady states gives the following equation for 
the collector thermal capacity: 
  ()
1 21 1 1022
12 2
21
m mt
tt
ta mt
tft
t
t tdt dttt U dt mc Gdt
C−⎥
⎦⎤
⎢
⎣⎡ΔΤ+−Α−ΔΤ−Α
=∫∫∫∫η
        ( G . 1 0 )  
From the test records, ( tm – ta), ΔT and G are plotted as a function of time. The areas under the curves, 
between the two steady states, are: 
  () dtttt
ta m∫−2
1,  ∫ΔΤ2
1t
tdt and  ∫2
1t
tGdt 
respectively. 
The y intercept η0 and the slope U of the linear form of the instantaneous efficiency η are known from testing. 
A value for the effective thermal capacity is determined by inserting these experimental values in Equation 
(G.10). 
 
  
 
  FDUS 854-2: 2009 
98 © UNBS 2009 – All rights reserved
 Annex H 
(informative) 
 
Comparison of the collector model of  6.1 to the collector model of 6.3 
hin 
In a comparison, we start to describe the present stati onary collector model used in 6.1. This model has been 
widely used both in testing (ISO 9806-1 and ASHRAE 93- 77) and for simulation. The basic equation is a 
stationary model for near normal incide nce angle operation and can be written as: 
 () ( ) ( )2
2 1.
* ' / a m a m en ttcttc G FAQ −−−− =τα           ( H . 1 )  
The irradiance is denoted G*, but could instead be denoted Gb to point out that only high irradiance levels are 
accepted in the test sequence, and thus a low diffuse fr action. No correction for non stationary conditions is 
made, so very stable inlet and radiation conditions are needed for each test point. 
Furthermore, it is assumed that the incidence angle is near normal, so that incidence angle effects can be 
neglected. 
In 6.1, there are furthermore optional test proc edures also for the determination of incidence angle 
dependence of the zero loss efficiency and the effective t hermal capacitance of the collector. Therefore the full 
instantaneous equation from all the options in 6.1 can be written as: 
() ( ) ( ) ( ) dtdtc ttcttc G K FAQ m a m a m b en / * ' / 52
2 1.
−−−−− = θτα θ        ( H . 2 )  
As a first step in the approach of 6.3,  the first term of the equation is divided into two parts, giving the sum of 
the zero loss efficiency for beam radiation and the diffuse radiation, or F’(τα )enKθb(θ)G* is divided into        
F’(τα )enKθb(θ)Gb + F’(τα )enKθd(θ)Gd while the rest of the equation is kept unchanged. 
To make it possible to test a much wider range of sola r collectors, but also to achieve a much more complete 
characterization of the collector from the same test and with the same method, just another correction is 
added to the collector model, namely the correction for the wind-dependence. In the 6.1-method, this is 
handled by limiting the test requirements for wind-spe ed to 2-4 m/s for glazed collectors during test. For 
unglazed, their sensitivity to wind-speed has resulted in  the need for a test method (6.2) of their own, which 
prescribes that 3 different efficiency functions have to  be measured for each collector at 3 given wind-speeds. 
A fact that makes testing of each colle ctor expensive and time consuming. For test sites with varying climate, 
such tests can also be difficult to conduct outdoors. 
In this approach, wind-dependence is modeled by 2 te rms added to the basic equation. One gives the effect 
on the zero loss efficiency (- c6uG*) and the other the effe ct on the heat losses (-c 3u(tm-ta)). After a final 
addition of the Long-wave irradiance dependence of the heat losses (+c 4(EL–σ Ta4)), modeled as done for 
unglazed collectors (see Annex F), the collector model is  completed and is written as in Equation 7. Equation 
7 gives the output power of the collector per  square meter of the reference area used 
  
 
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  99
  
 FDUS 854-2: 2009 
100 © UNBS 2009 – All rights reserved
 Annex I 
(informative) 
 
Properties of water  
1.1   Density of wate r (at 1 bar) in kg/m3 
ρ(ϑ) = a0 + a1ϑ + a2ϑ2 + a3ϑ3 + a4ϑ4 
(0 ≤ ϑ ≤ 99.5 °C) 
with 
a0 =   999.85 
a1=   6.187 × 10-2 
a2=   -7.654 × 10-3 
a3=  -3.974 × 10-5 
a4=   -1.110 × 10-7 
The deviation of the polynomial to the values published in tables is always smaller than 0.02 %. R2 equals 
0.99998 
1.2   Specific heat capacity of water (at 1 bar) in kJ/(kg K) 
cp(ϑ) = a0 + a1ϑ + a2ϑ2 + a3ϑ3 + a4ϑ4 
(0 ≤ ϑ ≤ 99.5 °C 
with 
a0=   4.217 
a1=   -3.358 × 10-3 
a2=   1.089 × 10-4 
a3=   -1.675 × 10-6 
a4=   1.309 × 10-8 
a5 =  -3.884 × 10-11 
The deviation of the polynomial to the values published in tables is always smaller than 0.02%. R2 equals 
0.9994   FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  101
 Annex J 
(informative) 
 
Performance test report summary for quasi dynamic test method 
Identification: 
Manufacturer: 
Brand Name: ............. ................... ................... .................. ................ ................ .............. .................. Serial No: 
Collector Type: ......................................................................................................... Drawi ng document No: 
Dimensions of Collector unit: 
Length:     mm      Absorber ar ea: .......... ................ .............. ........ m2 
Width:     mm      Aperture area: ................................................ m2 
Height:     mm       Gross area (casing): ....................................... m2 
General Specifications: 
Weight: ....................................................................................................................... ....................... kg 
Heat transfer fluid: Flow range: ................................................................................................................... .................... to l/h 
Operating pressure: ........................................................................................................... ................ bar 
Stagnation Temperature at 1000Wm
-2, 
and 30 °C ambient temperature ...... ................... ................... .................. .................. ................. ........ °C 
 
 
  
 
 
 
 
  
 FDUS 854-2: 2009 
102 © UNBS 2009 – All rights reserved
 Thermal performance based on: 
Aperture Area:                Absorber Area:  
  
 Value Standard 
deviation  Value Standard 
deviation 
F’(τα)en   F’( τα)en   
Kөd   K өd   
b0   b 0   
c1   c 1   
c2   c 2   
c3   c 3   
c4   c 4   
c5   c 5   
c6   c 6   
 
Table M.1 – Incidence angle modifier, Kөb(ө) 
ө 10 20 30 40 50 60 70 80 
Kөb         
This test report should be completed in conjunction with the test report according to Annex D or E. 
Tested by: 
Date: FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  103
 Annex K 
(informative) 
 
General guidelines for the assessment of uncertainty in solar collector 
efficiency testing  
K.1   Introduction 
The aim of this annex is to provide a general guidance fo r the assessment of uncertai nty in the result of solar 
collector testing performed according to the present standa rd. Testing laboratories are often invited to provide 
a statement of uncertainty in test results in quantitativ e tests, in the framework of their accreditation or of 
application of product certification schemes. It is not the aim of this annex to define whether and in which 
cases the calculation of uncertaint y in test results is necessary. 
This guidance concerns only in collector  efficiency testing due to i) the gr eat importance of the result of this 
testing for the user, and ii) the peculiarities of the calculat ions, since the final result of efficiency testing is not 
derived by a single measurement but by elaborati on of a large number of primary measurements. 
It is noted that the proposed methodology is one of  the possible approaches for the assessment of 
uncertainty, and other approac hes can be implemented. It is of the re sponsibility of each La boratory to choose 
and to implement a scientifically valid approach fo r the determination of uncertainties, following the 
recommendations of the accreditation bodies, where approp riate. For a more detailed review of the different 
aspects of determination of uncertainties in solar collect or testing see also (Mathioulakis et al., 1999; Sabatelli 
et al., 2002; Müller-Schöll and Frei, 2000). 
K.2   Measurement uncertainties in solar collector efficiency testing 
The basic target of solar collector efficiency testin g is the determination of the collector efficiency by 
measurements under specific conditions. More specifically , it is assumed that the behavior of the collector can 
be described by a M-parameter single node, steady state or quasi-dynamic model: 
η = c1 + c2 p2 +.... + cM pM ..................................................................................................... (K.1) 
where; 
η is the collector instantaneous efficiency. 
p1, p2,…., pN are quantities, the values of which are determined experimentally through testing 
c1, c2, ….., cM are characteristic constants of the colle ctor that are determined through testing. 
In the case of the steady state model, for example, M = 3, c1= η0 , c2 = U1: c3 = U2, p1 = 1, p 2  = (Tm - Ta)/G and 
p3 = (Tm - Ta)2/G 
During the experimental phase, the output, solar energ y and the basic climatic quantities are measured in J 
steady-state or quasi-dynamic state points, dependi ng the model used. From these primary measurements 
the values of parameters η , p1, p2, ..., pM are derived for each point of observation j, j = 1... J. Generally, the 
experimental procedure of the testing leads to a format ion of a group of J observations which comprise, for 
each one of the J testing points, the values of ηj, p1,j, p2,j, …., pM,j. 
For the determination of uncertainties, it is ess ential to calculate the respective combined standard 
uncertainties u(ηj), u(p1,j), …..., u(pM,j) in each observations point. It should be noted that in practice the 
uncertainties u(ηj), u(p1,j), …..., u(pM,j) are almost never constant and the same for all points, but that each 
testing point has its own standard deviation. FDUS 854-2: 2009 
104 © UNBS 2009 – All rights reserved
 For the calculation of the standard deviation (squared standard uncertainty) in each point j, the following 
general rules can be applied (ISO GUM): 
I.  Standard uncertainties in experim ental data are determined by taking into account Type A and Type B 
uncertainties. According to the recommendation of ISO GUM, the former are the uncertainties determined 
by statistical means while the la tter are determined by other means. 
II.  The uncertainty u(s) associated with a measurement s is t he result of a combination of the Type B 
uncertainty uB(s), which is a characteristic feature of the calibration setup, and of the Type A uncertainty 
uA(s), which represents fluctuation during sampling of  data. If there is more than one independent source 
of uncertainty (Type B or type A) uk, the final uncertainty is calculated according to the general law of 
uncertainties combination: 
2/1
2⎟
⎠⎞⎜
⎝⎛=∑
kkuU                   (K.2) 
III.  Type B uncertainty uB(s) derives from a combination of uncertain ties over the whole measurement chain, 
taking into account all available data, such as sens or uncertainty, data logger uncertainty, uncertainty 
resulting from the possible differences between the measured values perceived by the measuring device. 
Relevant information should be obtained from calibration certificates or other technical data related to the 
devices used. 
IV.  By nature, Type A uncertainties depend on the spec ific conditions of measur ement and they account for 
the fluctuations in the measured quantitie s during the measurement. Type A uncertainty uA(s) derives 
from the statistical analysis of experimental data. In  some cases (for example in the case of the steady-
state model), the best estimate of S is the arithmetic means s of the I repeated observations si(i = 1 … I) 
and its Type A uncertainty is the standard deviations of the mean: 
Ι=∑
Ι=I
iis
S ,  and ()()
()2/1
2
1⎟⎟⎟⎟
⎠⎞
⎜⎜⎜⎜
⎝⎛
−ΙΙ−
=∑Ι
Ι=Αiiss
su            (K.3) 
In some other cases (for example in the case of the quasi-dynamic model where no arithmetic mean of 
the repetitive measurement s is used) uncertainty uA(s) can be equal to zero. 
V.  The term combined standard uncertainty means the standard uncertainty in  a result when that result is 
obtained from the values of a number of other quant ities. In most cases a measured Y is determined 
indirectly from P other directly measured quantities X 1, X2, ..., X P through a functional relationship Y = 
f(X1, X2,, ...X P). The standard uncertainty in the estimate y is given by the law of error propagation: 
() ()() ()2/1
1
1122
, cov 2⎟⎟
⎠⎞
⎜⎜
⎝⎛
+⎟
⎠⎞⎜
⎝⎛=∑∑ ∑
=−
−+−P
jiP
iP
ijji
j ii
ixxdxdf
dxdfxudxdfyU         (K.4) 
An example of such indirect determination in the case of solar collector efficiency testing is the 
determination of instantaneous efficiency η, which derives from the values of global solar irradiance in the 
collector level G, fluid mass flowrate m, temperate difference, ΔΤ, collector area A and specific heat 
capacity c f. Thus, in this case the standard uncertainty u( η j ) in each value η j of instantaneous efficiency 
is calculated by the combinati on of standard uncertainties in the values of the primary measured 
quantities, taking into account their relation to the derived quantity η . FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  105
 K.3   Fitting and uncertainties in efficiency testing results 
During analyzing the data a least square fitting of the model  equation is performed, in order to determine the 
values of coefficients c1, c2, ..., cM for which the model of Equation (K.1) represents the series of J 
observations with the greatest accuracy. 
Since in reality the typical deviation is almost never c onstant and the same for all observations, but each data 
point ( ηj, p1,j, p2,j, ..., pM,j) has its own standard deviation σj, an interesting solution is the use of the weighted 
least square (WLS) method, which calculates, on the base of the measured values and their uncertainties, 
not only the model parameters but also their uncertainty . In the case of WLS, the maximum likelihood estimate 
of the model parameters is obtained by minimizing the chi-square function: 
()()∑
=++−=J
j jjMN j j j
upc pc pcx
122
, ,22 ,11 2 ... η           ( K . 5 )  
Where u j2 is the variance of the difference η j − (c1 p1, j + c2 p2 j + ...cN pM, j ) 
()() ()()()( ) () ( )2
,2 2
,1212
, ,22 ,112....... .... jM M j j jMN j j jj puc puc u pc pc pc Var u ++ += ++−= η η    (K.6) 
Finding coefficients c 1,c2, ..., cM and their standard uncertainties by  minimizing chi-square function is 
complicated, because of the non-linearity present in Equation (K.5). A strategy is therefore to find these 
uncertainties numerically. A method for the case of a M-parameter model is presented below (Press et 
al.,1996). 
Let K also be a matrix whose J×M components kj,m are constructed from M basic functions evaluated at the J 
experimental values of p1,..., pM weighted by the uncertainty uj,: 
JJM
JJM j
jim
mj
uP
uPuP
uP
KuPk
, ,11,1
1,1
,
, ,
−−−−−−−−−−−−
==              (K.7) 
Let also L be a vector of length J whose components I j are constructed from values of η j to be fitted, weighted 
by the uncertainty uj. 
J Jjj
j
uu
Lu
//
,1 1
ηη
η
−−==Ι                 ( K . 8 )  
The normal equation of the leas t square problem can be written: 
(KT • K) • INV(C) = KT •  L                ( K . 9 )  
where C is a vector whose elements are the fitted coefficients 
Given the fact that for the calculation of variances uj2 the knowledge of coefficients c1, c2,..., cM is needed, a 
possible solution is to use the values of coefficients ca lculated by standard least squares fitting as the initial 
values. These initial values can be used in Equation (K.6) for the calculation of  uj2 , J=1 ...J and the formation 
of matrix K and of vector L. FDUS 854-2: 2009 
106 © UNBS 2009 – All rights reserved
 The solution of Equation (K.9) gives the new values of coefficients c1, c2, ...., cM, which however are not 
expected to differ noticeably from those calculated by standard least squares fitting and used as initial values 
for the calculation of  uj2  . 
Moreover, Z=INV(KT • K) is a matrix whose diagonal elements zk,k are the squared uncertainties (variances) 
and the off-diagonal elements zk,I = zI,k, k ≠ I are the covariance between fitted coefficients: 
() M m z cu mm m ,....,1 ,,= =               (K.10) 
Cov (ck,cl ) = zk,l, k = 1,..., M and l = 1,..., M and k ≠ l         (K.11) 
It should be noted that the knowledge of covariance betw een the fitted coefficients is necessary if one wishes 
to calculate, in a next stage, the uncertainty u(η) in the predicted values of η using Equations (K.1) and (K.4). 
Equation (K.9) can be solved by a standard numerical met hod, for example, by Gauss-Jordan elimination. It is 
also possible to use matrix manipulation func tions of commonly used spreadsheet software. 
 
 
 
 
 
 
  
 
  
 
  
 
  FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  107
 Annex L 
(informative) 
 
Determination pf the pressure drop across a collector 
L.1    General 
The pressure drop across a collector may be of importanc e to designers of solar collector systems. The fluid 
used in the collector for the test shall be water or a mixture water: glycol (60:40), or a mixture recommended 
by the manufacturer. 
The temperature of the fluid shall be (20 ± 2) °C. 
L.2   Test installation 
The collector shall be mounted in accordance with 6.1. 1 and coupled to a test loop which conforms broadly 
with 6.1.3, although less instrum entation is required for pressure drop determination than for collector 
efficiency testing. 
The heat transfer fluid shall flow from the bottom to the top of the collector, and particular attention shall be 
paid to the selection of appropriate pipe fittings at the co llector entry and exit ports, as specified in 6.1.3.3. In 
the case of unglazed collectors the direction of t he fluid flow may be recommended by the manufacturer. 
L.3   Preconditioning of the collector 
The fluid shall be inspected to ensure that it is clean. 
The collector shall be vented of air by  means of an air bleed valve or other suitable means, such as increasing 
the fluid flowrate for a short period to force air from the collector. 
L.4   Test procedure 
a)  Glazed solar collectors 
The pressure drop between the collector inlet and ou tlet connections shall be determined for flowrates 
which span the range likely to be used in real oper ation. In the absence of specific flowrate 
recommendations by the manufacturer, pressure dr op measurements shall be made over the range of 
flowrates from 0.005 kgs-1 to 0.03 kgs-1 per square metre of collector area.  The origin of the pressure drop 
diagram — the point (0,0) should be considered as well. 
At least five measurements shall be made at val ues equally spaced over the flowrate range. 
b) Unglazed solar collectors 
The pressure drop between the collector inlet and outlet connections shall be determined with the 
collector and its fluid close to ambient air temperatur e, and for flowrates, which span the range likely to be 
used in the application for which the collector is intended. 
Because the arrangement of the strips and the manifold s used in the test usually differs from typical 
installations, the pressure drop of  a strip and of the manifolds shall be determined separately. This may 
be achieved by two consecutive pressure drop measurements of one short absorber strip including 
manifolds (at least 3 m) and one long absorber strip including manifolds (e.g. 15 m). 
The difference of both pressure drop curves may then be divided by the difference of strip lengths 
between both absorbers. The resulting curve is the undisturbed pressure drop per meter strip.  FDUS 854-2: 2009 
108 © UNBS 2009 – All rights reserved
 The test should be carried out at a constant pr essure which corresponds to the intended operating 
pressure. 
In the absence of specific flowrate recommendations  by the manufacturer, pressure drop measurements 
shall 1be made over the range of flowrates from 0.02 kgs-1 to 0.1 kgs-1 per square metre of collector area. 
At least five measurements shall be made at values  equally spaced over the flowrate range. The zero 
level shall be checked as well. 
L.5   Measurements 
The following data shall be measured in accordance with 6.1.2: 
a)  fluid temperature at the collector inlet; 
b)  fluid flowrate; 
c)  heat transfer fluid pressure drop between  the collector inlet and outlet connections. 
The heat transfer fluid pressure drop across the colle ctor shall be measured with  a device having a standard 
uncertainty of 5 % of the measured va lue or ±10 Pa whichever is higher. 
L.6   Pressure drop caused by fittings 
The fittings used to measure the fluid pressure may them selves cause a drop in pressure. A zero check on the 
pressure drop shall be made by removing the collector from the fluid loop and repeating the tests with the 
pressure-measuring fittings directly connected together . The pressure drop caused by the fittings shall be 
used to correct the measured pressure drop of the collector. 
L.7   Test conditions 
The fluid flowrate shall be held constant to within ± 1 % of the nominal value during test measurements. 
The inlet temperature of the heat transfer fluid shall be held constant to within ±5 K (±1 K for unglazed 
collectors) during test measurements. The test shall be carried out with the collector at a temperature which 
lies within ±10 K of that of the surrounding air. Pressu re drop tests at other temperatures may be important for 
oil-based heat transfer fluids. 
L.8   Calculation and presentation of results 
The pressure drop shall be presented graphically as a function of the fluid flowrate for each of the tests 
performed, using the format sheets given in Annex D (gla zed collectors) or in Annex E (unglazed collectors). 
 
 
 
  
 
  FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  109
 
 
 FDUS 854-2: 2009 
© UNBS 2009 – All rights reserved  
 Certification marking 
Products that conform to Uganda standards may be  marked with Uganda National Bureau of Standards 
(UNBS) Certification Mark shown in the figure below.  
The use of the UNBS Certification Mark is gove rned by the Standards Act, and the Regulations made 
thereunder. This mark can be used only by those licens ed under the certification mark scheme operated by 
the Uganda National Bureau of Standards and in co njunction with the relevant Uganda Standard. The 
presence of this mark on a product or in relation to a product is an assurance that the goods comply with the 
requirements of that standard under a system of supervision, control and  testing in accordance with the 
certification mark scheme of the Uganda National  Bureau of Standards. UNBS marked products are 
continually checked by UNBS for conformity to that standard.  
Further particulars of the terms and conditions of  licensing may be obtained from the Director, Uganda 
National Bureau of Standards.  
 
 FDUS 854-2: 2009 
 
ICS nn.nnn.nn 
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© UNBS 2009 – All rights reserved