^V' ''^‘.*

nj^ini me 111^

NBSIR 80-2087

Testing Flat- Plate Water-Heating

Solar Collectors in Accordan'ce

With the BSE and ASHRAE

Procedures

ill

Prepared for Department of Energy

Institutional Programs Branch

larket Development Division

of Solar Applications

fashington, D.C. 20545

NBSIR 80-2087

* A / TESTING FLAT-PLATE WATER-HEATING

SOLAR COLLECTORS IN ACCORDANCE

WITH THE BSE AND ASHRAE

PROCEDURES

NATIONAL BURBAU

OP aTAKDABOe

LIHRABY

HOV 2 4 1980

. U~ h

^0 5 7

f 1

John P. Jenkins and James E. Hill

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

August 1 980

Prepared for Department of Energy

Institutional Programs Branch

Market Development Division

Office of Solar Applications

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technolcgy, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TESTING FLAT-PLATE WATER-HEATING SOLAR COLLECTORS IN ACCORDANCE

WITH THE BSE AND ASHRAE PROCEDURES

by

J. P. Jenkins and J. E. Hill

ABSTRACT

Five solar collectors were tested according to the BSE and ASHRAE test procedures, and the results compared. All five collectors tested were modular,

flat-plate, and water-heating, and included single-and double-glazed designs

with and without selectively-coated absorbers. In both procedures, collector

efficiency curves are determined. The ASHRAE procedure consists exclusively

of outdoor testing, whereas the BSE procedure requires a combination of outdoor and indoor testing (no irradiation) to determine the collector's optical

and thermal loss characteristics, respectively. During the indoor testing in

this study, the environmental test conditions were controlled and regulated

by use of specially built environmental simulators to Investigate the effect

of wind and "sky" temperature on the thermal loss characteristics of the

collectors. The simulators provided stable, uniform wind speeds in the range

of 0 to 7.1 m/s across the collectors and "sky" temperatures above the collector ranging from t^ (ambient air temperature) to t^ - 19°K.

Key Words: Instantaneous efficiency; optical efficiency; solar collectors;

thermal losses; thermal performance testing.

iii
TABLE OF CONTENTS

Page

ABSTRACT ill

1. INTRODUCTION 1 2. BSE COLLECTOR TEST PROCEDURE 2

3. RELATIONSHIP BETWEEN THE BSE COLLECTOR TEST PROCEDURE AND

ASHRAE STANDARD 93-77 10

4. EXPERIMENTAL EQUIPMENT AND INSTRUMENTATION 17

5. EXPERIMENTAL TESTS 42

6. TEST RESULTS AND DISCUSSION 47

7. SUMMARY AND CONCLUSIONS 64

8. RECOMMENDATIONS 66

9. REFERENCES 67

10. NOMENCLATURE 69

APPENDIX A. ANALYSIS OF PROPAGATED RANDOM MEASUREMENT

UNCERTAINTIES 71

Iv
1. INTRODUCTION

In May 1978, the European Bundesverband Solarenergie (BSE) Working Group

completed and adopted "Guidelines and Directions for Determining the Usability of Solar Collectors" [1]. Part A of [1] describes tests and procedures

for determining the thermal performance of solar collectors.

The BSE procedure prescribes that the optical efficiency and thermal loss

characteristics of a collector be determined independently through a series

of outdoor and indoor tests. The optical efficiency is determined through

outdoor testing in which the operating conditions are regulated so that the

collector experiences negligible heat loss. The thermal loss characteristics

are determined under indoor laboratory conditions with zero irradiance, by

circulating the working fluid through the collector over a range of operating

temperatures above ambient air temperature. The two separately determined

properties are then used to construct the collector's normal incidence efficiency curve as a function of various operating conditions.

The BSE collector test procedure developed out of a need to improve the

reproducibility of collector test results and decrease the time required for

testing. Reproducibility of test results is improved by reducing the outdoor

testing to a minimum and combining it with indoor testing under controlled

laboratory conditions. The indoor testing also reduces the required test

time by minimizing the dependence of testing upon outdoor weather conditions.

As part of the National Bureau of Standards effort to develop and refine

collector test standards, a review and experimental evaluation of the BSE

procedure was undertaken. The approach was to compare the results from using

the BSE procedure with those obtained from using ASHRAE Standard 93-77.

Five commercially available collectors were tested according to both 'procedures and the resulting near-normal-incidence instantaneous collector efficiency curves were compared. The determination of collector time constant

as well as the incident angle modifier was not investigated due to the near-

“ Identical test requirements for each in both procedures.

Conceptually, the BSE procedure is not new. Prior research completed by

Symons [3], Christie [4], Smith [5], Whllller [6], and Reed [7] have all

involved similar approaches. However, there has been a reluctance to adopt

such a technique because of the uncertainty associated with indoor detenrined

thermal losses accurately reflecting collector thermal losses experienced

outdoors. Within this study, a wide spectrum of collectors were tested and

analyzed to assess the magnitude of such thermal loss discrepancies. In

addition, realizing that discrepancies will exist, collector thermal losses

were experimentally evaluated under a variety of indoor environmental conditions to determine, if possible, what indoor test conditions would give test

results which would best reflect outdoor collector thermal losses.

1
2. BSE COLLECTOR TEST PROCEDURE

General Principles

The useful energy output under quasi-steady state conditions per unit time

of solar collector, 0^, is the difference between the incident solar radiation absorbed by the absorber surface, Q^, and the rate of thermal energy

dissipated into the environment, Qj^.

Qu Based upon the usual definition of collector efficiency*. n

( 1 )

" = • Aa) ( 2 )

where

G = hemispherical solar irradiance

= collector aperture area

When = 0

n„ = Q„/(G • A^), (3)

Substituting equations (2) and (3) into (1) yields

1 =

"o - • A^) (4)

The two terms on the right side of equation (4), and Q^/CG • A^) , represent the collector optical efficiency and overall thermal losses, respectively. The significance of each term can be visualized by looking at the

1000 W/m^ collector efficiency curve in Figure 1. The collector optical

efficiency, tIq, is represented by the curve intercept whereas the slope is

indicative of the collector overall thermal losses.

The BSE procedure prescribes a series of tests for determining the collector

efficiency by independently measuring the optical efficiency, tIq, and the

thermal losses, experimentally through a combination of outdoor and

Indoor testing.

*ASHRAE Standard 93-77 uses gross collector area in equation (2)

.

Aperture

area is used throughout this paper in order to compare the results of the

ASHRAE and BSE procedures on the same basis.

2
If)

n

«M

r>

uo

IE

3

Figure

1.

Collector

efficiency

using

the

BSE

procedure

vs.

the

difference

between

collector

mean

fluid

temperature,

t

,

and

ambient

air

temperature,

t^.
Optical Efficiency Determination (Outdoor Testing)

Collector instantaneous efficiency, In general, Is determined by testing a collector outdoors and monitoring the useful energy output and Incident solar

radiation. The collector is operating under quasi-steady operating and environmental conditions and measurements of mass flow rate and temperature rise across the collector are required to determine the collector useful energy

output.

In the BSE procedure to obtain the collector optical efficiency, n^, the collector mean absorber fluid temperature, t^, defined by

t ^ ^f,o + ^f,i (5)

m 2

where

tj

Q = collector outlet fluid temperature

t£*^ = collector inlet fluid temperature

is regulated such that it is within + 10°C of the ambient air temperature,

t^. The result is negligible heat loss from the collector to the surrounding

environment. Since = 0, can be calculated from equation (3), using

- tf,i) dT (6)

where

m = mass flow rate through collector

C = specific heat of the collector working fluid

T = time

When I t_ - t- I > 10°C, n_ must be corrected to account for the collector

thermal losses, The thermal losses are added to the measured collector

useful energy output, Q^, to obtain Q^. The correction value of can be

determined for a specific (tjj^ - t^) following the procedures outlined in the

next section.

Thermal losses Determination (Indoor Testing)

The collector thermal losses, Qj^, are determined within a controlled indoor

laboratory environment under zero (<lW/m^) solar irradiation conditions. A

working fluid is circulated in reverse through the collector over a range of

4
operating temperatures above ambient, specifically (t^ - t^) = 30°, 50°, 70°

and 90°C, while the fluid flow rate and temperature drop across the collector

are monitored. Qj^ is then determined by:

~ '^1

is then plotted as a function of (t^j^ - t^) as in Figure 2.

(7)

During the indoor testing, the environmental test conditions are regulated

such that the air temperature remains between 15 to 25°C (59 to 77°F) while

the wind speed 7-9 cm (2. 7-3. 5 in) above the collector stays above 4 m/s

(9 mi/h). In addition, the apparent environmental radiance temperature*,

tenv> ®ust not deviate from the air temperature by more than + 3°K (+ 5.4°R).

Collector Test Requirements

The BSE procedure test requirements regarding the allowable environmental test

conditions, measurement uncertainties, and range of operating test conditions

are tabulated in Tables 1, 2 and 3. The BSE test requirements are separated

into outdoor, (Pq), and indoor, (Ql)> testing and are presented along with

ASHRAE Standard 93-77 collector test requirements. Notice that the requirements for outdoor testing following the BSE and ASHRAE Standard 93-77 are

near Identical in all categories.

Collector Thermal Performance - Data Display

Using the results obtained from both outdoor and indoor testing, a family of

collector efficiency curves are produced as shown in Figure 1 . The collector

efficiency is plotted as a function of (t^^^ - t ) for various solar radiation

levels. The collector curves are generated using equation (4). is

obtained as a function of (tj^ “ t^) from the experimental indoor heat loss

tests, A^ from the collector geometry, while G, the solar irradiance level,

is selected for the required curve. The solar irradiance levels required to

be used are 1000, 800, 600, 400 and 200 W/m^.

* Temperature of a perfectly black environment which would radiate the same amount of thermal radiation as the real environment.

5
Ml 6

Figure

2.

Collector

overall

thermal

losses,

Qx,

using

the

BSE

procedure

vs.

the

difference

between

collector

mean

fluid

temperature, t,

and

ambient
Table 1

Environmental Test

Environmental Parameter

Ambient air temperature

Wind velocity across collector

Total solar Irradlance

within collector plane

Beam solar Irradlance

Incident angle

Foreground reflectance

Apparent environmental

radiance temperature

Conditions Allowed Within The ASHRAE 93-77 And

BSE Collector Test Procedures

ASHRAE Standard

93-77

BSE*

r) -Determination,

Outdoor Testing

BSE*

•Qj^-De termination;

Indoor Testing

range < 30°C no limits 15 - 25°C

should be < 4.5

m/

s

> 4 m/s > 4 m/s

> 630 W/m^ no minimum < 1 W/m^

< 30° < 30°

< 0.20 < 0.20

< t^ + 3°K

a —

*BSE Guidelines and Directions for Determining the Usability of Solar Collectors, [1]
Measurement

Uncertainties

Allowed

Within

The

ASHRAE

93—77

And

BSE

Collector

Test

Procedures cO 60

•H

C *Wn

4J

CO

(U HMoo 0 T3 1 C

hJM

•O'a 60 0 c

•H -H

+J 4J

(0 CO C (U

K -H E-i H S

CO ^

pa Qj o

•u o

0) ’C3 Q 4J 1 d

oo c o

COu3ooco

CO

>-l 3

CJo

<3o

cO

>-l 3

CJ

CJ c u+ 1

CJ o+ 1

CJ+ 1

CJ o+ 1

CJ 6+ 1

CJ o+ 1 3

(U

0)

4-1 <U 4J

(U

CO 4-1

CJ oeg• o+ 1 ao • o+ 1

CJ 0cso+ 1 ooin• o+ 1

ego+ 1

CJ oin• o+ 1

1-^ + 1

S'? + 1

8s? + 1 M

(U 4J

0)S 6^ o cn 33

l-l fn

PX4 rH+ 1

CO 3 m 3 3 "H •H 0

CJ o3

4Jmu3

4J30 6>« o cn 33

»-l

34 + 1 m 3

0) c 3 •H rH 0 u o3

4-1 CO 3 3 3 O O O 3 3 3 3 t3 V4 3 U C 3 •H 3 3 4J X)

>4-1 3 3

>4-4 4-1 3 i-i •H 3 3 3

T3 iH 3- 4J •H 3 0 3

4-1 3 -H 3 i-i >-l 3 V4 4-1 3 3 3 M iH 0 4-1 O 3 O O 3 3 4-1 4J iH 3 3 l-> CJ 3 >4-1 3 3 3 iH rH

CO D- iH

O CO 3 3 0 rH in 3 4J 3 3 O 3 O S H O < X H 8
Table 3

Operating Test Conditions Required Within The ASHRAE 93-77 And

BSE Collector Test Procedures

BSE* BSE*

ASHRAE Standard nQ“Determination, Qt -Determination,

Operating Parameter 93-77 Outdoor Testing Indoor Testing

Setpoint Stability Setpoint Stability Setpoint Stability

Inlet fluid temperature

to collector

< + 0.5°C < + 0.5°C < + 0.5°C

Mass flow rate through 0.02 < + 1% 0.02 < + 1% 0.02 < + 1%

collector kg/s kg/s kg/s

or or or mfg. mfg. mfg.

spec

.

spec

.

spec.

Solar irradiance within

collector plane

> 630 W/m^ variation of

< + 3%

Specific heat of working

fluid

known + 0.5% Well Known Well Known

Density of working fluid known + 0.5% no requirement no requirement

Temperature difference

between collector inlet

and outlet

no requirement < 20°C none

Collector tilt angle no requirement 45° 45°

Stabilization interval 15 min 30 min 60 min

before testing

9
3. RELATIONSHIP BETWEEN THE BSE COLLECTOR TEST PROCEDURE

AND ASHRAE STANDARD 93-77

Transposing the BSE Thermal Performance Results to an ASHRAE Efficiency

Curve

The BSE and ASHRAE test procedures each prescribe a different format for

reporting the collector thermal performance results. The BSE results consist of a family of efficiency curves (for different values of Irradlance

levels) plotted as a function of the operating temperatures,

*^ni”^a‘ ASHRAE

Standard 93-77 prescribes that a single efficiency curve be plotted

with (t£ i“ba)/G as the x-axls variable.

In order to obtain a meaningful comparison between the BSE and ASHRAE collector performance results, It Is essential that the results be presented In

a common format. Consequently, the procedure below outlines a technique

for transposing the BSE efficiency curves Into an ASHRAE efficiency curve. The basic difference between them Is the x-axls term, (t -t„), for BSE and

(t£ for Standard 93-77. Therefore, the technique for transposing

efficiency curves Involves transposing the x-axls terms.

By definition

n =

Therefore

Aa G

(<^f,o " ^f,l)

n (G) A, JtC„

Considering

tm =

('"f ,o *"f , 1^

then

(^f,l ~ ^m)

n (G) A, 2(A)C„

Finally

n (G) A

[t - t ] - ; i

® ^ 2(m) C (t - t ) P = f . 1 a

( 8 )

(9)

(5)

( 10 )

( 11 )

G G

When transposing the BSE efficiency curves Into an ASHRAE curve, data points

are selected along one of the BSE efficiency curves, each with a corresponding

n and (tjjj - t^). Knowing n and (tjjj - t^) for each point and G, A^, A, and C

for the BSE efficiency curve, (tf^^ - t^)/G can be calculated for each data
point using equation (11). Finally, by replotting each data point using

the original efficiency value and the calculated value of (t^

^ - t^)/G for

each point, the ASHRAE curve results. *

Uncertainties Associated with the BSE and ASHRAE Collector Thermal

Performance Results

Figures 3 through 6 illustrate the range of expected uncertainties and

possible extremes associated with the thermal performance curves for collector no. 3* using the BSE and ASHRAE 93-77 collector test procedures.

Figure 5 is not required by the BSE procedure but is included here to

address the uncertainty in the BSE efficiency curve after being transposed

into an ASHRAE format.

In each figure, the overall uncertainty is segmented into two components, the

meteorological and measurement uncertainty. Meteorological uncertainty,

illustrated by the separation of the solid curves, represents the possible

extremes due to the allowable variation in meterological conditions during

testing. The measurement uncertainty, illustrated by the separation of the

dashed lines superimposed upon the meterological extremes, is a representation of the random uncertainty associated with individual measurements of

solar irradiance, fluid flow rate, temperature, and working fluid thermophysical properties in determining the plotted parameters. The rectangles

bound the measurement uncertainty associated with of both the x and y axis

parameters for selected points along each of the curves.

The meterological extremes were determined for both the BSE and ASHRAE curves using a computer model [8,9] based upon the traditional heat transfer relations for flat plate collectors [10] . Minor modifications were made to

allow for an input of zero solar irradiance and to provide proper convergence when iteratively determining The complete details of the computer

model and equations and how they were solved are given in the Appendices of

[8,9]. The meterological test conditions used as program inputs are indicated

on each of the curves. Considering the lack of BSE requirements for levels

of total solar irradiance within the collector plane G, and diffuse percentage

G^ typical extremes of 200 and 1000 W/m'^ and 0 and 50%, respectively, were

selected. Likewise for the Standard 93-77 efficiency curve, G, extremes

were selected at 0 and 50%. To ensure the accuracy of the analytical modeling

used to determine collector thermal performance at the meteorological extremes,

the results were first validated against experimental data at NBS. Complete

experimental data was available from the testing of collector no. 3 according

to the BSE and ASHRAE test procedures. The two sets of meteorological extremes

of Figure 4 represent the extremes associated with the BSE efficiency curves

of 1000 and 200 W/m^. Realizing that the BSE efficiency curves are constructed

from independent determinations of and Qj^, the worst case combination of

the two was used to obtain the high and low meteorological extremes.

*Collector no. 3 is a flat plate single glass glazed collector with a black

chrome selective absorber coating. A description of all collectots used in

this study is given in Table 7.

11
uncertainty

<3 5

12

Figure

3.

Uncertainty

associated

with

the

BSE

thermal

loss

curve

for

collector

no.

3,
'A3U3|P|||SI iSAlSepQ

13

Figure

4.

Uncertainty

associated

with

the

BSE

efficiency

curves

for
o>

%' ' A3U8I3I||8 J0PQ|03 T

14

Figure

5.

Uncertainty

associated

with

one

of

the

BSE

efficiency

curves

after

being

transposed

into

an

ASHEIAE

93-77

format

for
'A3U9|3i||a joiaspo

15

Figure

6.

Uncertainty

associated

with

the

ASHRAE

93-77

efficiency

curve

for

collector

no.

3
The measurement uncertainty associated with each of the curves was quantitatively determined using the propagated measurement uncertainty functions

developed in Appendix A. Applying the individual measurement uncertainties

of Table 2 and allowable operating conditions of Table 3 to the developed

uncertainty functions of Appendix A for the collector being examined, the

absolute measurement uncertainty was determined for each of the plotted

parameters. The horizontal and vertical dimensions of the rectangles superimposed upon the meterological extremes reflect the measurement uncertainty

of the plotted x and y parameters, respectively. The uncertainty functions

used to generate the rectangles are referenced on enlarged rectangles on

each figure.

Several observations can be made regarding the comparison of overall uncertainty associated with the BSE and ASHRAE collector test results. The BSE

and ASHRAE collector efficiency curves with related uncertainties are shown

in Figures 5 and 6 using a format consistent with Standard 93-77 for collector no. 3. An obvious difference is the smaller spread in the BSE efficiency

curve meteorological extremes when compared to the ASHRAE efficiency curve. The smaller spread, reflecting a smaller meteorological uncertainty, is

expected considering the tighter control over meteorological test conditions

as a result of indoor testing. For practical purposes, outdoor collector

testing must allow for a wider range of environmental test conditions. The

measurement uncertainties on Figure 5 and 6 are nearly identical (nominally

+ 5 percentage points) because of the near identical individual measurement

inaccuracies allowed within the ASHRAE and BSE documents. The dominating

factor contributing to the + 5 percentage points is the uncertainty of + 3%

related to the measurement of solar irradiance. Overall, the compounded

uncertainty (both meteorological and measurement) associated with ASHRAE

efficiency curve will always be greater than for the corresponding BSE

efficiency curve for the same collector.

16
4. EXPERIMENTAL EQUIPMENT AND INSTRUMENTATION

Test Facility

The collector testing loops designed, built, and used in the experiments

described in this report are housed at an former NIKE Missile Site adjacent

to the NBS Laboratories in Gaithersburg, Maryland. One of the underground

bunkers built originally for storing of missiles is used as the solar laboratory with the test loops mounted on an elevator used originally for transporting the missiles up to ground level. This unique facility allows NBS

personnel to conduct outdoor tests as well as indoor heat losses tests using

the same test equipment.

Figure 7 shows the elevator door closed over the underground bunker occupied

by the NBS collector testing group. Figure 8 shows the equipment on the

elevator just after the doors have opened. Figure 9 shows the equipment in the

outdoor testing configuration with the two liquid collector test loops in the

foreground, and the adjacent room containing the data acquisition system.

The primary equipment used during all of the experimentation Included the

collector test loops, the axial fan wind simulators, and the environmental

simulators. The collector test loops provided the proper collector operating

conditions and equipment to monitor collector thermal performance. Both the

axial fan wind simulators and the environmental simulators were used to control the environmental test conditions during indoor testing.

Collector Test Loops

Two separate test loops were built for the water-heating collectors. Both

test loops are essentially identical and consist of a portable supporting

frame for a test collector with the entire flow loop housed underneath the

collector within the framework. The collector can be adjusted over a wide

range of tilt angles (0-70®) and orientation (0-360®) and easily added to or removed from the structure. All the instrumentation (except sensors mounted

in the flow loop) is housed in the adjoining instrumentation room.

Figure 10 shows a close-up of one of the test loops with a flat-plate collector (collector no. 4) mounted to the top of the frame. Figure 11 is a schematic drawing of the test loop. As can be seen, the loop is closed and was designed to be able to control and stabilize the collector fluid inlet temperature to within + 0.5®C (+ 1.0®F) and the fluid flow rate to within + 1%.

Pure water was used as the working fluid in order to eliminate uncertainities

and possible errors associated with not knowing the working fluid properties.

Figure 12 shows the loops from the north and, as can be seen, all equipment

is sufficiently protected from the environment by having it enclosed within

the housing. Plumbing and hardware within the test stand base are encased

with 1.2 cm (0.5 in.) thick foam rubber insulation while all exposed pltimbing

is wrapped with 3.8 cm (1.5 in.) thick foam rubber Insulation. The following

paragraphs describe the details of the equipment used in the flow loops starting with the collector and moving clockwise in Figure 11. The specifications

for the equipment and sensors are given in Table 4.

17
Figure

7.

Elevator

doors

covering

an

underground

storage

bunker

at

the

NBS

annex,

Gaithersburg,

Maryland.
Figure 8. Solar collector testing equipment mounted on the elevator and

lowered to the floor of the underground storage bunker at the

NBS annex, Gaithersburg, Maryland.

19
20

Gaithersburg,

Maryland.
Figure 10. Collector no. 4 on test stand during outdoor testing

21
TEMPERATURE

MEASUREMENT

SECTION

K Ui

to S

i>i oou

(V £c

T-l J=

cc3

tocou

tocX

01

O.X

C ”3 o c

f—I (0 T3 to

0>

CO

U

O to 1—( <u U 4J 0) 3

60

22
EO .

(-1 XJ <4H C

CC

C r-1 <U

(U >-l W JC w

cc oc

CO l-< TJ a C X)

CO CO •u u

CD (U -C u 4-1 CD -r-l .<V CC

23
1

The temperature measurement sections are located immediately upstream and

downstream of the collector. Both inlet and outlet sections are located

as close as possible to the connection of the collector and are insulated

with 3.8 cm (1.5 in.) thick foam rubber insulation in order to minimize

thermal heat losses and to ensure a temperature change of less than 0.05"C

(0.1°F) between the sections and the collector. Proper thermal homogeneity

is provided by insuring a Reynolds number of at least 5000 and allowing

the fluid to pass through at least one right angle bend or static screen

flow mixers immediately before entering the measurement section. The

sections are constructed so that two temperature sensors can be Inserted

simultaneously through compression fittings. To provide proper bleeding

of trapped air from the entire system, air bleed valves are located

immediately adjacent to the wells. Sheathed platinum resistance thermometers (PRT) and linear bridge amplifiers are used to determine the inlet

and outlet absolute temperature across a collector. Two separate PRT's are inserted within each of the inlet and outlet temperature measurement

sections in order to ensure a check on temperature measurement. The PRT's

were calibrated in two matched sets to insure an absolute temperature measurement to within + 0.05“C (0.09“F) and a temperature difference resolution

of + 0.1“C (0.18“F). The X crossover network connected between the inlet

and outlet of the collector outside of the temperature measurement sections

is utilized for fluid flow reversal through the collector. During indoor

thermal loss tests, the flow is reversed so that the fluid flows down

through the collector.

A 11.3 liter (3 gal) expansion tank is in the closed flow loop to allow

for full expansion of the working fluid and to dampen pressure fluctuations.

A water-to-air heat exchanger is used to dump the collected energy during

outdoor testing. The water-to-air heat exchanger is a 36 cm (14 in) square

by 2.5 cm (1.0 in) deep fluid radiator. For adjustment purposes, the

water-to-air heat exchanger uses ganged ball valves to serve as a bypass

and modulate the heat dump. The ganged bypass valves produce a constant

back pressure and consequently a steady flow rate over the full range of

adjustment.

Although not shown in the schematic drawing of Figure 11, the flow loop can be opened in order to provide a means of periodically calibrating the

flow meters in place against a weigh tank and stopwatch. In addition,

the flow meters can also be easily removed for cleaning, checking, and

additional calibration.

Further downstream are the storage and recovery tanks including a pressure

relief valve. The 38 liter (10 gal) water storage tank can be seen in Fig- ure 11 and is incorporated in the flow loop to act as a buffer and eliminate

thermal cycling which tends to occur within a closed system. In addition,

the storage tank contains a 1.5 kW immersion heater which can be used to

increase the overall flow loop temperature.

24
Table 4

Specifications For The Equipment And Sensors Used In The

Collector Test Loops

Equipment/Sensor Specifications

Temperature measurement

section

Water-to-air heat

exchanger

Storage tank

Pressure relief valve

Filter

Electric Heater

Outer shell assembly is constructed from

1.27 cm (0.5 in.) i.d., 0.317 cm (0.125

in. ) wall hard-drawn copper tubing and

fittings; Compression fitting with Polytetraf luoroethylene ferrule allow for

insertion of sheathed temperature probes;

flow rate and flow configuration produces

turbulent flow and fluid swirling after

passing through one right angle bend and

static screen flow mixers. (See Figure

15.)

Automative radiator; effective surface

area of 1.85 m^ (20 ft^); U. value of

181.5 W/(m2*“C) (32 Btu/(h*ft^*°F))

37.8 ^ (10 gal.) glass-lined domestic

hot water tank; 1500 W (5118 Btu/h)

Immersion heater

2.06kPa (30 psi) spring-loaded valve

5 micron cartridge filter for hot

water applications

1000 W (3410 Btu/h), cartridge resistance immersion heater

Temperature Controller Adjustable proportional set point

controller; 1000 W (3410 Btu/h) load

capability using a triac assembly

and a type-T thermocouple sensor

25
Table 4 (cont.)

Specifications For The Equipment And Sensors Used In The

Collectors Test Loops

Equipment/ Sensor Specifications

Pump Self-priming, eccentric-disc positivedisplacement pump; variable pumping

capacity of 0-3.45 x 10 m^/s (0-5 gal./

min.), forward or reverse and a static

pressure head of 3.44 x 10^ Pa (50 psi)

Flow meters Turbine meter; Paddle-wheel type incorporating an orifice upstream and downstream providing a linear span of

0-6.3 X 10 m /sec (0-1 gal. /min.)

using ball bearings and a magnetic pickup; specially linearized to within + 0.9%

over the intended ranges of temperature

and flow rates

Temperature sensors 100 ohm, 3 wire platinum resistance

thermometers interfaced with linear

bridge amplifiers for a 1 mv/°C output

Thermocouples, type-T, 24 gauge incorporating stainless steel or copper

sheathing with and without grounded

thermocouple junctions

Temperature difference

sensors Platinum resistance thermometers - same

as for absolute temperature measurement

Pressure drop sensor Inverted U-tube manometer; 0.635 cm (0.25 in.) i.d. and 25.4 cm (10 in.)

long, filled with water and allowing

the upper U to be trapped with air

26
Mercury-in-glass thermometers calibrated from 0-100“c (32-212°F) and accurate

to + 1*C (+ 2®F) are inserted into oil filled 0.93 cm (3/8 in) diameter copper wells in line with the inlet and outlet to the storage tank in order to

monitor the respective temperatures. The inlet and outlet temperatures are

required for adjusting the heat exchanger bypass in order to properly maintain steady-state conditions in the flow loop. To obtain steady-state conditions, the heat exchanger bypass is adjusted so that the storage tank inlet

and outlet temperatures are equal to within 2°C (4®F).

A provision for make-up fluid in the flow loop serves several purposes. It can be used for filling or draining the loop or for pressurizing the entire

system. Make-up water is obtained from a demineralized water storage tank.

A 0-0.4 A/sec (0-6 gal/min) 5-mlcron particulate fluid filter is incorporated

within the loop in order to protect the system from foreign particles. The

filter is located upstream of the pump and flow meter since they are both

very susceptible to damage by particulate matter.

The controlled 1 kW heater downstream of the filter serves to stabilize the

inlet fluid temperature to the collector. Likewise the heater provides make

up energy into the loop to offset the energy losses during the collector

indoor heat loss tests. The input power to the heater is adjusted so that

the fluid temperature downstream of the pump is held constant to within +

0.1®C (+ 0.2“F). The heater power is controlled by a proportional temperature controller which senses the temperature downstream of the heater immediately after the pump with a type-T thermocouple and then proportionally

controls the heater power in response to the sensor's deviation from a set

point temperature. The proportionality of power output to set point deviation

is adjustable in order to minimize temperature cycling within the flow loop.

The flow rates encountered within the loop are very low (0-0.063 i/s (0-1 gal/

min)) requiring a stability of + 1% while operating against high back pressures. For such circumstances, a low-flow 0-0.32 i/s (0-5 gal/min) positivedisplacement eccentric-disc pump capable of operating against back pressures

of up to 340 kPa (50 psi) was selected over several centrifugal pumps. Most

typical centrifugal pumps are limited to higher flow rates and lower working

pressures. This pump is further capable of adjusting the flow rate to within + 0.0013 i/s (+ 0.02 gal/min) while a 1.3 cm (0.5 in.) diameter needle valve

downstream of the pump allows for fine adjustment.

A low flow paddle type turbine meter with a passive magnetic transducer is

used to measure flow rate. The flow meter produces a digital output proportional to volumetric flow.

During this test program, the flow meter was always calibrated in-situ with

a typical open-loop weight tank arrangement and in conjunction with any

related electronic displays or readouts.

The low flow turbine meter and electronic display were initially calibrated

over a flow and temperature range of 0-0.063 H/s (0-1 gal/min.) and 20 - 100°C (68-212®F) respectively to an uncertainlty of + 0.9%.

27
A visual flow meter is located immediately downstream of the electronic flow

meter in each loop. Functioning as a coarse flow indicator, the visual flow

meter also serves as a sight glass for determining whether air is trapped

within the system. Because the flow meter is only used to make coarse adjustments in the flow rate, the accuracy and resolution could be quite

low. The flow meter used, a rotameter, is capable of measuring flows of

0-0.07 l/s (0-1.1 gal/ min) with an uncertainity of + 2% of full scale.

For safety purposes, a 0.210 kPa (0-30 psi) static pressure gauge is con- nected at the solar collector inlet. The purpose is to visually indicate

the system pressure and warn of collector and system overpressurization.

Axial Fan Wind Simulator

The axial fan wind simulator used during indoor testing is comprised of two

68.5 cm (27 in) floor circulation fans. The fans provide a simulated wind

across the collector surface as in Figure 13 and during this test program

were either directed horizontally at the collector midsection or directed

longitudinaly along the collector plane as illustrated in Figure 14. The fan

speed and likewise wind speed can be stepwise controlled by use of a speed switch. Average wind speeds as high as 7.1 m/s (15.8 mi/h) can be

simulated.

Environmental Simulators

Two environmental simulators, ESI and ES2, were fabricated during this test

program in order to provide a more uniform wind velocity across the collector than possible with the axial fan wind simulator and to investigate the

effect of low "sky" temperature during indoor testing for collector thermal

loss. Environmental simulator 1 (ESI) was the initial prototype. The experience obtained from ESI lead to subsequent development of environmental simulator 2 (ES2). Within each simulator, wind speeds were produced ranging from

0 to 6.4 m/s (0 to 14.3 mi/h) and apparent environmental radiance temperatures, t._„, equal to the ambient air temperature, t„, and as low as t„-19°K

ESI is illustrated in Figure 15 and 16 and shown in Figure 17 with a collector under test. In use, it formed a rectangular duct enclosure typically

10 cm x 91 cm (4 in x 36 in) in cross-section over the glazing surface of

the collector under test. In cross-section, the bottom side of the duct

enclosure was formed by the collector glazing, the sides using wood framing,

and the top with a 0.05 mm (0.002 in) polyethylene plastic film. In operation, room air was pulled or pushed through the air duct to simulate a wind

across the surface of the collector. The air was transported through the

duct enclosure using a blower rated at 0.471 m^/sec (1000 ft^/min) at 25.4 cm (10 in) of H

2 O and regulated by butterfly or bayonet dampers upstream or downstream of the blower.

To effectively simulate a low apparent environmental radiance temperature,

a chilled tube and fin plate backed by insulation was suspended 1 cm (0.39

in. ) over and parallel to the polyethylene surface of the duct enclosure.

28
Figure

1.

Collector

efficiency

using

the

BSE

procedure

vs.

the

difference

between

collector

mean

fluid

temperature,

t

,

and

ambient

air

temperature,

t

.
AXIAL

FAN

DIRECTED

HORIZONTALLY

AT

COLLECTOR

MIDSECTION

AXIAL

FAN

DIRECTED

LONGITUDINALLY

ALONG

COLLECTOR

PLANE

(D

30

Figure

14.

Axial

fan

wind

simulator

orientations
CJ

a:o

t-<32

01

-j<

t-z >o

tr.

a:o

I-<

_i32

(/> UJ

31

Figure 1

j.

Environmental

simulators 1

and 2

used

to

simulate

winds

and

low

effective

sky

temperatures
PRESSURE

GAUGE

Ul

CN

32

Figure

16.

Cross-sectional

view

of

environmental

simulators

1

and
33
The chilled plate completely blanketed the polyethylene surface and

viewed the collector through the plastic film with a view factor of 0.85

(based upon plate and collector dimensions and spacing) . As a result of

the high IR transmittance of the polyethylene (80-83%*), the chilled plate

and collector glazing could exchange energy by radiation. The polyethylene

film was to isolate the air flow within the duct from the chilled plate

to ensure as much uniformity as possible of surface temperature over the

chilled plate (typically 1-2“C (2-4°F)) and eliminate any convective cooling

effects upon the air within the enclosed duct.

The chiller-loop in Figure 16 was used to provide a refrigerated working

fluid, water, to the tube and fin chilled plate.

The chilled water was provided by melting ice within the melt tank which

was continuously fed by a process ice machine. The ice water was then

circulated through a water-to-air heat exchanger and past a 500 W in line

immersion heater to elevate and control the inlet temperature of the

chilled plate to within + 1“C (+ 2"F) of a setpoint.

Environmental simulator 2 (ES2) is also illustrated in Figures 15 and 16 and

is shown in operation during collector testing in Figure 18. It was similar

in design to ESI but Included two modifications used to improve the stability

and uniformity of environmental control during testing. First, the duct

enclosure was altered to provide air flow along the collector edges and

improve the view factor between the simulator and collector. For ESI, air

passed only along the collector top glazing. Secondly, the 0.05mm (0.002 in)

polyethylene plastic used in ESI was not included. It was originally felt

that with the chilled plate in direct contact with the airstream, hot and

cold spots would appear upon the chilled plate surface and result in excessive cooling of the airstream. But by ensuring good thermal coupling between

the refrigerated fluid and the chilled plate, the surface temperature extremes were minimized and normally ranged only 4°C (9°F). Good thermal coupling was provided by utilizing a tube and fin plate with a high fin efficiency (0.97)

and maintaining a high flow rate of 220 mi /sec (3.5 gal/min) through the

chilled plate. In operation, room air was pulled through the U-shaped duct

enclosure longitudinally along the collector surface and edges. Likewise a refrigerated fluid was circulated through the U-shaped tube and fin plate to

reduce the plate surface temperature and effectively create a low apparent

environmental radiance temperature.

As a result of both the improved view factor and not including the polyethylene plastic within ES2, the necessity of mapping the apparent environmental

radiance temperature was eliminated. The apparent environmental radiance

temperature was determined knowing the average surface temperature and optical properties of the tube and fin plate. To verify the calculated apparent

environmental radiance temperatures ES2 was mapped similar to ESI during

the initial stages of simulation.

Based upon sample measurements at NBS using a spectrophotometer.

34
35
Monitoring Environmental Conditions

The specifications concerning the instrumentation used to monitor environmental conditions are given in Table 5. During outdoor testing, two types

of ambient air temperature sensors were used in conjunction with a standard

calibrated total-immersion ASTM liquid-filled thermometer. One was a type-T

calibrated thermocouple and the other an industrial platinum resistance

thermometer. The liquid-filled and resistance thermometer were accurate

to within + 0.1°C (+ 0.2°F) while the calibrated thermocouple uncertain! ty was + 0.2°C (+ 0.4°F). All were housed within a well ventilated small

instrument shelter located 1.25 m (4.1 ft) above the ground with its door

facing north. During indoor testing with the axial fan wind simulator, a resistance thermometer was located Immediately adjacent to the cup anemometer

as shown in Figure 13. For the environmental simulators, the air temperature

within the enclosed duct was determined using three methods. First, the exit

exhaust temperature from the simulator was monitored by a platinum resistance

thermometer located within the exhaust plenum as shown in Figure 15. Secondly, a resistance thermometer attached to a probe was periodically Inserted

within the air duct to spacially "map" the air temperature. Thirdly, an

array of thermocouples shown in Figure 15 were located at 15.2 cm (6 in)

intervals running longitudinally along the midsection of the enclosed duct.

The platinum resistance thermometer and thermocouples had an associated

uncertainty of + 0.1“C and + 0.2®C, respectively.

During outdoor testing and indoor testing with the axial fans, the wind speed

was measured by a cup anemometer delivering a direct current output proportional to the wind speed. Being portable, the wind anemometer could be

located close to any tested collector and adjusted in height. As a result of

a wind tunnel calibration, the resulting uncertainty of the wind velocity

measurement was + 0.35 m/s (+0.8 mi/h). When using the environmental simulators during indoor testing, a Blrams pattern anemometer was used to map the

air velocity profile across the enclosed air ducts. During the mapping, the

anemometer was sequentially positioned at 10 cm (4 in) increments along the

air intake or exhaust of the simulator as shown in Figure 15. Positioned at

each location, air velocity readings were obtained after integrating the air

flow over a 30 second interval. As many as 16 readings were sometimes taken

during a single run in order to average the results and determine an equally

weighted average wind speed through the simulator.

During outdoor testing, the wind direction was recorded by visual observation

of a directional wind vane during each test interval for the first part of

the test program. Later in the program, a weather vane producing an analog

output proportional to wind direction was installed to continuously monitor

wind direction.

36
Table 5

Specifications For The Instrumentation Used To Make The

Meteorological Measurements

Measurement Specification

Ambient air

temperature

Calibrated type-T, 24 gauge thermocouple

or precision platinum resistance thermometer housed within a vented weather

shelter

Wind speed Standard 3-cup wind anemometer incorporating a d.c. generator; output of 0.0581

volts/m/s (0.026 volts/mph) directly

proportional to wind velocity

Biram's Pattern Anemometer with jeweled

movement, disconnector, automatic zero

setting, 10 cm (4 in) in diameter for measuring air velocities from 1 to 15.3 m/s

(200 to 3000 ft/min)

Wind direction Direction vane with a two-wiper

potentiometer (0-540“

)

Total solar radiation

incident on the

collector tilted

surface

Precision spectral pyranometer utilizing

an all black thermopile detector and

temperature compensation; class 1 pyranometer as classified by the World

Meteorological Organization

Diffuse component of

the solar radiation

incident on the

collector tilted

surface

10 cm (3.93 in) shadow disc attached to a

1 m (3.28 ft) long support rod

Direct beam solar

radiation

Normal incidence pyrheliometer with a collimated view of 5.7“ and an automatic

tracker

Apparent environmental

radiance temperature

Pyrgeometer; precision infrared radiometer

capable of undirectional measurement of

either incoming or outgoing long wave

terrestrial radiation; a modification of

the precision spectral pyranometer using a silicon hemisphere with a transmittance of

0.50 from 0.35 to 50 ym

Collector IR radiative

losses

Thermographic camera with a black and white

and color display. The camera spectral

sensitivity is from 6-15 ym by incorporating germanium optics with a minimum temperature difference resolution of 0.2“C

(0.4“F) at an object temperature of 30“C

(86“F)

37
For measuring total solar irradiance incident on the solar collector, two

types of pyranometers were used. Initially, a "black-and-white" model 8-48*

pyranometer was used and was later replaced with an "all-black" Model PSP . Both pyranometers have a thermopile sensing element and similar protective

dome glazings but differ in the receiver optical coatings and patterns. The

pyranometer was mounted on an adjacent surface parallel to the collector in

such a manner that it did not cast any shadow onto the collector. Care was taken to ensure that the pyranometer was at the same tilt as the solar collector and to minimize reflected and reradlated energy from the solar collector onto the pyranometer. The typical mounting scheme used is shown in

Figure 10. The diffuse component of the incident solar radiation was determined for each efficiency test point by shading the pyranometer following the

recommended technique of reference [11] . In addition to monitoring the total

solar radiation, the direct normal radiation was measured using a pyrheliometer. The pyrheliometer tracked the sun and thereby measured the direct

normal incident solar radiation throughout the day.

The apparent environmental radiance temperature occurring within ESI was determined by use of a commercially available infrared radiometer (pyrgeometer). Based on a calibration against a blackbody source, the pyrgeometer was capable of determining the absolute incoming long wave irradiance to

within + 3% Integrated over the spectrum 3.5 to 50 pm. As a result, the

apparent environmental radiance temperature could be determined to within

+ 0.75“C (+ 1.35°F). When mapping, the pyrgeometer was inserted into the

duct enclosure through the air intake. The pyrgeometer was oriented with

the radiometer base flat against the collector glazing with the infrared

detector viewing in a direction away from the collector. The Intent was for

the radiometer to sense the same radiative environment as seen by the collector. The pyrgeometer was translated across the length and width of the

collector surface forming a matrix of 20 cm (8 in) squares. In practice,

the pyrgeometer was attached to a support rod as in Figure 15 and physically

positioned at each point within the matrix. During a normal mapping run, as many as 44 readings were sometimes necessary to adequately map the entire

collector surface. The average apparent environmental radiance temperature

for a particular mapping run was calculated by averaging all of the various

matrix segments equally.

The apparent environmental radiance temperature occurring within ES2 was determined from knowing the average surface temperature and optical properties of the tube and fin plate. The average surface temperature of the plate was determined using a matrix of thermocouples soldered to the blackened surface. To verify the calculated apparent environmental radiance temperatures,

ES2 was mapped similarly as with ESI during the Initial stages of use.

The collector overall radiative losses were qualitatively observed and documented using the thermographic equipment in Table 5. The equipment was com-

*Eppley Laboratories, Newport, Rhode Island. Identification of commercial

instruments does not imply recommendation or endorsement by the National

Bureau of Standards.

38
prised of two CRT displays, black and white and color, and a thermographic

camera sensitive to IR radiation over the spectral range of 6-15 pm.

Data Acquisition

A variety of data taken during this test program was monitored and recorded

by a data acquisition system and other recorders. Data such as flow rates

and temperatures from the liquid collector test loops, as well as meteorological data, were fed into one central instrumentation room. The room is a permanent part of the facility and is environmentally controlled and located

adjacent to the test loops as shown in Figure 9.

The input signals from the various test loop transducers consist of analog

voltage, digital voltage, or variable resistance. Analog signals are produced by thermopiles, pyranometers, anemometers, digital signals by liquid

flow meters; and variable resistance from resistance thermometers. The digital and variable resistance signals are converted into analog signals before

input to the data acquisition system. A flow rate monitor converts the flow

transducer digital signals into analog form while separate individually

matched and calibrated bridge amplifiers convert the resistance thermometer

signals into analog form before entering the data aqulsition system.

After conditioning the input data the information is fed into the equipment

shown in Figure 19. The equipment is comprised of a combination of strip

chart recorders and electronic integrators connected in parallel with a data

logger. The purpose of the strip chart recorders is to monitor pertinent

specific information on a continuous basis while the data logger scans and

records all the input data on a periodic basis. Information such as irradiance, flow rates, and inlet-outlet temperature differences are continuously

recorded on strip chart recorders in order to readily observe any transients.

Electronic integrators are sometimes used to integrate quantities such as solar radiation or fluid temperature rise across a solar collector. The data

logger scan interval was most often one minute during this test program. The

data logger is capable of calculating and recording the arithmetic average of

up to 16 Inputs over a specific time interval; thus, the scan rate can be once per minute, whereas the average of the inputs can be automatically calculated and printed every five-minutes. Both the scan rate and the averager

time interval are independent and adjustable. Typically input quantities

such as wind speed, air temperature, and collector fluid flow rate are

averaged using the built-in averager and printed on paper tape for later

analysis. Approximately half-way through the experimental study described in

this report, a magnetic tape drive recording system was interfaced with the

data logger system to expedite the data reduction on the NBS central computer

facility.

During a specific test, the collector absorber temperature as well as the

simulator air and chilled plate temperatures were manually recorded separately from the automatic data acquisition system. The required thermocouples

were interfaced through a selector switch into a thermocouple reader.

Table 6 Includes the specifications for the strip chart recorders, integrators, and data logger.

39
w

(U

J-l ou

0;ou

)-< tzo

•ac

n)>>

0)‘ s:

tc

r* >-l 3

*H

Xw

Ul c 0) o r*

o: 4^ 3 •H

CC OJ C3 CaE X

c. 0; •H C 3 C

cr TC 0)

c/: c cc o 2: •H

4^ o

•1—1 W 4-»

•f—< 3 4^ CT c UC w

t:

C c

4-) c c 4-) o C/)

(Tn

o;

UiD

bC i

40
Table 6

Specifications For Strip-Chart Recorders, Integrators, and The

Data Logger

Strip-Chart Recorders

2 Pen - Multi Range Input Span

Uncertainty; + 0.25% of span

Input Impedance: 0 - 5 V, 30 mega ohms; 5V-10V, 2.5 mega ohms

Time Constant: 0.5s

Input Spans: 0.1 mV - 10 V, multiple range

Type Inputs Monitored: thermopiles, pyranometers, flow rates

2 Pen - Fixed Input Span

Uncertainty: + 0.05% of reading

Input Impedance: 5 mega ohms

Time Constant: 0.75s

Input Span: 0 - 5 mV

Type Inputs Monitored; referenced thermocouples

3 Pen - Fixed Input Spans

Uncertainty; + 0.5% of span

Input Impedance: 5 mega ohms

Time Constant: Is

Input Spans: 0 - 5 mV, 0 - 100 mV, 0 - 5 V

Type Inputs Monitored: thermopiles, flow rates, resistance

thermometers

Integrators

Uncertainty; +0.5% of reading, + 2 digit s/h

Input Impedance: 1 mega ohm

Input Span: 0 - 30 mV

Type Inputs Monitored; thermopiles, pyranometers

Data Logger

Uncertainty; + 0.01 mV, Max Error: 0.005% full scale, + 0.2“C

Input Imedance: 100 mega ohms

Input Ranges: 0 - 400 mV; type-T thermocouples

Type Inputs Monitored; All input data thermocouples, 0 - 100® C; analog signals, 0 - 400 mV

Special Features; 16-channel arithmetic averager with separate

averaging time base

Magnetic Tape Drive; Incremental tape drive, 800 bpl, 9 track with

a DTL/TTL plus voltage true interface

41
5 . EXPERIMENTAL TESTS

Five flat-plate liquid heating solar collectors were experimentally tested

according to both the BSE procedure and ASHRAE Standard 93-77 . The collectors are described in Table 7 and were representative of various generic

types of commercially available collectors. The collector parameters

and were determined from the intercept and slope of the ASHRAE efficiency

curve, respectively. The Hottel, Whillier, and Bliss analytical formulation

for (xa)g and F" using the collector geometry and thermophysical properties

[12,13] were used to calculate F' and Uj^.

Collector no. 3 had the highest overall thermal performance and collector

no. 5 the poorest. The overall performance of the remaining three collectors

tended to fall between the extremes of collectors no. 3 and no. 5. Collectors no. 1 and 5 were identical in construction except for their types of

glazing. Both exhibited poor thermal coupling between the working fluid and

collector absorber due to a low fin efficiency and bond conductance resulting

from a thin foil copper absorber that was mechanically clamped to the fluid

risers as shown in Figure 20. Collectors no. 1 and 5 were selected to accentuate the differences between collector thermal losses measured indoors and

experienced outdoors. In particular, collector no. 5 should have experienced

the largest difference because of its high overall heat loss coefficient,

(U^) , and low collector efficiency factor, (F').

All five collectors were tested in direct conformance with both the ASHRAE

and BSE collector test procedures.* Table 8 summarizes the environmental

control configurations for all five collectors during the indoor testing.

Each of the five collectors were tested using the axial fan wind simulators

which were either directed horizontally at the collector midsection (configuration A) or longitudinally along the collector aperture plane (configuration B) as illustrated in Figure 14. Axial fan configuration B was an improvement over A because more uniform wind speeds resulted but was only

used with collectors no. 4 and 5. Collectors no. 3, 4 and 5 were also tested

with the environmental simulators, ESI and ES2.

Pulling the air through either simulator resulted in better uniformity in wind

speed compared with pushing it through. The air was pulled through the

environmental simulators for collectors no. 4 and 5 only.

Various indoor environmental test conditions were simulated using each of the

environmental control configurations. Using the axial fans, the wind speed across a collector was normally step controlled at 0, 3.5 and 6 m/s (0, 7.8,

13.4 ml/h) while the apparent environmental radiance temperature was allowed

to float around room temperature (t^ + 3“K) . Using the envorinmental simulators, three different wind speeds were simulated for each of two different

* When testing outdoors in accordance with the ASHRAE procedure, some data

points on the efficiency curves for collectors no. 1, 2, and 3 were obtained

when the wind speed exceeded 4.5 m/s (10 mi/h). The average wind speed for

all the points for collectors no. 1 and 3 was less than 4.5 m/s (10 mi/h).

42
Table 7

Description of Collectors Tested

Collector

No. 1 Collector

No. 2 Collector

No. 3 Collector

No. 4 Collector

Mo. 5

Manufacturer Commercial Solar

Energy PPG Industries Chamberlain Mfg.

Company Lennox LMSC 18-1 Commercial Solar

Energy

2 Gross Area (m ) Aperture Area (m^)

2.47

2.29

1.65

1.60 1.96

1.79

1.53 1.40

2.47

2.29

Glazing

Material Plate glass Tempered glass Tempered lowiron glass

Tempered lowiron glass with

antl-ref lectlve

coatings

Polyf luoroethylene

Number 2 2 1 2 1

Solar Transmittance 0.87

(single glass)

0.85 0.90 0.96 0.92

Absorber

Material Copper Foil Aluminum Roll

Bond

Mild Steel Mild Steel Copper Foil

Flow

configuration

10 parallel

risers

13 parallel

risers

Pillow absorber

conq)letely wetted

surface

10 parallel 10 parallel

risers

Coating Flat Lacquer Flat Lacquer Black Chrome

Nickel Substrate

Black Chrome

Nickel Substrate

Flat Lacquer

Solar Absorptance 0.95 0.94 0.94 0.94 0.95

I.R. Emlttance 0.92 0.92 0.12 0.10 0.92

Insulation

Material Low Density

Polyurethane

Glass fiber Glass fiber Fiberglass-board Low Density

Polyurethane

Thickness (cm) 6.35 7.62 7.00 7.9 6,35

* Fr 0.58 0.74 0.80 0.72 0.63

*Fr Ul w/(m^ • ”C) -4.71 -5.14 -4.43 -3.62 -6.56

** f 0.74 0.97 0.95 0.86 0.75

** Ul * ’c) 6.06 5.44 4.63 4.24 9.10

* Derived from slope or Intercept of ASHRAE 93-77 efficiency curve

** Determined from (to)^ and and HWB analytical formulations of (xa)g and F"

43
4A

Figure

20.

Cross-sectional

diagram

of

collector

no.

5

showing

the

thin

foil

absorber

and

mechanical

bonding

to

the

fluid

risers.
Table 7

Description of Collectors Tested

Collector

No. 1 Collector

No. 2 Collector

No. 3 Collector

No. 4 Collector

Mo. 5

Manufacturer Commercial Solar

Energy PPG Industries Chamberlain Mfg.

Conq>any Lennox LMSC 18-1 Commercial Solar

Energy

2 Gross Area (m ) Aperture Area (m^)

2.47

2.29

1.65

1.60

1.96

1.79

1.53 1.40

2.47

2.29

Glazing

Material Plate glass Tempered glass Tempered lowiron glass

Tempered lowiron glass with

antl-ref lectlve

coatings

Polyf luoroethylene

Number 2 2 1 2 1

Solar Transmittance 0.87

(single glass)

0.85 0.90 0.96 0.92

Absorber

Material Copper Foil Aluminum Roll

Bond

Mild Steel Mild Steel Copper Foil

Flow

configuration

10 parallel

risers

13 parallel

risers

Pillow absorber

completely wetted

surface

10 parallel 10 parallel

risers

Coating Flat Lacquer Flat Lacquer Black Chrome

Nickel Substrate

Black Chrome

Nickel Substrate

Flat Lacquer

Solar Absorptance 0.95 0.94 0.94 0.94 0.95

I.R. Emlttance 0.92 0.92 0.12 0.10 0.92

Insulation

Material Low Density

Polyurethane

Glass fiber Glass fiber FI berglass-board Low Density

Polyurethane

Thickness (cm) 6.35 7.62 7.00 7.9 6.35

* ^R <^“)e 0.58 0.74 0.80 0.72 0.63

*Fr Ul w/(m^ • “o -4.71 -5.14 -4.43 -3.62 -6.56

** 0.74 0.97 0.95 0.86 0.75

** Ul W/(m^ • *C) 6.06 5.44 4.63 4.24 9.10

* Derived from slope or Intercept of ASHRAE 93-77 efficiency curve

** Determined from Fj^ (To)g and Fj^U^ and HWB analytical formulations of (to)^ and F"

43
44

Figure

20.

Cross-sectional

diagram

of

collector

no.

5

showing

the

thin

foil

absorber

and

mechanical

bonding

to

the

fluid

risers.
Table 8

Environmental Control Configurations Used During The

Collector Thermal Loss Tests

Collector Axial Fan

Environmental

Simulator

1

Environmental

Simulator

2

No. 1 (Configuration A)

Horizontally at

collector midsection

No. 2 (Configuration A)

Horizontally at

collector midsection

No. 3 (Configuration B)

Horizontally at

collector midsection

Pushing air

through simulator

No. 4 (Configuration B)

Longltudlally In

collector plane

Pulling air

through simulator

No. 5 (Configuration B)

Longitudinally In

configuration collector

plane

Pulling air

through simulator
apparent environmental radiance temperatures. Wind speeds of 0, 3.5,, and

6 m/s (0, 7.8, and 13.4 mi/h) were simulated for apparent environmental radiance temperatures equal to room temperatures t„, and t_ - 19°K.

di a

46
6. TEST RESULTS AND DISCUSSION

The results of testing the five collectors according to the BSE and ASHRAE

test procedures are presented in Figures 21-31. The efficiency has been

plotted based on the Standard 93-77 format. The heat loss curves and family

of collector efficiency curves recommended in the BSE procedure are not

included. The solid curves on each figure are test results using the BSE

procedure and the dashed curve, results from testing in accordance with

Standard 93-77. Only the solid curves which represent test conditions

with wind speeds above 4 m/s (9 mi/h) and apparent environmental radiance

temperatures of t^ + 3®K are in strict conformance with the BSE procedure.

The additional curves are Included to show the effect of other environmental

conditions. The ASHRAE Standard 93-77 curves were constructed from a linear

least squares fit to the test data whereas the BSE curves were constructed

from utilizing the indoor measured collector thermal losses which were nonlinear and thus a second order efficiency curve resulted. Each figure contains a summary of the applicable outdoor and indoor test conditions. In

every case, the efficiency curves from the BSE tests are based on the average

outdoor solar Irradiance experienced during the Standard 93-77 test on the

same collector.

The results are discussed below in four parts. First, a conq)arison is made

between the BSE efficiency curves generated for all five collectors using

the axial fan wind simulators and the corresponding ASHRAE 93-77 curves.

Secondly, the BSE efficiency curves resulting from testing collectors no.

3 and 4 using environmental simulator 1 and for collector no. 5 using environmental simulator 2 are conqiared against the Standard 93-77 curves for the same collector. Thirdly, a comparison is made between the BSE efficiency

curves determined using the axial fan wind simulator and using the environmental simulators. Finally, experimental results are shown and discussed

concerning the control of environmental test conditions using the various

simulators during Indoor testing.

Collector Efficiency Curves Using Axial Fan Wind Simulators

Figures 21 through 25 give the experimental results for testing each of the

five collectors strictly according to the currently adopted BSE and ASHRAE

Standard 93-77 collector test procedures.* During all of the Indoor testing,

axial fans were used to simulate various wind speeds. Either fan configurations A or B of Figure 14 were used and are specified in each figure. The

apparent environmental radiance temperature was not regulated but was allowed

to float at or near room tenq>erature (t^ + 2“K).

In general, the agreement between the Standard 93-77 collector efficiency

curves and the BSE curves (wind speed > 4 m/s) (9 mi/h) for all five collectors is equal to or less than the measurement uncertainty associated with

the Standard 93-77 curve of 4-5 efficiency points. The only exception is

for collector no. 3 where the curves differ greatly in slope. However, the

*except as noted above

47
10

s

CM

48

Figure

21.

Comparison

of

ASHRAE

93-77

and

BSE

efficiency

curves

for

collector

no.

1

using

the

axial

fan

wind

simulators

during

Indoor

testing.
CM

IME

uo

49

Figure

22.

Cooq)arlson

of

ASHRAE

93~77

and

BSE

efficiency

curves

for

collector

no.

2

using

the

axial

fan

wind

simulators

during

indoor

testing.
C9

un cn CM

% J«|3t||03

50

Figure

23.

Comparison

of

ASHRAE

93-77

and

BSE

efficiency

curves

for

collector

no.

3

using

the

axial

fan

wind

simulators

during

indoor

testing.
CM

N

E

u

%

* Ashmmii* M|9tH«3

51

Figure

24.

Comparison

of

ASHRAE

93-77

and

BSE

efficiency

curves

for

collector

no.

4

using

the

axial

fan

wind

simulator

during

indoor

testing.
s

I

t

I X 8 e 8

X

8

JS|3§||9Q

9

O

52

Figure

25.

Comparison

of

ASHRAE

93-77

and

BSE

efficiency

curves

for

collector

no.

5

using

the

axial

fan

wind

simulators

during

indoor

testing.
efficiency curve discrepancies for collector no. 3 are still less than the

total uncertainty (both meteorological and measurement) associated with the

companion ASHRAE curve. For each collector except no. 3, the best agreement

between the BSE and ASHRAE efficiency curves occurs for wind velocities above

3.7 m/s (8.2 mi/h). The ASHRAE efficiency curve for collector no. 3 may be

high because of the low air temperature (around 4®C (40®F)) encountered

during most of the outdoor testing.

It was generally expected, based upon collector construction, that collector no. 5 would be the most sensitive to wind speed followed in order by

collectors no. 3, 2, 1, and 4. The collectors followed this trend except

for collector no. 2. Collector no. 2 exhibited a larger sensitivity to

wind speed than expected. After using thermographic equipment on it during

the various indoor heat loss tests, several thermal short circuits were

observed between the absorber plate and collector glazing or casing. The

short circuits were caused by direct contact of the collector absorber

with the outer edge flashing and the absorber buckling against the inner

glazing due to Inadequate allowance for thermal expansion.

The close agreement between the Standard 93-77 and BSE collector efficiency

curves for collector no. 5 in Figure 25 is significant. Collector no. 5

had a high overall thermal loss coefficient (9.1 W/(m^*®C)) and a low

collector efficiency factor (0.75). Both factors should tend to accentuate the differences between thermal losses indoors and outdoors. Consequently, of all the five collectors, the agreement between the two procedures

should have been the poorest for collector no. 5. In reality, the agreement

is quite good, less than the Standard 93-77 measurement uncertainty.

Collector Efficiency Curves Using Environmental Simulators 1 and 2

Figures 26 and 27 are the result of testing collectors no. 3 and 4, respectively using environmental simulator 1 (ESI) during the Indoor testing. Likewise, Figure 28 is the result of testing collector no. 5 with environmental

simulator 2 (ES2). For the BSE curves the apparent environmental radiance

temperature ranged from near ambient air temperature, t^, to t^ - (15 to

19 ®k). This range of radiance temperature, t , to t^ - (15 to 19“k) typically reflects the extremes of outdoor effective sky temperatures. Due to

the construction of ESI (allowing air flow only across the collector glaz

ing), the BSE efficiency curves of Figures 26 and 27 reflect the collector

sensitivity to wind only across the glazing. Since ES2 provided air flow

along both the collector top and edges. Figure 28 includes wind sensitivity

of collector no. 5 to both top and edge losses.

In figures 26-28, the agreement between the ASHRAE Standard 93-77 and BSE

efficiency curves over the full range of simulated environmental conditions

is better than or equal to the measurement uncertainty associated with the

Standard 93-77 curve. Collector no. 4 was very insensitive to variations in

both apparent environmental radiance temperatures and wind speed. This was expected considering collector no. 4 had a very low overall heat loss coefficient and because ESI used during the indoor testing accounted for top losses

only. Based upon these experimental results, no definitive conclusions can

53
r>

CM

54

Figure

26.

Comparison

of

ASHEIAE

93-77

and

BSE

efficiency

curves

for

collector

no.

3

using

environmental

simulator

1

during

Indoor

testing.
UR

CO

€*»<

C9

55

Figure

27.

Comparison

of

ASHRAE

93-77

and

BSE

efficiency

curves

for

collector

no.

4

using

environmental

simulator

1

during

Indoor

testing.
la

oo

C9

n

04

56

Figure

28.

Comparison

of

the

BSE

efficiency

curves

for

collector

no.

5

using

the

axial

fan

wind

simulator

and

environmental

simulator

2

during

indoor

testing.
be made about which indoor test conditions produce the best agreement between

the BSE and ASHRAE efficiency curves. This should have been expected

considering the total uncertainty associated with each of the efficiency

curves. In addition, the indoor environmental conditions which best reflect

outdoor performance will depend upon individual collector design.

Comparison Between BSE Efficiency Curves Obtained Using the Axial Fan Wind

Simulators and Environmental Simulators 1 and 2

Figures 29, 30 and 31 are the results of comparing the BSE curves determined

using the axial fan wind simulators and environmental simulators 1 and 2

during Indoor testing of collectors no. 3, 4 and 5, respectively. The

environmental simulators are summarized on each figure under BSE Indoor Test

Conditions.

The agreement between using ESI and the axial fans in Figures 29 and 30 are

excellent at 0 m/s but very poor at higher wind speeds. This is due to the

curves reflecting only the collector top thermal losses and not overall

losses as with the axial fans. The effect is higher efficiency curves for

wind speeds above 0 m/s when using ESI because of the lower collector heat

losses. The agreement between the curves in Figure 31 where ES2 is compared with the axial fan wind simulator is extremely good. The development

of ES2 to include air flow along the collector edges as well as the top

glazing results in more realistic collector thermal losses to the environment.

Regulation and Control of BSE Indoor Environmental Test Conditions

The sequential implementation of the axial fan wind simulator, environmental

simulator 1, and environmental simulator 2 was part of a progressive effort

to refine and improve the control of experimental indoor environmental test

conditions. A variety of measurement and mapping techniques were used to

monitor the indoor wind velocity and apparent environmental radiance temperatures. As a result, any improvements in control and uniformity of environmental conditions due to altering the axial fan configuration or environmental simulator construction were readily apparent and documented.

Figure 32 illustrates typical wind velocity profiles mapped transversely to

the air flow across a collector for axial fan configurations A and B while

pushing or pulling air through environmental simulators 1 or 2. Definite

Improvement in air flow uniformity resulted when the axial fans were directed

longitudinally along the collector plane and when the air was pulled through

either environmental simulator.

Figure 33 shows the results of mapping the apparent environmental radiance

temperatures using the pyrgeometer within environmental simulator 1. The

results are also typical of conditions within environmental simulator 2.

The matrix represents the special distribution of apparent environmental

radiance temperatures longitudinally and transversely as viewed out from

the collector surface. As a result of the collector-simulator view factor

and likewise increasing fluid temperature from bottom to top within the

simulator, higher apparent environmental radiance temperatures were observed

57
cu

' C9

58

Figure

29.

Comparison

of

the

BSE

efficiency

curves

for

collector

no.

3

using

the

axial

fan

wind

simulator

and

environmental

simulator

1

during

indoor

testing.
CO

% Ajwimm* J«ia*||*3

59

Figure

30.

Conq>arison

of

the

BSE

efficiency

curves

for

collector

no.

4

using

the

axial

fan

wind

simulator

and

environmental

simulator

1

during

indoor

testing.
T

60

Figure

31.

Comparison

of

the

BSE

efficiency

curves

for

collector

no. 5

using

the

axial

fan

wind

simulator

and

environmental

simulator 2

during

indoor

testing.
a

>

o

o

5n / TRANSVERSE DISPLACEMENT

\ ACROSS AIR F4OW

3-f

PUSHING AIR THROUGH ENVIRONMENTAL SIMULATOR I

UJ>

QC

<

5 -,

T

1 n—I r

PULLING AIR THROUGH ENVIRONMENTAL SIMULATOR I

LEFT SIDE RIGHT SIDE

intake INTAKE

PULLING AIR THROUGH ENVIRONMENTAL SIMULATOR 2

Figure 32. Typical wind speed transverse flow profiles mapped while using the

axial fan wind simulators and environmental simulators 1 and 2.

61
TOP

15 14 14 14

12 12 12 12

1

1

1

1

II II

1 1 10 10 1 1 10 9 10 10

10 9 9 10

9 9 9 9

9 9 9 9

9 9 9 9

9 9 9 9

10 1 1 12 13

BOTTOM

COLLECTOR NO. 3

Average apparent environmental ri^iance temperature = irc

Ambient air temperature = 23.1°C

TOP

12 12 12 12

12 II 1

1

12

12 1 1 II II

II II 1

1

II II 1

1

1

1

1

II II

1

1

1

1

1

1

II

1 1 II 10 II

1

1

II 1

1

1

1

BOTTOM

COLLECTOR NO. 4

Average apparent environmental radiance temperature = irc

Ambient air temperature =

Figure 33. Typical apparent environmental radiance temperatures mapped

within environmental simulators 1 and 2.

62
around the matrix perimeter and from bottom to top. The Increased uniformity

of apparent environmental radiance temperature as viewed by collector no.

4 was primarily the result of Increasing the fluid flow through the tube and

fin plate of the environmental simulator. The Increased flow rate decreased

the fluid temperature rise and consequently the ten^erature extremes from

top to bottom along the tube and fin sheet. The edge effects around the

matrix perimeter were essentially eliminated by using environmental simulator

2 which had a view factor near unity. Air temperature within the environmental simulators under zero wind conditions would stratify by as much

as 30**C (54**F) because of the stagnant enclosed air space. Consequently

during 0 m/s test conditions, a very low air flow (< 0.05 m/s (0.11 ml/h))

was maintained to minimize stratification. The tenq>erature stratification

was normally reduced to less than 5®C (9*F).

63
7. SUMMARY AND CONCLUSIONS

This report has briefly described the BSE solar collector test procedure.

A test facility has been fabricated at NBS in compliance with the BSE

test procedure in addition to existing test facilities capable of collector testing in accordance with ASHRAE Standard 93-77. A variety of five

different flat-plate liquid heating solar collectors were tested in

accordance with both procedures and the results compared. Slight refinements in the control of the BSE indoor environmental test conditions were

also investigated. The main conclusions of the study are:

1. For all five flat-plate liquid heating collectors tested in this

program, the differences between the efficiency curves determined

strictly using the BSE and the ASHRAE collector test procedures

were less than the total uncertainty, measurement and meteorological, associated with the ASHRAE Standard 93-77 curve. For

four of the collectors, the agreement was better than + 4-5 efficiency points which is typically less than or equal to the measurement uncertainty associated with the ASHRAE 93-77 curve.

2. The overall uncertainty in determining collector thermal efficiency

is reduced and repeatability improved when using the BSE collector

test procedure compared to the ASHRAE procedure. The uncertainty

in determining the collector optical efficiency is identical

between the procedures but the uncertainty associated with the

variation in meteorological test conditions is reduced when determining collector thermal losses under the BSE procedure.

3. For flat-plate liquid-heating collectors, the determination of the

near-normal-incidence instantaneous collector efficiency using the

BSE procedure can be completed in two days, one for outdoor testing,

and one for indoor testing. Only one day is dependent upon outdoor

environmental conditions in contrast to 4 days of outdoor testing

normally required by ASHRAE Standard 93-77 when using a fixed

orientation test stand.

4. The following are conclusions regarding the control of Indoor

environmental test conditions which give results that best reflect

actual collector thermal performance outdoors for flat-plate

liquid-heating collectors;

a. The simulated wind velocity across the collector surface

should be maintained at or above 3.7 m/s (8.2 mi/h).

b. The apparent environmental radiance ten5>erature can be allowed

to range from room temperature to as low as 16-19®K (28-32”R)

below room temperature, and the statement in conclusion 1 above

will still be satisfied.

64
c. The axial fan wind simulators are sufficient for simulating

controlled Indoor wind conditions. In order to maintain a uniformity In wind speed of + 9 percent, directing the axial

fans longitudinally along the collector aperture plane Is

preferred over directing the fans horizontally at the collector

midsection.

d. Environmental simulators 1 and 2 were successful In Increasing

uniformity of Indoor environmental test conditions In order

to parametrically examine collector thermal performance as a function of both wind speed and apparent environmental

radiance teiiq}eratures. Environmental simulator 2 Is preferred

because It provided simulated wind along both the collector

top and edges. Both simulators were capable of simulating

wind speeds from 0 to 6 m/s (0 to 13.4 ml/h) with a transverse

flow uniformity of + 4 percent and apparent environmental

radiance temperatures ranging from ambient air temperature

to 16“19*K (28-32*R) below ambient air temperature with a uniformity of + 2*K (3.6*R) as viewed by the collector.

65
8 . RECOMMENDATIONS

1. The BSE collector test procedure is reconmended as an alternative to

ASHRAE Standard 93-77 for testing flat-plate water heating solar collectors. Based upon the results of this study, the procedure improves

repeatability and reduces the time and overall uncertainty associated

with determining collector thermal performance.

2. The BSE procedure should be evaluated for possible use in testing

unglazed collectors. Considering that the thermal performance of

unglazed collectors is extremely sensitive to variations in environmental conditions, the BSE procedure should provide tighter regulation of the environmental test conditions to both ensure quasi-steadystate conditions and minimize the overall test result uncertainty.

3. An evaluation of the BSE procedure should be extended to include non- flat-plate collectors. Possible problems asociated with measuring

low collector thermal losses may prevent the application of the procedure to evacuated tubular or concentrating collectors.

4. The following are recommended modifications or additions to the present BSE procedure:

a. During the Indoor test, the wind speed across the collector surface

should be a setpoint value of 4 m/s (8.9 mi/h) with a specified

uncertainty and uniformity.

b. A standard procedure should be adopted for the measurement of indoor

simulated wind speeds. Likewise, a standard technique should be

Included for calculating the average wind speed.

c. The thermophysical properties of the collector working fluid should

be known to within +0.5 % and not vary by more than 0.5 % over

the operating temperature range within a test period. Presently,

the BSE procedure lacks any requirement for fluid density and

prescribes that the specific heat not vary by more than 5 % over

the operating temperature range between the collector inlet and

outlet.

d. The stability requirements for the collector inlet fluid temperature during the Indoor testing should be tightened to + 0.2°C

from the present + 0.5®C. Also, an inlet fluid temperature cycle

rate of less than 0.1“C/min should be specified. In practice,

the tighter control of the inlet tenq)erature is not any more difficult under stable laboratory conditions. The result will

be less uncertainty associated with the low temperature difference

measurements.

66
9. REFERENCES

1. BSE Guidelines and Directions for Determining the Usability of Solar

Collectors, A. Solar Collector Efficiency Test , BSE, Kruppstrasse 5,

4300 Essen 1, Federal Republic of Germany, May, 1978.

2. "Method of Testing to Determine the Thermal Performance of Solar

Collectors," ASHRAE Standard 93-77 , ASHRAE, 345 East 47th Street,

New York, New York, 10017, 1977.

3. Symons, J.G., "The Direct measurement of Heat Loss from Flat-Plate Solar

Collectors on an Indoor Testing Facility", CSIRO, Division of Mechanical

Engineering, Technical Report No. TR7 , 1976.

4. Christie, E.A., "A Method of Measuring the Performance Characteristics

of Flat-Plate Solar Collectors", CSIRO, Dlvlson of Mechanical Engineering, Technical Report No. TR6, 1976.

5. Smith, C.C., and T.A. Weiss, Design Application of the Hottel-WhillierBllss Equation", Solar Energy , Vol. 19, No. 2, pp. 109-114, 1977.

6. Whillier, A., "The Thermal Performance of Solar Water Heaters", Solar

Energy , Vol. 9, No. 1, 1965.

7. Reed, K. A., "Solar Collector Test Plan", Argonne National Laboratory,

9700 South Cass Avenue, Argonne, Illinois 60439, 1977.

8. Streed, E.R., and Thomas, W.C., "Results and Analysis of a Round-Robin

Test Program for Liquid-Heating Flat-Plate Solar Collectors", NBS

Technical Note 975 , August 1978.

9. Streed, E.R., Thomas, W.C., Dawson, III A.G., Wood, B.D., and J.E. Hill

"Results and Analysis of Round Robin Test Program for Liquid-Heating

Flat-'Plate Solar Collector," Solar Energy , Vol. 22, pp. 235-249, 1979.

10. Duffie, J.A., and W.A., Beckman, Solar Energy Thermal Processes , John Wiley and Sons, New York, 1974.

11. Guide to Meteorological Instrumentation and Observing Practices , Secretariet of the World Meteorological Organization, Geneva, Switzerland,

4th Edition, 1971.

12. Bliss, R.W., "The Derivation of Several Plate Efficiency Factors Useful in the Design of Flate-Plate Solar Heat Collectors", Solar Energy , Vol. 3, No. 4, December, 1959.

13. Whillier, A., "Prediction of Performance of Solar Collectors", Applications of Solar Energy for Heating and Cooling of Buildings , ASHRAE

GRP 170, ASHRAE, 345 East 47th Street, New York, ]0017, 1977.

67
14. Hill, J.E., Jenkins, J. P., and D.E., Jones, "Experimental Verification

of Standard Test Procedure for Solar Collectors", NBS Building Science

Series 117, January, 1979.

68
10. NOMENCLATUBIE

Aa - collector aperture area

S

- specific heat working fluid

F* - collector efficiency factor

F" - collector flow factor

Fr Ul - slope of collector efficiency curve

FR(xa)e - intercept of collector efficiency curve

G - hemispherical solar irradiance

Gd - diffuse hemispherical solar irradiance

•m - working fluid mass flow rate

n - collector efficiency

"^ASHRAE

- collector efficiency ASHRAE

’^BSE

- collector efficiency BSE

- collector optical efficiency

Ql “ collector overall thermal losses

Qo

- collector useful energy output at the optical efficiency

Qu

- collector useful energy output

R - dependent parameter

- ambient air temperature

^env

- apparent environmental radiance temperature

- collector inlet fluid temperature

’'f.o - collector outlet fluid temperature

- collector mean fluid temperature

Atf - temperature difference across the collector

«L

- collector overall heat loss coefficient

V - independent variable

69
~ wind velocity

W - random mesurement uncertainty

T - time

0 - incident angle - solar radiation

70
11. APPENDIX A

Analysis of Propagating Random Measurement Uncertainties

From the data recorded during the BSE and ASHRAE collector tests, the results

are reduced and plotted to formulate the following curves:

BSE

% vs (t^ - t^)

^BSE (*"m " ^a>

\

ASHRAE 93-77

^ASHRAE (^f,i

"

The above parameters used to plot each curve have a random and systematic

uncertainty associated with their determination. Systematic errors serve

to bias the plotted curves and produce a relative shift In the absolute

result, whereas random errors are responsible for the observed data scatter.

Considering that systematic errors will be assumed correctlble and do not

contribute to the observed data scatter, the subsequent analysis will only

address random errors and their propagation.

Each of the plotted parameters Qj^, (t^j^ - t^), nggg, and (tf ^ - t^)/G

are determined from Individual measurements of temperature, fluid flow rates,

solar irradlance, and the working fluid thermophysical properties. The random

uncertainty associated with each of the Individual measurements when combined

to calculate a plotted parameter will result In compounding the parameter

uncertainty. In order to assess the magnitude of the parameter uncertainty,

measurement uncertainty functions must be developed for each of the plotted

parameters as a function of the Individual measurements and their allowable

Inaccuracies. The uncertainty functions are based upon a root-mean-square (RMS)

uncertainty propagation and the measurement Inaccuracies allowed within the BSE

and ASHRAE procedures In Table 2. The results of testing collector no. 3 will

be used to demonstrate the utilization of the uncertainty functions to obtain

the parameter uncertainty and to illustrate the expected scatter on the

plotted curves.

In general, the relation for RMS propagated uncertainty due to random independent uncertainties is

where

W = random measurement uncertainty which could be either positive

or negative. The sign of W is indeterminant.

71
R =

V =

dependent parameter

independent variable

or

Propagated Measurement Uncertainty Functions for and ~

^a^

The corresponding formulations for each parameter are

Ql = m Cp (Atf)

Dq = i Cp (Atf)/G

<"m - ta = X

where

m = mass flow rate through the collector

Cp = specific heat of the working fluid

Atj = temperature difference across the collector

G = solar irradiance within the plane of the collector

'm = arithmetic mean temperature between collector inlet and

outlet fluid temperatures

= ambient air temperature

The RMS uncertainty functions are ^ = J!iY + (Sy +

Ql “/ V Cp/ /

72
V ^

From Table 2 Che allowable measurement uncertainties or Inaccuracies

specified within the BSE procedure are

''a . +± 0.01

W, + 0.005 assumed considering that the thermophysical properties

of the working fluid are well known

iiS - + 0.03

G -

W^. =» + 0.5®C

*Tn ~

= + 0.5*0

*-a ~

WAtf “ ±

When determining

itf - Ql/oJ Cp)

when determining

Atf = t1 o(G) A^/Cm Cp)

where

= aperture area of collector

Then, after substituting the individual terms into the uncertainty functions

( 1 )

73
( 2 )

For collector no. 3, the following are typical conditions.

= 1.79

m = 0.0358 kg/s

\

Cp = 4.187 kJ/(kg*"C)

G 200 W/m^

Substituting the conditions into equations (1), (2), and (3)

Propagated Measurement Uncertainty Function for\sE

The relation for riggg is

^SE “

'\)

Consequently the uncertainty function is

W

(3)

(4)

(5)

( 6 )

74
/ w \

/ n \

Substituting equation (1) and (2) Into eqns. (7)

Applying the appropriate conditions for collector no. 3, equation (8) becomes

Wnggg 1^1 (0.00102) t,„2 +

0.0000390 (Ql)

^ + 69.2

+ 0.00174 (9)

Propagated Measurement Uncertainty Functions for ASHRAE and (^f,l ~ ^a)/G

Realizing that the allowable measurement uncertainties with the ASHRAE

procedure are Identical to the BSE requirements for outdoor testing

the equations for determining and are Identical, therefore

Wn W_

^ASHRAE , J}o

^ASHRAE ^o

W

’^ASHRAE

'^ASHRAE

0.001025 +

0.1(m)Cp

Defining x' =

( 10)

75
w

= +

(tf.l - ta) ^ "^Vg /

Using the allowable ASHRAE measurement uncertainties from Table 2.

W 0.5

X

X

t ^/ ~

^a^

2 + 0.0009 ( 11 )

Applying the typical conditions fpr collector no. 3 to equation (10) with

G > 630 W/m^,

^ASHRAE

ASHRAE

( 12 )

76
NBS-114A (REV. 0-76)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2087

4. TITLE AND SUBTITLE

Testing Flat-Plate Water Heating Solar Collectors In

Accordance with the BSE and ASHRAE 93-77 Procedures

7. ^IJTHOR(S)

John P. Jenkins and James E. Hill

5. Publication Date August 1980

8. Performing Organ. Report No. 742.02

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP)

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington. D.C. 20234

13. Type of Report & Period Covered Final

15. SUPPLEMENTARY NOTES

, I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

18. ABSTRACT (A 300-word or leea tactual aanunary ot moat aignilicent inlormation. It document includes a aigniticant bibliography or

literature survey, mention it here.) Five solar collectors were tested according to the BSE and ASHRAE test procedures and

the results compared. All five collectors tested were modular, flat-plate, and water

heating, and included single- and double-glazed designs with and without selectivelycoated absorbers. In both procedures, collector efficiency curves are determined.

The ASHRAE procedure consists exclusively of outdoor tes ting, whereas the BSE procedur

requires a combination of outdoor and indoor testing (no irradiation) to determine

the collector's optical and thermal loss characteristics, repectively. During the

indoor testing in this study, the environmental test conditions were controlled and

regulated by use of specially-buLlt environmental simulators to investigate the effec^

of wind and "sky" temperature on the thermal loss characteristics of the collectors.

The simulators provided stable, uniform wind speeds in the range of 0 to 7.1 m/s across the collectors and "sky" tenperatures above the collector ranging from t

(ambient air temperature) to t^-19®K.

17. KEY WORDS (six to twelve entriea; alphabetical order; capitalize only the lirat letter ot the firat key word unleaa a proper name;

aeparated by aemicolona)

Instantaneous efficiency; optical efficiency; solar collectors; thermal losses;

thermal performance testing.

18. AVAILABILITY EUnlimited

1 I For Official Distribution. Do Hot Release to NTIS

I I Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

[X] Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

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21. NO. OF

PRINTED PAGES

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22. Price

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