AlllDD TTbEn

NATL INST OF STANDARDS & TECH R.I.C.

A1 11 0099621

9

Hill, James Edward/Experimental verlfica TA435 .U58 V117;1979 C.I NBS-PUB-C 1979

i|35

.U58

NO. 117

1979

C.2

NBS BUILDING SCIENCE SERIES 117

Experimental Verification of a

Standard Test Procedure

for Solar Collectors

U.S. DEPARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS *

»•

rrrTTJT.T. TT tttt 1 m~v I
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities' — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering' — Mechanical

Engineering and Process Technology' — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. 'Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
JAN Z W3

NBS BUILDING SCIENCE SERIES 117

1^1

Experimental Verification of a Standard Test Procedure

for Solar Collectors

James E. Hill

John P. Jenkins and

Dennis E. Jones

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Prepared for The Department of Energy

Research and Development Branch

for Solar Heating and Cooling

Office of the Assistant Secretary for Conservation and Solar Applications

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued January 1979
Library of Congress Catalog Card Number: 78-600138

National Bureau of Standards Building Science Series 117

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 117, 127 pages (Jan. 1979) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. 003-003-02008-7 Price $3.00

(Add 25 percent additional for other than U.S. mailing).
CONTENTS

Page

Abstract iv

1. Introduction 1

2. ASHRAE Standard 93-77 3

3. Description of NBS Test Facility 11

4. Test Results 25

5. Summary and Conclusions 41

6. Recommendations for Future Work 45

7. References 47

Tables 51

Appendix A - Calculation of Air Flow Rate Using a Nozzle

and Pressure Difference Measurements 61

Appendix B - Description of a Radiometer to Determine Sky

Temperature 64

Figures 67

iii
Experimental Verification of a Standard Test Procedure

for Solar Collectors

by

James E, Hill, John P. Jenkins and Dennis E. Jones

Abstract

A proposed procedure for testing and rating solar collectors based

on thermal performance was published by the National Bureau of Standards

(NBS) in 1974, Subsequently, the American Society of Heating, Refrigerating, and Air Conditioning (ASHRAE) developed a modified version of

the NBS procedure which was adopted in early 1977 as ASHRAE Standard

93-77. A test facility for water-heating and air-heating collectors has

been built at NBS and was used to support the development of Standard

93-77, The purpose of this report is to describe the recently adopted

test procedure, the NBS test facility, and the tests that were conducted

to support the development of the procedure.

Keywords: Measurement; solar collector; solar energy; solar radiation;

standards; standard test; testing.

In discussing this experimental program, certain commercial components

were used and are identified in order to provide a descriptive characterization of their features. Inclusion of a given component is this report

in no case implies a recommendation or endorsement by the National Bureau

of Standards, and the presentation should not be construed as a certification that any component would provide the indicated performance. Similarly,

the omission of a component does not imply that its capabilities are less

than those of the included components. This report is intended to be

informative and instructive and not an evaluation of any commercially

available components.

iv
1. INTRODUCTION

A proposal for testing and rating solar collectors based on thermal performance was published by the National Bureau of Standards (NBS) in 1974

[1-3]," The procedure prescribed that a series of outdoor steady-state

tests be conducted to determine the near-normal-incidence efficiency of

the collector over a range of temperature conditions. The American

Society of Heating, Refrigerating, and Air Conditioning Engineers subsequently developed a modified version of the NBS procedure which was adopted in February, 1977, as ASHRAE Standard 93-77 [4]. It is similar

to the NBS procedure but calls for additional tests to determine the

collector time constant as well as an incident angle correction factor

that can be applied to the near-normal-incidence efficiency to determine

collector performance both early in the morning and late in the day.

Three test loops have been built at NBS in accordance with ASHRAE Standard 93-77, two for modular water-heating collectors and the other for

Figures in brackets indicate literature references at end of the report,

1
air heaters. A major part of the NBS collector testing program since

mid-197 6 has been devoted to using these facilities to support the development of Standard 93-77, The purpose of this report is to describe

the recently adopted test procedure, the NBS test loops, and the tests

that were conducted during 1976 and 1977 to support the adoption of

the procedure.

A second major part of the NBS collector testing work has been to con- duct a round robin program in which two flat-plate liquid-heating

collectors were tested by 21 organizations around the United States

during 1976 and early 1977. The purpose of the program was to have a

a variety of testing laboratories attempt to utilize the NBS test procedure* and then determine the extent to which the results differed

or were comparable. The test data and subsequent analysis for this

program are available in separate publications [5, 6],

*ASHRAE Standard 93-77 was not adopted at the time this program was

initiated.

2
93

BASBDO^^^'>'^'^OE DEVICES

2. ASHRAE STANDARD 93-77

The testing procedure recommended by NBS in reference [1-3] called for

the determination of the thermal efficiency of the solar collector by

passing the heat transfer fluid through it at a steady rate with the

collector mounted outdoors under clear sunny conditions. Measurements

were required to be made during the middle of the day and consisted

primarily of determining the fluid flow rate, temperature rise in the

fluid as it passed through the collector, and the incident solar radiation (also called irradiance). These data could then be used to

compute collector efficiency. The tests were to be made so that at

least 16 "steady-state" efficiency values could be determined over a range of temperature differences between collector fluid and ambient

air in order to draw an efficiency curve for the collector. A

significant amount of detail was included on the recommended

apparatuses, the instrumentation, and the procedure to follow. The

testing procedure specified in ASHRAE Standard 93-77 consists of the

3
same series of efficiency tests (with some modifications to be noted

below) plus additional tests which allow one to determine the transient

response of the collector as well as how efficiency changes with

increasing incident angle (between the direct solar beam and outward

drawn normal to the plane of the collector aperture).

The major changes in the conduct of the efficiency tests as specified in

Standard 93-77 compared to that specified in [1, 2] are as follows:

1, The testing apparatus for water-heating collectors has been modified

to include a storage tank for damping out thermal transients and a bypass to permit periodic calibration of the flow meter in place.

Figure 1 is a schematic drawing of the recommended closed-loop from

[4], In addition, two other alternate open-loop configurations are

also given (see Figure 2 and 3) and considered acceptable provided

the test requirements and specifications can be met,

2, The testing apparatus for air heaters has been rearranged so that

air is "pulled" through the collector instead of being blown through

it (slight negative gauge pressure in the collector). Alternately,

an open-loop test configuration similar in concept to Figure 3 can

be used,

3, More stringent requirements have been included for the measurement

of incident solar radiation. Pyranometers are required to meet or exceed the characteristics of a first class pyranometer as classified by the World Meterological Organization [7],

4, In conducting the test, data must be taken when the solar incident

angle is less than 30° (compared to 45° in [1, 2]),

5, The time period required for the integration of energy quantities

to compute one efficiency value has been decreased from 15 minutes

to either 5 minutes or one time constant, whichever is larger,

6, In computing efficiency, the gross frontal area of the collector

is used instead of aperture area.

7, The efficiency curve is drawn by plotting efficiency as a function

of the difference between inlet fluid temperature and ambient temperature divided by the incident solar radiation. (Average fluid

temperature was used in [1, 2].) Inlet fluid temperature was chosen

to be used in the plot because the primary characteristics of the

collector required for the system design procedures of [8, 9] can be

determined directly from the slope and the intercept of the curve.

The major completely new features of Standard 93-77 compared to [1, 2]

are:

1. The collector is required to undergo a preconditioning test prior

to the start of the thermal tests. The collector must be exposed

4
for three cumulative days with no fluid passing through it and

with the mean incident solar radiation measured in the plane of

collector aperture exceeding 17,000 k.J/(m * day) (1500

/(ft^ • day)).

the

Btu

2. Prior to conducting the efficiency tests, the time constant is

determined (see below).

3. After completing the efficiency tests, a series of tests is con- ducted to determine the collector's incident angle modifier (see

below)

.

4. The entire group of tests may be made Indoors using a solar simulator

if desired. The specifications for the simulator are included and

follow closely those of references [10-12],

2.1 COLLECTOR TIME CONSTANT

When a solar collector is operating under steady-state conditions, the

relationship that governs its thermal performance is:

the rate

of useful

energy = extracted

from the

collector

In equation form:

the rate

of energy

absorbed

in the

collector

by the

absorber

the rate of

energy loss

from the

collector by

conduction,

convection

and radiation

mc.

the rate

of energy

carried

away by the

transfer

fluid

(1) ^ = FRl(Ta)e - FRUL(tf,i-ta) = ^(tf,e " tf,i)

where

*^u = rate of useful energy extracted from the collector, W

2 A = cross-sectional area of the collector, m

collector heat removal factor

I* = total solar energy incident upon the plane of the collector per unit

time per unit area, W/m

( ta)^ = effective transmittance-absorptance product for the collector

Uj^ = heat transfer loss coefficient for the collector, W/ (m2 . •c)

I - temperature of the transfer fluid entering the collector, **C

*In the technical literature, frequently the symbol E, irradiance or G,

global irradiance will be used instead of I [38].
t„ = ambient air temperature, °C

m = mass flow rate of the transfer fluid through the collector, kg/(s»m^)

Cp = specific heat of the transfer fluid, J/(kg»s)

tc = temperature of the transfer fluid leaving the collector, °C

Whenever transient conditions exist, the above equations are not valid

since part of the energy being absorbed is used for heating-up of the

collector and its components. The corresponding relationship for transient conditions is:

the rate

of change

of energy

stored in

the collector

and its

compo nents

the rate

of energy

absorbed

in the

collector

by the

absorber

the rate

of energy

loss from

the collector

by conduction,

convection

and radiation

the rate

of energy

carried

away by

the

transfer

fluid

In equation form [13,14] :

C cl

* -A_J. = FI(Ta) - F U (t -t ) - ^

(t -t ) ^2)

A de R e R L f,,i a A f,e f,i

where

= effective heat capacity of the collector, its components, and the

transfer fluid in the collector, J/°C

6 = time, s

Equation (2) can be solved for the exit temperature of the transfer fluid,

tf as a function of time, 9, after making the following assumptions: J. , e

The exit temperature, of the transfer fluid is related to the

average fluid temperature, t^, by

^ = K^hjL± (3)

de de

6
where [ 13]

K=!;P_ (II -1) (4)

and F' = collector efficiency factor

2. I, ( Ta)g, Uj^, t^, m, Cp, and t^ ^ are all constant for the period

covered by the transient solution.

The solution to equation (2) is then:

mc F^Kra)^ - Wtf,i -

^a) S ('f,e " ^f,i) "

^ KC^^ ® (5) = e F^Kxa)^ - W^f.i - ^a) (tf,e, initiart ,i>

A

It is conventional practice in the physical sciences that whenever an equation of the form of (5) describes the response of a particular

system, the quantity KC^/mCp is called the system's time constant.

The physical interpretation of the time constant is the time required

for the quantity on the left side of equation (5) to change from 1.0

to 0.368 where 0,368 = 1/e, It was felt that this concept should be

maintained in order to be consistent with the accepted interpretation

of time constant. Unfortunately, this has not always been done in

the solar collector field, Simon [15] has published the value of "time

constant" for nine different water-heating collectors using the 0 to

99% change time, Wijeysundera [16] has computed the "response time"

of typical one, two, and three cover air-heaters using the 0 to 90%

change time. Fortunately, the conventional time constant can be

computed from the published values by multiplying by an appropriate

constant in each case.

The time constant for a collector could be calculated theoretically

using equation (5), However, there is a large uncertainty in the value

of the effective heat capacity, C^. Duffie and Beckman [14] have shown

that the upper limit for this heat capacity is the sum of the products

of mass times specific heat for each of the components that make up the

collector (absorber, glass, insulation, etc), plus that of the transfer

fluid required to fill the collector. However, if this were used for C^,

the time constant would be overestimated. This is due to the fact that

the temperature of some materials in the collector only change a fraction

of the amount that the fluid temperature changes in such a transient

7
process. Consequently, the time constant is required to be determined

experimentally in ASHRAE Standard 93-77.

The actual test can be carried out in one of two ways. The most straightforward technique is to expose the collector to the solar radiation and

after the entering and exiting fluid temperatures have stablized, suddenly

shield the collector from the sun and record the exit fluid temperature on a strip chart recorder. The incident radiation must be above 790 W/m^

(250 Btu/(h • ft^)). The entering fluid should be within ±1°C C+Z^F) of

the ambient temperature for the duration of the test. This latter requirement simplifies the data reduction process using the left side of equation

(5).

A second technique that can be used is to shield the collector from the sun altogether (conduct the test inside for example). The inlet fluid

temperature is adjusted to 30°C (54°F) above the ambient and after the

exit temperature has stabilized, the inlet temperaure is suddenly

decreased to within ±1°C (+2°F) of the ambient, and the exit fluid temperature is again recorded as a function of time. It should be noted

that the time constant determined experimentally according to either of

the above procedures is valid only for the range of ambient temperatures

used in the test. Its value is temperature dependent as can be seen from equation (5) (U^^ is temperature dependent) and from Figure 5 of

reference [16]. However, since the main value of such a test is to

allow a relative comparison of collectors, it was felt that the one

test would be adequate.

2.2 COLLECTOR INCIDENT ANGLE MODIFIER

Simon and Buyco [17] have shown that the effective transmittanceabsorptance ] described by:

product, (Ta)^, of a solar collector can be satisfactorily

(xa), = K.^^-)e,n ^6)

where K^^ = incident angle modifier

(Ta)g ^

= effective transmi ttance-absorptance product for the

' collector at normal incidence

The incident angle modifier, K^^, is nothing more than a correction

factor which is a function of the incident angle, the angle between the

direct solar beam and the outward drawn normal to the plane of the collector aperture. It describes how the optical efficiency of the collector

changes as the incident angle changes. It is an essential factor for

predicting all-day efficiency for a stationary solar collector.

Figure 4 shows how the effective transmi ttance-absorptance product, (Ta)^

varies as a function of incident angle for two types of flat-plate

collectors exposed to 100% direct radiation [18]. Using the relationship

for the incident angle modifier as given in equation (6), the variation

8
of incident angle modifier with incident angle for these same collectors

is shown in Figure 5, Based on the optical characteristics of flat-plate

collectors, data from Figure 5 can be replotted as in Figure 6 and will

result in linear plots as shown. This same linear relationship may or may

not exist for non flat-plate collectors depending on their optical characteristics.

As with the time constant test, there are two ways in which the incident

angle modifier can be determined. The first technique is applicable when

the incident angle can be arbitrarily adjusted such as with a solar simulator or with an outdoor movable test rack. In this case, a thermal efficiency test is conducted in accordance with all the requirements of the

procedure used to get the near-normal-incidence efficiency curve except

that:

1, The inlet fluid temperaure is held within +1''C (±2°F) of the ambient

temperature.

2, The test is made at incident angles of 0, 30, 45, and 60°.

By comparing the above test results with those obtained in establishing

the efficiency curve at incident angles of less than 30° , values of

K can be computed as a function of incident angle.

The second technique is applicable for outdoor testing with a permanent

test rack where the collector orientation cannot be arbitrarily adjusted

with respect to the direction of the incident solar radiation (except for

perhaps adjustments in tilt ), The collector is tested for a complete day

with the inlet fluid temperature held constant as above. The efficiency

values are computed continuously and "pairs" of values are selected, one from the morning and one from the afternoon, which correspond to values

of 30, 45, and 60° in incident angle. The two efficiency values in each

pair are averaged to compensate for transient effects and then used as above to compute the incident angle modifier.

9

!

3. DESCRIPTION OF ^fBS TEST FACILITY

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

described in this report are housed at an abandoned NIKI Missile Site

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

underground bunkers built originally for storage of missiles was used

for fabrication of the test loops and they were mounted on the elevator

used originally for transporting the missiles up to ground level. This

unique facility allows NBS personnel to expose the test equipment to the

outdoor environment only on days when tests are run. In addition, the same test equipment can then be used for conducting indoor tests of

collector heat loss characteristics.

Figure 7 shows the elevator door closed over the underground bunker occupied by the NBS collector testing group*. Figure 8 shows the equipment

*Two additional underground bunkers are currently occupied and being used

by the NBS Fire Research Center.

11
mounted on the elevator just after the doors have opened. Figures 9

and 10 show the equipment being raised and stopped in a convenient position to clean the collector cover plate assembly on an air collector

array prior to the start of a test day. Figure 11 shows the equipment

in the testing configuration with the two water loops in the foreground,

the room holding the data acquisition system and associated instrumentation in the center, and the air loop on the far side. Some collector

arrays that have been tested were too large to mount on the test loop

structure and had to be kept permanently above ground and connected

to the test loops through flexible hoses. An example is the evacuated

tubular collector array in the foreground.

3.1 TEST LOOPS FOR WATER-HEATING COLLECTORS

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

contrast to a larger single loop which could accommodate 2-4 collectors.

This was done to be able to test more than one collector at a time and

still have the flexibility to make adjustments to individual collectors

during testing and yet not affect the other collectors being tested.

Both test loops are essentially identical and consist of an integral test

unit capable of supporting a typical flat-plate collector at a chosen

orientation while containing the flow loop within the enclosed base. The

tested collector can be adjusted over a wide range of tilt angles (0-70")

and orientations (0-360**) and easily added to or removed from the structure. As already indicated, a separate adjustable collector support

frame is used in cases where an exceptionally large collector is being

tested

.

Figure 12 shows a close-up of one of the test loops with a flat-plate

collector (collector No. 2) mounted to the top of the frame. Figure 13

is a schematic drawing of the test loop. As can be seen, it is a closed

loop similar in principle to that of Figure 1 and was designed to be

able to control and stabilize the collector fluid inlet temperature

to within tO.S^C (±1.0*F) and the fluid flow rate to within ±1%.

Pure water has been used as the working fluid in all collectors whose

characteristics are reported herein. This has eliminated uncertainties

and possible errors associated with knowing the working fluid properties.

Figure 14 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 and also insulated to minimize heat

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

with 1.3 cm (0.5 in.) foamed rubber insulation and all exposed extension

plumbing is wrapped with 3.8 cm (1.5 in.) foamed 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 13. The specifications for the equipment and sensors are included in Table 1.

The temperature measurement sections are located immediately upstream

and downstream of the collector allowing temperature sensors to be

inserted for measurement purposes. Figure 15 is a schematic drawing

of the temperature measurment section at the outlet of the collector

12
and Figure 16 is a photograph taken prior to insulating the pipe. These

sections are located as close as possible to the connection of the collector and are insulated with 3.8 cm (1.5 in.) foamed rubber insulation

in order to minimize thermal heat losses and to insure a temperature

change of less than 0.05°C (0.1°F) between the sections and the collector.

Proper fluid mixing is provided by allowing the fluid to pass through

at least one right angle bend immediately before entering the measurement

section. The sections are constructed so that two temperature sensors

can be inserted simultaneously. One temperature sensor can be a sheathed

thermocouple or resistance thermometer inserted through a compression

fitting while at the opposite end a set of thermopile junctions can be

inserted into a thin-wall copper oil-filled well. To provide proper

bleeding of trapped air from the entire system, air bleed valves are located immediately adjacent to the wells. An alternate technique for

insuring a well-mixed fluid stream at the temperature measuring station

is shown in Figure 17. This technique should work well but was not used

in either of the two loops described here.

Three types of temperature sensors have been used to monitor the absolute

temperature and the temperature difference between the inlet and outlet

of the collector. Only two types of temperature sensors are used simultaneously. Calibrated platinum resistance thermometers (PRT) or sheathed

type-T thermocouples are inserted in one end of the well for measuring

absolute temperatures while a six junction thermopile or PRT is used at

the other end to sense the temperature difference across the collector.

The choice between platinum resistance thermometers or thermocouples for

measuring absolute temperatures is primarily a matter of cost. Typically,

commercially available platinum resistance thermometers are accurate to

within ±0.1°C (±0.2°F). NBS calibrated 0.47 cm (3/16 in.), sheathed

platinum precision resistance thermometers with related electronic

bridges used in this study are accurate to within +0.05°C (+0.1°F).

In comparison, calibrated 0.64 cm (1/4 in.) sheathed type-T thermocouples

that were also used in this study are accurate to within ±0.1°C (lO.Z^F).

Therefore, both types of sensors meet the required accuracy for absolute

temperature measurements of within ±0.5°C (+1.0°F) as given in [4]. In

either case, caution should be taken to insure that the temperature

sensor immersion length into the temperature well agrees with the

immersion depth at calibration.

A type-T six-junction thermopile is used to measure the temperature

difference across the solar collector. At least six junctions were determined to be necessary in order to determine small temperature

differences to an accuracy of within +0,1 "C (+0,2''F) (assuming the

thermopile readout instrumentation accuracy is within +0,01 mv ), Currently the test procedure in [4] allows either resistance thermometers

or a thermopile to be used for determining the temperature differences

across the collectors. For reasons of accuracy and reliability, the

thermopile proved to be superior. Because of the large number of

junctions at either end of the thermopile, the wire gauge should be

as small as practically possible in order to minimize thermal conduction

13
losses along the thermopile leads away from the temperature measuring

junctions. For this installation, 30 or 36 gauge thermocouple wire

was found to be adequate. To further eliminate error due to thermal

conduction losses, the thermopile leads were doubled back along side

of the oil-filled temperature well as shown in Figure 15. The thermopile

is inserted into the well to at least a depth of 10 cm (4 in.). The

entire thermopile was constructed from the same spool of calibrated

wire

.

A water-to-air heat exchanger is used in each flow loop and an additional water-to-water heat exchanger is used in one of the loops in order to

allow for a wider range of heat dump for some of the higher performance

collectors being tested. The water-to-air heat exchanger is a 36 cm

(14 in,) square by 2,5 cm (1,0 in,) deep fluid radiator (see Figure 18)

while the other is a counter-flow single-pass water-to-water exchanger.

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. The heat dump of the waterto-water heat exchanger is adjusted by controlling the secondary side

fluid flow rate supplied from a local water main.

Although not shown in the schematic drawing of Figure 13, 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.) hot water storage tank can be seen

in Figure 10 and is incorporated within 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 is

periodically used to increase the overall flow loop temperature. In order

to further eliminate any air within the closed system, a fluid recovery

tank was substituted for the recommended expansion tank. Whenever the

flow loop pressure exceeds the 210 kPa (30 psi) relief valve setting,

the fluid is dumped into a 3 liter (0.8 gal.) recovery tank and later

recovered as the pressure in the loop drops,

Mercury-in-glass thermometers calibrated from 0-100°C (32-212°F) and

accurate to within +1°C (+2''F) are inserted into oil filled 0,93 cm (3/8 in,) 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 conditions in the flow loop. To

obtain a steady-state condition, the heat exchanger bypass is adjusted

so that the storage tank inlet and outlet temperatures are equal to

within 2°C (4°F)

.

14
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 provided from a local water main.

A 0-0.4 l/s (0-6 gal./min.) 5-raicron particulate fluid filter is incor

porated 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 500-watt heater downstream of the filter serves to stabilize the inlet fluid tempeature to the collector. 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 sensors* 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 with the flow loop are very low (0-0.63 l/s

(0-1 gal./min.)) requiring a stability of +1% while periodically operating against high back pressures. For such circumstances, a low-flow

0-0.32 Vs (0-5 gal./min.) positive-displacement eccentric-disc pump

capable of pressures 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 coarsely adjusting the flow rate to within +0.0013 ii/s (+0.02 gal./

min.) while a 1.3 cm (0.5 in.) needle valve downstream of the pump

allows for fine adjustment.

Different types of flow meters are used in the two different flow loops.

The first is a turbine type flow meter with a passive magnetic transduce

and the second, a positive-displacement oval-gear flow meter with an active transducer. Each flow meter produces a digital signal output pro

portional to the monitored flow rate. The low-flow omnidirectional

turbine meter and electronic display were initially calibrated over a flow and temperature range of 0-0.063 l/s (0-1 gal./min.) and 20 - 100°C

(68-212*'F) respectively with stated accuracy and linearly of within

±0.9%. Likewise, the low-flow oval-gear positive-displacement meter

was calibrated over a flow range of 0.01-0.07 Vs (0.17-1.1 gal./min.)

and temperature from 20-100°C (68-212''F) with a similar accuracy. When

calibrated on a monthly basis, each flow meter can retain the specified

accuracy of within +1%.

During this test program, each 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. Over a period of a year,

the accuracy of the turbine flow meter would have changed by + 2.5%

if it were not for frequent calibration. It is believed that the 2.5%

15
drift was primarily due to wear and scale building up within the turbine

ball bearings. In comparison, the oval gear flow meter was not subject

to any similar drift in calibration over the same time interval. However,

the oval gear meter did experience a gradual increasing internal flow

resistance due to bearing wear. Consequently with time, the pressure

drop across the flow meter became unstable resulting in a pulsating flow

and the use of the meter was discontinued.

A visual flow meter is located immediately downstream of the electronic

flow meter in each loop. Functioning as a course 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 used for

adjusting the coarse flow rate, the accuracy and resolution could be quite

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

0-0.07 Vs (0-1.1 gal./min.) with an accuracy of within +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 guard against collector and system overpressurization.

3.2 TEST LOOP FOR AIR-HEATING COLLECTORS

The test loop for air-heating collectors is shown in Figure 20 and schematically in Figure 21, It is divided into two major parts, the air

handling module and the collector stand. The air handling module is

mounted on a portable cart for ease of movement. The blower delivers

air to the air reconditioning apparatus where it is conditioned to the

desired temperature before entering the secondary flow measuring device.

The air then flows through flexible ducting to the collector inlet

measuring section which is attached directly to the collector array.

Leaving the collector array, the air passes through the collector outlet

measuring section and into another section of flexible ducting which is

connected to the primary flow measuring device. The air then passes to

the blower, thus completing the loop.

The primary details to note in the layout of the loop are the placement

of flow measuring devices on both the collector array inlet and outlet

with the primary flow measurement being on the collector outlet. Collector temperature and pressure measurements are made as close to the

collector array as possible in duct sections which approximate actual

installed duct sections. The blower is located on the downstream side

of the collector array with creates a negative gauge pressure in the

collector. This is generally the way most solar air-heating systems

are configured. The details of the equipment in the flow loop will be

described in the following paragraphs starting with the collector and

moving clockwise in Figure 21. The specifications for the equipment and sensors are included in Table 2.

16
The collector stand is shown in Figure 22 from the side and is designed

to accommodate a 2,5 m (8 ft) by 4.5 m (15 ft) collector array. The

collector array can be tilted from 0 to 65 degrees from the horizontal.

Casters on the base provide for rotation and mobility as shovm in

Figure 23. The stand is constructed of 7.6 x 7.6 cm x 0,64 cm (3 in.

X 3 in, X 1/4 in.) angle iron except for the pivot beams which are

10 cm X 10 cm X 0.95 cm (4 in. x 4 in. x 3/8 in.) angle iron. The base

is a solid welded unit mounted on locking casters. Two hand winches

provide a means for setting the collector tilt.

A 2.5 m (8 ft) by 4,5 m (15 ft) stud and plywood platform insulated with

glass fiber batts is mounted on top of the stand. The collectors are mounted on this platform as normally installed on a roof. The outside

edges of the collector array are also insulated to simulate the effect

of adjacent collectors. The collector measurement sections are attached

directly to the collector manifold and are suspended from the bottom of

the platform as can be seen in Figure 22, The collector measuring sections were constructed in accordance with reference [4] and are located

at both the collector inlet and exit. Collector inlet and exit temperature difference, and pressure drop are measured at these locations.

Reference [4] requires a length of 2,5 V a x b at the inlet and 6,5

Va X b at the exit between the temperature measuring station and the

collector manifolds where a and b are the cross-section dimensions of

the duct, A second requirement is that the air inlet and air outlet

ducts shall be insulated in such a manner that the heat loss to the

ambient air would not cause a temperature change for any test of more

than 0.3''C (0,5°F) between the temperature measuring locations and the

collector. In order to satisfy both of the above requirements, highly

insulated measuring sections were required.

2 The collector stand is designed to test collector arrays of up to 10 m

(100 ft^) in area. Assuming that the maximum flow rate used with a 10 m

(100 ft^) collector would be 0,20 m^/(s,m^) (4 ft^/min,ft^) for a total

of 2 m /s(400 ft^/min,), a 20 cm (8 in,) diameter circular duct was selected. This size provides duct velocities of approximately 5.9 m/s

(1150 ft/min.) for the maximum size collector array. The amount of insul

tion required was then calculated assuming a minimum flow rate of 0.03

m"^/s (64 ft^/min.)* and a maximum temperature difference between the air

stream and ambient of 70°C (126°F). Based on the properties of glass

fiber insulation and a length of 1,5 m (5 ft) between the collector

exit and the measuring section, it was found that 7 ft (2,1 m) of insulation was needed to meet the requirements of [4], A more reasonable

thickness of 20 cm (8 in,) was chosen and based on actual test conditions

for the collector tested in this study, it was calculated that the maximum error possible in collector efficiency due to duct heat loss between

the collector and measuring sections was less than 1%,

*This would correspond to a flow rate of approximately 0,01 m /(s'm ) (2 f t^/(min,f t^) for a two module array and is the minimum measurable

flow rate through the minimum size nozzle in the present test loop,

17
The duct sections were constructed with 20 cm (8 in.) inner diameter

thin-walled aluminum tubing. The "L" shape seen in Figures 22 and 23 was necessary due to physical dimension limitations imposed by the

collector stand. The flow duct and surrounding insulation are all

contained in the 61 cm (24 in.) square sheet metal box.

The pressure measuring stations consist of four 0.48 cm (3/16 in.)

nipples soldered to the duct and centered over 1 mm (0.04 in.) diameter

holes. The four static pressure taps located symmetrically around the

duct at each station are manifolded to a single tygon tubing pressure

line. The pressure taps inside the duct were smoothed using emery cloth

to remove any possible burrs. The pressure drop across the collector is

measured by using a 0-0.25 kPa (0-1.0 in. H2O) inclined manometer.

The temperature measuring stations consist of thermocouples mounted in

probes which are inserted into the air stream through compression fittings.

The probes were made using 0.3 cm (1/8 in.) o.d, brass tubing inserted

into 0.32 cm (1/8 in.) compression fittings which were threaded into the

duct wall. The thermocouples were made from 24 gauge type-T thermocouple

wire. Thermocouples are permanently positioned at three different points

in the duct in order to indicate any variation in temperature across the

duct. In addition, a thermocouple is mounted on a probe which can be

moved across the duct when desired to determine the temperature distribution. Temperature difference between the inlet and outlet of the

collector is determined using a six-junction type-T thermopile constructed

from a single spool of 24 gauge thermocouple wire. The thermopile junctions are located at the centers of equal cross-sectional areas as shown

in Figure 24.

Flow mixers were not installed in the duct. Flow conditions at the temperature measuring sections were found to be uniform (temperature traverses

were made). Flow mixers should only be used when really necessary. They

will cause a larger pressure difference between the collector and ambient

than normally experienced in an actual system installation, resulting in

increased collector air leakage and thus different performance.

The primary air flow measuring apparatus consists basically of a receiving

chamber, a discharge chamber, and an air-flow measuring nozzle as shown in

Figure 7 of reference [4]. The two chambers were formed separately using

24-gauge sheet metal to construct two boxes, one 100 x 40 x 40 cm (39 x 16

X 16 in.) and the other 70 x 40 x 40 cm (27 x 16 x 16 in.). The nozzle

mounting plate is a 0.5 cm (3/16 in.) steel plate with outside dimensions

of 50 cm (20 in.) square framing a center hole of 32 cm (12.5 in.) square.

The plate surface was ground flat and smooth where the nozzle is mounted.

Diffusion baffles which are decorative grills having approximately 40%

free area were purchased from a local hardware store and installed in the

two chambers at the locations specified in [4]. The two chambers and the

nozzle mounting plate are joined with gaskets and bolted together. An

air-tight window was formed in the duct to allow access to the nozzle.

Holes were cut in the back walls of the two chambers to allow connections

through duct work with the rest of the loop.

18
Nozzles were constructed in accordance with reference [4] and are fixed

to 36 cm (14 in.) squares with magnetic mounting strips. The magnetic

mounting strips and pressure of the air hold the nozzle in place and

form a tight seal. Five interchangeable nozzles were purchased, in sizes

of; 5, 6.5, 7.5, 10, and 12.5 cm (2, 2.5, 3, 4, and 5 in.) throat

diameters, which provide a range of flow rates from 0.03 to 0.41 m /sec

(70 to 950 ft /min,). Pressure taps are incorporated into the duct

walls as specified in [4] by centering and soldering 0.5 cm (3/16 in.)

nipples over 0.013 cm (0.04 in.) holes located on the four duct walls.

The two sides of the pressure sensing devices for the nozzle pressure

difference and the nozzle discharge gauge pressure each are connected

to four externally manifolded pressure taps. In addition, a pi tot tube

is used to sense velocity pressure at the nozzle throat. The pressure

difference across the nozzle is determined using a 0-1.2 kPa (0-5 in.

H2O) inclined manometer in parallel with an electronic pressure transducer. The pressure transducer is an elastic diaphragm type and produces

a 0 to 5 volt signal over the 0-1,2 kPa (0-5 in. H2O) pressure range.

The output signal is reduced to a 0 to 500 mv range using a voltage

divider network. The nozzle discharge gauge pressure is determined using

a 0-2,5 kPa (0-10 in. H2O) vertical manometer. The pi tot tube is

connected to a 0-1.2 kPa (0-5 in. H2O) inclined manometer. The procedure

for calculating air flow rate using these measurements is given in

Appendix A.

The blower is a centrifugal industrial model with a 3 phase 220 volt 3

horsepower motor capable of 0.47 m"^/s (1000 ft'^/min.) at standard conditions and a static head of 2,5 kPa (10 in, H2O). This is a great deal

more capacity than is needed for most test size collector arrays. However, the air handling module was designed to also be used for testing

full-sized thermal storage units. The blower must be sized to provide

the required flow rate over the pressure losses in the test loop. For

the present system, the major pressure loss occurs at the flow measuring

nozzle and is 0,25 to 0,75 kPa (1 to 3 in, H2O), The amount of air

delivered by the blower is controlled by the bayonet-type damper

on the blower outlet (coarse control) and stovepipe-type damper on the

inlet (fine control). The blower assembly is mounted on rubber vibration

isolators and duct connections to the blower are made using flexible

rubber connections which results in nearly complete vibration isolation

of the blower.

The air reconditioning apparatus was designed to deliver air at a set

temperature at its exit. Through the use of two hand-set dampers, part

of the hot air from the blower is allowed to pass on to the collector

while the rest of the hot air is exhausted to the atmosphere. Further

downstream, a third handset damper is used to bring in ambient make-up

air. Through proper positioning of the three dampers, the resulting

air stream temperature can be adjusted to a value slightly below the

desired temperature. The final temperature is attained and finely

controlled using 6 kW electric resistance heaters coupled to a proportional power controller. The heater provides the correct amount of

19
energy needed to bring the air-stream up to the desired temperature.

It is important that the temperature sensor be placed far enough away

from the heaters to sense the true temperature of the air stream. In

addition to the proportionally controlled heaters, two 18 kW industrial

duct heaters are also built into the loop. These duct heaters are con- trolled in 3 kW increments by hand set switches and are used primarily to

provide high temperature step inputs for thermal storage device testing.

Based on analysis and experience gained during this test program, it is

felt that steady-state conditions are easier to maintain if the temperature difference between inlet fluid and ambient air is held constant

rather than holding the inlet fluid temperature constant alone. Thus,

electric duct heaters with no controls (other than manual) can be used

when operating the test equipment in an open-loop configuration. Since

ambient temperature is fairly constant over typical test periods, the

test requirements in the current Standard 93-77 can still be met.

The dry bulb and wet bulb temperatures of the air stream are measured

just downstream of the air reconditioning apparatus in order to be able

to determine the specific volume and humidity ratio of the air. Both are measured using thermocouples, the latter with a saturated thermocouple

exposed to the air stream. The humidity ratio is used in the calculation

of air flow rate as explained in Appendix A and needs to be measured at

only one point in the loop.

The secondary flow-measuring apparatus is a commercial unit consisting of

air flow straighteners upstream of an averaging pi tot tube arrangement.

An aluminum honeycomb flow straightener precedes a pitot tube station.

The pitot tube station consists of several total and static pressure

taps carefully spaced to produce an average duct velocity reading. The

static and total pressure taps are connected to common static and

total pressure manifolds. The pressure difference between the two manifolds is read by a 0-0,25 kPa (0-1 in, H^O) slant gauge manometer and a "cfm meter". The "cfm meter" is a bourdon-tube pressure gauge with a cfm scale calibrated to match the specific design of the unit. This

dial reading was only used to adjust the blower and its dampers to the

approximate flow rate desired for a particular test. This secondary

flow-measuring apparatus was found to yield flow rates approximately

15% high when calibrated against the nozzle apparatus,

3,3 METEORLOGICAL MEASUREMENTS

The specifications for the instrumentation is given in Table 3. Two

types of ambient air sensors are being used in conjunction with a standard calibrated total immersion, ASTM liquid-filled thermometer. One

is a type-T calibrated thermocouple and the other a precision platinum

resistance thermometer. The liquid-filled and resistance thermometers

are accurate to within ±0,1°C (±0,2*'F) while the calibrated thermocouple

uncertainity is +0,1''C (±0,2''F), All are housed within a well ventilated

small instrument shelter located 1,25 m (4,1 ft) above the ground with

its door facing north.

20
The vd.nd speed is measured by a 3-cup wind anemometer delivering a dc

voltage proportional to the wind velocity. Being mounted upon a portable adjustable base, the wind anemometer can be located close to

any tested collector and adjusted in height. As the result of a wind

tunnel calibration, the resulting uncertainty in wind velocity measurement is +0.35 m/sec (+ 0.8 mi/h).

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 testing program, a weather vane producing an analog

output proportional to wind direction was installed to continuously

monitor wind direction.

For measuring total solar radiation incident on a 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. An intercomparison was made between two of the

model 8-48 instruments and a model PSP when all were tilted at several

angles from the horizontal up to 56**, typical of collector tilt angles

to be used in testing. The results are shown in Table 4. There was as much as a 7.4% difference even though the instruments agreed within 1.5%

in the horizontal position. Similar experiences by other researchers

in the field lead to the adoption of the requirement for using a first

class pyranometer as classified by WMO [5]. The model PSP pyranometer

is capable of an uncertainity of + 3.0% and has been shown to have

typical tilt errors of less than 0.9% for angles ranging from 0° to

50°. These data were obtained during an intercomparison against an Angstrom pyreheliometer .**

The pyranometer is mounted on an adjacent surface parallel to the

collector in such a manner that it does not cast any shadow onto the

collector. Care is taken to insure that the pyranometer is at the same

tilt as the solar collector and to minimize reflected and reradiated

energy from the solar collector onto the pyranometer. The typical mounting scheme is shown in Figure 25.

The diffuse component of the incident solar radiation is determined for

each efficiency test point by shading the pyranometer following the

recommended technique of reference [4] (see Figure 26).

*Eppley Laboratories, Newport, Rhode Island.

**Personal communications with the Solar Energy Research and Educational

Foundation, Washington, D.C.

21
In addition to monitoring the total solar radiation, the direct normal

radiation is measured using a pyrheliometer , The pyrheliometer tracks

the sun and thereby measures the direct normal incident solar radiation

throughout the day.

The sky temperature is being determined for some tests by the use of either of two instruments. One is a commercially-available

pj^geometer and the other is a net radiometer designed and built by

the French Building Research Center for NBS and described in Appendix B,

Both the pyr geometer and net radiometer are located within the collector

plane. Based upon a calibration against a blackbody source, the

pyrgeometer is capable of determining the absolute incoming long wave

radiation to an uncertainity of + 3% over the spectrum of 0,35 to 50^m.

The net radiometer has a greater degree of uncertainty (typically + 8%

in net radiation) when compared against the pyrgeometer. The principal

cause of the large uncertainty is suspected to be the variable effective

optical properties of the net radiometer in response to exposure and

solar spectral distribution.

Barometric pressure is determined periodically throughout the test

period using a precision barometer,

A special incident angle meter was designed and built for determining the

angle between the sun's direct beam and the outward drawn normal to plane

of the collector aperture, A schematic drawing of the meter is shown in

Figure 27, The principle of operation is very similar to that of a sun

dial. It consists of two flat 0,64 cm (1/4 in,) thick clear plastic

10 cm X 10 cm (4 in, x 4 in,) sections joined and braced orthogonally.

The vertical clear plastic member contains a quarter-circular graduated

scale in angular degrees identical to a protractor, and a shadow pointer

located at the center of curvature of the angular scale. In operation,

the meter base is placed on the surface of the collector and rotated

until the shadow cast by the pointer is located within the plane of the

vertical member. The incident angle is then determined by the intersection of the shadow and the graduated scale. The incident angle meter

has consistently agreed to within 1° to 2° when compared against the

analytically-predicted incident angle upon a tilted surface for typical

test conditions,

3,4 DATA ACQUISITION

A variety of data are monitored and recorded by the data acquisition

system and other recorders. Data such as flow rates, temperatures,

and pressures in the air and liquid collectors test loops as well as meteorological data are read into one central instrumentation room. The room is environmentally controlled and is located between the air

and liquid collector test loops as shown in Figure 11.

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, or pressure

22
transducers; 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 interprets and converts the

flow transducer digital signals into analog form while separate individually matched and calibrated bridge amplifiers interpret and convert

the resistance thermometer signals. Type-T thermocouples are provided

with an automatic electronic reference junction or an ice bath.

After conditioning the input data into either analog or digital signals,

the information is fed into the equipment shown in Figure 28. 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 insolation, flow rates,

and inlet-outlet temperature differences, are continuously recorded on strip chart recorders in order to readily observe any transients. Electronic integrators are periodically used to integrate quantities such

as incident solar radiation or fluid temperature rise across a solar

collector. The data logger scan interval should be as small as possible

and has most often been one minute. Of course, the rate of data scanning

should depend on the type and intensity of transients being monitored.

The data logger is capable of calculating and recording the arithmetric

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.

Table 5 includes the specifications for the strip chart recorders, integrators, and data logger used.

23
i
4. TEST RESULTS

During the period covered by this report (1976 and 1977), six different

collectors were tested. They are described in Table 6. Five were liquid-heating collectors, the other an air heater. The following

sections describe the tests conducted and results obtained.

4.1 LIQUID-HEATING COLLECTORS

Time Constant : The collector time constants were determined following

the procedures described in Section 2,1 of this report and reference [4],

To provide a radiation step-change, the collectors were either suddenly

shaded or unshaded with a white opaque cover suspended above the collector.

The opaque cover is shown in place above collector no. 4 in Figure 29.

In the case of the linear Fresnel lens tracking collector (collector

no. 5), the step-change was accomplished by rotating the collector off

the track, of the beam radiation toward the diffuse sky.

25
A graph of inlet and outlet temperature for collector no, 1 during a time-constant test when the collector was suddenly shaded is shown

in Figure 30. The time constant of each of the five liquid-heating

collectors is given in Table 7. The tabulated time constant was calculated by averaging the results from numerous tests where the collector

was both shaded and unshaded. On the average, the difference in the

measured time constant between shading and unshading was less than 6%,

In order of decreasing magnitude, the time constant was largest for

collector no. 4 then collectors no. 3, 2, 1, and shortest for collector

no. 5. Collectors no. 3, 2, and 1, being of the flat-plate design,

exhibited very similar time constants (on the order of 100 s) . The

short time constant (55 s) for collector no. 5 was due to a smaller

absorber mass and high flow rate compared to the flat-plate collectors.

The very large time constant (20 min.) for collector no, 4 was primarily

the result of a large collector fluid capacity (34 liter (9 gal,)) and

a very low flow rate (0.018 liter/s(0.3 gal. /min.)).

The alternate technique of conducting the test indoors and suddenly

changing the inlet fluid temperature was used for one collector and

found to give comparable results. However, this technique is much more

difficult to use experimentally.

During the deliberations that led to the adoption of ASHRAE Standard

93-77, there was controversy over what time constant is actually being

determined by this test. In reality, there can be more than one time

constant for a given collector since the various components of the

collector respond over a different time period to a sudden change in

the solar insolation conditions. Although not shown in Figure 30,

several strip chart recordings of the exit temperature from the flatplate collectors during a time constant test indicated another noticeable

but more gradual response after 6 or 7 min. which was an indication

that some parts of the collector were just beginning to respond. The

time constants indicated in Table 7 and determined in accordance with

Standard 93-77, are primarily a function of the fluid dwell time and

the absorber plate response.

Near-Normal-Incidence Efficiency : Collector no. 1 was the first collector tested. It was encased within a wooden frame on the edge in

order to simulate the effect of adjacent collectors that would exist

in an actual field installation. This was felt to be particularly

important for this collector due to a design that put the absorber

plate in intimate contact with the edge of the collector case. Throughout the testing of this collector, the "black-and-white" model

8-48 pyranometer was used for measuring the incident solar radiation

in the plane of the collector aperture. As already mentioned in Section

3.3 of this report, possible errors exist in the efficiency values due

to the collector being tested at tilt angles (from the horizontal) of

20-40°. Correction factors were not introduced to account for the

pyranometer error in analyzing the data for this collector since no reproducible correction factors have yet been determined for this

26
pyranometer. For example, the data from Table 4 indicates the correction

factor could be anywhere from 3.9% to 7.4% for the 45° tilt position

depending on the insolation level and the particular instrument used.*

A more comprehensive laboratory study has recently been completed [19]

in which the error is indicated to be only between 1.5 and 2.0%. This

pyranometer was not used for any further tests following the tests on collector no, 1,

The results of the efficiency tests are shown in Figures 31 and 32. The

efficiency values are based on the aperture area of the collector, 1.61 m^

(17,3 ft^). In Figure 31, all data were taken at solar noon on a number

of different test days whereas in Figure 32, the data were taken + 3/4 h

symmetrical with solar noon, A linear regression** curve fit of the data

in both figures resulted in identical curves. Therefore, for this type

of collector, the data scattering caused by collector thermal capacitance

can be averaged out if care is taken to make the measurements symmetrical

with solar noon as required by [4],

A solar screen was placed over the collector as shown in Figure 33 to

reduce the solar radiation incident upon the collector. The screen was

a flat-weave opaque glass fiber type and allowed efficiency data to be

taken at larger values of At/I, Normally, test data are obtained over

the entire range of At/ I values primarily by changing the operating

temperature of the collector, tj ^, This is because the ambient temperature and incident solar radiation are generally fixed (within limits)

for the location and time of year in which the test is being conducted.

However, Figure 32 shows that is was possible to establish part of

the efficiency curve by using the screen. This technique is not generally

recommended since in reality using lower insolation levels is not

equivalent to operating the collector at elevated temperatures. However,

the error is small for flat-plate collectors which operate at relatively

low temperatures (<80°C (175°F)),

Figures 34 and 35 show the results of the efficiency tests for collectors

no, 2 and no. 3 (again based on aperture area). The edges of these collectors were not encased in a wooden frame nor insulated on the edge

due to a good thermal design of the modules themselves.

Collector no. 4 was a single-glazed evacuated tubular collector with a back diffuse reflector. The results of the efficiency tests are shown

*This is assuming the calibration factor for the "all-black" model PSP

does not change with the tilt angle.

**It is felt that a linear plot is adequate for most collectors. Even

though collectors don't have a constant heat loss coefficient as is

indicated by a linear plot, data scatter caused by variable ambient

conditions and experimental uncertainties prevent the determination of

the "true" 2nd-order response of the collector.

27
in Figure 36 and a photograph of the collector under test is shown in

in Figure 25. The collector array consisted of two modules connected

in parallel. The large thermal time constant resulting from the large

collector fluid capacity and low operating flow rate coupled with a large temperature rise across the collector contributed to problems of

stabilization during testing. Extreme care had to be taken to eliminate

any perturbations within the collector to insure steady-state conditions.

At least one hour was allowed for temperature stabilization after reaching

operating conditions. The time interval over which the experimental

data are collected and integrated should be at least one time constant,

according to [4], amounting to a period of 20 min, for this collector.

Initially, a period of one hour was used for integration and it was found the 20 min, period resulted in efficiency values only 1,2%

different for data collected across solar noon.

The aperture area was used in computing the efficiency values in

Figure 36 and was interpreted as the area of the collector tubes and

the interstitial spacing between the tubes excluding the end brackets

and center manifold.

Collector no. 5 was a tracking, concentrating collector utilizing

a linear Fresnel lens and a selective-black-chrome-coated absorber.

The efficiency curve is shown in Figure 37. The tested collector

consisted of two modules connected in parallel, whose tilt angle could

be periodically adjusted and which automatically tracked the diurnal

movements of the sun. The tested modules are shown in Figure 38.

In Figure 37 two curves are shown, one representing a 0° incident angle

efficiency and the other a 23° incident angle efficiency. The efficiency

values in Figure 37 were calculated, only using the direct component of

the solar radiation and based on the aperture area of 1.8 m^ (19.4 ft^).

During three of the five days of testing, the collector tilt angle was'

such that the incident angle across solar noon was 23". On two other

days, the collector had an incident angle of 0 and 10° across solar noon. Based on results obtained by Pendleton [20], the incident angle has little

or no effect for angles up to 10 degrees. Therefore, a single curve was used for data collected at 0 and 10 degrees. Note that the efficiency

decreases by 8% for an incident angle of 23° at solar noon. This is

equivalent to the yearly declination variation for a collector tilt

angle equal to the latitude.

Figure 39 shows the results of the near-normal-incidence efficiency tests

for all five of the water-heating collectors. The efficiency values are based on the aperture area as given in Table 7. The data are replotted

in Figure 40 with the efficiency values based on gross collector area as required by ASHRAE Standard 93-77. The change in efficiency between

these two figures is small for collectors 1 through 4 since there is

only a relatively small difference in aperture and gross area for these

collectors (<15%). For collector no. 4, the gross area was assumed to

to include the end brackets and center manifold. There is a large

change in the efficiency of the concentrating collector (collector no, 5)

because of the authors' interpretation of gross area for this collector.

The aperture area of collector no. 5 was assumed to be the frontal area

28
of the linear Fresnel lens; the gross area was calculated based on

the center to center spacing between individual collectors.

During all tests, the pressure drop across the collector was measured

and recorded. Figures 41 and 42 show how the pressure drop varied as a function of flow rate for one of the flat-plate collectors (Figure 41)

and the evacuated tubular collector (Figure 42).

Incident Angle Modifier : Two different techniques were used to determine

the incident angle modifiers of the water-heating collectors. The first

technique involved keeping the collector test stand stationary with the

collector facing south, as was done when determining the near-normalincidence efficiency, and allowing the incident angle to change with

respect to the diurnal movement of the sun. In this procedure, the

instantaneous collector efficiency was determined throughout the day as a function of incident angle, and morning and afternoon values (for the same incident angle) were averaged to eliminate the thermal lag effect.

In this way, the incident angle modifier was determined for a wide range

of incident angles in one day. The second technique involved orienting

the test stand so that the collector faced north but tilted to a near- horizontal position with the tilt angle adjusted in order to obtain a specific incident angle across solar noon. With this procedure, it was only possible to obtain one value of incident angle modifier per test day.

The results for collectors no. 1, 2, and 3 are shown in Figures 43, 44,

and 45, respectively. The second technique described above was used to

obtain the curves shown in Figures 43 and 45. Both techniques were used for collector no. 2 and, as can be seen in Figure 44, gave comparable

results. The second technique or orienting the test stand to the north

was used to determine the solid line in the figure. The first technique was completed in one day, the data points indicated by circles taken

in the morning and those by triangles in the afternoon. The dashed line

is a curve fit through the average of these points. As can be seen by

the fact that the circles and triangles are relatively close together,

there was only a small thermal lag with this collector. This is consistent with its small time constant of 1,6 minutes. As can also be seen

in Figure 10, there was essentially a linear relationship between K^^

and the term (l/cos9) - 1, which is consistent with the theoretical

model for optical efficiency of a flat-plate collector [17],

Figure 46 shows test results for collector no, 4, the evacuated-tubular

collector. Note that the thermal lag effect is more pronounced in

Figure 46 than in Figure 44 which is consistent with the fact that

collector no. 4 has a time constant of 20.3 minutes. In addition, the

incident angle modifier increases with increasing incident angle, the

opposite from that of the flat-plate collector in Figures 43-45. Since

the tubes were not used with a concentrating reflector, this tubular

collector has its minimum efficiency at solar noon and larger values

at other times of the day. This is explained by the fact that the

tubes intercept a larger fraction of of the incoming solar radiation

at the large incident angles and also that the transmittance of solar

29
radiation through each glass cover tube does not change with time of

day since the sun is practically always normal to the glass surface.

With simple glazed flat-plate collectors, the incident angle modifier

varies in a smooth manner with incident angle and is basically independent of orientation (except for perhaps secondary effects due to changes

in tilt and hence convection heat loss or different shading of the

absorber by the collector frame at different orientations). The testing

procedure in Standard 93-77 for determining the incident angle modifier

was based on these characteristics and as a result is only really valid

for simple flat-plate collectors. The way in which the test should

be conducted for other types of collectors depends in general on the

specific collector being tested. For example, the tubular collector

tested and described in this report is sensitive to changes in incident

angle due to changes in both the sun's altitude angle and hour angle.

Therefore, determination of two incident angle modifiers would be appropriate; one describing how the collector's efficiency changes as the

angle between the outward drawn normal and direct beam increases in a direction parallel to the axis of the tubes and a second modifier for

the direction perpendicular to the tube's axis. The data in Figure 46 were determined by keeping the collector facing south in a fixed position

with the tilt adjusted so that the incident angle was zero at solar noon. Therefore, the data is representative of changes due primarily to a change in the diurnal hour angle.

A testing procedure to get the two incident angle modifier curves for a non-movable trough-type concentrating collector with an east-west axis

using a south-facing outdoor test stand has recently been described by

Thomas [21]. In a similar fashion, Johnson [22] has demonstrated the

difficulty of this type of testing for a single-glazed flat-plate

collector. The collector had a non-selective coating on a copper rollbond absorber. Mylar strips running the width of the collector were installed between the absorber and glazing to decrease convection and

reradiation losses. The strips were approximately 10 cm (4 in.) deep

and 0.95 cm (0.37 in.) apart. The optical characteristics of this collector were similar to that of a trough-type concentrator with the trough

axis running east-west.

Figure 47 shows the results of tests conducted by Johnson using an indoor

simulator at the NASA Lewis Research Center. With the collector tilted

at 57** and oriented normal to the simxilator source, the collector was then

rotated about a vertical axis and data taken to establish the upper dashed

curve. Then the tests were repeated for other tilt angles as indicated.

Unfortunately, by rotating the collector about a vertical axis, the angle

between the "sun's" beam and both principal axes in the plane of the

collector aperture were changing simultaneously. As a result, the effect

of incident angle changes perpendicular to each principal axis could not

be isolated. Notice in the figure that with each of the lower three

dashed curves, one of the data points lies far from the indicated curve.

30
If the tests were to be repeated, the collector orientation could be

changed in such a way as to establish two incident angle modifiers for

the collector. To show both effects, a family of curves should be

plotted for K^^ as a function of the incident angle projected into an east-west plane with Q-^ of Figure 48 as a parameter. The test itself

would be relatively easy to conduct using a simulator.

The experimental incident-angle-modifier curves for collectors no, 1-4

are shown collectively in Figure 49 and in Figure 50 in comparison with

the theoretically-predicted curves for single and double-glazed flatplate collectors with non-selective coated absorbers exposed to direct

beam radiation. The experimental results are in approximate agreement

with the theoretical curves. The differences are due to several reasons. Collector no, 2 was a double-glazed collector but its incident angle

modifier was not as dependent on incident angle as was expected from

the theoretical curve. The experiment was conducted under 18-40% diffuse radiation which should cause the collector's efficiency to decrease

less with incident angle than if it were exposed to 100% direct beam

radiation. In contrast, the efficiency of collector no. 3 decreased

more rapidly than would have been though since it was a single-glazed

collector and tested under 11-24% diffuse radiation. However, the

absorptance of the selective surface on the absorber could have been

a function of incident angle and hence caused the indicated response.

The angular response for collector no. 1 may be in error due to (1)

the use of the "black-and-white" pyranometer during the tests or (2)

as a result of 15-25% diffuse radiation during the test.

An incident angle modifier was not determined for collector no. 5, the

tracking, concentrating collector. As with collector no. 4, two incident

angle modifiers would be in order. The value of one of them, that which

describes the change in efficiency as the angle between the direct beam

and the outward drawn normal increases in a direction perpendicular to the

axis of the concentrator, is identically 1 since this collector tracks

the sun in the east-west direction to a tolerance of 0.2°. However, the

collector tilt angle is not normally changed during the year to reflect

changes in the sun's declination. Therefore, an incident angle modifier

should be used to account for the resulting change in performance. As

already indicated in Figure 37, efficiency data v^re taken on this collector for constant incident angles of both 0° and 23**. The incident

angle modifier was computed, based on these data, and is shown in Figure

51. Additional tests could be conducted at other angles to establish

a complete incident-angle-modifier curve for this collector.

4.2 AIR-HEATING COLLECTORS

Time Constant : Figure 52 shows results of the time constant test on collector no. 6, the air heater. The test was conducted outside under

a clear, sunny sky by shading the collector after steady-state conditions

had been reached. Initially the fluid entered the collector at 28.5''C

(83.3'*F) and exited at approximately 63°C (145°F) with a flow rate of

3L
0.01 m /(s.m ) (2 ft /(min. ft ). As can be seen, the time constant

was found to be approximately 12.7 minutes. This is much longer than

for flat-plate collectors which use water as the transfer fluid, but

less than that for the water-heating ev a cu a ted- tubular collector

(collector no. 4).

Near-Normal-Incidence Efficiency : Figure 53 shows the results of the

near-normal-incidence efficiency tests for the air heater. As can be

seen, curves are shown for two different flow rates. A recommendation

is given in Standard 93-77 that two different curves be established

because, unlike water-heating collectors, the efficiency of air heaters

is significantly affected by flow rate. The curves in Figure 53 are based on the gross area of 7.25 m^ (78.0 ft^) as recommended in the

Standard. Figure 54 shows how the curve for the test with the lower

flow rate changes if the aperture area were used to compute efficiency.

The efficiency values in Figure 53 are lower than expected, based on theoretical predictions for this collector. It was found during the

test that air was leaking into the four-module array. Air leakage in

air heaters is a troublesome problem that can affect test results,

as well as the actual performance of an installed system. During testing, the test loop can be sealed sufficiently well with duct tape,

caulking, etc. Leakage in and around the collector may occur as a result of the collector design and recommended installation practice.

This type of leakage of course should not be eliminated. However,

measurements should be made in such a way as to determine the true output of the collector array. In addition, the array should be tested

in a manner such that the air leakage occuring during the tests will

be indicative of the leakage that will occur in an actual installation.

Close and Yasoff [2 3] have recently published the results of an analysis

of air leakage in air-heating collectors. They assumed a constant leakage

rate along the length of the collector and determined its effect on the

efficiency measurements for all combinations of:

a. operating the collector under negative pressure (air leaking in) and

positive pressure (air leaking out), and

b. measuring the air flow rate before (upstream) the collector or after

(downstream) the collector.

The results are shown in Figure 55. The abscissa of the plots is the

leakage rate divided by the measured flow rate and the ordinate is the

ratio of the actual collector efficiency to the measured efficiency.

As can be seen, for three of the testing configurations, the discrepancy

between measured and actual efficiency is a direct function of the

leakage rate. In addition, for the case where the collector is operating under negative pressure and air is leaking in, the difference in

efficiency depends on the difference in temperature between the ambient

air and the entering air stream to the collector. Therefore, the error

in efficiency for this case is larger for the data points at the higher

32-
inlet fluid temperatures relative to ambient. Also note that when the

air leaks in, the actual efficiency is larger than indicated by the

measurements whereas just the opposite is true when air leaks out. This

is consistent with what one would intuitively expect. When air leaks

into the collector at a cooler temperature than is measured at the collector inlet, the collector is heating up the air over a larger temperature

difference than is indicated by the measurements. In addition, if the

air flow measurement is upstream of the collector, the collector is

heating a larger quantity of air than is indicated by the measurements.

When air leaks out of the collector, and the flow rate is measured upstream, the quantity of useful heated air is less than indicated by the

measurements. However, if air leaks out and the air flow measurement

is made downstream of the collector, the quantity of useful heated air

is precisely what is measured and as a result, there is no difference

between actual and measured collector efficiency. Data for this case

are not shown in Figure 55 since they would produce a horizontal

straight line with an ordinate value of 1.

The results of the above analysis have a direct implication for the

testing of air-heating collectors:

1. If the collector is normally operated under positive pressure, it

should be tested while operating under positive pressure and the air

flow rate measured downstream of the collector.

2, If the collector is normally operated under negative pressure, it

should be tested while operating under negative pressure and the air

flow rate measured both upstream and downstream of the collector in

order to quantify the leakage rate. In this manner, an estimate of

the actual collector efficiency could be made by the user of the

collector with the data of Figure 55 even though the "correction"

might not be made and published as part of the test results.

The data of Figure 53 were obtained with collector no, 6 operating under

negative pressure (normal installation practice for this collector) and

the flow rate measured downstream of the collector. Use of "smoke-bombs"

around the collector indicated air leakage into the collector. Therefore,

the actual collector efficiency was higher than that indicated in Figures

53 and 54.

As indicated by equation (1), the useful heat output of the collector is

directly proportional to the collector heat removal factor, F^. If the

collector test results are to be used without correction to predict the

performance of the collector in an actual field installation, then the

value of which occurs during the test should be approximately the same

as that which will occur in the field. The expression for F^ is [14]:

(7)

P

33
Therefore, the conditions during testing and in actual field operation

should be such that m/A, Cp, Uj^, and F' are approximately the same in

both cases. For liquid-heating collectors, this is easy to accomplish.

The flow rate per unit collector area, m/A, the type of fluid and

hence specific heat, Cp, and the collector tilt angle, temperature,

incident solar radiation, and ambient conditions are such that the

heat loss coefficient, Uj^, is approximately the same. In addition, the

collector efficiency factor, F' , is primarily a function of the geometry

of the absorber (material, thickness, distance between tubes, etc.)

and of course is the same for the tested and installed collectors at

any flow rate.

The situation is slightly different for air heaters. Although m/A, Cp,

and U-^ can be made the same in a similar fashion as with the liquidheating collectors, assuring the same value of F' is more difficult.

For air heaters, F' is primarily a function of the convection heat

transfer coefficient between the absorber and the air stream. Since

the air flow is nearly always in the turbulent flow range (to maximize

the heat transfer), the heat transfer coefficient is determined primarily by the value of the Reynolds number in the collector. For a collector designed with a simple rectangular channel under the absorber

such as in collector no. 6, the Reynolds number is given by:

u

where V = velocity of the air in the collector, m/s

= hydraulic diameter of the channel through which the air flows, m

u = kinematic viscosity of the air, m/s

Since the hydraulic diameter of the channel is equal to twice its width,

equation (8) can be rearranged to give:

Re = 2mL (9)

y A

where

y = dynamic viscosity, Pa's

L = flow path length of the collector array, m

Consequently, since the mass flow rate per unit area is being maintained

the same between the tested collector and the value to be used in the

field installation, the length of the collector or flow path length should

be maintained to insure the same Reynolds number, convection coefficient,

and hence F '

The particular collector tested in this program is designed to be

installed in the field with two modules in series and a flow path

34
length of approximately 4 m (13 ft). Therefore, four modules (two in

series and in parallel with two others) were used in obtaining the data

of Figure 53.

It would be useful to be able to test only one collector module and then

apply a correction technique to the data in order to predict the performance of an array consisting of two or more collectors in series. This

would simplify the testing of air heaters. Two techniques are suggested

here for doing this. In both techniques, the single module must be

tested at a flow rate that will result in the same flow velocity which

will occur in the array of collector modules in series. This will insure

the same Reynolds number, same convective heat transfer coefficient, and

hence same F'. As an example, if one collector module were being tested

and two are normally installed in series, by equation (9), the flow rate

per unit collector area to be used in the test would be twice the value

to be used in the actual installation in order to have the same Reynolds

number and the same flow velocity.

Technique No. 1*:

After the test on the single module is completed, the y intercept

of the efficiency curve is determined and set equal to Fj^(Ta)g.

By separate measurement or calculation, (xa)^ is determined for the cover plate assembly. is then calculated. In a similar fashion,

the slope of the efficiency curve is set equal to Fg^Uj^ and then

Uj^ is calculated from knowing F^, From the above information and

knowing mc for the test, equation (7) is used to compute F'.

Assuming F*^ and Uj^ to be the same for the collector modules in

series, the new value of mass flow rate per unit collector area is

used to calculate the value of Fj^ applicable for the modules in

series from equation (7). The new efficiency curve is then con- structed from the value of Fj^, (Ta)^, and Uj^.

Technique No. 2:

A correction factor is applied to the test data on the single module.

The correction factor is derived as follows. Equation (1) governs

the performance of the single collector. For two collectors in series

*Suggested by Bruce Cole-Appel, Solaron Corporation, Denver, Colorado.

#

[F^Kxa)

z R e

F U (t, , R L f ,i

(10)

35
where

Q l ~ the exit fluid temperature of the first collector

* ' equal to the inlet fluid temperature of the second, °C

Solving equation (1) for the exit fluid temperature of the first

collector,

'f,e,l = ^f,i +-A_ [F Kta) - F U (t^ - tj] ^^^^ mCp R

'

' ' e R f ,i

Substituting equation (11) into equation (10) and rearranging.

1 AFrUt

q^ = A[FRl(Ta)e " W^f.i " ta)] [1 " ^ ^-^]

mCp ^"-^^

The single module is tested and its efficiency curve determined. Then a new efficiency curve is established by correcting the collector output

for the single module by the multiplying factor

[1i - i

2

*mCp

In a similar fashion, the correction factor for applying the single

module test results to three modules in series can be shown to be,

AFrUt 1 AFrUt 2

[ 1 - _JLJi -f 1 ( R L)

] mcp i^Cp

and to four modules.

^ AFrUt AFrUt t AFrUt

mc mCn mc

P

p

""-p

The second technique above was verified by conducting an experiment on one-half of the four module array of collector no, 6, Two of the

modules were connected in parallel and efficiency data were gathered

for a flow rate per unit collector area of 0,02 va.^/s,Ta.^ (4 ft /min, ft^)

Therefore, the flow path length was only one-half of that used when

gathering the data shown in Figure 53 but the flow rate per unit area was twice the value of 0,01 m /s.m (2 ft /min, ft^) used for those

36
tests. The results of these second tests and comparisons were as follows

/

Corrected to

Two Module Four Module Four Module

Test Performance Test, Figure 53

0.52 0.46 0.46

(W/m2.°C) 5.61 4.95 4.69

The data from the two tests are shown together in Figure 56 along with

additional data for the two collector modules in parallel tested at a flow rate of only 0.01 m /s*m^ (2 ft /min'ft ), indicated by the data

in triangles and the long-short dash curve.

The discrepancy noted above between the heat loss factor, Fj^U^, for the

four module array and the corrected heat loss factor from the test on

the two modules in parallel at twice the flow rate per unit area could

have been due to the different leakage rates which occured during the

two different tests.

Incident Angle Modifier : Figure 57 shows the results of the incident

angle modifier test being run on the air heater using both of the techniques that were used with the liquid-heating collectors. As can be

seen, there was a large difference in the results using the two different procedures. The most reasonable result was obtained by keeping

the collector stationary, facing south and using the efficiency data

all through one test day to obtain the incident angle modifier values

indicated by the triangular data points in Figure 57. Notice the large

difference between the morning and afternoon values. This is consistent

with the large time constant of the collector and is the same characteristic that was observed for collector no. 4 and shown in Figure 46.

The second technique of orienting the test stand to the north and tilting

the collector so that a specific incident angle occurs across solar noon

gave completely erroneous results as indicated by the circular data

points in Figure 57. This is not considered an acceptable procedure

for collectors with large time constants.

4.3 ALL-DAY SOLAR COLLECTOR EFFICIENCY

Based on the tests run according to ASHRAE Standard 93-77, it is possible

to predict the all-day performance of the solar collector. In addition

to having the two curves of near-normal-incidence efficiency and incident

angle modifier, the following must be known as a function of time throughout the day:

1. inlet fluid temperature to the collector, t^ .

37
/ 2. ambient temperature, t„, and

3. incident solar radiation in the plane of the collector, I

The step by step procedure in Table 8 can then be carried out, A similar procedure has been used and reported by Zerlaut, Dokos, and Heiskell

[25].

Three separate all-day tests were conducted on collectors no. 2 and no. 3

in which the all-day efficiency was measured. The test days were com- pletely clear with steady insolation. The above procedure was used to

compute the all-day efficiency using the same test conditions ( and I)

and the incident angle modifier and near-normal-incidence efficiency curves

previously determined. The results are given in Table 9 and a comparison

of instantaneous efficiency throughout one of the test days for collector

no. 3 is shown in Figure 58. As can be seen, the agreement is very good.

The all-day collector performance can of course be determined by experiment as was done above. Many researchers over the past several years have

proposed that this would be a preferable way to collect data and report

performance instead of performing separate tests to determine the nearnormal-incidence efficiency and the incident angle modifier. Indeed, if

a large amount of daily efficiency data were collected for all types of

weather conditions and incident angle variations over perhaps a complete

year, then a plot of daily efficiency versus daily At/I would give a good

estimate of the expected long-term performance of the collector in

actual operation. This concept has been proposed by Tleimat, Howe, and

Buckland [26] and has been successfully used by then to characterize the

performance of solar stills. However, in solar collector testing, data

are usually collected over as short a period as possible and only a few

daily data points could lead to erroneous performance predictions.

It should be recognized that on a plot of daily efficiency versus daily

At/ 1, the y intercept of the plot is:

and not just

FR(^«)e,n

where day

f^T ^dT (13)

day ^J ^dx

Consequently, the resulting curve is only valid for days in which the

incident angle variation, percent diffuse radiation, and hence K ^ are approximately the same as on the test days, Simon and Miller [2^^ 28]

38
have used a technique for adjusting all-day collector efficiency obtained

outdoors to an effective efficiency value at normal incidence, in order

to compare the test data with data obtained on the same collector indoors

using a solar simulator. In essence, the all-day efficiency values and

the daily average value s of At/ I were both divided by a calculated value

of K^^ before plotting. K^^^ was calculated by knowing the incident

angle variations throughout the test days as well as the curve of

versus 0 from previous tests*. The resulting comparison with the inaoor

near-normal-incidence data was excellent.

Figure 59 shows data taken over several days for collector no. 6, the

air heater, and shows the kind of scatter that can exist when data other

than those collected on clear sunny days is used. The maximum integration

period used on any of the data in Figure 59 was four hours. Since this was a flat-plate collector tilted so that the incident angle was approximately 0 at solar noon and the data were all taken symmetrically with

solar noon, the maximum incident angle for any of the data was approximately 30° and hence K^^^ ~ 1. Even though the scatter is relatively

large, a mean curve through a large amount of such data should result

in a reasonable performance curve for the collector in actual operation.

*The actual technique was slightly more complicated since Simon and Miller

also accounted for the fact that the curve of K^^ was determined indoors

using the simulator and hence under 100% direct radiation and the outdoor

data was taken under condition of up to 50% diffuse radiation.

39

5. SUMMARY AND CONCLUSIONS

This report has briefly summarized the test procedure adopted In

February 1977 by the American Society of Heating, Refrigerating, and

Air Conditioning Engineers and known as ASHRAE Standard 93-77, "Method

of Testing to Determine the Thermal Performance of Solar Collectors."

A testing facility has been constructed at the National Bureau of Standards In Galthersbur g, Maryland, in accordance with this Standard. It

consists of two test loops for the liquid-heating collectors and one

for air-heating collectors. During 1976 and 1977, six different collectors were tested outdoors in accordance with the Standard or slight

variations of the procedures in the Standard. The main conclusions

of the study are:

1, The three primary thermal tests in the Standard, the time constant,

near-normal-incidence efficiency, and incident angle modifier tests

41
I

can be carried out with little difficulty on flat-plate liquidheating and air-heating collectors. The three day pre-conditioning

test was not evaluated during this study for any collectors,

2. The time constant and near-normal-incidence efficiency tests can be

carried out on an evacuated tubular water-heating collector and on a tracking, concentrating collector having a low concentration ratio

with little difficulty. For collectors with large time constants

(on the order of 20-30 min.) such as the evacuated tubular collector,

the near-normal-incidence efficiency curve is more difficult to

obtain because of the fewer test points that can be obtained during

a single test day due to problems of stability and larger test

intervals

3. The incident angle modifier test as currently specified in Section 8.3.3 of the Standard is only applicable for flat-plate

collectors. An entirely different procedure is required for other

types of collectors and it will vary depending upon the optical

characteristics of the collector. For example, a single-axis tracking

collector should have the incident angle modifier determined as a function of the angle between the outward drawn normal to the plane

of the collector aperture and the direct solar beam in a direction

parallel to the axis of the collector. In contrast, a stationary

evacuated tubular collector should have two incident angle modifier

curves determined; one as a function of the incident angle in a direction parallel to the tubes and one perpendicular to the tubes.

4. The procedure for conducting the time-constant test in Section 8.3.1,

Method (1) of the Standard, which involves suddenly shading the collector during an outdoor test, is much easier to complete and hence

preferable to Method (2), which involves suddenly changing the entering fluid temperature during an indoor test.

5. The procedure for conducting the incident angle modifier test in

Section 8.3.3, Method (2) of the Standard, which involves keeping

the test stand stationary facing due south and gathering all the

data in one all-day test, is preferable to Method (1), which requires

the test stand to be rotated and data collected across solar noon. Method (1) requires several test days to complete and can produce

erroneous results for collectors with large time constants (10-30

minutes)

6. A class 1 pyr anome ter> as classified by the World Me terological Organization [7]jis required to insure reasonably accurate test results.

7. It is necessary to collect thermal efficiency data symme triccilly with

respect to solar noon in order to eliminate collector-heat-capacity

effects and to insure an accurate near-normal-incidence efficiency

curve

42
Since the near-no rmal-lncldence efficiency data is always collected

on clear sunny days with the incident solar radiation in the plane

of the collector near 945 W/m^ (300 Btu/h'ft^), it is not necessary

to allow data to be taken at levels as low as 630 W/m^ (200 Btu/h'ft^)

as currently specified in Section 5.1.4 of the Standard. Such low

levels may result in additional unnecessary scatter in the data.

The definition of gross collector area and aperture area in the Standard allows too much interpretation to be made by the testing organization and consequently the possibility for error and misuse. In

addition, the aperture or absorber area should be used in the computation of collector efficiency since it is more closely related to

the performance of the collector than gross area. When gross area is

used, the y intercept and slope of the near-normal-incidence efficiency

A , . A curve are _a Fj^(Ta)^ and aF^^U^, respectively. The subscripts a and g

refer to aperture and gross, respectively. This fact must be realized

when relating the experimental data to the theoretical characteristics

of the collector as described in [14]. This possible confusion would

be eliminated if aperture area were used.

In the testing of air heaters to obtain efficiency values, care should

be taken to eliminate air leakage in the test loop and to properly

handle air leakage in the collector. If the collector is tested under

positive pressure, the air flow rate should be measured downstream of

the collector. If the collector is tested under negative pressure,

the air flow rate should be measured on both sides of the collector.

If a single module of an air heater is tested and the collector is

normally installed with two or more modules in series, it is possible

to adjust the test results to predict the performance of the fieldinstalled array by techniques outlined in this report.

The all-day collector performance of flat-plate collectors on clear

sunny days can be predicted with reasonable accuracy using the

test results obtained in accordance with the Standard.

The turbine flow meter used in one of the test loops for water-heating

collectors was found to drift in calibration by 2.5% over a period

of one year.

If any significant amount of collector testing is to be done, an automatic data logger with a magnetic tape drive recording system

is preferable to the use of strip chart recorders and hand recording

of data.

An accurate measurement of ambient air temperature is just as important as an accurate measurement of the transfer fluid entering the

collector. Both affect the abscissa value on the plot of near-normalincidence efficiency in the same way.

43
16. An open loop is preferable to a closed loop in testing air heaters,

since it's easier to maintain steady inlet temperature to the

collector.

17. The technique of determining the fraction of the incident solar radiation which is diffuse by periodically shading the pyranometer gave

results comparable vd.thin 2% to using a pyheliometer and obtaining

the diffuse fraction by deduction.

18. For flat-plate collectors having a time constant of less than five

minutes, the three primary thermal tests can be completed in six

complete test days using a stationary south-facing test stand. One

each for the time constant and incident angle modifier tests and four

to obtain the near-normal-incident efficiency data. It may be

possible to reduce the time to four or five days if the time constant and incident angle modifier data are taken on the same day.

19. The use of a solar simulator as specified in Section 7.3 of the

Standard was not evaluated during this study.

20. The use of integrators is not essential to obtain time averages of

variables such as fluid flow rate, t across the collector, and

incident solar radiation when determining the near-norraal-incidence

efficiency. Across solar noon, each of the variables typically

changes by less than 2% over a 1-minute interval and because

of linearity of variation, the arithmetic average of sampled data

agrees to within 1% with continuously monitored and integrated

data. This does not imply that the variable should not be continuously monitored by strip chart recorders.

44
6. RECOMMENDATIONS FOR FUTURE WORK

Based on the results of this study and consultation with researchers

around the world doing solar collector testing, the following recom- mendations are made for future work to improve the state-of-the-art

in collector testing:

1. A general procedure for establishing the incident angle modifier

curve(s) for a variety of non-flat-plate collectors should be

established

2. A method or procedure should be developed for using the time con- stant as experimentally determined in accordance with the Standard

for design purposes.

3. The feasibility of measuring air flow rate both upstream and downstream of an air collector during test to characterize its leakage

rate should be determined,

4. The feasibility of adopting the Standard to a wide range of concentrating collectors typically used in building applications should

be examined. A major part of this would be accomplished in 1. above,

5. The relationship between long-term collector performance exposed to a wide range of weather conditions and the performance indicated

by the tests of the Standard should be determined,

6. It would be desirable to obtain the same performance parameters for

the collector as is presently obtained by using a combination of

indoor and outdoor tests (without the use of a solar simulator).

This would reduce the time and costs of performing the tests, particularly in the less sunny parts of the U.S. Three different organizations have proposed the use of an indoor test and pumping hot water

through the collector to determine its heat loss characteristics

[29,30,31]. This would be an alternate way of establishing the slope

of the near-normal-incidence efficiency curve, thus requiring only

a short outdoor test to determine the ordinate intercept. This

technique should be evaluated to determine if an accurate efficiency

curve can be established. Work has already begun as a result of the

procedure in [31] being used in a round robin testing program being

conducted by the International Energy Agency (lEA) in 1978,

7. Because of the allowable range of environmental conditions in the

Standard during the test to establish the near-normal-incidence

efficiency curve, considerable scatter is possible in the data, A

correction technique has recently been demonstrated [5,6] whereby each

data point can be corrected to "standard environmental conditions"

using a mathematical model of the collector. This is a potentially

powerful tool for outdoor testing of collectors and should be experimentally verified and then "packaged" as a set of nomographs or com- puter program.

45
8, Several alternate testing configurations have recently been proposed

[32,33] that might enable the solar collector thermal output to be

determined more accurately than is now possible by the separate

measurements of m, Cp, and ^ - ^ , They basically involve

inserting an electrical heater in the test loop with a measurement

of At across the heater and eliminating the flow meter and a need

for knowing the specific heat of the transfer fluid. The accuracy

of this approach should be determined.

9. The liquid-heating collectors are frequently tested with one fluid

and one flow rate and used in actual installations with another fluid

and/ or flow rate. A correction technique for predicting performance

under other than test conditions needs to be developed from first

principles and verified for use in the field.

10. The potential for establishing a more accurate near-normal-incidence

efficiency curve by testing the collector side by side with a "standard collector" whose characteristics are well known should be investigated.

11. Determination and evaluation of a standard instrument and test procedure

for determination of effective sky temperature should be made.

12. For air-heating collectors, an additional test should be developed

which will result in the determination of how collector performance

varies with transfer fluid flow rate. This would enable a designer

to more easily select the optimum flow rate for his system.

46
7. REFERENCES

1. Hill, J.E. and T. Kusuda, "Methods of Testing for Rating Solar

Collectors Based on Thermal Performance," NBSIR 74-635 , December,

1974.

2. Hill, J.E. , Streed, E.R. , Kelly, G.E. , Geist, J.C., and T. Kusuda,

"Development of Proposed Standards for Testing Solar Collectors and

Thermal Storage Devices," NBS Technical Note 899 , February, 1976.

3. Hill, J.E., and E.R. Streed, "A Method of Testing for Rating Solar

Collectors Based on Thermal Performance," Solar Energy, Vol. 18,

No. 5, 1976.

4. "Methods 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.

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

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

Flat-Plate Solar Collectors," NBS Technical Note 975 , Augustjl978.

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

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

Flat-Plate Solar Collectors," Solar Energy , accepted for publication,

1978.

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

Switzerland, 4th Edition, 1971.

8. Klein, S.A, , Beckman, W.A. , and J. A. Duffie, "A Design Procedure for

Solar Heating Systems," Solar Energy, Vol. 18, No. 2, 1976.

9. Klein, S.A. , Beckman, W.A,, and J. A. Duffie, "A Design Procedure for

Solar Air Heating Systems," Solar Energy, Vol. 19, No. 5, 197 7.

10, Yass, K. , and H. B. Curtis, "Low-Cost Air Mass 2 Solar Simulator,"

NASA TM X-3509 , presented at the United States Section Meeting of the

International Solar Energy Society, held in Cleveland, Ohio, October

3-4, 1973.

11, Simon, F,F., and P. Harlamert, "Flat-Plate Collector Performance

Evaluation, the Case for a Solar Simulator Approach," NASA TM X-71427 , presented at the United States Section Meeting of the International

Solar Energy Society, held in Cleveland, Ohio, October 3-4, 1973.

12, Ramsey, J.W., Borzoni, J.T. , and T.H. Holland, "Development of FlatPlate Collectors for Heating and Cooling of Buildings," NASA CR-13804

June, 1975.

47
13. Klein, S.A. , Duffle, J. A., and W.A. Beckman, "Transient Considerations

of Flat-Plate Solar Collectors," ASME Journal of Engineering Power , Vol. 96A, p. 109, 1974.

14. Duffie, J. A., and W.A. Beckman, Solar Energy Thermal Processes , John

Wiley and Sons, 1974.

15. Simon, F.F., "Flat-Plate Solar-Collector Performance Evaluation with

a Solar Simulator as a Basis for Collector Selection and Performance

Prediction," NASA TMX-71793 , presented at the 1975 International Solar

Energy Society Meeting, held in Los Angeles, California, July 29-

August 3, 1975.

16. Wi jeysundera, N.W., "Response Time of Solar Collectors," Solar Energy , Vol. 18, pp. 65-68, 1976.

17. Simon, F.F., and E.H. Buyco, "Outdoor Flat-Plate Collector Performance

Prediction from Solar Simulator Test Data," NASA TM-71707 , presented

at the 10th AIAA Thermal Physics Conference, held in Denver, Colorado,

May 27-29, 1976.

18. "Solar Thermal Electric Power Systems-First Quarter Progress Report,"

Report No. NSF/RANN/SE/GI - 37815/PR/74/1, Colorado State University,

April, 1974.

19. Flowers, E. , "Test and Evaluation of the Performance of Solar Radiation Sensors at Inclinations from the Horizontal Under Laboratory

and Field Conditions," Department of Energy Report DSE/1041-1,

September, 1977.

20. Pendleton, R.L. , "Evaluating a Solar Energy Concentrator," ASHRAE

Journal , pp. 47-50, November, 1976.

21. Thomas, W.C., "Thermal Efficiency Test Procedures and Results for a Cyclindrical Concentrating Collector," Proceedings of the ERDA

Concentrating Solar Collector Conference , held in Atlanta, Georgia,

September 26-28, 1977.

22. Johnson, S.M., "Standardized Performance Tests of Collectors of Solar

Thermal Energy-A Flat-Plate Copper Collector with Parallel Mylar

Stripping," NASA TM X-73553 , November, 1976.

23. Close, D.J., and M.B. Yosoff , "The Effects of Air Leaks on Solar

Air Collector Behaviour," Solar Energy , Vol. 20, No. 6, pp. 459-

463, 1978.

24. Farber, E.A. and C.A. Morrison, "Clear Day Design Values,"

Chapter IV of Applications of Solar Energy for Heating and Cooling

of Buildings, ASHRAE Publication GRP 170, 1977.

48
25. Zerlaut, G.A., Dokos, W.T. , and R.F. Heiskell, "The Use of ASHRAE

Standard 92>-ll in Predicting All-Day Performance of Flat-Plate

Collectors," Proceedings of the 1977 Flat-Plate Solar Collector

Conference , held in Orlando, Florida, February 28-March 2, 1977,

26. Tleimat, B.N., Howe, E.D., and R.E. Buckland, "A Proposed Method

of Rating the Thermal Performance of Solar Collectors," presented

at the Joint Conference of the International Solar Energy Society

and the Solar Energy Society of Canada, held in Winnepeg, Canada,

August 15-20, 1976.

27. Simon, F.F., and Miller, D.M., "A Generalized Correlation of Experimental Flat-Plate Collector Performance," NASA TM X-71832 , presented

at the Conference on Standards for Solar Heating and Cooling, held

in Philadelphia, Pennsylvania, October 14-15, 1975.

28. Miller, R. , "Performance Correlations of Five Solar Collectors Tested

Simultaneously Outdoors," NASA TM X-73546 , presented at the Joint

Conference of the International Solar Energy Society and the Solar

Energy Society of Canada, held in Winnipeg, Canada, August 15-20, 1976.

29. Smith, C.C., and T. Weiss, "Design Application of the Hottel-WhillerBliss Equation," presented at the 1975 International Congress, "Solar

Use Now-A Resource for People," held in Los Angeles, California,

July 28 - August 1, 1975.

30. 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. TR 7, 1976.

31. "Guidelines and Directions for Determining the Feasibility of Utilizing

Solar Collectors, A. Efficiency Tests of Solar Collectors," A Proposed

Method of BSE, The German Solar Energy Industries Association,

September, 1976.

32. Carroll, J. and D. Wald, "Improved Instrumentation for Performance

Testing of Solar Collectors," unpublished note, Santa Clara Solar

Research Institute, Santa Clara, California, 1978.

33. Reed, K. A., "Interim Solar Collector Test Plan," Proceedings of the

ERDA Concentrating Solar Collector Conference , held in Atlanta,

Georgia, September 26-28, 1977.

34. Kusuda T. , "NBSLD, The Computer Program for Heating and Cooling Loads

in Buildings," NBS Building Science Series 69 , July, 197 6.

35. Buchberg, H. , and J.L. Cairns, "A Convective Conductance Meter: Design

and Evaluation," ISA Transactions, Vol. 10, No. 3, pp. 325-330, 1971.

49
36. Ito, N. Kumura K. , and J. Oko , "A Field Experiment Study on the

Convection Heat Transfer Coefficient on Exterior Surface of a Building," ASHRAE Transactions , Vol. 78, Part I, 1972.

37. Aagard R.L. , "Convection Free Instrument for Measuring Infrared

Radiation in the Atmosphere," The Review of Scientific Instruments , Vol. 29, No. 11, pp. 1011-1016, November, 1958.

38. Beckman, W.A., et. al,, "Units and Symbols in Solar Energy",

Solar Energy , Vol. 21, pp. 65-68, 1978.

50
Table 1. Specifications for the Equipment and Sensors Used in the

Liquid Collector Test Loops

Equipment/ Sensor Specifications

Temperature measurement

section/ temperature

well

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; 0.952 cm (0.375 in.) o.d.,

0.081 cm (0.032 in.), wall capped brass

tube provides for a thermopile well

while a 0.474 cm (0.187 in.) compression fitting with teflon ferrule allows

for insertion of sheathed temperature

probes; flow rate and flow configuration

produces turbulent flow and fluid swirling after passing through one right angle

bend (See Figure 15.)

Water-to-air heat

exchanger

Vega automative radiator; effective

surface area of 1.85 m^ (20 ft^);

value of 181.5 W/(m^*°C)

(32 Btu/(h*ft^»°F))

Water-to-water heat

exchanger

Single-pass counter-flow heat exchanger

Storage tank 37.81 (10 gal.) glass-lined domestic

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

immersion heater

Pressure relief valve 2.06 X 10-^ Pa (30 psi) spring-loaded

valve

Filter 5 micron cartridge filter for hot

water applications

Electric Heater 500 W (1705 Btu/h), rod 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

51
Table 1 (Cont.)

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 linerized to within + 0.9%

over the intended ranges of temperature

and flow rates

Temperature sensors

Positive Displacement meter; oval-gear

positive-displacement incorporating an active RF pickup functional over the flow

range of 1.07 x 10"^ to 6.30 x 10"^ m^/s

(0,17 to 1 gal. /min.)

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

Thermopile, type-T, 6 junctions formed

from 30 gauge wire with each junction

individually insulated with shrink

tubing

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

52
Table 2. Specifications for the Equipment and Sensors Used

in the Air Collector Test Loop

Equipment/ Sensor Specification

Nozzle Aluminum ASME long-radius nozzle;

factory-calibrated and traceable to

NBS standards; nozzle diameters of

5.1, 6.4, 7.6, and 10.2 cm (2.0,

2.5, 3.0, 4.0 in.)

Blower Industrial centrifugal blower; rated

at 0.47 m-^ (1000 ft^/min.) at 2500 Pa

(10 in. H2O); 746 W 2500 ( 3 hp)

electric motor

Duct heaters On/off; 39 kW in 3 kW increments

Proportionally-controlled; 6 kW

Temperature sensors Thermocouples ; laboratory-fabricated

from premium-grade type-T 24 gauge

thermocouple wire

Temperature difference

sensors

Six junction thermopile; laboratory

fabricated from premium-grade type-T

24 gauge thermocouple wire

Pressure sensors Nozzle pressure drop; 0 to 1000 Pa

(0 to 4 in. H2O) inclined manometer;

5 P^ (0.02 in.H20) smallest scale

division; also 0 to 1250 Pa (0 to

5 in. H2O) elastic diaphram pressure

transducer

Nozzle gauge pressure; 0 to 2500 Pa

(0 to 10 in. H2O) vertical manometer;

25 Pa (0.10 in. H2O) smallest scale

division

Pitot tube pressure; 0 to 1250 Pa

(0 to 5 in. H2O) vertical manometer;

5 Pa (0.02 in. H2O) smallest scale

division

53
Table 3. Specifications for the Instrumentation Used

to Make the Meteorlogical 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

Wind direction

Total solar radiation

incident on the

collector tilted

surface

Diffuse component of

the solar radiation

incident on the

collector tilted

surf ace

Standard 3-cup wind anemometer incorporating a d.c. generator; output

of 0.0581 volts/m/s (0.026 volts/raph)

directly proportional to wind velocity

Direction vane with a two-wiper

potentiometer (0-540°)

Precision spectral pyranometer

utilizing an all black thermopile

detector and temperature compensation;

class 1 pyranometer as classified by

the World Meteorological Organization

10 cm (3.93 in.) shadow disc attached

to a 1 m (3.28 ft) long support rod

Direct beam solar

radiation

Normal incident pyrheliometer with

a collimated view of 5.7° and an automatic tracker

Sky temperature Pyrgeoraeter; 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 transraittance of 0.50 from 0.35

to 50 ym

Incident angle between

sun's direct beam

and the outward

drawn normal to the plane of the

collector aperture

Shadow protractor; see Figure 27

and accompanying description

54
coo

e

a

3

(Uc

H

03 3

c c o o

•H •H U 00 4J

rt <f C 1 •H H 00 >

1-1 o

CJ CM Q

c CN

h-l U 00

OJ c

4-1 •H

<u T3 6o a) c 03

n) 3

a.

?^

eu 0-.

00 >

o

00

r-i O

+ +

55
Table 5. Specifications for Strip-Chart Recorders, Integrators,

and the Data Logger

Strip-Chart Recorders

Multi Range Input Span

Uncertainty: ±0,25% of span

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

Time Constant: 0.5s

Input Spans: 0.1 mV - 10 V, multiple range

Type Inputs Monitored: thermopiles, pyranometers , flow rates

Fix Input Span

Uncertainty: +0.05% of reading

Input Impedance: 5 megohms

Time Constant: 0.75s

Input Span: 0 - 5 mV

Type Inputs Monitored: referenced thermocouples

Fixed Input Spans

Uncertainty: +0.5% of span

Input Impedance: 5 megohms

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 digits/h

Input Impedance: 1 megohm

Input Span: 0 - 30 mV

Type Inputs Monitored: thermopiles, pyranometers

Data Logger

Uncertainty: +0.01 mV, Max Error: 0.005% FS, ±0.2 °C

Input Imedance: 100 megohms

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 bpi, 9 track with

a DTL/TTL plus voltage true interface

2 Pen -

2 Pen -

3 Pen -

56
o

PQ

T3 M cn u (U (U T3 4) to 0) c <u d x tn J3 •H to O T3 o >, ^ o 3 to c 00 O •H OO •r-l 1-1 4J H -H 4-1 to o «-l •H XI t4-4 in 4-1 •H tH 4-1 ^

•H a 0 l-l a a to > a CO T3 00 0 to O 1 u u V) 0 l4 rH to tn iH C b

fl 05 •H <u CO OJ 3 0 tn 3 •H a 01 rH CO a jQ o <9 tNl J3 0) to tn x; 0 X

3 tH (U •H 1-1 •H C c r-< C o; iH

Ul 00 00 (A >4-l 00 ro 14-1 1-1 1-1 00 1-1 X U-l CM

1 1-1 (C

4-1 (I) C tn c > 0 to a 0)

n) •H rH 01 tj

1 1 4-1 00 00 (U 1 1 ij to f-l 1 d (U 0) tj c tj 0) <u to a. l-l 01 0) •H iH m .H Ul <U •H OJ to .H to to 3 to to c CO N XI Ul XI U5 4-1 (U <4-l OO tfl 00 m o 0) tn tn (0 « 3 n) 3 to U-l to l-l u c to c to to l-l C 01 C 3 cd O iH o 7-1 01 o 3 •H •H > •H •H l-l 01 O iH O T3 00 T3 OO V4 o 4-1 to I/l 00 tfl 00 0) to r-l H T3 &0

tnC T3

CO -H U 3 H -H u

to

0) H

o

O

.H Z

o

4J X) tn a II d ••> 1 X) a

01 01 a* 0) M

-d 4J X u-i u 0) 0 CJ 0 01 0

0) c o 3 01 0 (U 0 C 4-1 cn a a 4J

4-1 3 a +J X 4-1 to 0) 0 0 0 l-l to 0 tn a

c O •H 3 rH tu 0. X 0 0) 0 cn H 1-1 <u

1-1 a X 00 u a in 4-1 H 4-1 1^ II CU to 0) II to 00 tH CJ 0 0 a l-l -C3 cx <u X T3 1-1 •- c II c 0 01 4-1 00 ?X (U H

o 01 X -d a 01 •H 0 tH CJ II 0 c: tH 4-1 X a OJ 01 to a in u 0 <u in r-l a; H >4-l to >-l 0 12

4-1 IH o 0) O 00 CU 3 CU U 00 0 tH OJ CJ 01 0 C

•H l-l X CI4 c X rH 4-1 X to c 0 rH to l-l C cn sO to X 3 o 3 X 0) 3 • 0) 3 M -H 0 3 >, a iH <U X X

3 cn tH 0) rH 4-1 in X 4-1 H 4-1 CN 0 4-1 X to to X CO iH CJ

•> c

1-1 §C to o a

c X

iH I ^

a tH o 3 tH (0 tH O tH

to >-l X

^ I O CJ 0)

0) Vi CO OJ CJ

4-) 0) tH tH 10

CO Q< X CU tM HO. in IJ O. O - 3

CJ 00 01 tn H C 0

OJ X iH O OJ

OJ 4-1 X l-l >

4-1 1-1 3 X 1-1 CO 3 4-1 CJ 4J

- I rH ^

CU CJ 01 to •U rH CU cn X in01

c

OJ CO

01 « 4J CJto

CO 01 UH

CO H l-l H OJ 3

00 cn CO

OJ1-KSI 3 a

4-1 rH U CO vD

CU 01 a u rH < <

l-l I iJ CJ CU CJ CO

13, C 01 t4-l Q. CO -H l-l O rH q; 3

c; X cn cn

0) -H

4J (d

(1) O.

org

CO 4J 0

in0 CU - 00 vO 00 m rH CO ON rH 01 0 rH 0) • 0 M rH rH rH cn

CJ <

iH

(d

57
Table 7. Time Constants for the Liquid-Heating Collectors Tested

Transfer Fluid

Flow Rate Time Constant

Collector No, m /s min.

1 3.2 X 10-5 1.7

2 2.8 X 10-5 1.6

3 3.5 X 10-5 1.8

4 3.8 X

10-5 20.3

5 5.0 X

10-5 0.9

58
Table 8. Computation of All-Day Solar Collector Efficiency

Calculation Steps Hour of the Day, Solar Time"'-

Daily

6-7 7-8 4-5 5-6 Total

1. Inlet fluid temperature to the

collector, t^ . , °C

2. Ambient air temperature, t^, °C

3. Incident solar radiation of a horizontal

surface, 1^^, W/m

4. Ratio of total incident solar radiation on the

tilted surface (normal to the

collector plane) to that on a horizontal surface, R

5. Incident solar radiation on the collector

plane, I, W/m , line 3 x line 4

6. Collector thermal efficiency at nor- mal incidence determined in accordance with Sections 8.3.2 and 8.5

of ASHRAE Standard 93-77 and using

data from lines 1, 2, and 5

7. Incident angle between the direct

solar beam and outward dr^wn normal

to the collector plane, 9

8. Incident angle modifier, determined

in accordance with Sections 8.3.3

and 8.6 of ASHRAE Standard 93-77

and using the value of 6 from line 7

2 9. Energy output from the collector, W/m , line 5 x [line 6 + Fj^CTa)^ ^ x (K^^ - 1)]

10. Collector thermal efficiency,

line 9/line 5

to convert standard time to solar time, see reference [14], pp. 18-19.

'to compute R, see reference [14], pp. 48-55.

'can be determined by direct measurement

using data from reference [24] or other

accepted methods.

^to compute 6, see reference [14], pp. 14-18.

59
Table 9. A Comparison of Measured and Predicted All-Day Collector Efficiency

Collector

tf,i-ta

''C

dayjid x

kJ

Measured

Daily

Collected

Energy

kJ

Predicted

Daily

Collected

Energy

kJ

Measured

Daily

Efficiency

%

Predicted

Daily

Efficiency

%

2 21 33,400 18,500 19,200 55.4 57.5

2 52 34,800 15,800 15,840 45.4 45.5

2 64 34,100 15,800 14,900 46.3 43.7

3 19 41,700 28,300 27,000 67.8 64.7

3 52 42,600 22,100 21 , 300 51.9 50.0

3 64 44,100 18,800 19,800 42.6 44.9

60
APPENDIX A

Calculation of Air Flow Rate Using a Nozzle

and Pressure Difference Measurements

The air flow rate through the nozzle apparatus is calculated byi

Qmi = 1-^1 (^Pn V^-^ (1)

where

Qjjjj^ = measured air flow rate, m /s

= nozzle discharge coefficient

2

Aj^ = nozzle throat cross-sectional area, m

Apjj = static pressure difference across the nozzle. Pa

f

-5 = specific volume of the air at the nozzle, m-^/ kg dry air

The nozzle discharge coefficient C^, is determined from the following

table

Reynolds Number

%e

20,000

50,000

100,000

150,000

200,000 and above

Discharge Coefficient

0.96

0.97

0.98

0.98

0.99

Alternately, a value of C^^ can be calculated from the following equation

for 20,000 £Nj^g ^200, 000:

= 0.95 + 3.7 X 10-7 N^^ (1 - ^-^fj^)

'Re (2)

61
The nozzle Reynolds Number is calculated from:

NRe = ft ^a % (3)

where

V- = velocity of the air at the nozzle throat, m/s

Dj^ = nozzle throat diameter, m

= temperature factor for equation ( 2 ) above

determined from the following table:

Temperature °C Temperature Factor

-6.7 78275

+4.4 72075

+15.6 67425

+26.7 62775

+37.8 58125

+48.9 55025

+60.0 51925

+71.1 48825

can be determined from the velocity pressure measured at the nozzle

throat with a pitot tube or calculated from the following equation:

= 1.40 (Ap^ (4)

The nozzle throat cross-sectional area, Aj^, is determined by measuring

its diameter in four places approximately 45 degrees apart around the

nozzle in each of two planes; one at the exit and the other in the

straight section up in the nozzle. The nozzle pressure difference /^jg,

is measured as specified in the body of the report; with both an inclined

manometer as well as an electronic pressure transducer.

The specific volume of the air at the nozzle, u ^, is calculated from:

^

= 10.1 X 10^ un/[PN (1 + W^)] (5)

62
where

= specific volume of the air at the wet bulb and dry

bulb temperature existing at the nozzle but at

standard barometric pressure, m'^/kg dry air

= absolute pressure at the nozzle throat, Pa

Wj^ = humidity ratio of the air at the nozzle,

kg H20/kg dry air

Alternately, can be calculated for:

. _ Ra % (1 + 1»608 W^

) (6) ^

where

R„ = gas constant for dry air, (P„*m )/(kg'''K)

= absolute temperature of the air at the nozzle, °K

Both and W^^ are determined from a psychroraetic chart or simplified

psychrometic computer routines such as those on pp. 150a - 158a of

reference [34]. The wet bulb and dry bulb temperature are measured

together at only one point in the test loop; Wj^ is computed based

on those measurements, and it is assumed to be constant throughout

the loop. The absolute pressure at the nozzle throat is calculated

as the sum of the barometric pressure and the nozzle discharge gauge

pressure, p^. is measured as indicated in the body of the report

using a vertical manometer.

Once the air flow rate, Qj„-j^ > is calculated, the following equation issued

to calculate the flow rate, Q . ^ in m /s at standard temperature and mi , s

^ pressure

Q . = Q ./(I. 2 u') (7) ^mi , s ^mi' * n'^ ^

'

63
APPEtlDIX B

Description of a Radiometer to Determine Sky Temperature

If an electronically-heated plate mounted in a frame with the upper

surface exposed, is placed outdoors and allowed to reach steady-state

conditions, the following energy balance applies:

the rate

of electrical

energy input

to the heater

the rate of

+ incident solar

radiation

absorbed by

the heater

the rate of the net rate of the rate of heat

heat transfer + heat transfer to + transfer through

to the ambient the ambient the edges and back

air by convection surroundings by of the mounting

radiation frame by conduction

In equation form:

^ % (1) + as I = h^(Tn - T;,) + eadn - To ) +

where

p— = rate of electrical energy input to the heater per unit of surface ^ area, W/m^

cxg = absorptance of the electrical heater surface to the solar

radiation

I = incident solar radiation on the heater surface, W/m

h„ = convection heat transfer coefficient between the surface of the

C 9 heater and the ambient air, W/(m C)

Tp = surface temperature of the electrical heater, °K

T^ = ambient air temperature, °K

e = emittance of the surface of the electrical heater

a = Stef an-Boltzmann Constant, 5.6697 x 10"^ W/(m^ -"K)

Tg = effective sky temperature, °K

Qk = rate of heat transfer through the edges and back of the mounting

A frame by conduction, W/m^

64
Using the above principle, a number of researchers have built instruments

to determine the value of one or more of the variables in equation (1)

with the rest predetemined , or eliminated, or measured. In some cases,

two identical electrical heaters have been built with all but one design

characteristic the same and by using the heaters or sensors side-by-side

exposed to the same environment, a simple relative measurement is used

to determine the value of an unknown variable. Buchberg and Cairns [3 5]

and Ito, Kimura, and Oka [36] have used the principle for determining

an experimental value of the convection coefficient, h, for air flow over a surface of specific geometry. Aagard [37] adapted the principle

to the determination of effective sky temperature at night with I = 0.

The radiometer designed and built by the French Building Research Center

and one of the instruments being used by NBS to determine the effective

sky temperature during a collector test also makes use of the principle

of the balance expressed by equation (1). Two flat receivers are mounted

side by side on the top of an insulating slab. Each receiver is a 0.093 (1.0 ft^) square surface with a central area of 0.01 m^

(0.11 ft^) which is backed by a resistance heater. As can be seen in

Figure 60, the first receiver has it outer perimeter area covered with

a white surface of low absorptance and high emittance and the central

heater covered with a shiney surface of low absorptance and high emittance. The second receiver is treated in just the opposite manner. A

star-shaped thermopile is mounted on each receiver to detect any temperature difference between the central heated area and the perimeter surface

area. A special electronics package was designed for and is used with

the receivers. The electronics supplies electrical energy to the central

heater of either receiver in order to keep its surface temperature equal

to the surface temperature of the surrounding perimeter area. The electronics package also integrates the input electrical energy over either

a 10 minute or 30 minute period. Only one of the receivers is used at

a time and the particular one is selected automatically.

Writing equation (1) for both the central heated area as well as the

perimeter area of a given receiver, two equations in several unknowns

result. If, however, it can be assumed that the convection coefficient,

h, and the back losses are the same for both areas and recognizing

A~

that the surface temperatures of two areas are controlled to the same value, then the two equations can be solved simultaneously for the

effective sky temperature, T^ , as a function of the following variables

which are either known or are measured during the test:

asl» as2'

65

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Angle of incidence.G, degrees

Effective Transmittance-Absorptance Product for Three Flat-Plate

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70
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60

Figure 5 Incident Angle Modifier for Three Flat-Plate Solar Collectors with

Non-Selective Coatings on the Absorber.

71
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72
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73
Solar Collector Testing Equipment Mounted on the Elevator and

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NIKI Site, NBS, Gaithersbur g, Maryland.

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79
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TEMPERATURE MEASUREMENT SECTION

Figure 15 Schematic Diagram of the Temperature Measuring Stations in the

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No. 2.

107
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108

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Legend : THEORETICAL PREDICTIONS, CONVENTIONAL FLAT-PLATE

COLLECTORS

EXPERIMENTAL RESULTS, FLAT-PLATE COLLECTOR,

SINGLE-GLAZED, COPPER ABSORBER WITH FLAT-BLACK

PAINT, PARALLEL MYLAR STRIPPING (Sunsav)

I I

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ANGLE OF INCIDENCE, O, DEGREES

75

Figure 47 Experimental Results for a Incident Angle Modifier Test for a WaterHeating Flat-Plate Solar Collector with Parallel Mylar Stripping [22].

113
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in Comparison with Theoretical Predictions,

116
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_ Legend : O LINEAR FRESNEL LENS

TRACKING CONCENTRATING COLLECTOR

COPPER ABSORBER

BLACK-CHROME SELECTIVE SORFACE

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Figure 51 Incident Angle Modifier for Water-Heating Solar Collector No. 5 at

One Angle of Incidence.

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126
NBS.114A (REV. 7.73>

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS BSS 117

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

EXPERIMENTAL VERIFICATION OF A STANDARD TEST

PROCEDURE FOR SOLAR COLLECTORS

5. Publication Date January 1979

6. Performing Organization Code

James E. Hill, John P. Jenkins, and Dennis E. Jones

8. Performing Organ. Report No.

9. PERFOllMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Worlc Unit No. 742 4417

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of Energy

Research and Development Branch for Solar Heating and Cooling

Office of the Assistant Secretary for Conservation and

Solar Applications, Washington, D.C. 20545

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number 76-6OOI3S

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A proposed procedure for testing and rating solar collectors based on thermal

performance was published by the National Bureau of Standards (NBS) in 1974.

Subsequently, the American Society of Heating, Refrigerating, and Air Conditioning

(ASHRAE) developed a modified version of the NBS procedure which was adopted in

early 1977 as ASHRAE Standard 93-77. A test facility for water-heating and air-heatingj

collectors has been built at NBS and was used to support the development of

Standard 93-77. The purpose of this report is to describe the recently adopted

test procedure, the NBS test facility, and the tests that were conducted to support

the development of the procedure.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Measurement; solar collector; solar energy; solar radiation;

standards; standard test; testing.

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UNCLASSIFIED

21. NO. OF PAGES.

127

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