f

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NBS

PUBLICATIONS

). ... NBSIR 82-2522

A Comparison of Unglazed Flat

Plate Liquid Solar Collector

Thermal Performance Using the

ASHRAE Standard 961 980 and

Modified BSE Test Procedures

A11100 T^S7TE

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Equipment Division

Washington, DC 20234

May 1982

Sponsored by:

Division of Active Heating and Cooling

ce of Solar Heat Technologies

Department of Energy

100 hington, DC 20585

. U56

02-2522

1902

c. 2

NBSIR 82-2522

A COMPARISON OF UNGLAZED FLAT

PLATE LIQUID SOLAR COLLECTOR

THERMAL PERFORMANCE USING THE

ASHRAE STANDARD 96-1980 AND

MODIFIED BSE TEST PROCEDURES

MATIOKAJ. «u*»au

or BTAKDAJUJB LIMJURT

jUL ? 1982

7 & &

lift

/

John P. Jenkins and Kent A. Reed

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Equipment Division

Washington, DC 20234

May 1982

Sponsored by

Division of Active Heating and Cooling

Office of Solar Heat Technologies

U.S. Department of Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES iv

ABSTRACT vi

1. Introduction 1 2. Test Procedures 3

3. Uncertainty Associated With Unglazed Collector Efficiency Results .. 8

4. Test Facility 9

5. Experimental Tests 18

6. Test Results and Discussion 22

7 . Summary and Conclusions 29

8. References 30

iii
ABSTRACT

This paper reviews the BSE procedure and summarizes the ASIIRAE Standard 96-1980

for testing unglazed solar collectors. The ASHRAE procedure consists exclusively of outdoor testing, whereas the BSE procedure requires a combination of outdoor and indoor testing (no irradiation) to determine the collector optical and thermal loss characteristics, respectively. Two unglazed

flat plate liquid solar collectors were tested according to ASHRAE Standard 96-1980 and BSE procedures and the results compared. During the indoor

BSE thermal loss tests blowers were used to simulate winds of 0-3.9 m/s

(0-8.72 mi/hr) to investigate the wind effect upon collector thermal losses.

The results demonstrate that the differences between the BSE and ASHRAE

Standard 96-1980 thermal efficiency curves were less than the uncertainty

associated with the curves.

Key Words: ASHRAE Standard 96-1980; BSE; collector efficiency; unglazed

collector

vi
1 . INTRODUCTION

In January 1980 the American Society of Heating, Refrigeration, and AirConditioning Engineers adopted ASHRAE Standard 96-1980, "Methods of Testing

to Determine the Thermal Performance of Unglazed Flat Plate Liquid-Type

Solar Collectors" [1]*. Standard 96-1980, which closely parallels ASHRAE

Standard 93-77 [2], defines test procedures for determining the instantaneous

thermal efficiency and incident angle modifier for unglazed solar collectors.

Standard 96-1980, like 93-77, allows testing either outdoors in natural sun- light or indoors under a solar simulator while requiring the establishment

of quasi-steady-state test conditions under high levels of "clear sky"

radiation. Because of the greater sensitivity of unglazed collectors to

environment conditions and high operating flow rates. Standard 96-1980 in

contrast to 93-77 imposes substantially tighter requirements on collector

test loop thermal stability, instrumentation accuracy, and allowable

environmental variations during a test.

In May 1978, a German industrial working group entitled Bundesverband

Solarenergie (BSE), completed and adopted, "Guidelines and Directions for

Determining the Usability of Solar Collectors" [3]Part A of the Guidelines

defines test procedures for determining the thermal performance of solar

collectors. Unlike the ASHRAE procedures, the BSE procedure requires both

outdoor (irradiated) and indoor (nonirradiated) testing in order to determine

independently the collector optical efficiency and thermal loss characteristics, respectively. From these separately determined collector properties,

instantaneous thermal performance is calculated for the operating conditions

expected

The BSE collector test procedure can potentially reduce the time required

for testing and improve the reproducibility of collector test results,

compared to the ASHRAE outdoor test procedures, without requiring an indoor

solar simulator as specified in the ASHRAE standard. However, the BSE procedure determines the collector heat loss under zero-irradiance conditions,

and it is not apparent that the thermal efficiency established by the BSE

procedure will be equivalent to the thermal efficiency determined by the

ASHRAE procedures.

In an earlier study conducted at the National Bureau of Standards [4,5],

five glazed flat plate water-heating collectors were tested according to

both the BSE procedure and the ASHRAE Standard 93-77 outdoor procedure, and

the results were compared. For each of the five collectors tested, the

instantaneous thermal efficiency curves determined by the two procedures

differed by less than the total uncertainties associated with the curves. This agreement of the test results was substantiated by a concurrent ana- lytical study that also investigated the influence of various collector and

environmental parameters on the residual differences [6]

*Numbers in brackets indicate references in section 8.

1
In the current study, two representative unglazed flat plate water-heating

collectors were tested according to both a modified BSE procedure and the

new ASHRAE Standard 96-1980 outdoor procedure. This report describes the

salient features of the procedures, the test apparatus, and the test results.

2
2 . TEST PROCEDURES

ASHRAE Standard 96-1980

The ASHRAE Standard 96-1980 test requirements for stability, accuracy, and

environmental conditions exceed those of ASHRAE Standard 93-77. Only the

salient features and differences are described here. According to ASHRAE

Standard 96-1980, if the collector to be tested has no back insulation, it is

to be mounted on a rigid insulating board having an insulating value of R = 0.88 m^C/W (5.0 f t2«h«F/Btu) . The irradiance in the plane of the collector

must exceed 630 w/m^ (200 Btu/ft2«h) and for near-normal-incidence testing the

angle of incidence must be less than 30°. The average wind speed for each

test condition must not exceed 1.3 m/ s (3.0 mph) , a factor of three tighter

than the ASHRAE Standard 93-77 requirement. The range of ambient temperature

for all reported test points making up the efficiency curve must be less

than 10°C (18°F), again a factor of three less than the ASHRAE Standard

93-77 requirement. In addition, the allowable ambient temperature is explicitly restricted to the range of 15 to 38°C (60 to 100°F).

The instrumentation specifications for an ASHRAE Standard 96-1980 test follow

closely those of ASHRAE Standard 93-77, with two exceptions. The limits

for accuracy and precision in the temperature difference measurements are now ± 0.01°C (± 0.018°F), a factor of ten lower. In addition, ASHRAE

Standard 96-1980 calls for measurement of the dew point temperature for each

data point.

The more restrictive temperature requirements are reflected in the apparatus

specification of ASHRAE Standard 96-1980 as well. Piping and mixing section

losses or gains at collector inlet and outlet must not cause a temperature

change for any test of more than 0.01°C (0.018°F) between each mixing device

and the collector. The reconditioning apparatus must control the temperature

of the fluid entering the collector to within ± 0.1°C (± 0.18°F) for each

data point. Both specifications are around a factor of five more restrictive

than their ASHRAE Standard 93-77 counterparts.

In the ASHRAE Standard 96-1980 procedure, the collector thermal efficiency

curve for near-normal-incidence is established by taking at least four data

points at each of four different values of inlet fluid temperature. The

ordinate of each data point is the efficiency which is calculated as the

average thermal energy delivered by the collector during a test interval

divided by the average total solar irradiation incident upon the gross collector area during a test interval. The thermal efficiency data are plotted

against the fluid parameter (tf ^-t a )/G, where tf ± is the inlet

fluid temperature, t a is the ambient air temperature, and G is the solar

irradiance measured in the plane of the collector. At least one reported

value of the fluid parameter must be in the negative range of approximately

-0.02 to -0.15 m^» °C/W (-.11 to -.84°F hr ft^/Btu). This major departure

from the ASHRAE Standard 93-77 procedure is dictated by the fact that unglazed

collectors are normally used in low temperature applications in which the

collector fluid may be below ambient air temperature resulting in heat gains

as well as solar heat gains. An equation for the thermal efficiency curve

3
is obtained from the data using the least-square fitting technique. A

typical ASHRAE Standard 96-1980 test result including the data and the fitted

thermal efficiency curve is shown in figure 1.

The experimental determination of the incident angle modifier for an unglazed

collector follows closely the ASHRAE Standard 93-77 procedure and so will not

be described here.

ASHRAE Standard 96-1980 does not require the determination of a collector time

constant

Modified BSE Procedure

The derivation and application of the BSE collector test procedure has been

described in detail previously [4,5]. The essence of the method is the separate determination of the collector optical efficiency (a number) , .n 0 , during outdoor testing, and the collector thermal loss (a function), Ql, during

indoor testing. The collector thermal efficiency, n , is then calculated by

n = no - QL/(G*An) , (1)

where G is the total solar irradiance measured in the collector plane and An

is the net aperture area to the collector. For the purpose of comparison

within this study, the BSE procedure was modified to use gross collector area instead of aperture area, in conformance with ASHRAE Standard 96-1980.

The collector optical efficiency is determined in the BSE procedure by measuring thermal efficiency under conditions where collector thermal losses are minimized. Specifically, for a valid data point, the mean temperature of the

fluid in the collector, t m , defined by

tf i - collector inlet fluid temperature

t f - collector outlet fluid temperature (2)

must be held within ± 10°C (18°F) of the ambient air temperature, t a

. The

measured thermal efficiency is then equal to the optical efficiency, n 0

. When

the mean fluid temperature cannot be held within ± 10°C of ambient air temperature, then the measured thermal efficiency must be corrected for collector

thermal losses to arrive at the optical efficiency. The appropriate correction must be calculated for the actual value of (tm-t a ) based on the measured

indoor thermal losses.

Using the BSE procedure, collector thermal losses are determined under

essentially zero (< 1 W/m^) solar irradiation conditions within a controlled

indoor environment. The environmental test conditions must be regulated so that the air temperature remains between 15 and 25°C (59-77°F) while the

wind speed 7 to 9 cm (2.75-3.5 in) above the collector surface is greater

than 4 m/s (8.9 mi /hr).

•f ,o + fc f ,i

4
100 UNGLAZED FLAT-PLATE COLLECTOR

90

Average flow rate = 13.96 Kg/s (3.69 gpmj

Average ambient temperature = 32.8°C

Average insolation = 1032.9 W/m2

Average wind speed = 1.65 m/s

Average relative humidity = 27%

)R

EFFICIENCY,

%

8

S3

5 40

UimJ

•J8 30 o \

20

10

1

i

i i i i i i i i i i i i i i •0.004 0.002 0 0.002 0.004 0.006 0.008 0.10 0.012 0.014 0.016 0.018 0.020 0.022 0.024 0.026 0.028

^^rc/w/m2

)

Figure 1. ASHRAE Standard 96-1980 collector efficiency versus the

difference between the inlet fluid temperature, tf >:^,

and ambient air temperature, t a , divided by the incident

solar irradiance G.

5
In addition, the apparent environmental radiance temperature* or effective

black body sky temperature must not deviate from the air temperature by more than ± 3K (5.4°R). Preconditioned fluid is circulated through the collector

over a range of operating temperatures above ambient, specifically at (tm-ta ) values of 30, 50, 70, and 90°C (86, 122, 158, 194°F). The thermal loss,

is calculated based on the measured fluid flow rate temperature drop

between the collector inlet and outlet. The heat loss is then plotted as a function of the parameter (t m-t a ) as shown in figure 2. The equation of

the heat loss curve can be obtained using the least-square fitting technique.

A family of collector thermal efficiency curves is constructed from the

optical efficiency, n 0 and the thermal loss function Ql, using equation

1. The curves are calculated for irradiance values of 200, 400, 600, 800,

and 1000 W/m^, (63.7, 126, 190, 253, 317 Btu/hr ft^) and plotted as a function of the parameter (tm

-t a ) as shown in figure 3.

For the purpose of this comparative study minor modifications have been

introduced into the BSE procedure to allow for direct comparison between

the BSE and ASHRAE test results. As mentioned already, the BSE procedure

was modified to use gross collector area instead of net aperture area. The same test facility built for the ASHRAE Standard 96-1980 testing was used

for the BSE testing, so that the more restrictive instrumentation and apparatus specifications in ASHRAE Standard 96-1980 were met automatically. The

collector thermal losses were measured over a range of (t m-t a) from about

— 10°C (-14°F) to about 40°C (104°F). As in the development of ASHRAE Standard 96-1980, operating temperatures below ambient temperature were included

to investigate the ability of unglazed collectors to gain energy from the

environment. The wind speed specification in the BSE procedure was also

changed. Because the simulated wind could be controlled easily in the laboratory, collector thermal losses were investigated at several wind speeds

from 0 m/s (still air) to 3.9 m/s (0-8.72 mi/hr). The BSE thermal performance

results have been presented within an ASHRAE Standard 96-1980 format. The

t m-t a x-axis variable has been transposed into the ASHRAE (tf } i-t a )/G

as discussed in [4]

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

6
o 10 20 30 40 50 60 70 80

tm ~ *a. °C

Figure 2. BSE collector thermal losses versus the difference

between collector mean fluid temperature, tm , and

ambient temperature, t a

.

Figure 3. BSE collector efficiency versus the difference between

collector mean fluid temperature, tm , and ambient air

temperature, t a

.

7
3. UNCERTAINTY ASSOCIATED WITH UNGLAZED COLLECTOR EFFICIENCY RESULTS

For unglazed collectors, variations in environmental test conditions contribute a large uncertainty in determining collector efficiency that is difficult

to quantify. Wind is the dominant environmental parameter. In order to minimize the influence of wind in outdoor thermal performance testing, the ASI1RAE

Standard 96-1980 requires that the average wind speed during any valid test

interval not exceed 1.35 m/s (3.0 mi/hr). If wind were a steady-state phenomenon, this wind speed restriction would be sufficient. For common unglazed

flat plate collectors, the uncertainty due to the allowable range of wind

speeds could be calculated. Unfortunately, wind is a dynamic phenomenon with

continual, transient fluctuations in both magnitude and direction. The

short-term fluctuations strongly influence the collector's instantaneous heat

loss coefficient. Because it has not been possible to adequately characterize

and correct for wind fluctuations during testing, the resulting thermal performance data exhibit apparently random scatter that increases with increasing

(tf ^-t a )/G. At high values of (tf a-t a )/G it is even possible for wind

fluctuations to preclude establishment of quasi-steady-state test conditions

although the average wind speed stays within the 1.35 m/s (3.0 mi/hr) specification.

The simulated wind generated during indoor testing is both steady-state and

reproducible. As a consequence, the indoor thermal loss data obtained using

the modified BSE procedure exhibit minimal scatter due to wind fluctuations.

There is, therefore, little uncertainty introduced into the resulting thermal

efficiency curves.

The contribution to the uncertainty in collector thermal efficiency due to

the propagation of random measurement uncertainties was analyzed for the BSE

and the ASHRAE Standard 93-77 test procedures in [4]The relative uncertainties in measurement are very similar in the case of ASHRAE Standard 96-1980, so the previous analysis is expected to apply. The analysis concluded that the

random measurement uncertainty is typically ±4 efficiency points (ri ± 4)

for both the ASHRAE and the BSE efficiency curves.

The total uncertainty in unglazed collector thermal performance is a combination of the measurement uncertainty and the uncertainty associated with

environmental test conditions. Since the BSE thermal loss tests are determined under stable controlled environmental conditions, the total uncertainty

associated with the BSE efficiency curve is essentially limited to the mea- surement uncertainty of ± 4 efficiency points. The total uncertainty associated with the ASHRAE thermal efficiency curve is expected to be larger. This

point is discussed further in section 6, Test Results and Discussion.

8
4. TEST FACILITY

The unglazed collector test facility is shown in figures 4-9. The facility

comprises a test stand, a fluid test loop, and data acquisition equipment.

The entire facility is mounted upon an elevator which is capable of transporting the equipment between indoor and outdoor test facilities. This unique

arrangement allows tests to be completed indoors and outdoors using the same

test equipment and without disturbing the collector undergoing test. The

test stand supports the test collector at any orientation, and contains the

fluid test loop. The collector mounting surface consists of asphalt shingles over 1.2 cm (1/2 in) plywood sheathing backed by fiberglas batts 10 cm (4 in)

thick simulating an actual roof surface.

Fluid Test Loop

The fluid test loop is shown schematically in figure 8. This closed fluid

loop is very similar to the suggested ASHRAE Standard 93-77 configuration

for glazed collectors but with several modifications to accommodate the more

restrictive ASHRAE Standard 96-1980 test requirements for unglazed collectors.

The test loop is capable of maintaining fluid temperatures to within ± 0.02°C

(0.036°F) at the inlet of the collector during a one minute test interval

and maintaining the fluid flow rate through the collector to within ±0.9

percent. Major differences between the unglazed collector test loop and a typical ASHRAE Standard 93-77 closed loop configuration include:

a. the capability of maintaining the collector fluid inlet temperature

below ambient temperature (tf j < t a );

b. the capability of delivering high fluid flow rates 0.126-0.504 i/s

(2 to 8 gpm) ; and

c. the capability of resolving small temperature differences ± 0.03°C

(0.054°F) between the collector inlet and outlet.

The fluid test loop was designed to operate at subambient temperatures in

order to obtain collector thermal performance at negative values of tm-ta and

( tf } j[-t a )/G as required in ASHRAE Standard 96-1980. The fluid circulating

in the test loop can be cooled below ambient air temperature using liquid-toliquid heat exchangers and chilled-water provided by a secondary loop. The

two heat exchangers have different thermal capacities and can be connected

singly or in tandem as required to meet the thermal load imposed by the test

collector

Most unglazed collectors operate at high fluid flow rates. Consequently, the

fluid test loop was designed to operate at flow rates from 0.126-0.504 1 /

(2 to 8 gpm) typically eight times greater than the flow rate for glazed

collectors of comparable area. The pump, hydronic system, and flow transducers were selected to accommodate the range of flow rates.

Because of these high fluid flow rates, unglazed collectors experience a small

temperature rise. At; for a single collector module typically 1-2°C. In order

9
Figure 1. Artifact Standards Typical of Those Now Calibrated by NBS
Figure A. View of the unglazed collector test facility showing the

fluid loop, test stand, and instrumentation building

during outdoor testing.

10

Figure 5. View of the unglazed collector test stand while

performing outdoor tests upon collector 1.

11
Figure 6. Close up view of collector 1 during outdoor tests showing

the simulated roof test bed and temperature measurement

sections at the collector fluid inlet and outlet.

12
Figure 7. View of the fluid test loop mounted upon the test stand base.

13
Figure 8. Schematic of the fluid test loop.

14
15
to minimize the uncertainty in the At measurement the apparatus must be able

to resolve small temperature differences on the order of ± 0.01°C (0.018°F)

as specified in ASHRAE Standard 96-1980. In practice the facility described

is capable of resolving temperature differences with an uncertainty of ± 0.03°C

(0.054°F) under "steady-state" conditions. The ± 0.03°C resolution is a combination of the ability to stabilize the collector inlet fluid temperature

and the ability to measure small temperature differences.

Stabilization of the collector inlet fluid temperature is attained through a combination of techniques. The fluid test loop is well insulated. Two 153 %

(40 gal) tanks located at the collector supply and return provide a large thermal mass to buffer temperature fluctuations within the loop. The tanks were designed to be well mixed. Under average outdoor test conditions, the tanks

dampen inlet temperature fluctuations to less than ± 0.02°C (0.03°F) over a one minute test interval. During indoor testing, greater temperature stability was possible with fluctuations less than ± 0.01°C (0.018°F) observed

over a typical test interval.

Temperature measurement sections were constructed to measure the bulk fluid

temperatures at both the collector inlet and outlet and the difference in

fluid temperature between the collector inlet and outlet. One of the

temperature measurement sections is illustrated in figure 8. Each temperature measurement section is thermally insulated with a wide-mouth vacuum

dewar capped with 7.5 cm (3 in) of foam insulation. The dewar contains and

thermally guards the platinum resistance thermometer (PRT) and the 10- junction

type-T thermopile sensor. Each PRT is strain-free, has a time response of

three seconds, and was calibrated at NBS to within an accuracy of 0.005°C

(0.01°F). The thermopile has a time response of 0.6 sec. It was calibrated

at NBS and found to be capable of resolving temperature differences of ± 0.02°C

(0.036°F). The thermopile was constructed to be capable of monitoring average

fluid temperature by using individual thermal wells for each of the 10-

junctions, distributed across the fluid stream. Static fluid mixers were placed upstream of the temperature sensors to ensure proper measurement of

bulk fluid temperature. During testing, the PRT and thermopile At measurements were cross checked for accuracy and temperature stability. If the

agreement was within 0.03°C (0.054°F) over a test interval, the At measurement

was considered valid. Before and after a series of experiments, the PRT

calibrations were checked at the triple point of water (0.01°C) by removing

the sensors and inserting them into a Triple Point Cell.

Instrumentation

The data taken during testing were sampled and logged at one minute intervals

by an automatic data acquisition system shown in figure 9. Critical measurements such as solar irradiance, temperature difference across the collector,

and wind speed were also continuously monitored using strip chart recorders.

The primary measurements and transducers monitored by the data acquisition

system include:

16
Measurement Transducer

Temperature Collector Inlet

and Outlet

-10 Junction Type-T Thermopile

-100 Ohm Strain Free Platinum

Resistance Thermometers

Ambient Air -100 Ohm Platinum Resistance

Thermometer Contained Within

an Aspirated Radiation Shelter

Flow Rate -In Line Axial Turbine Meter

-Positive Displacement Meters

with an Eccentric Woble Disc

Radiation Solar -Pyranometer PSP

Infrared -Pyrgeometer PIR

Wind Speed -3 Cup Wind Anemometer

Low Threshold with DC Generator

-Birams Pattern Vane Anemometer

The platinum resistance thermometers (PRT) were mutually connected in series

with a 1 ma constant current source and a 100 ohm NBS-type precision resistor.

The data acquisition system measured the voltage drops across each individual PRT and the precision resistor. The measured PRT voltage drops were

then ratioed against the precision resistor voltage drop to determine the

PRT resistances. PRT resistance-temperature relationships were then applied

to determine the measured temperatures.

The test loop fluid flow rates were continously monitored using three flowmeters an in-line axial turbine meter and two positive displacement meters

as shown in figure 8. Each of the flowmeters has a digital output which was totalized over a one minute test interval to determine an average flow rate.

Redundent flow meters were used to ensure the accuracy of flow measurement.

In addition to the required straight lengths of pipe upstream and downstream

of the turbine meter, a uniaxial filter was used upstream to eliminate flow

swirling and properly condition the fluid. All three flowmeters and the uniaxial filter were periodically removed in tandem and calibrated. A visual

flowmeter is located downstream of the three digital flowmeters. The visual

flowmeter, or rotameter, functions as a coarse flow indicator and sight

glass for determining whether air is trapped within the system.

17
5. EXPERIMENTAL TESTS

Two unglazed flat plate liquid-heating collectors were tested according to

both the modified BSE procedure and ASHRAE Standard 96-1980. The two collectors are described in table 1 and illustrated in figure 10. Collector 1 is an extruded polyolefin absorber impregnated with carbon black and collector 2 is a copper, tube and fin absorber with a high temperature black lacquer

coating. The collectors tested were mounted against a rear-insulated

asphalt shingle surface attached to the collector test stand. The simulated

roof surface can be seen in figure 6.

During the BSE indoor thermal loss tests, shown in figures 11 and 12,

axial-fan wind simulators were used to simulate average wind speeds of 0,

2.1, and 3.6 m/s (0, 4.6, 8.0 mi/hr). The collectors were oriented horizontally with the axial fans blowing longitudinally along the collector

plane. A vane anemometer was used to measure the air velocity of the simulated wind at 20 cm (3.93 in) increments across the collector midsection at

1 cm (0.4 in) above the surface. The air velocity reading at each location

was obtained by integrating the air flow over a 30 second interval. As many

as 13 readings were taken during a single run in order to determine an average wind speed. During a typical measurement sequence, wind speed uniformity

ranged ± 9 percent. To determine an average absorber plate surface temperature during the thermal loss tests, thermocouples were attached to the

absorber surface. The thermocouples were positioned transverse across the

collector midsection and located on and between the fluid risers.

During outdoor testing, shown in figure 5, the BSE optical efficiency, n 0 , ASHRAE instantaneous thermal efficiency, and incident angle modifier were determined. The collectors were mounted upon a fixed, south-facing test

stand and adjusted for normal incidence at solar noon. The ASHRAE thermal

efficiency and BSE optical efficiency angle modifier data were obtained

over solar noon at normal incidence ± 4° . Data to compute the incident

angle modifier (KaT ) were obtained over the day as the sun tracked

across the fixed, south-facing collector. Collector thermal performance

was measured at various angles of incidence both before and after solar noon

to determine values of . The outdoor wind speed was monitored

using a cup anamometer mounted on the collector plane as shown in figure 6.

18
Table 1

Collector 1 Collector 2

Manufacturer^-

Collector Area

Absorber Material

Flow Configuration

Coating

Solar Absorptance (AM=1.5)^

I.R. Emittance (25°C)^

F'FR

Fafco

1.28 m x 2.31 m Polyolefin

Parallel Risers

Carbon Black in

Polyolefin

0.96

0.92

0.81

0.80

Terra-Light

0.863 m x 2.30 m

Copper

Parallel Risers

Black Lacquer

0.95

0.89

0.87

0.85

F Identification of commercial products does not imply recommendation

or endorsement by the National Bureau of Standards.

2 Obtained from laboratory spectrophotometer measurements.

3 Determined knowing a and absorber plate temperature.

Figure 10. Cross-sectional view of unglazed collector 1 and 2 absorbers.

19
Figure 11. View of the unglazed collector test facility during indoor

thermal loss tests with the axial fan wind simulators.

20
Figure 12. Top view of unglazed collector 1 during indoor thermal

Loss tests with the axial fan wind simulators.

21
6. TEST RESULTS AND DISCUSSION

The BSE thermal curves for collectors 1 and 2 are presented # in figures 13

and 14 respectively. The collector overall thermal losses Ql are plotted

as a function of t m-t a for average simulated wind speeds of 0, 2.1, and

3.6 m/s (0, 4.6, 8.0 mi/hr). Note that the curves cross over at tm-ta = 0

and include negative thermal loss (i.e., a net gain) for negative t m-ta. This illustrates the fact that unglazed collectors readily gain thermal

energy from the environment when operating at below ambient conditions.

Environmental and operating conditions are noted on each figure. To compare

the losses, the loss coefficient, Up,, was calculated for each collector

using the relation

U

L ” «L /A<Vt a

) (3)

where t^ is the average plate temperature of the collector. The loss

coefficients at three wind speeds are plotted as functions of t -t

a in

figure 15. The close agreement of the heat loss coefficients for the two

collectors is expected considering the nearly identical absorber optical

properties and the consistent indoor environmental test conditions for both

collectors. The change in slope of the zero-wind Ul curve is probably

due to a laminar to turbulent transition in the free convection film

coefficient

Figures 16 and 17 show the outdoor test results used to determine the ASHRAE

Standard 96-1980 instantaneous thermal efficiency as well as the data used to

determine the BSE optical efficiency, q 0. In figure 16, the efficiency curve

for collector 1 is a linear least square fit to the data, and in figure 17,

the efficiency curve for collector 2 is a second order least square fit. The

encircled data in each figure indicate the outdoor data used to determine the

BSE optical efficiency for each collector.

The environmental and operating conditions listed on each figure summarize

the range of prevailing outdoor test conditions. The range of wind speeds

reported for each data set in the figures is based on integrated wind speed

averages from a cup anamometer mounted on the collector plane. Generally

speaking, the range of prevailing outdoor wind speeds was lower for collector

1 than for collector 2, especially for high values of (tf } f-ta )/G. This

difference in prevailing wind speeds is reflected in the lower slope of the

ASHRAE thermal efficiency curve determined for collector 1. The influence

of wind on the thermal performance is demonstrated clearly by the test

data taken at values of (t f .-t_)/G near 0.03°C/w*m^ (0 . 169°F-hr-f t^/Btu)

I j 1 3. The right-hand group of data points in figure 16 was taken in calm conditions, whereas the comparable data group in figure 17 was taken with average

wind speeds in the range 0.5-1. 5 m/s (0-3.35 mi/hr). The increased scatter

present in the latter data is striking. The influence of wind upon collector

thermal performance increases with (tf f-ta )/G since collector thermal

losses are directly a function of (tf f-t a ) . At high values of (tf f-t a )/G

reached during testing, the influence of wind perturbations nearly precludes

the attainment of quasi-steady-state test conditions, even though the average

22
Figure 13. Collector 1 indoor thermal losses, Ql, versus the difference

between collector mean fluid temperature, tm , and ambient

air temperature t a.

-20 -10 0 10 20 30 40 50

tm-ta, °C

Figure 14. Collector 2 indoor thermal losses, Qj__, versus the difference

between collector mean fluid temperature, tm , and ambient

air temperature, t a

.

23
Figure 15. Collector 1 and 2 loss coefficients, Up,, versus the difference

between collector mean plate temperature, tp, and ambient

air temperature, t a.

24
COLLECTOR

EFFICIENCY

|%)

rj

3

COLLECTOR

EFFICIENCY

|%)

-0.02 0.02

tf,i - t a

0.04

(°C/w/m2

)

0.06

e 16. ASHRAE Standard 96-1980 efficiency curve and BSE r) 0 data

set (encircled) for collector 1 determined during outdoor

testing

01 I I I I 1 ^ I l -0.02 0 0.02 0.04 0.06

PC/w/m’)

Figure 17 . ASHRAE Standard 96-1980 efficiency curve and BSE n Q data

set (encircled) for collector 2 determined during outdoor

testing

25
wind speeds were below the ASHRAE specified limit. No simple method was

found to characterize the instantaneous wind speed and correlate it to the

instantaneous thermal performance measurements in such a way as to reduce

the data scatter. Based on the BSE investigation, it is felt that wind

simulators could be used during outdoor testing. Although not attempted

during this investigation, the use of wind simulators should result in

more controllable and less transient wind conditions during the test.

The BSE thermal efficiency curves for the two collectors are shown in

figures 18 and 19. The optical efficiencies were extrapolated from the

ASHRAE thermal performance data encircled in figures 16 and 17. The thermal

losses were calculated for wind speeds of 0, 2.1, and 3.6 m/ s (0, 4.6, 8.0

mi/ hr). The solar irradiance used in the calculation was the average irradiance observed during the ASHRAE and BSE n 0 testing. For comparison, the

calculated BSE thermal efficiency curves are presented in the ASHRAE format

along with the ASHRAE curves. The influence of wind is clearly illustrated

in the BSE curves. For collector 1, the BSE curve for zero wind agrees

closely with the ASHRAE curve which represents test data obtained during

calm wind conditions (<0.5 mi/hr, 0.2 m/s). Likewise for collector 2 the

agreement is good because the ASHRAE curve determined under wind speeds of

0.3-2. 1 m/s (0.6-4. 6 mi/hr) falls between the BSE 0 and 2.1 m/s (0 and 4.6

mi/hr) curve. In both cases the agreement between the BSE and ASHRAE curves

is within the uncertainty of the efficiency curves.

The incident angle modifier curves, KaT , for collectors 1 and 2 are shown

in figures 20 and 21, respectively. Since the incident angle modifier is

determined in an identical manner using the BSE or ASHRAE procedure, a single Kqj curve is presented for each collector. The collector incident

angle modifier is plotted against the collector incident angle for data

collected using a fixed south-facing test stand. The solid and open circles

represent data obtained before and after solar noon, respectively. Typically, using a fixed test stand, collector thermal storage effects tend to

skew Kax low before solar noon and high after solar noon. This degree of

skewing or thermal lag effects is directly related to the collector time

response and the transient nature of solar irradiance within the collector

plane. The 99 percent step change time response of collectors 1 and 2 is

very short, approximately one minute. Consequently, the hysteresis effect is

negligible and is dominated by random measurement uncertainties. This is

illustrated by the near random distribution of KaT values before and

after solar noon. For collector 1, becomes non-linear at approximately

30° whereas collector 2 is at best fit linear. The non-linearity of KaT

for collector 1 is most likely due to the convoluted absorber surface as shown in figure 10.

26
Collector

efficiency

(%)

17

^

Collector

efficiency

(%) rf

- 0.02 0 0.02 0.04 0.06

t

f | - t a (°C/w/m2

) u

18. Comparison of ASHRAE Standard 96-1980 and modified BSE

efficiency curves for collector 1.

Figure 19. Comparison of ASHRAE Standard 96-1980 and modified BSE

efficiency curves for collector 2.

27
INCIDENT

ANGLE

MODIFIER,

K«r

INCIDENT

ANGLE

MODIFIER,

K«

r

Figure 20. Incident angle modifier, Kqj

for collector 1.

8NCIDENT ANGLE, Degrees

Figure 21. Incident angle modifier, , for collector 2.

28
7. SUMMARY AND CONCLUSIONS

This paper has reviewed the modified BSE procedure and summarized the ASliRAE

Standard 96-1980 Standard for testing unglazed solar collectors. An unglazed

collector test facility was constructed and two unglazed collectors tested

in accordance with both procedures. The BSE efficiency curves demonstrate

the strong sensitivity of collector thermal performance with wind. The

ASHRAE results illustrate the data scatter due to quasi-steady-state outdoor

test conditions and due to measurement uncertainties associated with resolving small temperature differences across a collector. For the two flat

plate liquid unglazed collectors tested, the differences between the BSE

and ASHRAE Standard 96-1980 thermal efficiency curves were less than the

uncertainty associated with the curves.

For testing unglazed collectors, the modified BSE technique is preferred

over the ASHRAE Standard 96-1980 for the following reasons:

The BSE procedure eliminates the large uncertainties in determining collector

thermal performance associated with transient outdoor test conditions by

determining steady-state collector thermal losses within a controlled

laboratory environment.

The BSE procedure allows collector thermal performance to be determined

under selected environmental conditions. Because of the environmental sensitivity of unglazed collectors it is essential that collector thermal performance be measured over a range of wind speeds and directions.

The BSE determination of the near normal incidence efficiency curve can be

completed in less test time than normally required for ASHRAE Standard 96-

1980. The combined indoor-outdoor BSE testing minimizes the dependence of

tests upon outdoor environmental conditions.

29
REFERENCES

1 . "Methods of Testing to Determine the Thermal Performance of Unglazed

Flat-Plate Liquid Type Solar Collector," ASHRAE Standard 96-1980 , ASHRAE, 1791 Tullie Circle, N.E., Atlanta7~GA 30329, 1980.

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

Collectors," ASHRAE Standard 93-77, ASHRAE, 1791 Tullie Circle, N.E.,

Atlanta, GA 30329, 1980.

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

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

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

4. Jenkins, J. P. and Hill, J. E., "Testing Flat-Plate Water-Heating

Solar Collectors in Accordance with the BSE and ASHRAE Procedures,"

NBSIR 80-2087 , August 1980.

5. Jenkins, J. P. and Hill, J. E., "A Comparison of Test Results for

Flat-Plate Water-Heating Solar Collectors Using the BSE and ASHRAE

Procedures," ASME Journal of Solar Energy Engineering , Vol. 102,

No. 1, pp 2-15, 1980.

6. Jenkins, J. P. and Bushby, S. T., "Differences in Determining Collector Thermal Performance Between the BSE and ASHRAE Collector Test

Procedures," National Bureau of Standards, Washington, D.C. 20234,

1981, submitted for publication to the ASME Journal of Solar Energy

Engineering

30
NBS«1 14A (REV. 2 -80 ) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 82-2522 May 1982

4. TITLE AND SUBTITLE

A Comparison of Unglazed Flat Plate Liquid Solar Collector Thermal Performance Using

the ASHRAE Standard 96-1980 and Modified BSE Test Procedures

5. AUTHOR(S)

John P. Jenkins and Kent A. Reed

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Division of Active Heating and Cooling

Office of Solar Heat Technologies

U.S. Department of Energy

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

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

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This paper reviews the BSE procedure and summarizes the ASHRAE Standard 96-1980 for

testing unglazed solar collectors. The ASHRAE procedure consists exclusively of outdoor testing, whereas the BSE procedure requires a combination of outdoor and indoor

testing (no irradiation) to determine the collector optical and thermal loss characteristics, respectively. Two unglazed flat plate liquid solar collectors were tested

according to ASHRAE Standard 96-1980 and BSE procedures and the results compared.

During the indoor BSE thermal loss tests blowers were used to simulate winds of

0-3.9 m/s (0-8.72 mi/hr) to investigate the wind effect upon collector thermal losses,

The results demonstrate that the differences between the BSE and ASHRAE Standard 96-

1980 thermal efficiency curves were less than the uncertainty associated with the

curves.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

ASHRAE Standard 96-1980; BSE; collector efficiency; unglazed collector

13. AVAILABILITY

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20402.

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