Several solar cells having dissimilar spectral response curves and cell construction were measured at various locations in the United States to determine sensitivity of cell performance to atmospheric water vapor and turbidity. The locations selected represent a broad range of summer atmospheric conditions, from clear and dry to turbid and humid. Cell short circuit current under direct normal incidence sunlight, the intensity, water vapor and turbidity were measured. Regression equations were developed from the limited data base in order to provide a single method of prediction of cell current sensitivity to the atmospheric variables

Hart, R. E.

Klucher, T. M.

English

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ERDA/NASA 1022/76/7
NASA CP -2010
Technical Report 11-8
SENSITIVITY OF SOLAR-CELL PERFORMANCE TO
ATMOSPHER I C VARIABLES
I I - DISSIMILAR CELLS AT SEVERAL LOCATIONS
Thomas M. Klucher and Russell E. Hart
Naticnal Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135	
^101.5 6 7 as 10
JUL 1977
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Prepared for	 -
ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION
Office of the Assistant Administrator ftr
Solar, Geothermal, and Advanced Energy Systems
Division of Solar Energy
Under Interagency Agreement E(49-26)-1022
Presented at Second Annual Photovoltaic Measurements Workshop
Baton Rouge, Louisiana, November 10-12, 1976
(NASA-CP-2010) SENSITIVITY OF SOLAR-CELL	 N77-26624
PERFORMANC E TO ATMOSPHERIC VARIABLES. 2:
DISSIMILAR CELLS AT SEVERAL LOCATI04S (NASA)Unclas
16 p HC A02/MF A01	 H1/44 35427
SENSITIVITY OF SOLAR-CELL PERFORMANCE TO ATMOSPHERIC VARIABLES
II. DISSIMILAR CELLS AT SEVERAL LOCATIONS
by Thomas M. Klucher and Russell E. Hart
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
ABSTRACT
Fifteen solar cells, having dissimilar spectral response curves
and cell construction, were measured at various locations in the United
States to determine sensitivity of cell performance to atmospheric water
vapor and turbidity. The locations selected represent a broad range
of summer atmospheric conditions, from clear and dry to turbid and humid.
Cell short-circuit current under direct normal incidence sunlight, the
intensity, water vapor and turbidity were measured. Regression equations
were developed from the limited data base in order to provide a single
method of prediction of cell current sensitivity to the atmospheric
variables.
SUMMARY
Fifteen solar cells, having dissimilar spectral response curves
and cell construction, were measured at various locations in the United
States to determine the sensitivity of their performance to atmospheric
variables. The locations selected represent a broad range of summer
atmospheric conditions, from clear and dry to turbid and humid. Schuepp
turbidity values from 0.04 to 0.259, water vapor from 0.70 cm to 2.35 cm
and air masses from 0.93 to 2.0 were measured during the trip. The
lI	 y
2calibration values--defined as the ratio of measured cell short-circuit
current to incident intensity--were compared with the atmospheric vari-
ables and inserted in a regression equation developed to predict cell
sensitivity. It was found that there was a correlation between the
regression constants obtained and the cell spectral response curves.
It appears that cell spectral response determines the regression constants
and hence the sensitivity of cell calibration value to atmospheric changes.
INTRODUCTION
solar cells, which are currently being used for power generation
applications on earth by a variety of manufacturers, differ markedly in
design and materials used. The cells, eventually combined into large
sized arrays and placed in the field, are subjected to environments which
may alter their performance. In order to determine their initial per-
formance and subsequent change, if any, the arrays are compared against
a reference or standard solar cell. These cells are identical in material
and construction to the cells in the array and are periodically calibrated
against a total incidence detector. In the case of the cells presently
used as standards in the ERDA National Photovoltaic: Program, the cali-
bration is performed relative to a normal incidence pyrheliometer under
direct normal incidence sunlight (Ref. 1). The standard cells are
measured at the short-circuit point of their I-V characteristic and the
calibration procedure results in a single calibration value--the ratio
of cell short-circuit current generated to the direct solar intensity
normally incident on the cell surface.
3Both the solar spectral irradiance distribution and the total
irradiance vary with changes in the atmospheric constituents. There-
fore, the solar cell calibration value can be expected to be sensitive
to the atmospheric constituents present during calibration because of
the different spectral response characteristics of cell and pyrheliometer.
In a previous report (Ref. 2), a simple regression equation was developed
for data on a single solar cell. This equation describes the sensitivity
of the particular cell's performance to atmospheric turbidity, water vapor,
and air mass. This being the case, it is highly desirable to determine
the sensitivity of available cells to atmospheric composition and to de-
termine how these changes are related to the basic spectral response of
each cell.
Fifteen solar cells, typical of types currently available for ter-
restrial and space use having dissimilar spectral response curves and/or
construction design, were measured within a two-week period at various
locations in the United States to determine sensitivity to water vapor,
turbidity, and air mass. Silicon, GaAs and Cu 2S/CdS solar cells were
included. The locations were selected to represent a broad range of
summer atmospheric conditi„ns, ranging from clear and dry to turbid and
humid. The sensitivity of the calibration value of each cell to water
vapor; turbidity, and air mass was evaluated by regression analysis and
t
the results are presented in this report.
1
4APPARATUS AND MEASUREMENT
The apparatus used to measure the calibration value of each cell
consisted of a pyrheliometer and solar cell collimating tube shown in
Figure 1. Measurement of the direct solar radiation at normal incidence
was obtained using a normal incidence pyrheliometer. The pyrheliometer
was mounted on a sun tracker to provide direct intercomparison intensities
during the measurement period. The pyrheliometer has a 10:1 collimation
ratio and is temperature compensated to within ± l q over the temperature
range -200C to +400C. The units were calibrated by Eppley Laboratories
with respect to IPS 1956 standard. During measurement, each cell was
inserted in the holder at the end of the 10:1 collimating tube and con-
nected to the J ohm t .1% load resistor. A multichannel voltmeter was
used to record cell output voltage, temperature, and pyrheliometer output
voltage.
The fifteen solar cells used in this study represented a wide variety
of cell constructions and spectral responses. There were 13 silicon
solar cells, one GaAs cell and one Cu 2 S/CdS cell. Some were developed
especially for terrestrial application, while others were space cells
which have been applied to terrestrial use or have some unique features.
This range of cell types was selected in this study to provide a com-
prehensive survey of available cells. The cells were all mounted in the
special holders described in Ref. 2.
Concurrent with measurements of cell short-circuit current,
Isc, cell temperature, and the normal incidence irradiance,
F
were measured by the sunphotometers shown in Figure 2. The procedures used
to determine 8 and W using the sunphotometer are described in Ref. 2. The
main differences between the work in this report and that of Ref. 2 are:
•	 (1) the altitude of each location visited was used to estimate the pressure
needed to determine the atmospheric variables in lieu of actual barometeric
measurements, and (2) cell short circuit currents were temperature corrected
to a common temperature (250
 C) in the absence of a temperature controller,
REGRESSION EQUATAION
The form of the regression equation used to determine the sensitivity
of the calibration value of each cell to water vapor, turbidity, and air
mass was as previously described in Reference 2, namely
1	 0.25	 -
log 1 s = log CO + log C I [10(B-0.045)+ I M + log C2 (WMr )	 eq. 11)
n
whe. ee CO , C I , and C2 are constants determined by a least square fit of the
data. C0 is the "extraterrestrial calibration value", C I is the turbidity
regression constant, and C 2 is the water vapor regression constant. B is
the Schuepp turbidity coefficient, W is the precipitable water vapor, M
r 
is
the relative air mass, and M is the absolute air mass. (M = M rp/po , where
p is the atmospheric pressure at the location and p0 is the standard atmos-
pheric pressure.)
RESULTS AND DISCUSSION
Table I indicates the locations visited and the range of atmospheric
parameters measured. As can be seen, the measured turbidities and precipi-
table water vapor represent a broad range of summer atmospheric conditions,
6from very clear sky (B - 0.04) to hazy (B - 0.259). Frecipitable water
ranged from a minimum of 0.70 cm to 2.35 cm. The variation in turbidity
does not appear to be strongly correlated with variations in water vapor
content. In all, a total of 25 measurements were made on each of 12 cells,
24 on 2 cells, and 23 on one cell.
Figure 3 summarizes the results obtained for the regression constants
C 1 and C2, where plotted points are indicated by cell number, for all the
cell studied. Values for the turbidity regression constant, C I , go from
0.98 to 1.018, while the water vapor regression constants, C2 , range from
1.05 to 1.115. As can be seen from Equation 1, regression constants with
values less than 1 indicate a decrease in cell calibration value with in-
crease in the associated atmospheric variable while constants greater than
1 indicate an increase in calibration value with increase in the atmospheric
variable.
Except for the GaAs cell (#110) there appears to be a correlation be-
tween water vapor and turbidity regression constants. 	 The Cu2S/CdS cell
067) appears indistinguishable from silicon solar cells. It can be seen
that cells with low C l constants have high C2 constants, and vice versa.
To qualitatively gain some understanding of this relationship between con-
stants, relative.spectral response curves were plotted in Figure 4 for
cells 72, : 26, and 43 (located at the extreme and middle portions of the
calibration plots); cell 110 was also plotted for comparison. It can be
seen from Figure 4 that there is a shift in peak response from blue to
red in going from cells 72 to 26 and then to 43.
Since losses in spectral irradiance due to increased turbidity occur
mainly in the blue region of the solar spectrum, cells with higher blue
relative spectral response should experience increased loss in short circuit
i
i7
current with increase in atmospheric turbidity. For all except 4 of the
cells studied herein, this loss in cell current outweighed the concurrent
loss in normal incident irradiance, resulting in turbidity regression constants
less than 1. It also follows that those cells with higher blue response have
a lower regression constant.
Similarly, cells with higher red response lose current with increased
atmospheric water vapor due to the loss o ► spectral irradiance in the near
infrared, primarily in the 0.940 dun region. However, in this case, the nor-
mal incident irradiance loss beyond the spectral region of cell response
far outweighs the loss 'of current, resulting in a water vapor regression
constant greater than 1 for all cells measured. It also follows that cells
with higher red response have a lower water vapor regression constant. As
might be expected the spectral response of the cell determines the sensi-
tivity of the calibration value to ,:he atmospheric variables.
Table 2 shows a comparison of results of regressior analyses performed
on cell 01 using the separate data bases obtained at Cleveland (of. 2) and
at the other locations visited. As can be seen, the regression constants
CO and C l obtained at Cleveland are 0.3 to 0.4% higher than the constants
obtained from data taken at the several locations visited. This difference,
is insignificant considering the limited amount of data. The standard de-
viation of the difference between measured and predicted calibration values
was,l% for the data taken at Cleveland and was 0.5% for data taken at the
_ 
different locations. For the other cells studied, the range of standard
deviation was 0.3% to 1%, except for GaAs, which had a standard deviation	 i
£.	 of 2%. Error analysis of the uncertainties in the measured values of Isc'
n M, Band W indicate precision errors of 1 to 1.5% are to be expected.
>
8The anomalous behavior of GaAs in the correlation curve between regression
constants, as well as the relatively high standard deviation, possibly may
be attributed to its drastically different spectral response, especially
the sharp cut off at about 0.88 um.
SUMMARY OF RESULTS
Measurements of 15 solar cells with dissimilar spectral response and
construction were made at five geographically separated locations in the
contiguous United States to determine the sensitivity of the cell calibra-
tion value to atmospheric water vapor and turbidity. A regresssion equation
was used to obtain regression constants associated with each atmospheric
variable. The following results were obtained:
1. There is a correlation between regression constants and solar cells,
cells with low turbidity regression constants had high water vapor regress-
ion constants and conversely.
2. The value of the regression constants and the sensitivity of cell
calibration value to the atmospheric variables, water vapor content and
turbidity appear to be closely related to spectral response of the silicon
and Cu2S/CdS solar cells.
3. The gallium arsenide solar cell data could not be correlated with
the other cells.
NOTE: The data set used in this report is available upon request. Mail
request to:
NASA-Lewis Research Center
Attn: Mr. T. M. Klucher, MS 302-1
21000 Brookpark Road
Cleveland, OH
	
44135
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I
TABLE I. - LOCATIONS VISITED AND RANGE OF
ATMOSPHERIC PARAMETERS
- NEWARK, DELAWARE
- GAINESVILLE, FLORIDA
- PASADENA, CALIFORNIA
-PHOENIX, ARIZONA
- ALBUQUERQUE, NEW MEXICO
RANGE OF ATMOSPHERIC PARAMETERS
T UR BIDITY	 0.04 - 0.259
WATER VAPOR 0.70 - 2.35 CM
AIR MASS	 0.93 - 2.00
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Sensitivity of solar-cell performance to atmospheric variables. 2: Dissimilar cells at several locations

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