Application development activities were designed to stimulate the market for photovoltaics so that as costs are reduced there will be an increasing market demand to encourage the expansion of industrial solar array production capacity. Supporting these application development activities are tasks concerned with: (1) establishing standards and methodology for terrestrial solar cell calibration; (2) conducting standard and diagnostic measurements on solar cells and modules; and (3) conducting real time and accelerated testing of solar cell modules and materials of construction under outdoor sunlight conditions

Deyo, J. N.

English

NASA Technical Reports Server

RECENT DEVELOPMENTS IN PHOTOVOLTAI C 
ENERGY B Y  ERDAINASA-LeRC 
by 
JamesN. Deyo 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 
69 
https://ntrs.nasa.gov/search.jsp?R=19770023333 2020-03-20T07:54:53+00:00Z
SUM MAR^ 
The Tests and Applications Project of the ERDA Photovoltaic Program is 
concerned with the testing of photovoltaic systems and the growth of their use in 
real terrestrial applications. This activity is an important complement to the 
development of low cost solar arrays by providing requirements based on applica- 
tion needs and stimulating markets to create demand to absorb increasing pro- 
duction capacity. A photovoltaic system test facility is now operational, market 
stimulation has been initiated through applications, and standards for terrestrial 
cell measurements established. 
INTRODUCTION 
The Tests and Applications Project is concerned with the development of 
terrestrial applications for photovoltaic systems as part of the ERDA National 
Photovoltaic Energy Conversion Program. This work is being conducted through 
a series of coordinated activities to identify practical real applications, build 
and demonstrate systems in such applications, conduct tests to understand and 
improve the characteristics and performance of photovoltaic systems, and 
provide requirements and data from this activity as guidance to ERDA ' 8  develop- 
ment of low cost solar arrays. Solar cell modules for these application systems 
will be purchased by the ERDA program as  part of the Low Cost Silicon Solar 
Array Project managed by JPL. These application development activities a re  
designed to stimulate the terrestrial market for photovoltaics so that as  costs 
a r e  reduced there will be an increasing market demand to encourage the 
expansion of solar array production capacity by industry. 
Supporting these application development activities are  tasks concerned 
with: (1) establishing standards and methodology for terrestrial solar cell 
calibration, ( 2) conducting standard and diagnostic measurements on solar cells 
and modules, and (3) conducting real time and accelerated testing of solar cell 
modules and materials of construction under outdoor sunlight conditions. 
Experience and information gained through this project will be used to 
evaluate emerging systems technology, provide systems-related guidance to the 
other projects of the national program, and develop information for future 
photovoltaic program planning. 
Specific activities in each of the major subprojects follow. 
70 
APPLICATIONS 
Near -Term Applications 
Near-term applications are  classified primarily as any terrestrial applica- 
tion which could utilize a photovoltaic power system and be cost-competitive on a 
life cycle basis within the next 10 years (by 1986). Some of these applications 
exist today but a r e  generally small in power level (<lo00 W) and usually remotely 
located. In other instances, the cost-competitive time for photovoltaics is still 
5 to 10 years away and users a re  unwilling or unable to pay the premium cost to . 
install a photovoltaic system today. In still other cases, the market is latent. 
Many potential users are  unaware or unsure of the benefits and the readiness of 
solar cell power for their applications. Unless such users a re  fully apprised of 
the solar electric option, their entry into the solar cell market may be greatly 
delayed. Experiments and demonstrations specific to the application of a 
potential user should aid and speed the incorporation of solar electric power into 
the users' operational systems as well a s  stimulating industry to produce the 
systems. 
To examine the possibilities for stimulating the near-term market, 
several thrusts have been initiated. 
Mini-applications. These involve the selection, design, assembly, and 
deployment of a small number of photovoltaic systems (100 to 1000 W) powering 
equipment which is both useful and generally understood by the public. A s  
examples of these systems, two photovoltaically powered small refrigerators 
have been assembled and deployed to Isle Royale National Park, Michigan (Fig. 1) 
and the Papago Indian Reservation, Arizona (Fig. 2 ) ,  respectively. Public 
information news releases thus far have generated many inquiries both nationally 
and foreign (Australia, Japan, South Africa, etc. ) . Another application to show 
the public the versatility and availability of photovoltaic power is shown in 
Figure 3. The 1.7 kW solar array shown was used during July, August, and 
September during the Festival of American Folk Life, in Washington, D. C. , to 
charge batteries on electric vehicles used by the National Park Service to manage 
the Festival. This display (whole not cost-effective) did provide the many 
visitors to the festival with an opportunity to see photovoltaics at work. Other 
mini-applications , such as water pumping, are  being considered for deployment 
in the near future. This activity serves to introduce photovoltaics to the public, 
expand public awareness, and stimulate ideas for applications both domestic 
and foreign. 
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Cost-shared Tests. A second thrust has involved implementation of 
cost-shared tests of photovoltaic applications relevant to near-term needs of 
user-agencies of government. Agreements have been reached with the National 
Weather Service ( N W S )  for tests of photovoltaic systems to power six remote 
automated meteorological observation stations (RAMOS) which NWS is developing 
( Fig. 4) . N W S  expects to deploy similar systems throughout the U. S. (many in 
remote areas) over the next several years. Solar eells appear to be attractive 
for powering many of these stations. Two systems have also been fabricated 
for use by the U. S. Forest Service on fire lookout towers in Lassen and Plumas 
National Forests, California (Figs. 5 and 6). These systems (300 W each) are 
providing power for lighting, water pumping, refrigeration, and radio communi-- 
cations for the tower operator who lives at  the lookout during the fire season 
( 6  to 8 months each year). Other cost-shared application tests a r e  being 
identified. In all cases the purpose of the cost-shared test is to provide a way 
for a user agency with a large number of applications to examine the possibilities 
of photovoltaics in his operational climate with the support and guidance of 
another agency. Following successful cost-shared tests the user agency will be 
expected to satisfy his operational needs for photovoltaic systems through 
purchases f rom commercial sources. 
Mail Solicitation. A third thrust has involved the identification of new 
applications in government through mailings of a descriptive brochure. The 
purpose of the mailing has been to solicit others for needs in order to identify 
a wide range of potential applications for development. Two mailings (800 each) 
have elicited a very encouraging interest and response. These responses are 
now in the process of followup and evaluation. 
Other ideas for stimulating the market for photovoltaic systems are being 
considered and additional thrusts will be implemented. 
Department of Defense Applications 
Energy resources and alternatives a re  an important concern in the 
operation of the Department of Defense ( DoD) . Part of DoD's interest in investi- 
gating energy alternatives is the consideration of photovoltaic power systems for 
military applications. To help DoD initiate their investigation of photovoltaics, 
support has been provided to design and construct photovoltaic arrays to power 
five initial applications which DoD has selected for demonstration. Peak power 
levels for these applications range k o m  160 W to 12 kW. Total power require- 
ments a re  approximately 26.5 kW. 
75 
Figure 4. National Weather Service (NWS) remote weather station 
(RAMOS) Mammoth Mountain, California. Similar to 
six additional units planned for ERDA 
application tests, 
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System Test Faci li ty 
The objective of the System Test Facility (STF) is to provide a facility 
where photovoltaic systems may be assembled and electrically configured, 
without true physical configuration or arrangement, in a breadboard fashion for 
operation and testing to evaluate their performance characteristics. The STF 
provides a vital support function to the overall Applications Subproject. A s  a 
%readboard" system it allows prompt preliminary investigation and checkout 
of components subsystems before they a re  mounted in more elaborate and visible 
experiments o r  demonstrations, 
The STF will be one of the first of the large-scale installations (40 kW 
peak in CY77) of photovoltaic systems, and the test results and experience will 
be invaluable to the planning and execution of the later experiments for the larger 
applications that will be identified by the mission analysis and system definition 
projects. Operating experience and system test data a re  needed to corroborate 
the results of the system analysis studies and to validate and refine the computa- 
tional models used in the analyses. The facility will be available for conducting 
tests needed by all projects of the ERDA Photovoltaic Program. 
' 
The facility is presently operational with approximately 10 kW peak of 
solar cell modules installed (Fig. 7 ) .  Construction is complete to extend the 
capacity for installation of an additional 30 kW of array. 
Facility Configuration. The facility has been designed to test systems 
both with and without battery storage. For the system with energy storage, a 
54 cell, 48 kW-h lead-acid battery is used in parallel with the solar array. 
The approach chosen was to consider that all photovoltaic power generated would 
be used for test loads, battery charging, or, i f  in excess, dissipated as heat 
through a shunt regulator. Power to load banks will be furnished in the form of 
a two-wire, 120-V single-phase 60 H z  ac developed through a self-commutating 
dc/ac inverter. This arrangement provides instantaneous back-up power in the 
event of any photovoltaic power system interruption or to cover large peak test 
loads. 
The facility also is capable of testing a system without energy storage by 
using a dc/ac inverter in parallel with the incoming utility line. Inverters of 
this type a re  designated line-commutated inverters. In this case, excess solar 
array power appearing at the inverter output is fed back through the incoming 
utility line to be used elsewhere in the utility system. In this mode the inverter 
will be controlled to extract power from the solar array at a voltage corresponding 
to maximum power generation. 
79 
80 
A flexible wiring arrangement has been provided between the arrays and 
power conditioning equipment so that; the field of solar cell modules may be con- 
figured into a variety of array arrangements of different voltage and power levels. 
A s  a first system test, the field array was connected to an assortment of 
loads through an 8 kW line-commutated inverter designed to share the load with 
a utility power source. The test confirmed that when the loads required more 
power than the array could provide, the extra power was drawn from the utility. 
When excess power was generated by the array it was fed to the utility grid, 
The efficiency of the inverter (ac power "out" divided by dc power "in") was 
measured to be approximately 70 percent at 20 percent of rated power and 90 
percent at 50 percent of rated power. No attempt was made in this early test 
to filter out the harmonic distortion in the output of the inverter. This is planned 
for future tests. 
. 
Electromagnetic interference tests indicated that the level of radiated 
frequencies from the array was indistinguishable from background radiation when 
further than 20 f t  from the solar array. In the vicinity of the line-commutated 
inverter there was some interference picked up on the AM broadcast band. This 
disappeared when the AM receiver was removed a few feet from the control 
center where the inverter was located. There was no noticeable difference in 
the quality of T V  or FM reception when the power was switched from the utility 
grid to the output of the inverter. Further tests on both radiated and conducted 
interference a re  planned and will be done a s  a function of array capacity. 
The effect of shadows on solar cells is being investigated. It has been 
observed that the typical terrestrial solar cell has a very high dark leakage 
current. It has also been found that it is rather difficult to get a true dark 
shadow on a single cell in a module. The cells, as they are encapsulated, in 
modules are typically mounted so that they a re  approximately 1/8 to 1/4 of an 
inch from the module surface. Some light gets to the shadowed cell from the 
edges in a normal installation. Therefore, the effects of shadowing are not 
likely to be a s  severe as in space applications where the shadows are deeper. 
STANDARDS AND MEASUREMENTS 
The objective of this subproject is to ensure the availability of reliable 
performance test methods and information for all participants in the ERDA 
National Photovoltaic Conversion Program. To carry out these objectives, a 
number of interrelated tasks are employed to acquire inputs from participants 
in the program, develop test methods, and provide guidance and information back 
to the program participants. 
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Workshops 
Workshops are being employed as a primary means of acquiring technical 
input from program investigators with respect to terrestrial solar cell measure- 
ments. A first workshop was held in March of 1975 and an interim measurement 
procedure manual issued, A second workshop was held November 10-12, 1976, 
at Baton Rouge, Louisiana to refine the interim procedures. Topics addressed 
were (1) terrestrial sunlight and its effect on solar cell performance, (2) solar 
simulation, and (3)  techniques for cell and array measurement and standard 
cell calibration. 
Reference Condit ions and Methodology 
Activities in this task have investigated the variation of standard cell 
calibration coefficients with atmospheric conditions and the e r ror  in cell measure- 
ments introduced by spectral mismatch between sunlight, simulator, test cells, 
and standard cells, 
In an effort to determine the sensitivity of solar cell performance to a 
range of atmospheric conditions and insulation components, an insolation 
measurement facility has been set up. The approach is to measure the insulation 
using a solar cell and a black body detector in each of several different orienta- 
tions. Detectors a re  in place in several orientations (horizontal, horizontal- 
shadow banded, inclined, 37" above horizontal, etc.) . This facility is now 
operational and data a re  being acquired. 
The cell measurement facility has been automated with a HP 9830 
calculator-interface bus system. I-V curves, spectral response curves, cell 
dark forward I-V characteristics, and cell V characteristics can readily 
be obtained. The data are  acquired, calculations made, and results presented 
or  plotted using the 9830. Over 4000 measurements have been made for more 
than 60 different organizations in the past year. 
-I oc s c  
During the last seven months, over 2600 modules from the 46 kW ERDA/ 
JPL buy have been delivered to Lewis. T-V measurements have been made on 
approximately 13 percent of the modules with a pulsed solar simulator. The 
simulator target area is being expanded from 32 in. in diameter to 5 ft by 9 ft. 
Module and array measurements can also be made outdoors using the HP calcu- 
lator system. Temperature and irradiance are measured and corrections made. 
82 
I nterim Reference Cells 
Reference cells are being calibrated and distributed to investigators to 
use in their own facilities so as to standardize measurements and permit call 
characteristics to be compared between investigators more accurately. 
Both silicon cell and cadmium sulfide cell standards have been delivered. 
The calibration coefficients are affected significantly by atmospheric water vapor 
content and turbidity. An updated standard cell holder has been designed and 
fabricated. The main improvement is the field-of-view which allows the cell to 
be used in any orientation. These holders will be distributed as  replacements 
for the earlier unit. 
Field I nsulation Measurements 
One problem in assessing the performance of photovoltaic systems in 
field applications is the lack of local insulation data. Generally, systems a r e  
sized based on national insolation data for the region of the test. Local condi- 
tions at the application site may not match the national data. Therefore, a low 
cost insulation recording instrument is being developed. The instrument is 
self-contained and uses a silicon solar cell as a sensor. Field instruments are 
being made for distribution to a variety of test sites. These include all applica- 
tion sites and all endurance testing sites. 
ENDURANCE TEST I NG 
The objective of this subproject is to determine the endurance of solar 
cell modules and module materials under both accelerated and real-time environ- 
mental conditions of intended use. 
Accelerated outdoor exposure will be used to test cells, modules, arrays, 
and their component materials in concentrated sunshine. Accelerated testing of 
a number of solar cell encapsulant materials was started in July 1974 by the 
Lewis Research Center. Work in this area will be expanded with tests being 
made over a series of controlled environmental conditions (i. e. , temperature 
and humidity) and in the presence of atmospheric pollutants. 
Real-time outdoor exposure tests provide the most accurate measurement 
of life for solar cell arrays and materials. Established testing companies with 
solar insolation and climatic reporting capability are conducting the real-time 
83 
tests. Several hundred samples a re  presently being tested under both accelerated 
and real-time conditions. Some solar cell modules from the JPL 46 kW buy are 
showing delamination of the encapsulant around the edges. So far, this has not 
resulted in any noticeable decrease in electrical output. 
Of the polymeric materials being tested {Table 1) , those that show 
promise a re  FEP Teflon, perfluoro alkoxy, polymethylmethacrylate, polyvinyli- 
dene fluoride , and a sandwich of Tedlar-vexar-polyethylene. 
TABLE 1. SOME MATERIALS UNDERGOING ENDURANCE TESTS 
Accelerated Tests 
FEP-A Teflon with GE 574 adhesive 
FEP-C Teflon with GE 574 adhesive 
Polymethylmethacrylate 
Polycarbonate - UV stabilized 
Real-Time Tests 
Polyurethane 
Quartz 
Kapton 
TIV silicone 
Polyester 
Polyvinylidene fluoride 
Polyvinylidene fluoride copolymer 
TVP - Sandwich of 1 mil UV stabilized Tedlar 
Vexar plastic grid - 3/4 in. mesh UV inhibited 
polyethylene 
CONCLU S I ON S 
Several near-term applications have been fabricated and installed at field , 
sites for testing. Arrays for DoD applications have been delivered and testing 
initiated. The System Test Facility at NASA-Lewis Research Center has become 
operational. Considerable progress and understanding has been gained regarding 
the problems and variables of terrestrial measurements of solar cells and 
modules. During the coming year, increased emphasis will be placed on the 
\ 
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84 
stimulation of the market for photovoltaics and the fielding of additional near-term 
application tests. Emphasis will also be increased on breadboard system testing, 
endurance testing, and the refinement of measurement techniques. A l l  of these 
activities will provide a greatly increased base of real terrestrial data concerning 
photovoltaic systems to help guide and focus the ERDA Program on the critical 
problems to be solved. 
85 


Recent developments in photovoltaic energy by ERDA/NASA-LeRC

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