PDF Design Considerations of Solar Arrays

[Pages:8]DESIGN CONSIDERATIONS OF SOLAR ARRAYS FOR TERRESTRIAL APPLICATIONS*

R. G . Ross, Jr.**

Jet Propulsion Laboratory Pasadena, C a l i f o r n i a 91103

U.S.A.

Abstract

The use of p h o t o v o l t a i c s o l a r a r r a y s f o r t e r r e s t r i a l a p p l i c a t i o n s poses a number of unique requirements which must be addressed by t h e a r r a y designer and user. The requirements r e s u l t from t h e p h y s i c a l c h a r a c t e r i s t i c s of p h o t o v o l t a i c devices, t h e n a t u r e of t y p i c a l terrest r i a l p h o t o v o l t a i c a p p l i c a t i o n s , t h e demands of t h e t e r r e s t r i a l environment, and t h e competitive forces a s s o c i a t e d w i t h t h e t e r r e s t r i a l power supply market. Various t e r r e s t r i a l a r r a y requirements and design cons i d e r a t i o n s a r e discussed both i n terms of t h e c u r r e n t state-of-the-art and i n terms of t h e goals of t h e U.S. National Photovoltaics Program. Included a r e discussions of the r e l a t i v e performance of various a r r a y module designs currently on t h e market together with thoughts f o r f u r t h e r design improvements.

*This work p r e s e n t s t h e r e s u l t s of one phase of research c a r r i e d o u t by t h e J e t Propulsion Laboratory, C a l i f o r n i a I n s t i t u t e of Technology under NASA c o n t r a c t NAS7-100, sponsored by t h e Energy Research and Development Administration.

**Engineering Manager, Low-cost S i l i c o n S o l a r Array Project.

Proceedings of the 1976 International Solar Energy Society Conference Winnipeg, Canada, A77-48910 23-44, v.6 (1976), pp. 48-56.

Introduction

Though s o l a r p h o t o v o l t a i c a r r a y s have been used f o r many y e a r s

a s a primary s o u r c e of power f a r space v e h i c l e s , t h e t e r r e s t r i a l use

of p h o t o v o l t a i c s h a s come i n t o prominence only w i t h i n t h e l a s t

couple of years. I n support of decreased dependence on our l i m i t e d

f o s s i l and nuclear f u e l s u p p l i e s , t h e U.S. Energy Research and

Development Administration (ERDA) h a s a n a c t i v e program which addres-

ses t h e g r e a t l y expanded development and u t i l i z a t i o n of t e r r e s t r i a l

p h o t o v o l t a i c systems( l ) . A major p a r t of t h e National P h o t o v o l t a i c

Program i s centered a t t h e Jet Propulsion Laboratory and is r e f e r r e d

t o a s t h e Low-cost S i l i c o n S o l a r Array (LSSA) P r o j e c t .

The p r i -

mary o b j e c t i v e of t h e LSSA p r o j e c t i s t h e timely development of low-

c o s t commercial-quality photovoltaic a r r a y s through an a c t i v e program

of i n d u s t r i a l and academic involvement. Analysis of t h e c u r r e n t

s t a t e of t e r r e s t r i a l photovoltaics technology and t h e d e f i n i t i o n of

future a r r a y requirements i s an important and necessary s t e p toward

meeting t h i s objective.

11. Array Requirement D e f i n i t i o n

The examination and development of a r r a y requirements i s a cooperative e f f o r t involving a l l elements of the National Photov o l t a i c s Program t o g e t h e r w i t h t h e a r r a y manufacturers, power system designers, and p o t e n t i a l users.

Understanding t h e u s e r and power system needs i s c r i t i c a l t o t h e success of t h e a r r a y development e f f o r t . To t h i s end, t h e ERDA program c o n t a i n s t h r e e major system-oriented a c t i v i t i e s , a mission analysis a c t i v i t y t o i d e n t i f y and evaluate p o t e n t i a l user groups and t h e i r system needs, a system analysis a c t i v i t y t o examine t h e development of system designs t o meet t h e user needs, and a demonstration program a c t i v i t y t o evaluate and demonstrate f i n a l system performance. An a c t i v e attempt i s a l s o being made t o work w i t h p r e s e n t and p o t e n t i a l users i n the development of requirements t o meet t h e i r specific needs.

A second important ingredient i n t h e d e f i n i t i o n of t e r r e s t r i a l array requirements involves understanding the solar array design f a c t o r s and s e n s i t i v i t i e s which r e s u l t from t h e physical charact e r i s t i c s of t h e photovoltaic devices themselves and the manufacturi n g f a c t o r s a f f e c t i n g t h e i r production. To achieve t h i s understanding t h e LSSA p r o j e c t has a broad program d i r e c t e d toward t h e design, manufacturing, and marketing f a c t o r s a f f e c t i n g a r r a y p r i c e and performance. These a c t i v i t i e s include cost studies of various manufacturing approaches, experimental programs aimed a t both device

and manufacturing process improvement, and the procurement and t e s t i n g of present state-of-the-art s o l a r arrays.

A t h i r d major i n g r e d i e n t of t h e requirement d e f i n i t i o n process involves characterizing the natural t e r r e s t r i a l environment and developing appropriate environmental test requirements. These requirements must be consistent with both t h e operational l i f e t i m e needs of the potential user groups and the economical and technical constraints associated with available manufacturing methods.

The remainder of t h i s paper p r e s e n t s an overview of a r r a y requirement t r e n d s beginning t o evolve from t h e various ERDA a c t i v i t i e s and c o n t r a s t s these trends with t h e current s t a t e of the t e r r e s t r i a l photovoltaics technology.

111. Array Design Considerations

For present t e r r e s t r i a l arrays the primary requirement is t o g e n e r a t e power f o r small, o f t e n remote electric-power a p p l i c a t i o n s . Typical examples include radio relays, navigational l i g h t s and horns, and remote climatological and environmental instrumentation. A s shown i n Table I, t h e n a t u r e of t h e s e a p p l i c a t i o n s , t h e i r s i z e , t h e i r d i v e r s i t y and t h e i r technical needs strongly influences t h e c h a r a c t e r i s t i c s of current arrays. Likewise, the applications a r e limited t o those which can profitably u t i l i z e current array characteristics a t their present high price.

To meet an o b j e c t i v e of i n c r e a s e d energy independence r e q u i r e s t h a t p h o t o v o l t a i c s become economically v i a b l e f o r l a r g e energy consumption of t h e f u t u r e . The obvious a p p l i c a t i o n s w i t h s i g n i f i c a n t energy usage include c e n t r a l e l e c t r i c u t i l i t i e s , i n d u s t r i a l and commercial applications, and r e s i d e n t i a l applications. As indicated i n Table 11, t h e s e a p p l i c a t i o n s p l a c e s i g n i f i c a n t l y d i f f e r e n t requirements on f u t u r e s o l a r arrays.

The l a r g e s t requirement change i s i n t h e a r e a of c o s t . Whereas small remote a p p l i c a t i o n s can achieve c o s t e f f e c t i v e n e s s w i t h a r r a y p r i c e s of $30/watt and higher, l a r g e s c a l e applications require array p r i c e s of 5 0 ~ / w a t tand less t o compete w i t h conventional means of e l e c t r i c a l energy production. A major t h r u s t of t h e ERDA program i s directed a t achieving t h i s cost reduction through a combination of t e c h n o l o g i c a l advances and mass production methods. Approximately t h r e e f o u r t h s of t h e c u r r e n t c o s t of s o l a r a r r a y s i s involved i n t h e s o l a r c e l l s themselves. This i s spread more o r less e q u a l l y between the c o s t of t h e s i l i c o n , t h e c o s t of making a s i n g l e c r y s t a l wafer, and the cost of implanting the diode junction and metalization.

Table I. P r e s e n t Array C h a r a c t e r i s t i c s Versus Present Photovoltaic Power Application C h a r a c t e r i s t i c s

I

Present Application Characterist i c s

Present Array Characterist i c s

Small diverse markets, o f t e n requiring custom treatment, b u t i n d i v i d u a l l y incapable of supporting a significantly different array design o r mass production methods-

Widely varying designs between manufacturers. Each manufactuerrs l i n e e a s i l y adaptable t o widely varying a p p l i c a t i o n s and geared t o hand

. assembly operations. L i t t l e o r no interchange-

a b i l i t y between manufacturers

Consume l e s s than 1000 w a t t s of low voltage ( ................
................

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