14 Photochemical Aspects of Solar Energy Conversion and
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Storage JAMES R. BOLTON Photochemistry Unit, Chemistry Department, University of Western Ontario, London, Ontario, Canada N6A 5B7
Most of the current and proposed applications of solar energy involve i t s conversion to heat for space and water heating or to drive a Carnot engine to produce mechanical work or electricity. There are, however, some applications of solar energy which involve its conversion directly into electricity or to be stored as chemical energy without any thermal step in the process. These applications are quantum processes in that solar photons are employed to drive photophysical and photochemical processes. In this article, I w i l l define qualitatively and quantitatively the thermodynamic and kinetic limits on the photochemical conversion and storage of solar energy as it i s received on the earth's surface, evaluate a number of possible reactions with particular emphasis on the generation of solar fuels such as hydrogen from water and the generation of electricity. A.
General Requirements on the Photochemical Reaction
Many authors have considered the general requirements for useful solar photochemical reactions (1, 2, 3, 4, 5). In summary, they are: 1.
The photochemical reaction must be endergonic.
2.
The process must be cyclic.
3.
Side reactions leading to the irreversible degradation of the photochemical reactants must be totally absent.
4.
The reaction should be capable of operating over a wide bandwidth of the visible and ultraviolet portions of the solar spectrum with a threshold wavelength well into the red or near infrared.
5. The quantum yield for the photochemical reaction should be as high as possible. This chapter not subject to U.S. Copyright. Published 1979 American Chemical Society. Tomlinson et al.; Chemistry for Energy ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
14.
Solar
BOLTON
Energy
Conversion
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Storage
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I n a d d i t i o n , t h e r e a r e some r e q u i r e m e n t s w h i c h a p p l y p a r t i c u l a r l y t o chemical energy storage r e a c t i o n s : 6.
The b a c k - r e a c t i o n must be e x t r e m e l y s l o w u n d e r a m b i e n t conditions t o permit long-term storage, but should pro ceed r a p i d l y under s p e c i a l c o n t r o l l e d c a t a l y t i c c o n d i t i o n s o r e l e v a t e d t e m p e r a t u r e s s o as t o r e l e a s e t h e s t o r e d e n e r g y when needed.
7.
The p r o d u c t ( s ) o f t h e p h o t o c h e m i c a l r e a c t i o n s h o u l d be e a s y t o s t o r e and t r a n s p o r t .
8.
The r e a g e n t s and any c o n t a i n e r m a t e r i a l s h o u l d be cheap a n d n o n - t o x i c a n d t h e r e a c t i o n s h o u l d be u n a f f e c t e d b y oxygen.
A t p r e s e n t , the only photochemical storage system which s a t i s f i e s nearly a l l o f these conditions i s the r e a c t i o n o f photo s y n t h e s i s ; h o w e v e r , c e r t a i n p h o t o v o l t a i c c e l l s s u c h as t h e s i l i c o n a n d GaAs c e l l s , s a t i s f y many o f t h e r e q u i r e m e n t s f o r d i r e c t c o n version to e l e c t r i c i t y . In addition, there are several possible s y s t e m s w h i c h have a p o t e n t i a l t o s a t i s f y most o f t h e r e q u i r e m e n t s and w i l l be c o n s i d e r e d i n t h i s a r t i c l e . B.
S o l a r E n e r g y A v a i l a b l e a t t h e B a n d Gap
Wavelength
D i r e c t c o n v e r s i o n systems a r e t h r e s h o l d d e v i c e s , t h a t i s , t h e r e i s a minimum p h o t o n e n e r g y w h i c h can i n i t i a t e t h e p h o t o c h e m i c a l r e a c t i o n . T h i s i s c a l l e d t h e hand-gap e n e r g y E w i t h a c o r r e s p o n d i n g band-gap w a v e l e n g t h Xg. Eg u s u a l l y corresponds to t h e 0-0 t r a n s i t i o n t o t h e l o w e s t e x c i t e d s i n g l e t s t a t e o f t h e a b s o r b e r (see F i g . 1 ) . H e n c e , i t i s i m p o r t a n t t o know what f r a c t i o n o f t h e i n c i d e n t s o l a r p o w e r i s a v a i l a b l e a t t h e band-gap energy. I f N (X) i s the i n c i d e n t s o l a r photon f l u x i n the wavelength b a n d f r o m X t o X + dX ( i n p h o t o n s m~ s " nm~l) and α(λ) i s t h e a b s o r p t i o n c o e f f i c i e n t o f t h e a b s o r b e r i n t h e same b a n d , t h e n t h e a b s o r b e d f l u x o f p h o t o n s w i t h λ
