5 Solar Energy Conversion through Photosynthesis? RODERICK K. CLAYTON
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Division of Biological Sciences, Cornell University, Ithaca, Ν. Y. 14853
Ultimately the use of sunlight offers the clearest way to meet our needs for energy without running afoul of serious environmental problems. Direct solar heating is approach ing technical and economic feasibility on a large scale. Generation of fuel from organic wastes can be economically profitable now, especially when combined with the waste -assisted growth of algae. Growth of energy-efficient plants such as sugarcane and harvesting of wild plants such as water hyacinth as sources of fuel are attractive possibilities. Schemes to modify normal photosynthesis or to use ex tracted parts of photosynthetic tissues to generate hydrogen or electricity from sunlight are visionary, but they are in their infancy and should be developed along with all other reasonable approaches.
We
estimate t h a t i n t h e U n i t e d States w e h a v e e n o u g h f o s s i l f u e l , i n one f o r m o r another, t o m e e t o u r e n e r g y r e q u i r e m e n t s f o r m o r e t h a n
a t h o u s a n d years at t h e present rate of c o n s u m p t i o n , b e t w e e n 1 0 10
2 0
J (or about 1 0
1 7
1 9
and
B t u ) a n n u a l l y . R o u g h estimates o f energy reserves,
m o d i f i e d f r o m d a t a l i s t e d b y H a m m o n d ( I , 2 ) , are s h o w n i n T a b l e I . H o w e v e r , t h e use of a l l o u r fossil f u e l reserves w i l l r e q u i r e n e w solutions to economic a n d environmental problems.
I n t h e l o n g r u n , o n l y solar
e n e r g y is r e l a t i v e l y free f r o m t h e r m a l a n d c h e m i c a l p o l l u t i o n . E v e n t h e i n c r e a s e d a b s o r p t i o n o f solar r a d i a t i o n b e c a u s e of its c o l l e c t i o n as a n e n e r g y source c o u l d b e offset b y c o m p e n s a t i n g reflectors, so t h a t t h e a b s o r p t i o n of heat b y t h e p l a n e t as a w h o l e r e m a i n s u n a l t e r e d . C u r r e n t l y the U . S . E n e r g y Research a n d Development
Administration ( E R D A )
projects t h a t 6 % o f o u r e n e r g y needs w i l l b e m e t w i t h solar systems b y t h e y e a r 2000, a n d 2 5 % b y 2020 ( 3 ) . 93
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
94
SOLID STATE
Table I.
CHEMISTRY
Rough Estimates of Energy Reserves in the United States at the Present Rate of Consumption Energy
Source
Years
P r e s e n t l y developed o i l a n d gas E s t i m a t e d t o t a l o i l a n d gas Fission (conventional) C o a l a n d o i l shale Geothermal Fusion Solar
>
2
3
3
α
q
>
6
9
9
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L e t us list e n e r g y sources t h a t c a n b e m a d e q u a n t i t a t i v e l y i m p o r t a n t i n t h e U n i t e d States, a n d t h a t are d i r e c t l y or i n d i r e c t l y of
contemporary
solar o r i g i n . W e s h a l l t h e n r e v i e w these b r i e f l y a n d c o n s i d e r i n m o r e d e t a i l those t h a t i n v o l v e photosynthesis. 1.
W i n d - p o w e r e d turbine.
2.
O c e a n t h e r m a l g r a d i e n t s ; heat engine.
3.
D i r e c t solar h e a t i n g ; h e a t engine or c h e m i c a l r e a c t i o n c y c l e p r o
ducing hydrogen from water. 4.
P h o t o s y n t h e t i c systems,
p l a n t s i n c l u d i n g algae,
( a ) V a r i a t i o n s of a g r i c u l t u r e ; g r o w t h of
and conversion
to fuels,
(b)
Photosynthetic
hydrogen production. 5.
Photoelectric
devices,
possibly
using materials
derived
from
p h o t o s y n t h e t i c tissues. I n m a n y of these systems h y d r o g e n is l i k e l y to p l a y a c e n t r a l r o l e (4)
b o t h as a c l e a n f u e l a n d as a means of s t o r i n g a n d t r a n s p o r t i n g
energy.
A s a f u e l , the o n l y c h e m i c a l p o l l u t a n t m i g h t b e oxides of n i t r o
g e n i f a t m o s p h e r i c n i t r o g e n is exposed to a h i g h c o m b u s t i o n t e m p e r a t u r e . A s a m e d i u m of exchange h y d r o g e n c a n be c o n v e r t e d to e l e c t r i c i t y w i t h about 4 0 %
efficiency, a n d e l e c t r i c i t y t o h y d r o g e n at a b o u t 8 0 %
effi
c i e n c y . I t has b e e n e s t i m a t e d ( 5 ) t h a t t h e t r a n s p o r t of energy as h y d r o gen is c h e a p e r
t h a n the t r a n s m i s s i o n of e l e c t r i c i t y i f the d i s t a n c e is
greater t h a n a b o u t 250 m i l e s . Wind W i n d - p o w e r e d e l e c t r i c generators c a n be g o o d d e c e n t r a l i z e d sources of p o w e r i n r u r a l areas, a l t h o u g h the o p t i m u m size f o r efficiency is e s t i m a t e d at s e v e r a l megawatts.
T h e cost of s u c h a p o w e r p l a n t is e s t i m a t e d
( 3 ) to b e t w o to f o u r times greater t h a n t h e present cost, a b o u t $400 p e r k i l o w a t t , of a n u c l e a r p o w e r p l a n t . A n extensive a r r a y of m a j o r w i n d - p o w e r e d p l a n t s , a b s o r b i n g m u c h of the surface w i n d over a large r e g i o n , c o u l d h a v e a n effect o n t h e l o c a l w e a t h e r t h a t m e r i t s serious s t u d y .
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
5.
CLAYTON
Sohr Energy
Ocean Thermal
Gradients
95
Conversion
A t e m p e r a t u r e difference of 20 ° C exists v e r t i c a l l y t h r o u g h a d e p t h of 1000 m i n t r o p i c a l w a t e r s , a n d a s i m i l a r difference is f o u n d at the e d g e of t h e G u l f S t r e a m n e a r the coast of F l o r i d a . T h i s difference a r o u n d 3 0 0 ° Κ could provide ~ 6 %
efficiency for a heat engine w i t h a C a r n o t c y c l e ,
a n d a n efficiency greater t h a n 2 % h y d r o g e n c o u l d b e expected. problems
(2)
concerned
f o r the g e n e r a t i o n of e l e c t r i c i t y or
There w o u l d be formidable
engineering
w i t h thermal conductivity between
the
sea
w a t e r a n d the w o r k i n g fluid, p e r h a p s a m m o n i a , w h e n t h e d i f f e r e n t i a l i n t e m p e r a t u r e is so s m a l l . T h e c o r r o s i v e a c t i o n of sea w a t e r m u s t also b e Downloaded by MONASH UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch005
c o n s i d e r e d i n e s t i m a t i n g cost a n d l i f e t i m e of s u c h a p o w e r p l a n t . A b s t r a c t i o n of heat f r o m the o c e a n w o u l d b e c o m p e n s a t e d f a c t t h a t the o p t i c a l a b s o r b a n c e
b y the
of c o l d w a t e r is greater t h a n t h a t of
w a r m e r w a t e r . T h u s there w o u l d b e a net increase i n the flow of h e a t f r o m the s u n to t h e earth's surface, w h e r e the e n e r g y is e v e n t u a l l y r e leased, b u t t h e l o c a l t e m p e r a t u r e of
t h e ocean
would
remain fairly
constant. W i t h a v e r t i c a l g r a d i e n t the u p w e l l i n g of d e e p e r w a t e r w o u l d c a r r y o r g a n i c nutrients t h a t m i g h t b e u s e d to f e e d p h y t o p l a n k t o n at t h e surface. A l s o , t h e system c o u l d b e a r r a n g e d to p r o v i d e f r e s h w a t e r , a c o m m o d i t y that m a y soon b e c o m e c r i t i c a l l y scarce i n F l o r i d a . Direct
Solar
Heating
T h e t o t a l flux at the earth's surface f r o m the s u n at its z e n i t h is a b o u t lkW/m
2
(6).
T h e a n n u a l m e a n flux i n the U n i t e d States is a b o u t 200
W / m . T h e c o n s u m p t i o n of p o w e r i n t h e U n i t e d States amounts to a b o u t 2
0.2 W / m . 2
T h u s a solar p o w e r p l a n t o p e r a t i n g at 5 %
o v e r a l l efficiency
w o u l d r e q u i r e a b o u t 1/50 of the country's surface, o r t h e average
size
of one state. R o o f t o p solar h e a t i n g of w a t e r i n i n d i v i d u a l houses a l r e a d y w i d e s p r e a d use, e s p e c i a l l y i n t r o p i c a l regions.
finds
E v e n i n the northeastern
U n i t e d States one c a n r e a d i l y p r o v i d e 2 0 % of the t o t a l d o m e s t i c r e q u i r e m e n t i n this w a y , at a cost c o m p e t i t i v e
power
w i t h electric heating
d e r i v e d f r o m the costlier fuels ( a b o u t t w i c e the cost of e n e r g y f r o m a nuclear or a coal b u r n i n g p l a n t ) . T h e p r a c t i c a l i t y of d i r e c t solar h e a t i n g d e p e n d s o n t h e
"greenhouse
effect." T h e glass of a greenhouse transmits n e a r l y a l l the solar r a d i a t i o n . W h e n a b s o r b e d b y the e a r t h , most of this r a d i a t i o n is c o n v e r t e d to heat, c o r r e s p o n d i n g to q u a n t a i n t h e i n f r a r e d . T h e r e - r a d i a t e d heat at these w a v e l e n g t h s is almost e n t i r e l y reflected o r a b s o r b e d b y the glass of t h e greenhouse,
a n d is t h e r e b y t r a p p e d i n s i d e e x c e p t for t h e p a r t t h a t is
r a d i a t e d f r o m the outer surface of the glass. A n i d e a l m a t e r i a l f o r t r a p -
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
SOLID STATE
96
CHEMISTRY
p i n g h e a t i n this w a y w o u l d t r a n s m i t a l l w a v e l e n g t h s of t h e solar s p e c t r u m b e l o w a b o u t 2000 n m a n d reflect a l l greater w a v e l e n g t h s .
This can be
a c h i e v e d w i t h a m u l t i l a y e r i n t e r f e r e n c e filter a n d c a n b e a p p r o x i m a t e d b y d e p o s i t i n g a t h i n film of s i l i c o n o n glass. H o w e v e r , t h e use of s u c h high-performance "greenhouse"
m a t e r i a l s entails u n s o l v e d p r o b l e m s
of
cost a n d l o n g t e r m s t a b i l i t y . T h i s t y p e of solar h e a t t r a p c a n r e a c h t e m p e r a t u r e s
approaching
3 0 0 ° C w i t h n o f o c u s i n g of t h e s u n . F o c u s i n g t h e s u n i n t o a l i n e a r i m a g e w i t h a b o u t t e n f o l d c o n c e n t r a t i o n of i n t e n s i t y c a n g i v e a b o u t 5 0 0 ° C a n d a n a p p r o x i m a t e p o i n t focus c a n g i v e t e m p e r a t u r e s
(7),
greater t h a n
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1 0 0 0 ° C . O f course the f o c u s i n g systems ( a r r a y s of m i r r o r s t h a t t r a c k the sun's m o v e m e n t )
e n t a i l g r e a t l y i n c r e a s e d costs. A l s o , t h e u n f o c u s e d sys
t e m c a n f u n c t i o n w i t h a c l o u d y sky. "greenhouse"
F i n a l l y , the p e r f o r m a n c e
material becomes poorer
of
the
as the stored t e m p e r a t u r e i n
creases, b e c a u s e t h e s p e c t r u m of t h e r m a l r a d i a t i o n is s h i f t e d to shorter w a v e l e n g t h s a n d overlaps m o r e w i t h t h e s p e c t r u m of s u n l i g h t . D e g r a d a t i o n of the m a t e r i a l s h o u l d also b e c o m e a greater p r o b l e m at the h i g h e r temperatures. T h e t r a p p e d solar h e a t c a n b e t r a n s f e r r e d to a l a r g e r e s e r v o i r b y means of a c i r c u l a t i n g fluid o r a "heat p i p e " w h i c h relies o n gaseous convection.
T h e r e s e r v o i r c a n t h e n d r i v e a n e n g i n e to p r o d u c e
t r i c i t y , h o p e f u l l y w i t h o v e r a l l efficiency t r i c i t y ) as great as 2 0 % . cally w i t h comparable
elec
( f r o m solar r a d i a t i o n to
elec
H y d r o g e n c o u l d then be p r o d u c e d electrolyti-
efficiency.
A l t e r n a t i v e l y , the heat c o u l d b e u s e d to d r i v e a c y c l e of c h e m i c a l reactions t h a t p r o d u c e
hydrogen and oxygen from water.
M a n y such
cycles h a v e b e e n c o n c e i v e d ; s o m e h a v e b e e n b r o u g h t to a s e m b l a n c e t e c h n i c a l f e a s i b i l i t y , a n d n o n e has b e e n
established on a large
A single e x a m p l e p r o p o s e d b y A b r a h a m a n d S c h r e i n e r ( 8 )
of
scale.
follows.
300°K LiN0
2
+ I
2
+ H 0 2
>LiN0
3
+
> I +
H
>L i N 0
2
2HI
700°K 2HI
2
2
750°K LiN0 Net :
3
H 0 2
> i0
2
+
+ H
\0
2
2
T h i s r e a c t i o n s e q u e n c e m i g h t b e d r i v e n at temperatures a t t a i n e d w i t h a l i n e focus of t h e s u n . A p r i m i t i v e b u t effective t y p e of solar heat c o l l e c t o r is the solar p o n d ( 6 ) . T h e p o n d s h o u l d b e a b o u t l m d e e p a n d 5 0 m or m o r e i n
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
5.
CLAYTON
Solar Energy
97
Conversion
d i a m e t e r , t o m i n i m i z e h e a t loss at the edges i n t o t h e e a r t h . H e a t t r a n s f e r r e d t o e a r t h t h r o u g h t h e b o t t o m is l a r g e l y r e t a i n e d a n d c a n b e r e t r i e v e d at n i g h t . T h e b o t t o m of the p o n d is b l a c k to m a x i m i z e a b s o r p t i o n , a n d t h e surface is c o v e r e d to r e t a r d e v a p o r a t i o n a n d c o n s e q u e n t c o o l i n g . A n i n g e n i o u s a p p r o a c h ( 9 ) is to use b r i n e c o v e r e d b y a s h a l l o w l a y e r of f r e s h w a t e r . T h e surface l a y e r r e m a i n s c o o l b u t does n o t m i x w i t h t h e denser b r i n e b e l o w .
D a y t i m e t e m p e r a t u r e s at t h e b o t t o m c o u l d r e a c h
about 100°C. I n c o n c l u s i o n , d i r e c t solar h e a t i n g appears t o b e e s p e c i a l l y f a v o r a b l e as a m e t h o d f o r large-scale solar energy c o n v e r s i o n , w i t h m u c h of t h e Downloaded by MONASH UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch005
technology already proven. Y aviations
of
Agriculture
T h e o v e r a l l efficiency w i t h w h i c h solar e n e r g y is s t o r e d as o r g a n i c matter (mostly carbohydrate)
i n crops a n d forests is i n t h e r a n g e
0 . 1 - 0 . 4 % for " C " p l a n t s [those t h a t use t h e C a l v i n - B e n s o n c y c l e 3
of (10)
f o r c a r b o n a s s i m i l a t i o n ] , a n d a b o u t t w i c e t h a t f o r t r o p i c a l grasses a n d o t h e r " C " p l a n t s , u s i n g t h e H a t c h - S l a c k (11) 4
p a t h w a y for
fixing
C0 . 2
I n t h e latter category sugarcane c a n s h o w a n a n n u a l y i e l d of 5 0 tons d r y w e i g h t p e r acre a n d o v e r a l l efficiency a b o u t 2 . 5 % f o r s t o r i n g the e n e r g y of s u n l i g h t i n o r g a n i c m a t t e r , a b o u t h a l f of w h i c h is sucrose. T h e s u g a r c a n b e f e r m e n t e d to e t h y l a l c o h o l at a n e s t i m a t e d cost less t h a n $1 p e r g a l l o n (12).
T h e same c o u l d b e d o n e w i t h o t h e r r e a d y sources of c a r b o
h y d r a t e , s o m e i n the category of w a s t e , s u c h as s p o i l e d g r a i n i n o u r m a j o r w h e a t f a r m i n g areas. P l a n t m a t e r i a l s as w e l l as o r g a n i c wastes c a n also b e c o n v e r t e d to fuels b y o t h e r means ( 2 ) .
P y r o l y s i s , effected b y a n a e r o b i c h e a t i n g to
a b o u t 5 0 0 ° C at a t m o s p h e r i c pressure, p r o d u c e s a m i x t u r e of c r u d e o i l , l o w - g r a d e c o m b u s t i b l e gas, a n d " c h a r . " T h e gas a n d c h a r c a n b e b u r n e d to m a i n t a i n the h i g h t e m p e r a t u r e a n d some of the gas r e c y c l e d to p r e serve a n a n a e r o b i c atmosphere. A m o r e efficient process, b u t t e c h n i c a l l y m o r e difficult, is hydrogénation, c o n d u c t e d at a l o w e r t e m p e r a t u r e ( a b o u t 3 0 0 ° C ) b u t at ~ 200 a t m . C a r b o n m o n o x i d e a n d steam are i n t r o d u c e d to p r o v i d e a r e d u c i n g e n v i r o n m e n t ; t h e o r g a n i c m a t t e r is c o n v e r t e d a l m o s t e n t i r e l y to c r u d e o i l . A t h i r d a l t e r n a t i v e is a n a e r o b i c b a c t e r i a l f e r m e n t a t i o n . T h i s a v o i d s the use of h i g h pressures o r t e m p e r a t u r e s a n d gives p r i n c i p a l l y a v e r y d e s i r a b l e f u e l , m e t h a n e . A d r a w b a c k of b a c t e r i a l f e r m e n t a t i o n is t h a t a b o u t 4 0 % of the o r g a n i c m a t t e r is left as a r e s i d u a l s l u d g e t h a t m u s t b e d i s p o s e d of or t r e a t e d b y one of t h e other m e t h o d s . I f a l l t h e r e a d i l y c o l l e c t e d w a s t e w e r e p r o c e s s e d i n o n e of these w a y s , we could provide 3 %
of o u r present o i l use, or 6 %
of the o u t p u t of
present p o w e r stations. T h e s e schemes as a p p l i e d to w a s t e are c o m -
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
SOLID S T A T E
98
CHEMISTRY
m e r c i a l l y a d v a n t a g e o u s n o w , w h e n t h e cost of a l t e r n a t i v e d i s p o s a l ( l a n d fill)
has b e e n d i s c o u n t e d . W h e n these schemes are a p p l i e d to p r o d u c t s of
a g r i c u l t u r e , t h e c o n v e r s i o n to f u e l m u s t c o m p e t e w i t h use as sources of f o o d , fiber, a n d c h e m i c a l s .
H o w e v e r , w i t h some p l a n t s t h e c o n v e r s i o n
to f u e l is c l e a r l y a d v a n t a g e o u s .
O n e s u c h p l a n t is t h e w a t e r h y a c i n t h ,
w h i c h g r o w s so p r o l i f i c a l l y i n s h a l l o w t r o p i c a l waters that i t is a n u i s a n c e to n a v i g a t i o n a n d at the same t i m e is easy to harvest.
Its w h o l e s a l e
r e m o v a l m i g h t , h o w e v e r , t h r e a t e n the e x t i n c t i o n of some a n i m a l s , s u c h as t h e manatee. A l g a e a n d photosynthetic bacteria comprise a special category i n
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" e n e r g y f a r m i n g " ( 1 3 ) . I n p o n d s , w i t h t h e i r g r o w t h assisted b y o r g a n i c wastes a n d w i t h a d e q u a t e s u n l i g h t , algae c a n y i e l d i n a f e w d a y s w h a t a c a n e field c a n y i e l d i n a n e n t i r e c r o p , w i t h a b o u t 4 % of t h e solar e n e r g y s t o r e d as o r g a n i c m a t t e r , m a i n l y l i p i d s a n d p r o t e i n .
( U n l i k e t h e algae,
t h e p h o t o s y n t h e t i c b a c t e r i a cannot g r o w w i t h o u t a source of
reduced
c o m p o u n d s , as t h e y c a n n o t use w a t e r as a source of h y d r o g e n f o r t h e fixation
of c a r b o n d i o x i d e . A great v a r i e t y of o r g a n i c c o m p o u n d s
serve this p u r p o s e . )
can
A one-acre " w a s t e p l u s a l g a e " p o n d , w i t h the h a r
vest c o n v e r t e d to f u e l a n d t h e n c e t o e l e c t r i c i t y , m i g h t g i v e a b o u t 15 k w at a b o u t one cent p e r k i l o w a t t h o u r . C a t t l e f e e d lots p r o v i d e a l a r g e c o n c e n t r a t e d source of w a s t e t h a t c a n b e c o u p l e d to a n algae f a r m . S o m e of t h e algae m i g h t b e f e d to t h e cattle. A n a e r o b i c f e r m e n t a t i o n of the algae w o u l d y i e l d m e t h a n e , s l u d g e , a n d some b y - p r o d u c t s ( C 0 , N , a n d P ) t h a t c o u l d b e r e c y c l e d to t h e 2
algae. T h e a m o u n t of e a s i l y c o l l e c t e d w a s t e is n o t e n o u g h to p r o v i d e t h e greater p a r t of o u r needs f o r f u e l a n d e n e r g y b y these m e a n s , b u t t h e greatest p o s s i b l e d e v e l o p m e n t of the f o r e g o i n g systems is c l e a r l y a d v a n tageous a n d e c o n o m i c a l l y s o u n d . M u c h of t h e l a n d a n d m o n e y r e q u i r e d f o r s u c h projects w o u l d b e n e e d e d f o r w a s t e d i s p o s a l i n a n y case. 'Photosynthetic Hydrogen
Production
A f a r m o r e v i s i o n a r y v a r i a t i o n of a g r i c u l t u r e is f o u n d i n schemes, p r e s e n t l y b e i n g e x p l o r e d , to r e d i r e c t t h e p h o t o s y n t h e t i c process to y i e l d h y d r o g e n i n s t e a d of c a r b o h y d r a t e . Photosynthesis i n g r e e n p l a n t s a n d algae i n v o l v e s t w o d i s t i n c t p h o t o c h e m i c a l systems (14).
I n one of these, c a l l e d s y s t e m 2, a q u a n t u m of
light energy absorbed by chlorophyll induces a photochemical o x i d a t i o n r e d u c t i o n r e a c t i o n w h i c h generates a s t r o n g o x i d a n t a n d a w e a k r e d u c t a n t . T h e o x i d a n t is s t r o n g e n o u g h to r e m o v e electrons f r o m w a t e r , r e l e a s i n g o x y g e n f r o m t h e w a t e r . T h e r e d u c t a n t feeds electrons, t h r o u g h a c h a i n of e l e c t r o n t r a n s p o r t i n g m o l e c u l e s , to t h e s e c o n d p h o t o c h e m i c a l s y s t e m (system 1).
I n s y s t e m 1 a p h o t o c h e m i c a l process, a g a i n d r i v e n b y t h e
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
5.
CLAYTON
Solar Energy
Conversion
99
energy of l i g h t a b s o r b e d b y c h l o r o p h y l l , generates a s t r o n g r e d u c t a n t a n d a w e a k o x i d a n t . T h e w e a k o x i d a n t is n e u t r a l i z e d b y the electrons flowing
f r o m system 2. T h e s t r o n g r e d u c t a n t p r o d u c e d b y s y s t e m 1 is
s t o r e d i n i t i a l l y i n the f o r m of r e d u c e d f e r r e d o x i n , a n i r o n - c o n t a i n i n g p r o t e i n t h a t c a n i n t u r n r e d u c e other substances. the r e d u c t i o n of C 0
2
T h e n o r m a l e n d r e s u l t is
to c a r b o h y d r a t e . T h e p l a n of schemes f o r p r o d u c
i n g h y d r o g e n t h r o u g h photosynthesis is to a l t e r the n o r m a l u t i l i z a t i o n of r e d u c e d f e r r e d o x i n . I n s t e a d of flowing to the e n z y m e s t h a t c a t a l y z e C 0 fixation,
t h e electrons f r o m r e d u c e d f e r r e d o x i n cause r e d u c t i o n of H
2 +
ions to H :
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2
2H
+ 2e"
+
H
2
T h i s r e a c t i o n is c a t a l y z e d b y e i t h e r of t w o e n z y m e s t h a t o c c u r i n m a n y algae a n d b a c t e r i a : h y d r o g e n a s e a n d n i t r o g e n a s e
(15).
H y d r o g e n a s e is f o u n d i n g r e e n algae a n d i n some b a c t e r i a . I n the algae its n a t u r a l f u n c t i o n m a y b e to rid the o r g a n i s m of excess r e d u c t a n t s that c a n arise u n d e r c e r t a i n c o n d i t i o n s , t h e r e b y r e s t o r i n g a d e s i r a b l e b a l a n c e b e t w e e n o x i d i z e d a n d r e d u c e d states of t h e c h a i n of carriers
electron
(16).
Nitrogenase, f o u n d i n blue-green algae a n d i n b o t h photosynthetic a n d some n o n - p h o t o s y n t h e t i c b a c t e r i a , has the p r i m a r y f u n c t i o n of n i t r o gen
fixation,
reducing N
to a m m o n i a . H o w e v e r , this e n z y m e c a n also
2
m e d i a t e t h e r e d u c t i o n of H
+
to H . Its a c t i v i t y r e q u i r e s a d e n o s i n e t r i 2
p h o s p h a t e a n d is i n h i b i t e d b y N ; h y d r o g e n a s e is n o t subject to these 2
complications. B o t h h y d r o g e n a s e a n d nitrogenase are i n h i b i t e d b y o x y g e n , w h i c h i n t r o d u c e s a serious p r o b l e m since the photosynthesis of g r e e n p l a n t s generates o x y g e n a l o n g w i t h r e d u c i n g p o w e r .
I n fact, the
hydrogenase
a c t i v i t y of algae w a s d i s c o v e r e d i n c u l t u r e s t h a t h a d b e e n k e p t i n the d a r k u n d e r a n a e r o b i c c o n d i t i o n s f o r s e v e r a l h o u r s ( 17).
U p o n illumina
t i o n , t h e h y d r o g e n a s e a c t v i t y d e c l i n e d as the a l g a e b e g a n t o oxygen.
produce
N o one has f o u n d c o n d i t i o n s u n d e r w h i c h i n t a c t algae p r o d u c e
h y d r o g e n a n d o x y g e n c o n c o m i t a n t l y at rates e x c e e d i n g a m i n i s c u l e f r a c t i o n of the n o r m a l rate of photosynthesis
(18).
W e are thus l e d to c o n s i d e r schemes, p r o p o s e d b y K r a m p i t z a n d others (15,
19),
i n w h i c h t h e p l a n t cells a r e b r o k e n a n d t h e i r p h o t o
c h e m i c a l c o m p o n e n t s are d i s s e c t e d a n d r e a r r a n g e d t o p r o d u c e h y d r o g e n w h i l e s h i e l d i n g t h e h y d r o g e n a s e o r nitrogenase f r o m o x y g e n .
T h e r e are
s e v e r a l reasons w h y the o x y g e n - e v o l v i n g s y s t e m m u s t b e k e p t separate f r o m the r e d u c t a n t s a n d catalysts t h a t p r o d u c e h y d r o g e n . genase a n d nitrogenase are sensitive to o x y g e n .
First, hydro
S e c o n d , the r e d u c t a n t s
m u s t n o t b e a l l o w e d to r e a c t w a s t e f u l l y w i t h o x y g e n b e f o r e t h e y c a n d r i v e t h e c o n v e r s i o n of H
+
t o H . T h i s a p p l i e s to n a t u r a l r e d u c e d p r o d 2
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
100
SOLID STATE
CHEMISTRY
u c t s a n d also a r t i f i c i a l ones t h a t m i g h t u s e f u l l y b e i n t e r p o s e d t o l i n k the r e d u c i n g side of system 1 to t h e e v o l u t i o n of h y d r o g e n . T h i r d , the o x y g e n e v o l v i n g system m u s t b e s h i e l d e d f r o m the s t r o n g r e d u c t a n t s w h i c h c o u l d o t h e r w i s e p r e v e n t the o p e r a t i o n of s y s t e m 2.
F i n a l l y , the o x y g e n a n d
h y d r o g e n m u s t b e s e p a r a t e d i f t h e h y d r o g e n is t o b e s t o r e d a n d u s e d . T h e a l t e r n a t i v e , b u r n i n g the e v o l v e d h y d r o g e n i m m e d i a t e l y w i t h
the
e v o l v e d o x y g e n , w o u l d b e h a z a r d o u s o n a l a r g e scale a n d w o u l d r e s t r i c t t h e options f o r use of t h e h y d r o g e n . T h r e e examples c a n b e c i t e d of p h o t o s y n t h e t i c systems i n w h i c h t h e e v o l u t i o n of h y d r o g e n is c o u p l e d w i t h t h e release of o x y g e n f r o m w a t e r . Downloaded by MONASH UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch005
(a)
Chloroplasts from
s p i n a c h leaves
are m i x e d w i t h
bacterial
h y d r o g e n a s e , a n d the i n h i b i t o r y a c t i o n of o x y g e n is m i n i m i z e d w i t h a n o x y g e n - a b s o r b i n g system c o m p o s e d of glucose p l u s glucose oxidase
(20).
H y d r o g e n is t h e n e v o l v e d as a p r o d u c t of photosynthesis. (b)
The
filamentous
a l g a Anabena
has cells w h i c h p e r
cylindrica
f o r m photosynthesis, a n d other cells ( h e t e r o c y s t s )
t h a t are s p e c i a l i z e d
f o r n i t r o g e n fixation. R e s p i r a t i o n i n t h e heterocysts keeps the c o n c e n t r a t i o n of o x y g e n l o w i n those cells. R e d u c e d p r o d u c t s c a n diffuse f r o m t h e p h o t o s y n t h e t i c cells i n t o the heterocysts, w h e r e t h e y p r o m o t e e v o l u t i o n u s i n g t h e nitrogenase as c a t a l y s t (21, (c)
T h e w a t e r f e r n Azolla
fives
hydrogen
22).
symbiotically w i t h nitrogen-fixing
b l u e - g r e e n algae. W h e n p r o v i d e d w i t h n i t r a t e a n d s h i e l d e d f r o m n i t r o g e n , the system g r o w s , a n d h y d r o g e n evolves
(23).
I n n o n e o f these systems is the r a t e of h y d r o g e n e v o l u t i o n m o r e t h a n a s m a l l f r a c t i o n , c a . 0 . 2 - 3 % , of " n o r m a l " photosynthesis. S o m e c u r r e n t efforts a n d p r o p o s a l s t o i m p r o v e t h i s a p p r o a c h 19, 24)
w o u l d b e g i n w i t h t h e i s o l a t i o n of t h e essential c o m p o n e n t s
tems 1 a n d 2 a n d h y d r o g e n a s e )
(15, (sys
f r o m t h e i r n a t i v e tissues. T h e c o m p o
nents c a n b e k e p t separate b y means of s e m i p e r m e a b l e m e m b r a n e s , b y a d s o r p t i o n o n s o l i d p a r t i c l e s , or b y m i c r o e n c a p s u l a t i o n . F u n c t i o n a l c o m m u n i c a t i o n b e t w e e n t h e c o m p o n e n t s is m a i n t a i n e d b y d i f f u s i b l e e l e c t r o n carriers i n aqueous m e d i a .
S o m e of the necessary b u t f r a g i l e e n z y m e
systems m i g h t b e s t a b i l i z e d b y c r o s s - l i n k i n g t h e i r p r o t e i n s
internally
t h r o u g h g l u t a r a l d e h y d e fixation, o r b y a d s o r p t i o n o r m i c r o e n c a p s u l a t i o n . Obviously
these
approaches
raise difficult problems,
especially
with
r e g a r d to s t a b i l i t y , b u t one c a n at least e n v i s i o n a c o m p l e t e f u n c t i o n a l s y s t e m w i t h i n the b o u n d s of present t e c h n o l o g y . W e h a v e y e t to c o n s i d e r questions of efficiency a n d r a t e i n p h o t o s y n t h e t i c systems t h a t p r o d u c e h y d r o g e n .
O f a l l the solar e n e r g y t h a t
falls o n a leaf o r a d e n s e c u l t u r e of algae, o n l y the p a r t w i t h w a v e l e n g t h s b e l o w 680 n m is a b s o r b e d b y c h l o r o p h y l l a n d o t h e r p i g m e n t s .
O f the
r a d i a t i o n t h a t is a b s o r b e d , t h e q u a n t a of shorter w a v e l e n g t h s h a v e r e l a -
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
5.
CLAYTON
Solar Energy
101
Conversion
t i v e l y m o r e energy, b u t this is q u i c k l y d e g r a d e d to the l e v e l of the l o n g w a v e a b s o r p t i o n b a n d of c h l o r o p h y l l n e a r 680 n m .
This degradation,
w i t h the excess e n e r g y d i s s i p a t e d as heat, h a p p e n s before t h e l i g h t e n e r g y can be used for photochemistry.
Because
of the c o m b i n e d
effects
of
n o n - a b s o r p t i o n at w a v e l e n g t h s b e y o n d 680 n m a n d d e g r a d a t i o n of e n e r g y a b s o r b e d at shorter w a v e l e n g t h s , a b o u t 4 0 % of the i n c i d e n t solar e n e r g y b e c o m e s a v a i l a b l e for p h o t o c h e m i s t r y i n t h e f o r m of 680 n m q u a n t a . A t 680 n m e a c h q u a n t u m has a n energy of 1.8 e V . W h e n systems 1 a n d 2 c o o p e r a t e to r e m o v e a n e l e c t r o n f r o m w a t e r a n d p r o m o t e i t to the l e v e l of h y d r o g e n , a n energy of 1.2 e V is stored (14, 2 5 ) .
This requires an i n p u t
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of t w o q u a n t a , one i n e a c h system, or a t o t a l i n p u t of 3.6 e V . T h e m a x i mum
efficiency t h e o r e t i c a l l y a t t a i n a b l e i n this w a y , f o r t h e storage
solar energy as h y d r o g e n w i t h the c o n c o m i t a n t release of o x y g e n w a t e r , is therefore ( 1 . 2 / 3 . 6 ) X 4 0 % or 1 3 % .
I n p r a c t i c e one w o u l d d o
w e l l to r e a l i z e 5 % , a n d i f the h y d r o g e n w e r e b u r n e d to p r o d u c e t r i c i t y , a n o v e r a l l efficiency of 2 %
of
from elec
m i g h t b e a t t a i n e d . T h i s is close to
the efficiency of g r o w i n g sugarcane, b u t e l e c t r i c i t y is a f a r m o r e d e s i r a b l e e n d p r o d u c t t h a n sugar. W e w o u l d n e e d to c o m m i t 1 / 1 0 of the area o f t h e c o u n t r y , o n l a n d o r i n the n e i g h b o r i n g ocean, to satisfy o u r t o t a l e n e r g y r e q u i r e m e n t f r o m s u n l i g h t at a n o v e r a l l efficiency of
2%.
A n o t h e r p r o b l e m has to d o w i t h t h e rate at w h i c h a p h o t o s y n t h e t i c system c a n k e e p p a c e w i t h i n c o m i n g l i g h t energy. t h e p h o t o s y n t h e t i c tissue is s u c h (14)
T h e a r c h i t e c t u r e of
that e a c h p h o t o c h e m i c a l r e a c t i o n
center, w h e t h e r of s y s t e m 2 o r system 1, is s e r v e d b y a b o u t 200 m o l e c u l e s of ' l i g h t h a r v e s t i n g " or " a n t e n n a " c h l o r o p h y l l . T h e a n t e n n a c h l o r o p h y l l m o l e c u l e s d o not p a r t i c i p a t e d i r e c t l y i n the p h o t o c h e m i s t r y ; t h e y a b s o r b l i g h t a n d d e l i v e r the energy to t h e r e a c t i o n centers.
If every q u a n t u m
a b s o r b e d b y the a n t e n n a i n f u l l s u n l i g h t w e r e u s e d p h o t o c h e m i c a l l y at the r e a c t i o n centers, electrons w o u l d b e
flowing
t h r o u g h the
complete
c h a i n , f r o m w a t e r to f e r r e d o x i n , at a rate of a p p r o x i m a t e l y 2000 p e r sec. I n fact t h e e l e c t r o n t r a n s p o r t c h a i n c a n t r a n s p o r t n o m o r e t h a n a b o u t 200 electrons p e r sec, so i n f u l l s u n l i g h t o n l y 1 / 1 0 of t h e i n c o m i n g q u a n t a c a n b e u s e d . T h e p r o s p e c t of i n c r e a s i n g the rate at w h i c h the e l e c t r o n t r a n s p o r t system c a n operate is l i m i t e d , b u t t h e r e is a better w a y to solve this p r o b l e m ( 16).
I f e a c h r e a c t i o n center w e r e s e r v e d b y a n a n t e n n a of
o n l y 20 c h l o r o p h y l l m o l e c u l e s r a t h e r t h a n 200, q u a n t a w o u l d b e d e l i v e r e d to the r e a c t i o n centers at a rate of 2 0 0 / s e c i n f u l l s u n l i g h t , r a t h e r t h a n 2 0 0 0 / s e c . T h e e l e c t r o n t r a n s p o r t m a c h i n e r y c o u l d t h e n k e e p pace.
The
o n l y q u a l i f i c a t i o n is t h a t t h e system b e o p t i c a l l y dense e n o u g h to a b s o r b most of t h e i n c i d e n t q u a n t a at w a v e l e n g t h s b e l o w 680 n m . I t s h o u l d n o t b e difficult to delete most of the a n t e n n a c h l o r o p h y l l s f r o m a n i n v i t r o p r e p a r a t i o n , or to start w i t h m u t a n t p l a n t s o r algae t h a t h a v e a n a b n o r m a l l y l o w r a t i o of a n t e n n a c h l o r o p h y l l s t o r e a c t i o n centers.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
102
SOLID STATE
CHEMISTRY
T h e v i s i o n of g e n e r a t i n g h y d r o g e n b y photosynthesis w i t h r e a s o n a b l e efficiency is m o s t a t t r a c t i v e . T h e r e a r e f o r m i d a b l e p r o b l e m s t o b e o v e r c o m e , b u t efforts t o solve t h e m h a v e just b e g u n . Photoelectric
Devices
T h e d i r e c t c o n v e r s i o n of solar e n e r g y t o e l e c t r i c i t y has great p o t e n t i a l advantages of h i g h efficiency, m i n i m a l m a c h i n e r y , a n d
flexible
use
of t h e p r o d u c t . I n s p e c i a l a p p l i c a t i o n s s u c h as the p r o v i s i o n of p o w e r to i n s t r u m e n t s o n spacecraft, w e a l r e a d y use p h o t o v o l t a i c s i l i c o n cells.
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C o n t e m p o r a r y solar batteries are s e m i c o n d u c t i v e devices t h a t o p e r ate i n t h e m a n n e r suggested b y F i g u r e 1.
L i g h t promotes a n electron
i n t h e m a t e r i a l f r o m a g r o u n d state ( v a l e n c e b a n d ) to a n e x c i t e d state, leaving a n electron vacancy or hole
( - f ) i n the valence band.
e x c i t e d e l e c t r o n loses s o m e e n e r g y a n d enters a c o n t i n u u m of
The
excited
states, the c o n d u c t i o n b a n d . T h e c e l l is d i v i d e d b y a j u n c t i o n i n t o t w o regions of different c o m p o s i t i o n ; i n one r e g i o n t h e e l e c t r o n has a h i g h m o b i l i t y a n d i n t h e other the m o b i l i t y of t h e h o l e is h i g h . T h e difference i n c o m p o s i t i o n is d e s i g n e d t o shift t h e energies of the v a l e n c e a n d c o n ELECTRON
ENERGYο CD
