8 Wavelength-Selective Surfaces JOHN C. C. FAN
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Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Mass. 02173
Spectrally selective coatings have potentially important applications in solar/thermal/electric conversion, solar heating, and window insulation. These coatings can be divided into two classes: transparent heat mirrors and selective-black absorbers. Transparent heat mirrors transmit solar radiation and reflect thermal radiation; selective absorbers absorb solar radiation and have low infrared emissivity. We have prepared both transparent heat-mirror films (TiO /Ag/TiO2, Sn-doped In O , and Sn-doped In O microgrids), and cermet absorbers (MgO/Au) that have excellent wavelength-selective properties. In addition, the cermets promise to be stable at the elevated temperatures required of the absorbers. 2
2
3
2
3
n p e r r e s t r i a l solar r a d i a t i o n is a l o w - i n t e n s i t y , v a r i a b l e - e n e r g y source a r r i v i n g at a b o u t 800 W / m . 2
Its e c o n o m i c f e a s i b i l i t y d e p e n d s o n
efficient c o l l e c t i o n , c o n v e r s i o n , a n d storage. T h i s p a p e r concentrates o n the use of w a v e l e n g t h - s e l e c t i v e surfaces to i m p r o v e t h e efficiency of solare n e r g y collectors a n d to p r o v i d e i n s u l a t i o n to d o m e s t i c w i n d o w s . F i g u r e 1 illustrates t h e b a s i c p r i n c i p l e of a flat-plate solar collector. It consists of a t r a n s p a r e n t c o v e r p l a t e a n d a n absorber s e p a r a t e d b y a v a c u u m or a gas. Solar r a d i a t i o n is t r a n s m i t t e d t h r o u g h t h e c o v e r p l a t e a n d c o n v e r t e d b y t h e absorber i n t o t h e r m a l energy, p a r t of w h i c h is t r a n s f e r r e d to a w o r k i n g fluid s u c h as w a t e r o r a i r , a n d p a r t of w h i c h is lost.
F o r efficient o p e r a t i o n t h e r m a l losses f r o m t h e h e a t e d
absorber
m u s t b e r e d u c e d . T h e s e losses c a n o c c u r b y c o n d u c t i o n , c o n v e c t i o n , a n d infrared radiation.
W a v e l e n g t h - s e l e c t i v e surfaces r e d u c e t h e r a d i a t i o n
losses b y t a k i n g a d v a n t a g e of t h e s p e c t r a l s e p a r a t i o n of solar r a d i a t i o n a n d t h e t h e r m a l r a d i a t i o n e m i t t e d b y objects at t e r r e s t r i a l t e m p e r a t u r e s . F i g u r e 2 shows s c h e m a t i c a l l y t h e a b s o r b e r a n d c o v e r p l a t e s e p a r a t e d b y a i r . R a d i a t i o n losses, w h e n n o w a v e l e n g t h - s e l e c t i v e surfaces are u s e d , 149
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
150
SOLID STATE Transparent
CHEMISTRY
/
Coverplate „
Heated Fluid
Absorber
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Insulation
Figure 1. are c o m p a r a b l e
Schematic of a flat-plate solar collector
w i t h those b y c o n v e c t i o n
a n d conduction.
Traditional
strategy reduces t h e r a d i a t i o n losses w i t h a w a v e l e n g t h - s e l e c t i v e
absorber
that has h i g h solar a b s o r p t i v i t y b u t l o w i n f r a r e d e m i s s i v i t y . A n alternate strategy uses a t r a n s p a r e n t h e a t - m i r r o r film c o a t e d o n t h e i n s i d e of t h e coverplate.
These wavelength-selective
films
t r a n s m i t solar r a d i a t i o n t o
t h e absorber b u t reflect t h e i n f r a r e d r a d i a t i o n e m i t t e d b y t h e absorber b a c k t o t h e absorber.
T h e r m a l s t a b i l i t y of t h e heat m i r r o r is easier t o
a c h i e v e since i t r e m a i n s at a m u c h l o w e r t e m p e r a t u r e t h a n t h e absorber. T h e o p t i c a l K i r c h o f F s l a w states t h a t t h e s u m of t r a n s m i s s i o n , reflec t i v i t y , a n d a b s o r p t i v i t y m u s t e q u a l one. T h e r e f o r e , f o r a heat m i r r o r a h i g h solar t r a n s m i s s i o n r e q u i r e s a l o w reflectivity a n d a b s o r p t i v i t y i n t h e solar s p e c t r u m .
H e a t m i r r o r s m u s t also h a v e h i g h i n f r a r e d r e f l e c t i v i t y
a n d h e n c e a l o w t r a n s m i s s i o n a n d a b s o r p t i v i t y i n t h e i n f r a r e d . F o r selec t i v e - b l a c k absorbers a h i g h solar a b s o r p t i v i t y r e q u i r e s a l o w r e f l e c t i v i t y a n d t r a n s m i s s i o n i n t h e solar s p e c t r u m .
T h e absorbers m u s t also
have
l o w i n f r a r e d e m i s s i v i t y t o e m i t m i n i m a l heat. A p e r f e c t e m i t t e r is a p e r fect absorber; a l o w i n f r a r e d e m i s s i v i t y m e a n s a l o w i n f r a r e d a b s o r p t i v i t y .
13 T
C
Air 4 Radiation
Figure
2.
Convection
Λ Conduction
Mechanisms of heat loss in a flat-plate collector
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
8.
FAN
Wavelength-Selective
151
Surfaces
S i n c e absorbers are n o r m a l l y o p a q u e ( a n d h e n c e h a v e n o t r a n s m i s s i o n ) , b y K i r c h o f f s l a w a l o w i n f r a r e d a b s o r p t i v i t y is o b t a i n e d b y a h i g h i n f r a r e d reflectivity. T h e r e f o r e b o t h the a b s o r b e r a n d t h e t r a n s p a r e n t heat m i r r o r s h o u l d h a v e h i g h i n f r a r e d reflectivity. I t is difficult to t a i l o r s i m u l t a n e o u s l y b o t h t h e solar a n d t h e i n f r a r e d p r o p e r t i e s of a film. Nevertheless t h e i n f r a r e d losses f r o m a t y p i c a l s o l a r collector can be reduced w e l l b e l o w conduction a n d convection b y e i t h e r transparent heat m i r r o r s or s e l e c t i v e - b l a c k absorbers. the conduction
a n d convection
losses
Because
losses are t h e n d o m i n a n t , i t is a d v a n
tageous to h a v e heat m i r r o r s w i t h a h i g h solar t r a n s m i s s i o n o r absorbers Downloaded by MONASH UNIV on November 23, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch008
w i t h a h i g h solar a b s o r p t i v i t y , e v e n at the expense of s o m e r e d u c t i o n i n t h e i n f r a r e d reflectivity.
H o w e v e r , i f the collectors are e v a c u a t e d ,
the
r a d i a t i o n losses are d o m i n a n t , a n d a h i g h i n f r a r e d r e f l e c t i v i t y b e c o m e s important.
I n this case, the solar t r a n s m i s s i o n o r a b s o r p t i v i t y c a n
be
somewhat lower. F i g u r e 3 shows the n o r m a l i z e d solar r a d i a t i o n of a i r mass 2 a n d the b l a c k b o d y r a d i a t i o n f r o m a h e a t e d b o d y at a t e m p e r a t u r e of 6 0 0 ° K .
The
t w o s p e c t r a h a r d l y o v e r l a p , a n d a selective surface h a v i n g a n i d e a l i z e d reflectivity s p e c t r u m ( d o t t e d l i n e i n the
figure)
w o u l d p r o d u c e almost
t o t a l w a v e l e n g t h separation. I d e a l transparent heat m i r r o r s a n d selectiveb l a c k surfaces w o u l d b o t h h a v e this reflectivity s p e c t r u m . H o w e v e r , the heat m i r r o r s w o u l d
t r a n s m i t solar r a d i a t i o n , a n d the
selective-black
absorbers w o u l d a b s o r b solar r a d i a t i o n .
WAVELENGTH
(am)
Figure 3. Normalized spectra for solar radiation of air mass 2 and for the radiation from a bfockbody at 600°K. Dashed line represents the reflectivity spectrum for an idealized selective surface.
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
152
SOLID STATE
Transparent
Heat
CHEMISTRY
Mirrors
T r a n s p a r e n t heat m i r r o r s c a n b e u s e d i n flat-plate collectors for solar h e a t i n g a n d c o o l i n g of b u i l d i n g s , w i t h receivers of c o n c e n t r a t e d s u n l i g h t f o r s o l a r / t h e r m a l / e l e c t r i c c o n v e r s i o n , a n d o n w i n d o w s as t r a n s p a r e n t t h e r m a l i n s u l a t i o n for e n e r g y c o n s e r v a t i o n a n d for heat s h i e l d s . T a b l e I shows a n estimate of e n e r g y savings i f h e a t - m i r r o r films are c o a t e d o n w i n d o w panes. T h e c a l c u l a t i o n s are b a s e d o n a 9 0 - d a y h e a t i n g p e r i o d p e r y e a r , a n average t e m p e r a t u r e difference of 20 ° C
between
i n s i d e a n d outside, a n d a 1 0 - m p h w i n d . A s i n g l e glass p a n e w o u l d lose
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a n n u a l l y a b o u t $2.75 w o r t h of heat p e r s q u a r e m e t e r .
I f a heat m i r r o r
r e f l e c t i n g 9 0 % of the t h e r m a l r a d i a t i o n w e r e c o a t e d o n this s i n g l e glass p a n e , the heat losses w o u l d b e c u t i n h a l f . T h e heat losses w o u l d b e e q u i v a l e n t t o those of u n c o a t e d double-glass panes s e p a r a t e d b y a n a i r g a p of a b o u t t w o inches, as i n t h e s t o r m - w i n d o w c o n f i g u r a t i o n . I f d o u b l e panes are u s e d , a h e a t - m i r r o r film o n t h e i n s i d e surface of t h e i n s i d e p a n e w o u l d r e d u c e the heat loss to o n l y a b o u t 9 O 0 / m / y e a r . 2
Optimal per
f o r m a n c e r e q u i r e s , i n a l l cases, p l a c i n g the heat m i r r o r o n the surface f a c i n g the i n s i d e of the b u i l d i n g . P l a c e m e n t of the film o n the i n s i d e surface of the o u t s i d e p a n e is a l i t t l e less effective, b u t h e a t - m i r r o r films o n the i n s i d e surfaces of b o t h i n s i d e a n d o u t s i d e panes c a n r e d u c e t h e heat loss to a b o u t 6 O 0 / m / y e a r . 2
A c c o r d i n g to a s t u d y p u b l i s h e d b y the
A m e r i c a n I n s t i t u t e of P h y s i c s ( I ) , ca. 4 - 5 %
of the n a t i o n a l e n e r g y c o n
s u m p t i o n is lost t h r o u g h the w i n d o w s i n r e s i d e n t i a l a n d c o m m e r c i a l b u i l d i n g s . T r a n s p a r e n t heat m i r r o r s c o u l d save 1 - 2 % of t h e t o t a l n a t i o n a l energy consumption. T r a n s p a r e n t heat m i r r o r s c a n b e d i v i d e d i n t o three t y p e s : a m u l t i l a y e r film c o n s i s t i n g of a m e t a l film s a n d w i c h e d b e t w e e n t r a n s p a r e n t d i e l e c t r i c layers, a s i n g l e - l a y e r t r a n s p a r e n t c o n d u c t o r h a v i n g a c o n t r o l l e d c o n c e n t r a t i o n of c h a r g e c a r r i e r s , a n d a c o n d u c t i n g m i c r o g r i d . T h e b a s i c p r i n c i p l e of t h e m u l t i l a y e r films f o l l o w s f r o m K i r c h o f F s law.
I t is w e l l k n o w n t h a t metals h a v e h i g h i n f r a r e d reflectivity. U n f o r Table I.
Estimate of Savings with Heat Mirrors on Domestic Windows 0
Window
Heat Value $/m /year
Heat Losses
2
Single glass pane H e a t m i r r o r on single pane D o u b l e panes H e a t m i r r o r on inside pane H e a t m i r r o r on outside p a n e H e a t m i r r o r s on both panes "Based on: Δ Τ = 40^/gal heating oil.
20°C, 10-mph wind, R
2.75) 1.26 \ 1.45/ 0.87) 0.94 0.62 iT
=
Savings with Heat Mirror 1.49 0.58
0.90, 90 days heating per year, and
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
8.
FAN
Wavelength-Selective
0.4
0.3
153
Surfaces
0.5
0.8
1.0
2.0
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WAVELENGTH
3.0
(/xm)
Figure 4. Absorptivity of Ag, Au, and Cu films (each 200 A thick) vs. wavelength, cal culated from bulk optical constants t u n a t e l y t h e y also h a v e h i g h solar reflectivity.
If t h e i r solar r e f l e c t i v i t y
can be suppressed a n d if they have l o w intrinsic absorptivity, then b y K i r c h o f F s l a w t h e y w o u l d h a v e h i g h solar t r a n s m i s s i o n . F i g u r e 4 s h o w s t h e i n t r i n s i c a b s o r p t i v i t y of 2 0 0 - A - t h i c k layers of three m e t a l s : A u , A g , a n d C u . B o t h A u a n d C u h a v e too h i g h absorptance at 0.5 μχη, w h e r e t h e solar i n t e n s i t y is a m a x i m u m .
However,
A g has o n l y a f e w
percent
a b s o r p t i v i t y i n the w h o l e solar s p e c t r u m ; c l e a r l y it is t h e m o s t p r o m i s i n g m e t a l . I f the A g reflectivity i n the solar s p e c t r u m c a n b e s u p p r e s s e d
by
antireflection coatings, s u c h as T i 0 , t h e n A g s h o u l d i n d e e d b e a g o o d 2
t r a n s p a r e n t heat m i r r o r . W e h a v e p r e p a r e d s u c h m u l t i l a y e r films. F i g u r e 5 s h o w s t h e m e a s u r e d o p t i c a l reflectivity a n d t r a n s m i s s i o n of a 180-A T i O / 1 8 0 - A 2
180-A T i 0
2
film (2,3)
Ag/
d e p o s i t e d o n glass b y r f s p u t t e r i n g . T h e t r a n s m i s -
WAVELENGTH (μπ\) Figure 5. Measured optical transmission and reflectivity of a 180-A TiO /180-A Ag/180-A TiO film on Corning 7059 glass t
B
In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
154
SOLID STATE
CHEMISTRY 100
100
.SUBSTRATE TRANSMISSION^ _ y*
^
—
—
—yi
95 ± 1.0 percent
330ATi0
ζ g
_
80
o
60
130 A Ag 3 3 0 A T i 0 p p ^
CO CO
40
s CO
CG 7059 GLASS (~1 mm thick)
ζ
