Radiative cooling to low temperatures with selectivity IR-emitting surfaces

187

Thin Solid Films, 90 (1982) 187-190 ELECTRONICS AND OPTICS

RADIATIVE C O O L I N G TO LOW T E M P E R A T U R E S WITH SELECTIVELY IR-EMITTING SURFACES* C. G. GRANQVIST, A. HJORTSBERG AND T. S. ERIKSSON

Physics Department, Chalmers University of Technology, S-412 96 Gothenburg (Sweden) (Received August 12, 1981 ; accepted September 20, 1981)

Radiative cooling to low temperatures is possible with surfaces which radiate primarily in the 8-13 gm atmospheric "window" range. We outline a theoretical analysis of the required radiative properties and report some small-scale experiments using evaporated SiO and "Si3N4" films.

I. INTRODUCTION

Spectrally selective surfaces are required for many different applications of solar and terrestrial radiation 1. Surfaces for efficient photothermal conversion 2, transparent heat mirrors 3, and leaves of living plants 4 are known examples. In this paper we discuss a fourth example: surfaces for radiative cooling to low temperatures 5. These surfaces take advantage of the fact that the atmospheric downward radiation is very low in the 8-13 gm "window" range, particularly when the air is dry. Figure 1 shows some typical spectral radiance data 6. A surface which radiates predominantly in the 8-13 gm band therefore reaches a thermal equilibrium governed by the weak counter-radiation in this range and by conductive and convective heat inflows. The cooling power is usually not more than 100 W m -2 at near-ambient temperatures. Hypothetically, considering radiation balance only, we predict a minimum temperature of about 50 °C below that of the ambient. In this paper we discuss the radiative surface properties required for efficient cooling to low temperatures. 2. THEORETICALANALYSIS We consider radiation balance between a horizontal freely radiating surface and the clear night sky. The incoming and outgoing radiative power densities are Pi. ~ f o d2 W(T~,2) eaa(2) ens(2)

(1)

* Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea,Israel, September 21-25, 1981. 0040-6090/82/0000-0000/$02.75

© ElsevierSequoia/Printedin The Netherlands

188

C . G . G R A N Q V I S T , A. HJORTSBERG, T. S. ERIKSSON

Pout = J o d2 W(T~,2) ells(A)

(2)

where W is the Planck function, T is the temperature, e n is the hemispherical emittance, 2 is the wavelength, and the subscripts a and s refer to atmosphere and cooling surface respectively. The radiative cooling power is Pc - P o u t - Pir, f 13ttm ~ ' H s O ' ( T s 4 - - Ta':l') --~-( | - - ~'l'la2)eHs2

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(3)

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(5)

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SPECTRALLY SELECTIVE FILMS A N D P R A C T I C A L TESTS

SiO films on aluminium provide an illustrative example of a selectively IRemitting surface 5'7. Figure 2 shows measured reflectance spectra for resistively 10

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RADIATIVE COOLING WITH IR-EMITTING SURFACES

189

evaporated films of various thicknesses. The reflectance and transmittance data were combined in a novel fashion a to derive the complex dielectric function of SiO 9 from which we computed the basic cooling parameters ~"s2 and ~/. as functions of the film thickness. The highest spectral selectivity occurred for films 1.0 I~m thick (see Fig. 2), which yielded ~s2 = 0.45 and r/" = 2.12 at T, = 0 °C. A polished aluminium plate of diameter 10 cm was coated with a 1.0 ~tm film of electron-beam-evaporated SiO. The plate was mounted under three IR-transparent polyethylene convection shields in a thermally isolated polystyrene box. An identical plate was painted black, so that it served as a black body radiating reference, and was positioned in an identical box. The cooling performance was tested during clear nights. The minimum temperature reached with the IR-selective surface was about 14 °C below that of the ambient. A somewhat smaller temperature difference was obtained with the black body surface. The reason for the rather small difference in performance of the two test panels lies in an unfavourably low value of ells2 for the SiO coating and in insufficient thermal isolation of the polystyrene boxes. In order to make surfaces with a significantly higher value of ~"s2, we recently prepared films of "Si3N4" by electron beam deposition onto pre-evaporated aluminium. The films were not stoichiometric and were contaminated with oxygen. Figure, 3 shows a reflectance spectrum from a preliminary experiment. It is evident that the atmospheric "window" range is covered to a much greater extent than with SiO. 100--

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4.

CONCLUDING REMARKS

Passive cooling by making use of the atmospheric "window" appears to give prospects for several practical applications. Selectively IR-emitting surfaces are required for the full potential of this source of cooling to be realized. Our work on evaporated films of SiO 5,7 and "Si3N+" represents two steps towards the development of a practically useful surface. A different approach to radiative cooling uses selective IR emission from flowing C2H + gas encased in an IR-transparent container lo.

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C . G . GRANQVIST, A. HJORTSBERG, T. S. ERIKSSON

ACKNOWLEDGMENTS This work was supported financially by grants from the Swedish Natural S c i e n c e R e s e a r c h C o u n c i l a n d t h e N a t i o n a l S w e d i s h B o a r d for T e c h n i c a l Development. REFERENCES 1 2 3 4 5 6

7 8 9 10

C.G. Granqvist, Appl. Opt., 20 (1981) 2606. B.O. Seraphin, Solar energy conversion: solid state physics aspects, Top. Appl. Phys., 31 (1979) 5. J.L. Vossen, Phys. Thin Films, 9 (1977) 1. D.M. Gates, H. J. Keegan, J. C. Schleter and V. R. Weidner, Appl. Opt., 4 (1965) 11. C.G. Granqvist and A. Hjortsberg, J. Appl. Phys., 52 (1981) 4205. P. Berdahl and M. Martin, in D. Prowler, I. Duncan and B. Bennett (eds.), Proc. 2nd Natl. Passive Solar Conf., Vol. 2, American Section of the International Solar Energy Society, Newark, 1978, p. 684. C.G. Granqvist and A. Hjortsberg, Appl. Phys. Lett., 36 (1980) 139. A. Hjortsberg, Appl. Opt., 20 (1981) 1254. A. Hjortsberg and C. G. Granqvist, Appl. Opt., 19 (1980) 1694. A. Hjortsberg and C. G. Granqvist, Appl. Phys. Lett., 39 (1981) 507.