Radiative cooling of TiO2 white paint

SolarEnergy.Vol.20.pp. 185-188. PergamonPress 1978. Printedin GreatBritain

TECHNICAL

NOTE

Radiative cooling of TiO2 white paint A. W. HARRISON and M. R. WALTO~ The University of Calgary, Department of Physics, Calgary, Alberta, Canada T2N 1N4 (Received 27 April 1977: in revised form 15 July 1977)

INTRODUCTION Selective surfaces have been used extensively in solar energy collectors[l], but to date have not been exploited to any great extent in radiation cooling devices in which advantage is taken of the atmospheric window region 8-13/~m. As noted by Johnson[2] both natural and man-made structures are known to cool below ambient temperature under certain environmental conditions. In a recent experimental study Catalanotti et al.[3] have shown that substantial cooling of a selective surface below ambient temperature can be achieved provided certain conditions are satisfied. The work reported here is similar to that of Catalanotti et al. but has been carried out under different environmental conditions and with a different selective surface. The underlying theory has been given in the earlier study[3] and shows that the power radiated from a selective surface of spectral emissivity ~,(A) and which obeys Lambert's cosine law is P r=J0f./2 7r sin 2 0 ~ : [1-R(A,O)]~r(A)B(A, T)dAd0

(I)

where 0 is the angle between direction of radiation and normal to the surface, R(A, 0) is the spectral reflectivity of the polyethylene protective cover (see below), and B(A, T) is the black body spectral radiance for a temperature T. The power absorbed by the radiator from the polyethylene cover and the atmosphere depends on the angular dependent spectral radiance of the atmosphere N(A, 0) and on the spectral emissivity a()L, 0) and spectral transmissivity T(A, 0) of the polyethylene cover. This power is given by f .~/2 f® ~" sin 20Jo [N(A, 0) T(A, 0) +B(h, To)a(h, 0)]~(~)dAd0

Po = Jo

(2) where it is understood that R(A, 0) + T(A, 0) + a(A, 0) = 1, where TO is the ambient temperature in the immediate vicinity of the radiator and where the atmospheric spectral radiance depends on atmospheric temperature and humidity gradients as discussed below. For a large radiator insulated on all sides except that

facing the sky and where edge effects and conduction of heat to the radiator from outside can be neglected, we have in the steady state

e r - Po = 0.

EXPERIMENTALRESULTS A schematic diagram of the radiation cooling unit constructed for this experiment is shown in Fig. 1. It consists essentially of a 6 mmx 50 cmx 50 cm aluminum plate coated with a thin layer of the surface being investigated. Thermistor probes inserted into the aluminum plate and almost touching the radiator surface are used for measuring the temperature T of the surface while a similar probe housed in a Stevensons screen is used for measuring the ambient temperature To. The aluminum plate is insulated on all sides except that facing the sky with 2.5 cm styrofoam and 2 cm plywood. Approximately 3 cm above the radiator surface is stretched a thin polyethylene sheet which provides the necessary convection cover. Whereas in the unit constructed by Catalanotti et al. [3], conduction of heat from the surroundings to the radiator surface were significant due to the small size of their cooling unit, in the study here with a much larger unit and better insulation this effect is considered insignificant. A survey of readily available materials having high emissivities in the 8-13 ~m region showed that the oxides and carbonates of titanium, aluminum, calcium and zinc are likely candidates for the required white-black selective surface since they also possess high reflectivities in the visible region. Many commercially available white paints contain these compounds, especially titanium dioxide. After several trial experiments with different commercially available paints and laboratory prepared mixtures of these compounds, it was decided to use a white paint manufactured by the Perma Paint Company, Calgary, Alberta, and containing 35 per cent of titanium dioxide. While this paint has been used to obtain the results presented below, it is the opinion of the authors that in all probability most good quality white paints will produce a similar performance. In the study here, several aluminum plates were coated with an optically thick

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Fig. 1. Diagram showing essential features of the radiation cooling unit used in this investigation. Not drawn to scale. 185

186

Technical Note

layer of white paint and tests performed to determine effects of aging, weather, etc., but so far no noticeable changes in performance have been observed. Three identical radiation cooling units were located on the open roof area of a tall building on The University of Calgary campus (lat. 51°N) and have been monitored continuously over a four month period Nov. 1976--Mar. 1977. Comparison of one unit with another confirmed that the results reported here are typical of those to be expected from such a simple radiator. While different amounts of cloud cover and local humidity conditions were noted throughout the observing period and corresponding cooling effects recorded, the emphasis in this report is on the cooling produced under clear skies and at night when direct and scattered solar radiation are absent. Continuous monitoring of the radiation cooling produced with the white paint radiator surface described above showed that maximum cooling below ambient temperature is achieved in the absence of direct sunlight, under clear skies and for low ground level absolute humidity Wo. In Fig. 2 are sample calibrated records of AT = To- T taken over 24 hr periods. In Fig. 2(a) can be seen the effect of exposure to direct sunlight on a clear day. At local noon the value of AT is approximately 10°C less than at night when there is no sunlight. Figure 2(b) illustrates the effects of variable cloud cover showing how the atmospheric window 813 #.m is masked due to the near black-body emission from dense cloud in this wavelength region. Maximum values of AT as high as 15°C were recorded on some occasions under clear skies and low absolute humidity as shown in Fig. 2(c).

S , L,0HT

FEBRUARY 7, 1977

An attempt has been made to analyse all the results obtained to date with respect to the effect of humidity on the cooling achieved. Only data obtained under clear skies and indirect sunlight conditions were used for this purpose. Averages of both AT and Wo were taken over 6 hr periods and then plotted as in Fig. 3 in which the straight line has been drawn using the principle of least squares. It is clear that the presence of water vapour in the atmosphere has a strong influence on the thermal emission of the atmosphere in the 8-13p.m window as expected[4]. A correlation analysis of AT and Wo showed that the simple product moment correlation coefficient R=0.59. Application of Student's t test shows that the chance T of obtaining this calculated coefficient for a true value R = 0 is <0.1 per cent. If a value of T < 5 per cent is taken as an indication of a significant correlation as is usually done, then it must be concluded here that AT and Wo are strongly correlated. DISCUSSION Equations (1)--(3) above can be used to derive an estimate of AT to be expected from the radiator used in this experiment provided certain factors in the equations are either known or are assumed. Detailed reflectivity or emissivity data for the white paint used here is not available. However it is known that white paints can have an emissivity of 0.92 or more in the 8--13#m region and an emissivity of 0.05 or less in the visible region [5-7]. In the calculations here it was assumed that ,,(A)= 0.92 inside the atmospheric window and ¢r(A) = 0.05 on either side of the window. Data for the polyethylene cover was taken partly from

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Fig. 2. Sample 24 hr records of radiation cooling AT(°C) for various envh'onmental conditions.

Technical Note 4.5

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187

N(A, 0) for a clear sky and W 0 = 2.5 g m-3 is shown in Fig. 4 and

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has been used in the numerical evaluation of T from (3) for TO= 10°C. The calculated value of AT ( = To - T) obtained is 21°C and should be compared with the experimentally observed value of 15°C on 19 February, 1977, when the absolute humidity was also -2.5 g m-3. The agreement between calculated and observed values of AT is fair considering the uncertainty of the assumed values for er(A) and the likely imperfect behaviour of the insulation and convective cover around the radiator. Aside from the comparison of calculated and measured values of aT, it is satisfying to find that a commercially available and economically attainable material can be used to produce a significant radiation cooling. Values of AT-15°C can be achieved with this unit in Calgary (elevation = 1.1 km). However it must be noted that at lower altitudes and in more humid climates AT would be smaller.

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CONCLUSION It has been shown experimentally that a high content Ti02 white paint as used on the exterior of buildings can under certain conditions cool to 15°C below ambient temperature. An approximate estimation of the cooling to be expected under local conditions prevailing in Calgary where the experiment was performed showed that this is a reasonable value to expect. An increase in ambient humidity or cloud cover, or the presence of direct sunlight can all degrade the performance of this simple selective surface. Investigations are continuing to determine in more detail the effect of varying humidity on the cooling AT.

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Fig. 3. The effect of ambient absolute humidity Wo (gm -3) on AT(°C) under clear sky conditions and absence of direct sunlight I

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TYPICAL ATMOSPHERIC CALGARY, ELEVATION =

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Acknowledgements--The authors are grateful to The University of Calgary for making available research facilities and funds for I

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Fig, 4. Empirical atmospheric spectral radiance data measured at Calgary and used in the estimation of A T from

(3). the work of Catalanotti et al.[3] and partly from direct measurement. The dependence of RUt, 0), T(A, 0) and a(A, 0) on wavelength and angle was assumed to be the same as that measured by Catalanotti et aL[3], while absolute values of these factors were deduced from direct measurements on the actual polyethylene film used here which was thicker ( ~ 20 t~m) than the one used by them. The most important factor in the calculation of AT is the atmospheric spectral radiance N(A, 0). This can vary markedly with altitude, climate, etc. Fortunately data for Calgary is readily available due to parallel investigations of N(A, 0) which have been made recently[4]. Empirical data for this project. They are also grateful to Mr. L. Murdock for his

technical advice and assistance without which this work could not have been completed. ~CES 1, A. B. Meinel and M. P. Meinel, Applied Solar Energy, An Introduction. Addison-Wesley (1976). 2, T. E. Johnson, Radiation cooling of structures with infrared transparent wind screens. Solar Energy 17, 173 (1975). 3. S. Catalanotti, V. Cuomo,G. Piro, D. Ruggi, V. Silvestrini and G. Troise, The radiative cooling of selective surfaces. Solar Energy 17, 83 (1975).

188

Technical Note

4. A. W. Harrison, Atmospheric thermal emission 7-15 #m. Can. J. Phys. 54, (14), 1442 (1976). 5. W. L. Wolfe (editor), Handbook of Military Infrared Technology. Office of Naval Research, Department of the Navy, Washington, D.C. (1965).

6. R. V. Dunkle, Thermal Radiation characteristics of surfaces, In Theory and Fundamental Research in Heat Transfer (Edited by J. A. Clark). Pergamon Press, New York (1963). 7. M. M. Koltun, Selective surfaces and coatings for solar technology. Geliotekhnika 7 (5), 70 (1971).