Solar Energy Materials 5 (1981) 129-140 129 North-Holland Publishing Company SPECTRALLY SELECTIVE COPPER SULPHIDE COATINGS S. B. GADGIL, R. THANGARAJ, J. V. IYER, A. K. SHARMA, B. K. GUPTA and O. P. AGNIHOTRI Department oJ' Physics and Center oJ Eneryy Studies. Indian Institute of Technoloyy, New Delhi-110016, India Received in revised form 20 March 1981 In the present investigation, copper sulphide films were chemically deposited on aluminium substrates and their optical properties were studied for their application in photothermal conversion of solar energy. The films are stable, have good adhesive properties, are fairly uniform and found to possess spectral selectivity. The maximum solar absorptance :t s (0.89) and minimum thermal emit- tance ex (0.25) are found for a film deposited at 250'C and having thickness of 1.6 #m. The selective coating gave a stagnation temperature of 124 C which givesthe improvement in collector efficiency of 17'I.~, over the black paint. The filmsare found to be stable up to 200" C. Differentaging tests such as temperature stability, temperature cycling and ultraviolet irradiation have been done. The films essentially provide a low cost selectivecoating for photothermal conversion of solar energy. The cost of the coating per square meter area has been estimated to be around $2.5. 1. Introduction The flat plate solar energy collector is the simplest type of collecting device which is used for photothermal conversion of solar energy. It essentially consists of a black- ened absorber plate to which are attached tubes for energy removal. Good conversion efficiencies are obtained if the losses from the absorbing surface are minimised. The losses are due to conduction, convection and radiation. The radiation losses from the plate, which is blackened in order to absorb solar energy, become dominant as the temperature of the plate goes up. However, if the receiving surface has such selective properties, that for sunlight (~ < 2#m) it is a poor reflector (good absorber) whilst for thermal radiation (2 > 2/~m) it is a good reflector (poor emitter) then such a surface will attain higher temperature (as compared to black surface) when exposed to sun- light [1, 2]. If such a selective coating having high solar absorptance a s and low thermal emittance ~T is used as receiver surface then it will certainly result in an increase in the efficiency of collection. The performance of a solar selective coating is characterised by the ratio ~s/eT. There are many methods of preparing selective coatings such as spraying, vacuum evaporation, electroplating, etc. One simple technique is to make use of a semiconductor with band gap between 0.5 and 1.2 eV which absorbs in the solar region (0.25 to 2.5 #m) and transmits above this region. By coating such a semiconducting material on highly polished metal substrates, a selective absorber can be obtained. The good transmission of such a semiconducting 0165-1633/81/0000-0000/$02.50 ~)1981 North-Holland