Platinum-Based High Temperature Selective Absorber Coatings

In photothermal energy conversion the absorbed solar flux is converted to heat which is then used directly or is converted to mechanical or electrical energy. Efficient receiver design requires that thermal losses from the absorber surface, which is the hottest element of the system, be minimised. Receivers are, in fact, generally classified according to their operating temperature ( I , 2). Thus one has low temperature collectors (T < 2oo0C), which are used primarily for space and water heating, m e d i u m t e m p e r a t u r e c o l l e c t o r s (zoo < T < 5oo0C), which can be used for absorption air conditioners and to heat the working fluid for low temperature heat engines, and high temperature collectors (T > 5oo0C), which can be used to drive high temperature engines for electric energy conversion. Convection and conduction losses can generally be reduced to acceptable levels by proper mechanical design. Radiation losses are minimised by the use of spectrally selective surfaces (3). The concept of the spectrally selective surface is based on two facts. The first is that about 98 per cent of the solar flux is in the wavelength range below 2000 nm, while thermal radiation losses are at longer wavelengths (Wien’s law), except at temperatures above 700OC. The second is Kirchhoffs law, which states that the total hemispherical spectral absorptance, a(1), is equal to the total hemispherical spectral emittance, &(A), at any given wavelength 1. A spectrally selective surface is therefore one that possesses a high spectral absorptance at wavelengths within the solar spectrum-typically 300 to I 500 nm-and a low spectral absorptance, and therefore a low spectral emittance, at longer wavelengths. Such surfaces can have a pronounced effect on the performance of photothermal collectors operating at temperatures below about 700 to 8ooOC (3) . Bulk materials with highly selective surfaces do not appear to exist. Therefore coatings must be used. An effective coating consists of an optical filter which passes the long wavelength radiation (d > 1500 nm) while absorbing the short wavelength radiation, deposited over a low emittance base layer. The requirement is therefore for a multi-layer optical coating, with unusual thermal stability as far as optical coatings are concerned, and with economy of large scale production. This paper describes a platinum-alumina cermet coating which is believed to be the thermally most stable selective absorber coating developed thus far. The coating was first studied by Professors Sievers and Buhrman and their students at Cornell University in the late