High temperature; stable, spectrally selective solar absorbers for thermochemical hydrogen production

Abstract

The thermochemical reduction of water by reactions similar to the Mark I process requires a processing temperature of 730°C. The efficient utilization of solar photothermal energy conversion, in distributed collector systems, to attain this temperature will require the use of suitable spectrally selective surfaces which are stable at the operating temperature. A coating system with demonstrated high temperature capability has been developed at the Optical Sciences Center under NSF/RANN sponsorship. A silicon thin film absorber is deposited by chemical vapor deposition (CVD) on a silver thin film reflector. This optical stack is fabricated at temperatures in excess of 800°C, and the CVD technology is amenable to large scale production in a flow-through system. At 500°C the present Si-Ag system has typical solar absorptance and total normal emittance values of 0.75 and 0.06, respectively. Samples were fabricated which maintained their high spectral selectivity after 2000 thermal cycles between 150°C and 450°C, and after 100 h at 600°C. Further process studies now underway indicate that the solar absorptance can be improved to better than 0.85 by employing a Si-Ge multilayer absorber and that the operating range of the stacks can potentially be raised to the 800–900°C range by employing a refractory metal thin film reflector.