Control of thermal emittance of stainless steel using sputtered tungsten thin films for solar thermal power applications

Low thermal emittance is the key factor of a solar collector. For high temperature solar thermal applications, low emittance is an important parameter, because the thermal radiative losses of the absorbers increase proportionally by T-4. Our primary motivation for carrying out this work has been to lower the thermal emittance of stainless steel substrate (intrinsic emittance=0.12-0.13) by coating a thin film of high infrared (IR) reflecting tungsten (W). Tungsten thin films were deposited on stainless steel substrates using a glow discharge direct current magnetron sputtering system. Emittance as low as 0.03 was obtained by varying the thickness of W coating on stainless steel substrate. The influences of structural, morphological and electrical properties of the W coating on its emittance values are studied. The effect of substrate roughness on the emittance of W coating is also examined. Thermal stability of the W coatings is studied in both vacuum and air. In order to demonstrate the effect of W interlayer, solar selective coating of AlTiN/AlTiON/AlTiO tandem absorber was deposited on W coated stainless steel substrates, which exhibited absorptance of 0.955 and emittance of 0.08 with a thermal stability up to 600 degrees C in vacuum. (C) 2014 Elsevier B.V. All rights reserved.