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 T4. 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 °C in vacuum.