Accelerated Ageing of Solar Reciever Coatings: Experimental Results for T91 and VM12 Steel Substrates

Abstract

This paper reports experimental results for accelerated ageing test campaigns performed on four different solar receiver coatings applied on T91 and VM12 steel substrates. VM12 tubular samples are exposed at a dish test facility in order to perform thermal cycles under concentrated solar flux. No significant optical degradation could be observed on coated samples after 100 thermal cycles (50 hours at 650 °C), while bare and polished reference substrates oxidized during testing. Three solar receiver coatings achieved a stable solar weighted absorptance αs above 95 % after exposure. A selective coating further showed a thermal emittance of 25 % at 650 °C, instead of 65-75 % for non-selective coatings, thus achieving a thermal efficiency above 90 % at 250 kW/m2, instead of 85 % for non-selective coatings. T91 coated metal coupons were tested in four standard climate test chambers for condensation, damp heat, humidity freeze and neutral salt spray. Two non-selective coatings passed all climate tests without significant optical degradation, while the selective coating did not pass the neutral salt spray test due to severe corrosion.