Effect of thickness and temperature on the thermal emittance of solar absorber coatings at room and high temperatures

The emittance of solar absorber coatings is a crucial parameter that determines the radiative heat loss from concentrating solar thermal (CST) receivers. However, measuring emittance of CST coatings at typical operating temperatures (up to 800℃) is technically challenging. Currently there is no available data on the effect of coating thickness and degradation on emittance at these temperatures. We develop a method for measuring emittance at high temperatures. We compare emittance measured at ambient temperature and temperatures up to 800℃ for Pyromark 2500 with coating thickness (from 10 µm to 80 µm). We found that the emittance of Pyromark increases with coating thickness, particularly for wavelengths below 8 µm. In thin coatings, this trend is attributed to the contribution of materials within coating (oxide layers or the substrate) to the total emissive power received by the detector. The total emittance of Pyromark increases monotonically with temperature, with a steeper increase for temperatures below 400℃ and for thinner coatings. This study reveals the significance of measuring emittance at temperatures representative of the operating temperatures of receivers rather than at room temperature. The difference in the estimated emission loss for emittance measurements at room temperature and 800℃ can be as large as 10%.