Thermographic measurements on high temperature solar absorber coatings under concentrated solar flux

Kurzfassung

In the course of the Raiselife project, three uncoated and twelve black coated tubular solar Receiver samples are investigated in an accelerated ageing test campaign to evaluate their durability . Thermal cycling tests are carried out, in which sample surfaces are exposed to concentrated solar flux at a modified solar dish system located at PSA in the south of Spain. Sample surface temperature needs to be controlled during cycling tests to be maintained below a maximum temperature setpoint, characterize possible degradation mechanisms and improve knowledge about the thermal cycling test procedure. Two approaches for surface temperature measurement are applied. The existing temperature measurement system consisting of thermocouples and a tool to estimate the temperature Gradient between the embedded thermocouple and sample surface. This method is enhanced by a simulation using ANSYS Thermal, confirming its applicability. A second temperature measurement system consisting of a solar blind IR camera is implemented. After confirming its calibration and applicability under the desired circumstances, an IR image is taken during cycling. Measured raw values of sample surface temperature are weighted with their specific thermal emittance in the wavelength range of the camera. Coated samples show stable thermal emittance values throughout cycling tests. Uncoated samples show optical degradation influencing the outcome of IR image processing, allowing the applicability of precise thermographic measurements to only be ensured for coated samples. A refinement of ANSYS Thermal properties promises more reliable simulation results. An improvement on the optical characterization process of sample surfaces promises more reliable thermographic measurements, in order to decrease its sensitivity with respect to emittance.