Steady state calorimetric measurement of total hemispherical emittance of cylindrical absorber samples at operating temperature

At DLR’s QUARZ Center a test bench has been established to measure, using steady state calorimetric method, the total hemispherical emittance of cylindrical solar thermal absorber samples at temperatures up to 450 °C. Emittance measurement of solar absorber surfaces is commonly performed by direct-hemispherical reflectance measurements with spectrophotometers. However, the measurement of cylindrical samples with spectrophotometers can be considered still a challenge as integrating spheres, reference samples and calibration services by national metrology institutions are optimized for flat sample measurement. Additionally samples are typically measured at room temperature. The steady state calorimetric method does not rely on reference samples and the measurement is performed at operating temperature. In the steady state calorimetric method electrical power input used to heat the sample is equated to the radiative heat loss from a heated sample to the environment. The total emittance can be calculated using the Stefan-Boltzmann equation from radiative heat loss power, the defined sample surface area and measured surface temperature. The expanded uncertainty (k=2) of the total hemispherical emittance has been determined to ±13 % for a typical parabolic trough absorber sample at a temperature of 300 °C and a heating power of 100 W. The test bench was validated by the measurement of three samples with the spectrophotometer and the steady state calorimetric method