Radiant emittance calculated by heat transfer analysis of a PTC receiver tested with vacuum versus measurement of an absorber sample using spectrophotometer

Heat loss of receiver tubes is related with the radiant emittance of absorbers. This parameter can be obtained using an IR spectrophotometer to measure the spectral hemispherical reflectance of absorber tube samples which requires the breaking of a receiver tube. Nevertheless, radiant emittance can be also obtained indirectly testing a complete receiver tube in a test device to measure its heat losses by radiation. This paper shows results of tests performed to different receivers applying both methodologies to calculate their emittance. One conclusion is the importance of performing measurements in an evacuated atmosphere when the non-destructive method is applied; this is to eliminate convective heat transfer to the surroundings, especially at low temperature when convective heat losses may have a much greater influence on the accuracy of emittance results.Heat loss of receiver tubes is related with the radiant emittance of absorbers. This parameter can be obtained using an IR spectrophotometer to measure the spectral hemispherical reflectance of absorber tube samples which requires the breaking of a receiver tube. Nevertheless, radiant emittance can be also obtained indirectly testing a complete receiver tube in a test device to measure its heat losses by radiation. This paper shows results of tests performed to different receivers applying both methodologies to calculate their emittance. One conclusion is the importance of performing measurements in an evacuated atmosphere when the non-destructive method is applied; this is to eliminate convective heat transfer to the surroundings, especially at low temperature when convective heat losses may have a much greater influence on the accuracy of emittance results.