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Heat loss of parabolic trough receivers is measured in the laboratory at Steady State. Criteria for the Steady State shall ensure sufficient stabilization of the measurands without being excessive. The IEC TS 62862-3-3:2020 provides such criteria. This paper presents a model for the assessment of Steady State criteria in heat loss tests. Assuming constant heating power, lumped heat capacity and linearized heat loss, the model shows, that the IEC TS 62862-3-3 allows for significant deviations at low temperatures and large heat capacity. For the investigated configuration of low heat loss and large heat capacity, the model shows allowed deviations of 12.6 K at 100 °C, 5.5 K at 200 °C and 2.5 K at 300 °C. The paper discusses the weaknesses of Steady State criteria in the IEC TS 62862-3-3 and proposes to clarify the meaning of ±0.5 K and ±1% and, to expand the stabilization period to 60 minutes for measurements below 300 °C and to limit the scope of the standard to measurements > 240 °C. [ABSTRACT FROM AUTHOR]

Heat loss testing of parabolic trough receivers with heating cartridge is well-established. In that technique temperature is measured by pressing thermocouples against the inside surface of the absorber tube. As the temperature of the thermocouple is influenced by radiation temperature and air temperature in the annulus, the temperature of the thermocouple is higher than the absorber temperature. The paper presents a measurement of absorber temperature over-prediction as function of heating power between 89 °C and 577 °C for three different insulation configuration yielding over-prediction between 4.4 K and 15.3 K. This shows, that absorber temperature over-prediction is at a relevant order of magnitude for heat loss measurements of parabolic trough receivers. Interpolation using 2nd- and 3rd-order 2d polynomials with the additional condition of disappearing over-prediction at HL=0 for correction of heat loss measurements is demonstrated. [ABSTRACT FROM AUTHOR]

This paper reports heat flux measurements on parabolic trough receivers, which are integrated to calculate the receiver overall heat loss. Integrated heat fluxes yield an overall heat loss value which agrees with DLR ThermoRec heat loss measurements within ± 5 %, in the simplified case of assuming a homogeneous heat flux distribution around the receiver circumference. Heat flux measurements also allow separating glass envelope and shielded bellow heat loss terms for receiver thermal modelling. Shielded bellows only represent 3 to 4 % of the receiver total length at ambient temperature, however heat flux measurements show that their contribution to the overall receiver heat loss ranges from 5 to 15 %, depending on the shield construction and the absorber temperature. The receiver specific heat loss thus tends to lie above the glass envelope specific heat loss. Almost homogeneous heat flux values are measured along the glass envelope length (variation below 5 %), while more inhomogeneous heat flux values are observed over circumference of the glass envelope (variation between top and side of 17 %). The measurement setup analysis shows a good repeatability, although a proper mounting of heat flux sensors is important in order to avoid systematic biases. [ABSTRACT FROM AUTHOR]

A round robin for parabolic trough receiver heat loss and optical efficiency in the laboratory was performed between five institutions using five receivers in 2015/2016. Heat loss testing was performed at three cartridge heater test benches and one Joule heating test bench in the temperature range between 100 °C and 550 °C. Optical efficiency testing was performed with two spectrometric test bench and one calorimetric test bench. Heat loss testing results showed standard deviations at the order of 6% to 12 % for most temperatures and receivers and a standard deviation of 17 % for one receiver at 100 °C. Optical efficiency is presented normalized for laboratories showing standard deviations of 0.3 % to 1.3 % depending on the receiver. [ABSTRACT FROM AUTHOR]