An evacuated receiver partially insulated by a solar transparent aerogel for parabolic trough collector

A modified evacuated receiver partially insulated by a solar transparent aerogel was proposed for the parabolic trough collector. An optical-thermal model was developed and validated to investigate the optical and thermal performance of the modified receiver and a traditional evacuated receiver. Firstly, it was revealed that different heat transfer modes among the aerogel, absorber and glass can obviously influence the heat loss from the absorber. Based on adjusting these modes through varying the aerogel thickness, an optimal receiver design was obtained. Then, the effects of the direct normal irradiance and the coating emittance were analyzed. Finally, the optimal receiver was compared with two existing modified receivers which are insulated by an opaque material and a radiation shield, respectively. Optical study shows that the optical efficiency of receiver just decreases by 0.16% when the aerogel thickness increases from 0 to 24.5 mm, which indicates the aerogel influences slightly on the optical performance. Thermal study indicates the modified receiver with the aerogel thickness of 20 mm has the best insulating performance. Compared with the traditional receiver, it can reduce the heat loss from the absorber by 7.3-10.1% when the inlet fluid temperature is within 583-823 K, and the direct normal irradiance is within 400-1000 W.m(-2). As a result, the receiver efficiency can be improved by 0.01-2.92%, and the collector efficiency is improved by 0.01-2.75%. Moreover, it is found that the higher the coating emittance or the fluid temperature is, more obvious the improvement in the receiver efficiency becomes. Finally, comparison of different receivers indicates the optimal receiver with the aerogel can achieve higher receiver efficiency than the two existing modified receivers under all examined typical conditions.