Optical analysis of an evacuated tube collector with built-in compound parabolic concentrator for process heat applications

Abstract

To date, insufficient attention has been paid to the potential of renewable energy resources in industrial applications. Up to 21% of final energy demand and feedstock-use in the manufacturing industry sector could be of renewable origin by 2050. Process temperatures found in industrial processes range from low (T<100°C), medium (100°C < T < 250°C) to high (T > 250°C) temperatures. Whereas low temperature applications are already addressed by well-established collector technologies, the medium temperature applications are still in an early development stage. The aim of this study is to evaluate the optical performance of an Evacuated Tube Collector with Compound Parabolic Concentrator (ETC-CPC) used for medium temperature industrial process heat. To this end, an optical simulation model is developed in Tonatiuh ray-tracing software and is validated towards the theoretical fully developed curve. The parametric analysis performed investigates the impact of the truncation origin, the reflectivity of the CPC mirror and the absorber radius on the optical efficiency and the power production of the solar collector. It is found that the collector with the biggest truncation origin (0.025m) performs better under small Sun angles (0°-20°), but worse under greater Sun angles (40°-90°). This happens because the collector with the greatest truncation origin has the biggest mirror area and under small Sun deviations this leads to more gains and higher efficiency. The quality of the mirror plays an important role in the optical efficiency of the collector, but mainly for incident angles less than 20°. Moreover, increasing the radius of the absorber, the power production per aperture area is increased. The variation of the absorber radius shows that the radius of the base scenario compensates most of the optical errors but without being big enough to have excess heat loss. The results of this study are valuable for the design, simulation and performance analysis of ETC-CPC for delivering medium temperature heat.