Modeling the external flow of a novel HorseShoe receiver and the evaluation of thermal performance

Abstract

The linear receiver of a Parabolic Trough Collector is the most critical element in the entire system. The Universal Vacuum Air Collector concept is the most extended type of receiver in both experimental and industrial facilities. Besides their considerable cost, their efficiency usually drops as operation time passes. This is mainly due to a partial loss of vacuum in the evacuated annulus between the absorber and the glass cover. An alternative design called HorseShoe receiver is proposed in this work, whose main goal is to maintain the thermal performance throughout its entire lifespan. This innovative receiver is indicated for low-to-medium temperature ranges, which is particularly suitable for solar heat for industrial processes. It consists of a horseshoe-like cavity absorber having its upper border insulated. In addition, two main advantages can be taken by using two symmetric lenses as glass cover: reconcentrate solar radiation into the cavity (improvement of the intercept factor) and protect stratification conditions (reduction of thermal losses). A transient numerical model with customized boundary conditions has been implemented to evaluate both thermal performance and temperature difference in the absorber domain, which is critical for the thermal stress conditions. For that purpose and as a key contribution, not only the Heat Transfer Fluid (HTF) temperature but also the heat transfer coefficient in the duct are set. In particular, HTF temperature ranges from 80 °C to 220 °C and the inner heat transfer coefficient from 600 W/(m 2 ⋅ K) to 1800 W/(m 2 ⋅ K). Results show that numerical thermal performance is above 96%, which is mainly due to the reduction of thermal radiation losses, where the absorber active surface emittance is ϵ a b s = 0 . 3 . Since no inclination of the receiver has been considered in this initial study, it is shown that the air inside the cavity is stratified, which reduces convective losses. Moreover, temperature difference between the coldest and the hottest spot of the absorber is no higher than 30 K, which is affordable from a technical point of view. A polynomial heat losses correlation is also provided, whose parameters are on the same order of those by conventional receiver correlations available in the literature. • A new concept of solar linear cavity receiver (HorseShoe receiver) is proposed. • Thermal losses have been evaluated numerically. • Thermal performance is above 96% under the selected operation conditions. • The symmetric lenses enhance air stratification. • Most of the thermal losses are due to thermal radiation.