Performance comparison of organic fluids for a novel low temperature solar thermal electricity system

In order to obtain acceptable cost-effectiveness and higher heat collection efficiency, two-stage collectors and heat storage units are proposed for the innovative low temperature solar thermal electricity system. The organic fluid is preheated by flat plate collectors (FPC) prior to entering a higher-temperature heat exchanger connected with compound parabolic concentrators (CPC). The two-stage heat storage units are composed of two kinds of phase change material (PCM) with diverse melting temperatures. Five widely or newly used fluids, R123, R113, R245fa, pentane and butane are selected for the Organic Rankine Cycle (ORC). Fundamentals of the proposed system are illustrated. Mathematic models are built and the numerical simulation is carried out based on distributed parameters. The performance of the five fluids on ORC, heat collection and the optimal proportion of FPC area to the total collector area is investigated and compared. According to simulation results, the regenerator influences fluid performance on both the ORC and heat collection. And fluids of lowest ORC efficiency on the contrary will offer the highest collector efficiency. Thus the fluid selection criteria are different from that for a solo ORC. The optimal FPC proportion is strongly correlated with fluid properties especially when less effective CPC collectors are used. The optimal FPC proportion is maximized on the use of R245fa and the overall collector efficiency is about 14.1% higher than that of single-stage collectors.