Performance investigation of a novel zeotropic organic Rankine cycle coupling liquid separation condensation and multi-pressure evaporation

Abstract

The decrease in fossil energy reserves and the increase in energy costs have resulted in a strong interest in power generation using renewable heat sources or waste heat. Organic Rankine cycle (ORC) is a promising heat-to-power conversion technology. Although the ORC using zeotropic mixture is superior to ORC using pure fluid in thermodynamic performance due to low irreversibility during the evaporation and condensation of zeotropic mixture, the improvement in thermodynamic performance is usually achieved at the cost of poor economic performance. The studies on improving the thermo-economic performance are limited. In the present study, a novel zeotropic ORC coupling liquid separation condensation and multi-pressure evaporation is proposed. Multi-pressure evaporation is presented to improve heat match between the heat source and working fluid. Liquid separation condensation is applied to control the composition of the mixtures that enter the evaporation processes. Thermodynamic analysis and optimization model of the novel ORC is developed. The superiority of the proposed novel ORC over the traditional simple zeotropic ORC and traditional multi-pressure evaporation zeotropic ORC is elaborated for different mixtures. The contribution of mixture composition adjustment and condensation enhancement through liquid-vapor separation on the cycle performance improvement is investigated. Sensitivity analysis of heat-sink inlet temperature, heat source inlet temperature, vapor quality, heat source specific heat capacity, and heat-sink temperature rise on the cycle performance are conducted. The case results show that net power output of the novel ORC is 13.05–26.18% higher than that of the simple zeotropic ORC. The contribution of mixture composition adjustment on improving the net power output can be up to 3.57% compared with traditional multi-pressure evaporation ORC for mixture R245fa–R365mfc. When the heat transfer enhancement through liquid separation is incorporated into the thermodynamic optimization, the net power output of the novel zeotropic ORC can be increased by 8.22% compared with the traditional multi-pressure evaporation ORC under the same economic constraint.