Off-Design Performance Comparative Analysis Between Dual-Pressure Organic Rankine Cycles Using Pure and Mixture Working Fluids

Dual-pressure Organic Rankine Cycles (ORCs) driven by the low temperature heat source usually work under part-load conditions, and it is therefore essential to predict the off-design performance of such ORCs. This paper presents the off-design performance prediction of the dual-pressure ORC on the basis of the model including plate heat exchangers, axial turbines and a centrifugal pump. Pure working fluid R600a and the mixture R245fa/R600a are compared. The sliding pressure operation strategy is considered under off-design conditions. The results indicate that under the design hot water parameters (hot water 140 °C, 64.87 kg/s), compared with the single-pressure ORC using R600a, the dual-pressure ORC using R600a shows a 9.57% higher net power and a 17.32% higher heat transfer area. Furthermore, the dual-pressure ORC with the mixture R245fa/R600a (0.42/0.58 mass fraction) shows a 1.04% higher net power and a 3.87% higher heat transfer area than the dual-pressure ORC using R600a under the design hot water parameters. In the dual-pressure ORC, the rotational speed of the high-pressure pump is more strongly influenced by the inlet temperature of hot water than that of the low-pressure pump. In addition, when the mass flow rate ratio of hot water or the inlet temperature of hot water increases, the difference of the net power between the dual-pressure ORC using the proposed mixture R245fa/R600a (0.42/0.58 mass fraction) and that using pure R600a increases.