Optimal design of ORC systems with a low-temperature heat source (Q406000)

Summary: A numerical model of subcritical and trans-critical power cycles using a fixed-flowrate low-temperature heat source has been validated and used to calculate the combinations of the maximum cycle pressure \((\mathrm P_{\mathrm{ev}})\) and the difference between the source temperature and the maximum working fluid temperature (DT) which maximize the thermal efficiency \((\eta_{\mathrm{th}})\) or minimize the non-dimensional exergy losses \((\beta)\), the total thermal conductance of the heat exchangers \((\mathrm{UA}_{\mathrm t})\) and the turbine size (SP). Optimum combinations of \((\mathrm P_{\mathrm{ev}})\) and DT were calculated for each one of these four objective functions for two working fluids (R134a, R141b), three source temperatures and three values of the non-dimensional power output. The ratio of \((\mathrm{UA}_{\mathrm t})\) over the net power output (which is a first approximation of the initial cost per kW) shows that R141b is the better working fluid for the conditions under study.