Optimal Design of ORC Systems with a Low-Temperature Heat Source

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 (P-ev) and the difference between the source temperature and the maximum working fluid temperature (DT) which maximize the thermal efficiency (eta(th)) or minimize the non-dimensional exergy losses (beta), the total thermal conductance of the heat exchangers (UA(t)) and the turbine size (SP). Optimum combinations of P-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 UA(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.