Thermo-economic performance evaluation and multi-objective optimization of a screw expander-based cascade Rankine cycle integrated with parabolic trough solar collector

In the present study, a cascade power generation system, consisting of a partial evaporation Rankine cycle (PERC), and an organic Rankine cycle (ORC) with a parabolic trough solar collector (PTSC) as the driver coupled with a storage tank, has been studied and optimized from energy and exergoeconomic viewpoints. Instead of using conventional steam turbine, a screw expander has been utilized in PERC, in order to facilitate coupling the PERC with the PTSC and the storage tank, operating near the recommended temperature of 300 degrees C, as well as to avoid producing superheated vapor in the upper Rankine cycle. This configuration is appropriate specifically because of the pressure limit at the inlet of expander (4 MPa), corresponding to a saturation temperature of 250 degrees C for steam. With toluene as the selected organic fluid among 4 candidates, parametric analysis was performed to investigate the effect of variations in solar radiation intensity, collector aperture area, PERC evaporation and condensation temperatures, and expander inlet steam quality on the thermoeconomic performance of the system, followed by a tri-objective optimization using genetic algorithm considering net power output, exergy efficiency and total cost rate as objective functions. The obtained results show that for the optimum design point, the studied solar power plant with aperture area of 5540 m(2) and storage tank volume of 184.7 m(3) can produce 782 kW of power with exergy efficiency of 18.61% and total cost rate of 228 $.h(-1). The optimum design case indicates an improvement in net power output, exergy efficiency, and unit cost of electricity by 65%, 2.9%, and 27.26%, respectively, compared to the base case. Furthermore, at the optimal point, the power output of ORC (440.6 kW) is higher than that of PERC (349.2 kW). However, the unit cost of electricity production is lower for PERC (22.82 $.GJ(-1)) compared to ORC (37.19 $.GJ(-1)).