Exergy-economic study and multi-objective optimization of a geothermal-based combined organic flash cycle and PEMFC for poly-generation purpose

Using the organic flash cycle (OFC) instead of the organic Rankine cycle can mitigate some loss associated with the heat transfer process in the heating heat exchanger. On the other hand, OFC still needs some modifications to improve its performance. In the present study, an absorption chiller (for cooling production) and a proton exchange membrane fuel cell (PEMFC) (for extra power generation) are incorporated with the OFC to enhance its performance. Also, the waste of the high-temperature throttle valve and heat rejection stage in OFC are recuperated with an expander and two thermoelectric generators, respectively. The geothermal energy is used as a heat source of OFC and the residual heat is consumed in a hot water heat exchanger to produce heating. Furthermore, the output power of PEMFC is sent to a reverse osmosis desalination system for freshwater production. A parametric analysis is done to assess the effect of changes in geothermal hot water temperature, the minimum temperature difference in the heater heat exchanger, the separation temperature of OFC, and the condensation temperature of OFC on the output performance. The three-objective optimization shows that exergy efficiency, payback period, and net present value can be obtained at 25.47%, 0.299 years, and 80.19 M$, correspondingly. At this optimum point and with a mass flow rate of 15 kg/s for the geothermal stream, the proposed system can produce 544.4 kW power, 1889 kW heating, 698.3 kW cooling, and 13.7 kg/s freshwater.