Thermoeconomic analysis and optimization of a regenerative two-stage organic Rankine cycle coupled with liquefied natural gas and solar energy

Abstract

This study investigates the thermoeconomic performance of a new integrated system including a regenerative two stage organic Rankine cycle which is coupled with a parabolic trough collector via a thermal storage tank. The cold energy of liquefied natural gas (LNG) is used to absorb the heat duty of condenser. The LNG subsystem not only allows the ORC cycle to produce more power by reducing its condensate pressure, but also provides the system with extra power via the LNG expander and chilled water. The system is capable of producing power with solar fraction of a hundred percent during the day. The thermoeconomic analysis is performed to optimize the system for design point conditions. The analysis also reveals the exergoeconomic criteria on system components. Results show that solar collector has the most value of Z˙+C˙D which is due to both high exergy destruction and high investment costs of solar collector. Also, storage tank and condenser are the second and third important components with respect to exergoeconomic criterion. Parametric analysis is performed on the system to show the effects of eleven key thermodynamic parameters on system performance. In order for optimization, the product cost rate and exergy efficiency are chosen as the objectives. Eleven decision variables including inlet temperature and pressure of the turbines, heat exchanger minimum temperature differences along with the mass flow rate of storage tank, condensate pressure and LNG pressure were chosen according to parametric analysis. With the aid of TOPSIS decision making technique, the optimal point was selected among the Pareto frontier of the genetic algorithm. Results show that system can reach the efficiency of 19.59% and product cost rate of 3.88 million dollars per year.