Thermodynamic optimization of the operating parameters for a combined power cycle utilizing low-temperature waste heat and LNG cold energy

This paper deals with the optimization of a novel combined power system, which can effectively recover low-temperature waste heat and fully utilize the cold energy of LNG as well, based on the first thermodynamic law and the second thermodynamic law respectively. Parametric analysis has been performed to study the effects of heat source temperature, ammonia turbine inlet pressure, LNG turbine inlet and outlet pressures, as well as ammonia mass fraction of basic solution. The simulation results show that the system performance can be improved by applying optimization techniques. The optimization is conducted under a certain set of constraints by using the differential evolution (DE) algorithm to maximize the first and the second law efficiency respectively. Through parallel direct search over the whole feasible region, it is found that a maximum first law efficiency of 39.33% can be obtained when variable vector V1 = [423.70 K, 1.8 MPa, 3.904 MPa, 0.3 MPa, 0.52]; while a maximum second law efficiency of 55.62% can be obtained when variable vector V2 = [423.93 K, 1.874 MPa, 3.493 MPa, 0.8 MPa, 0.48]. In addition, the irreversibilities in various components of the cycle under typical operating conditions and exergy efficiency optimum condition have been compared through detailed exergy analysis. -- Highlights: • The combined power cycle utilizes low-temperature waste heat and LNG cold energy. • Parametric analysis results recommended that the cycle may be optimized. • Thermal and exergy efficiency were selected as objective functions separately. • Differential evolution algorithm was applied to reach the maximum efficiency. • Optimization of operating parameters improved the cycle performance significantly