Exergy analysis of a combined power cycle using low-grade heat source and LNG cold energy

This paper presents a comparative exergy analysis based on the second law of thermodynamics for a combined power generation cycle using a low grade heat source and LNG cold energy. The combined cycle consists of a LNG power cycle and an ammonia-water Rankine cycle with (R) and without regeneration (S). The effects of system parameters on the exergy destruction at each component as well as the total exergy efficiency are investigated extensively. A characteristic exergy efficiency-work diagram is proposed to consider the two system variables simultaneously. The results indicate that the thermodynamic performance of the combined system is strongly affected by the system parameters. Under the typical operating conditions for a heat source of 200°C, calculated optimal exergy efficiencies are 22.0% (S) and 21.4% (R) for the condensation temperature at T_c = 40°C, and 27.0% (S) and 28.5% (R) for T_c = −20°C.