Thermodynamic performance simulation and design optimisation of trilateral-cycle engines for waste heat recovery-to-power generation

The trilateral cycle (TLC) is one of the most promising alternatives among the heat recovery-to-power technologies, due to its compact system configuration and high performance at relatively low compression work and low-to-moderate expander inlet temperature. These feats make the TLC beneficial for off-grid applications particularly in remote or offshore applications where power-to-weight ratio of the power plant is of significance. This study presents the thermodynamic performance simulation and design optimisation of the TLCs using unconventional working fluid for heat recovery-to-power generation from low-grade waste heat, which is considered for process development and integration of the TLC. Four system configurations, comprising the simple TLC, recuperated TLC, reheat TLC and regenerative TLC are analysed and compared their performance metrics. Based on the theory of steady-state steady-flow thermodynamics, the simulation models of the TLC power plants, corresponding to their thermodynamic processes are established and implemented using engineering equation solver. The results show that the thermal efficiencies of the simple TLC, recuperated TLC, reheat TLC and regenerative TLC employing n-pentane are 11.85-21.97%, 12.32-23.91%, 11.86-22.07% and 12.01-22.9% respectively at subcritical operating conditions with low-grade heat in the temperature limit of 393-473 K. These suggest that the thermal integration of the optimised design of the simple TLC enhanced heat exchange efficiencies as well as the performance metrics. A comparative study among these cycles shows that the proposed recuperated TLC achieved the best performance metrics.