Exergo-economic analysis of different power-cycle configurations driven by heat recovery of a gas engine

Abstract

In the present work a 34.3 kW stationary gas engine considered as prime-mover of bottoming thermodynamic cycles. The waste heat recovery cycles of engine exhaust gas consist of several configurations of organic Rankine cycle and combination of Rankine/Kalina cycles, also the engine jacket water heat is recovered by trilateral flash cycle. The performance of bottoming cycles was investigated from thermodynamic and thermo-economic points of view. For the organic Rankine cycle, three fluids including Benzene, Toluene and Cyclohexane have been compared. Also, considering that the trilateral flash cycle is a suitable cycle for heat recovery from a low temperature heat source due to its better matching between the temperature profiles of cycle working fluid and heat source fluid, this cycle is considered as bottoming waste heat recovery cycle for engine jacket water. In order to compare different working fluids and cycle configurations, three objective functions including energy efficiency, exergy efficiency and specific investment cost for the 18 studied cases are compared. The results reveal that, energy, exergy efficiency and specific investment cost of all configurations with toluene as working fluid of organic Rankine cycle is the highest and least compared to the other working fluids, respectively. But for configuration Rankine with Open Feed Heater-Kalina-Trilateral Flash Cycle, benzene fluid is better in terms of specific investment cost, however the energy and exergy efficiency of this cycle is also maximal with toluene fluid. Also, the results depict that configurations Rankine with Internal Heat exchanger – Trilateral Flash Cycle and Rankine with Open Feed Heater_Kalina_Trilateral Flash Cycle thermodynamically, and configurations Rankine with Internal Heat exchanger – Trilateral Flash Cycle and Rankine with Open Feed Heater – Trilateral Flash Cycle exergo-economically show the better performance. In this regard, employing configurations Rankine with Internal Heat exchanger – Trilateral Flash Cycle, Rankine with Open Feed Heater-Kalina – Trilateral Flash Cycle and Rankine with Open Feed Heater – Trilateral Flash Cycle in base mode leads to 6.85 kW, 6.48 kW and 6.54 kW increment in net power produced imposing 3318 $/kW, 4090 $/kW and 3377 $/kW specific investment cost, respectively.