In-depth analysis of the effects of turbo-expander and condenser pressures on the performance of an Organic Rankine Cycle (ORC) waste heat recovery system

Considering the environmental problems and energy prices, waste energy recovery is one of the subjects that should be given more attention. Currently, internal combustion engines (ICEs) are the most used heat engines. In the present paper, while introducing four different configurations with different equipment arrangements, the waste energy recovery using the organic Rankine cycle (ORC) from a widely used ICE in the shipping fleet is evaluated. All four designs include a single-loop ORC but with a different number of heat exchangers. Case 1 is simple, case 2 includes a recuperator, case 3 includes a preheater, and case 4 includes both a recuperator and a preheater. Due to the low temperature of wasted energy in ICEs, suitable working fluids are selected and for all fluids, the effects of inlet pressure to turbo-expander (TE), inlet pressure to the condenser, changes of ICE power, and TE isentropic efficiency are investigated. The results show that the set (ICE + ORC) best net energy and exergy efficiencies are related to case 4, and equal 42.77 % and 41.74 %, respectively. The amount of destroyed exergy in the cycle for cases 1 to 4 equals 2640 kW, 2595 kW, 2625 kW, and 2560 kW, respectively. Considering the exergy content of consumed fuel, the exergy efficiency of the cases equals 40.31 %, 41.53 %, 40.62 %, and 41.74 %, respectively. Increasing TE inlet pressure from 3 to 8 bar increases the avoidance of CO2 production from about 200 tons per year to about 700 tons.