An exergy analysis of various layouts of ORC-VCC systems for usage in waste heat recovery onboard ships

An exergy based analysis is carried out for waste heat recovery from a marine Diesel engine using various layouts of organic Rankine cycles (ORCs) for driving a vapor compression refrigeration cycle. The ORC layouts studied are a simple organic Rankine cycle (ORC), an organic Rankine cycle with internal heat exchanger (RORC) and a serial cascade ORC cycle (SCORC). In addition to the well-known fluid R134a, two hydrocarbon-based refrigerants, namely butane (R600) and isobutane (R600a), are considered in the present study. It is found that sensible improvements are attained using the cascade ORC and the ORC with internal heat exchanger configurations compared to simple ORC under certain conditions. This improvement depends on both the heat source temperature and the working fluid considered. Moreover, the results indicate that R600 as working fluid has the best performance from a thermodynamic point of view. Finally, though R600, a pure hydrocarbon, is completely accepted by the environment, more attention should be paid to its flammability.