Influence of exhaust heat distribution on the performance of dual-loop organic Rankine Cycles (DORC) for engine waste heat recovery

In this paper, two Dual-loop Organic Rankine Cycles (DORC) were used to recycle the waste heat from both engine exhaust and coolant. The two cycles were defined as S1 and S2, respectively. Both cycles have a high-temperature loop and low-temperature loop. In S1, both high-temperature and low-temperature loops recycled the waste heat from engine exhaust, while in S2, only the high-temperature loop recycled the waste heat. Water, methanol, toluene, R245fa, and pentane were the candidate working fluids of the high-temperature loop, while R143a was the working fluid in the low-temperature loop. The influence of exhaust heat distribution on the performance of dual-loop organic Rankine cycles(DORC) for engine waste heat recovery was studied in this paper. Net output power, exergy efficiency, sum of products of heat transfer coefficient and heat transfer surface area of the heat exchanger (UA), and unit electricity production cost of the system (EPC) were selected as the objective functions. The results show that the lower the proportion of waste heat from the exhaust distributed to low-temperature loop, the better the system performance. When the waste heat from exhaust was completely recycled by high-temperature loop, the system performance reached its best, namely, S2 performed the best. Methanol-based S2 cycle performed the best and the net output power, exergy efficiency, UA, and EPC were 117.46 kW, 57.15%,65.02 kW/K and 0.0532 $·(kW·h)−1, respectively. In DORC, irreversible losses mainly occurred in the evaporator, turbine and condenser of the high-temperature loop and the heat exchanger, turbine and condenser of the low-temperature loop, among which the losses in evaporator of high-temperature loop and condenser of low-temperature loop were the greatest.