Comparison of the performance of two different Dual-loop organic Rankine cycles (DORC) with nanofluid for engine waste heat recovery

To recover the heat from engine exhaust, coolant liquid and high-temperature loop, two different Dual-loop organic Rankine cycles (DORC) are studied in this paper. The two systems differ for the number of stages of heat recovery from engine exhaust, and both include high temperature loop and low temperature loop in each system. R123, R245fa, ethanol, R141b, and water are the candidate working fluids of HT loop, and R143a is the working fluid of LT loop. Because the coolant water in engines has lower temperature, it is more difficult to recover its heat. Therefore, in this study, graphene nanoparticles and carbon nanotubes are added to coolant water to enhance its heat transfer. Net output power, thermal efficiency, and exergy efficiency are selected as the objective functions. Results show that the single stage system (S1) is a little better than the other. Water-based S1 performs the best and the net output power, the thermal efficiency, and the exergy efficiency are 96.92 kW, 14.13% and 64.04%, respectively. High evaporation pressure and turbine inlet temperature are better for performance optimization. And when the coolant water contains 0.5 wt% carbon nanotubes, system reaches the max net output power increment of 3.84 kW.