Performance comparison and working fluid analysis of subcritical and transcritical dual-loop organic Rankine cycle (DORC) used in engine waste heat recovery

In this paper, a new dual-loop organic Rankine cycle (DORC) consisting of a high-temperature (HT) cycle and a low-temperature (LT) cycle is proposed to recover the waste heat of the exhaust and the engine coolant and the residual heat of the HT cycle. Two different systems are adopted. According to the cycle mode adopted in the LT cycle, the system can be defined as subcritical-subcritical (sub-sub) DORC or subcritical-transcritical (sub-trans) DORC. R125, R143a and R218 are chosen as the candidate working fluids in the LT cycle of the sub-trans DORC, while R124, R134a, R245fa, R600, R600a and R1234yf are the candidates of the sub-sub DORC. Net output power and exergy efficiency are selected as the objective functions. Results show that R143a-based sub-trans DORC system performs the best and the net output power and the exergy efficiency are 39.91 kW and 48.42%, respectively. Small condensation temperature of the HT cycle is advantageous for the performance optimization. Irreversibilities of evaporator EHT, condenser CLT and the two turbines (THT and TLT) are the top four, which are the optimization objectives. For R143a-based sub-trans DORC, system performs best with 100% operating load.