Analysis of regenerative dual-loop organic Rankine cycles (DORCs) used in engine waste heat recovery

• Low condensation temperature of the HT loop is beneficial to performance optimization.
• The influences of inlet temperature of turbine THT and regenerators are evaluated.
• System N adopting mode 1 (water/subcritical-R143a/transcritical) performs best.
• Condenser CLT and turbine TLT are the optimization objectives of exergy performance.

In this paper, three regenerative dual-loop organic Rankine cycle (DORC) systems are proposed to compare with the simple DORC system. Waste heat of the exhaust, engine coolant and residual heat of the HT loop are recovered in these four systems. In the HT loop, water and siloxane are chosen as working fluid candidates, transcritical cycle and subcritical cycle are evaluated. In the LT loop, R143a is used as the working fluid and transcritical cycle is adopted. Net output power and exergy efficiency are selected as objective functions. Based on the engine data and mathematic model, operating parameters are optimized and component irreversibility is analyzed. Results show that low condensation temperature of the HT loop is beneficial to performance optimization. The inlet temperature of turbine THT should be high for wet fluids in subcritical cycle and low for dry fluids in both subcritical and transcritical cycles. Maximum net output power and exergy efficiency are obtained when water is used as the working fluid of the HT loop and no regenerator is adopted in the system. Corresponding values are 39.67 kW and 42.98%. When siloxane is used as the working fluid of the HT loop, DORC with double regenerators performs better. For all systems evaluated, irreversibilities of condenser CLT and turbine TLT are large.