TRANSIENT DUTY CYCLE ANALYSIS FOR MOBILE ORGANIC RANKINE CYCLE APPLICATIONS

asme-orc2015 Tracking Number 114

Internal combustion engines have high exhaust energy at the tail pipe, and mobile organic Rankine cycle (ORC) systems have been proposed to harness this waste heat, thereby providing a significant opportunity for vehicle CO2 emissions reduction. This paper discusses the impact of transient duty cycles on mobile ORC system performance. A thermodynamic model of an ORC system is presented, which includes a detailed heat exchanger model based on the Effectiveness-NTU method, and which has the ability to account for the thermal capacitance of the ORC system – which is of particular importance during vehicle start-up. Thermodynamic system simulations from start-up were performed based on four transient engine test cycles applied to a simulated 11.7L heavy-duty diesel engine: the Constant-Speed, Variable-Load (CSVL) cycle, the European Transient Cycle (ETC), the Non-Road Transient Cycle (NRTC), and the World Harmonized Transient Cycle (WHTC). For a fixed heat exchanger size, working fluid and ORC mass flow rate, it was found that the relatively high-load CSVL cycle (acting as a surrogate for an off-road machine) produced a peak average power of 9.0 kW from start-up, while the WHTC (representative of on-highway driving conditions) only delivered 3.2 kW. Results also indicate a very narrow band (5.5–6.2%) of peak exhaust energy recovery across all duty cycles, implying a close to linear scaling of ORC system power output with duty cycle intensity and engine size. The choice of vehicle/application is thus constrained by the ability to design an acceptable low-power ORC expander, suggesting that vehicles running the most heavily-loaded duty cycles stand to gain the most benefit from mobile ORC systems.