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Session: Session 13: Waste heat recovery from engines II
Session starts: Tuesday 13 October, 15:00
Presentation starts: 15:40
Room: 1B Europe

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Miles Robertson (Imperial College London)
Aaron Costall (Imperial College London)
Peter Newton (Imperial College London)
Ricardo Martinez-Botas (Imperial College London)

Abstract:
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.