A system-level study of an organic Rankine cycle applied to waste heat recovery in light-duty hybrid powertrains

Abstract

The global transportation sector produces approximately 20% of human-induced greenhouse gas and pollutant emissions. Road transport accounts for almost 75% of that amount. As a result, the automotive industry has invested heavily in powertrain electrification as a long-term strategy for reducing the environmental impact of urban mobility. However, in-vehicle energy storage and access to charging infrastructure are still large bottlenecks to widespread uptake of battery electric vehicles (BEV). Therefore, hybrid electric vehicle (HEV) architectures are a short-to-medium-term solution that will enable a gradual transition to a cleaner transportation system. Current HEV platforms continue to rely on the internal combustion engine (ICE) for a significant proportion of tractive effort. Therefore, optimising the efficiency of the thermal power plant remains a key challenge. In fact, modern ICE's only convert up to 40% of the energy released in the combustion process into useful mechanical work. The remainder is lost in the form of high-enthalpy exhaust gases and engine cooling. Waste heat recovery (WHR) aims to reclaim a proportion of this energy to increase the overall efficiency of the vehicle and has been identified as a key enabler of real-world emissions reductions by the UK Advanced Propulsion Centre (APC). The organic Rankine cycle (ORC) has been identified as a promising technology for WHR in HEV powertrains. Basic thermodynamic principles were used to identify three key parameters that determine ORC power output: condenser pressure, evaporation pressure and fluid superheating. Matlab was used to develop a semi-dynamic model that takes into account the thermal inertia of the entire system to predict ORC performance under transient operating conditions. This was coupled with a Simulink vehicle model capable of capturing the impact of WHR integration, i.e. increased cooling load, on fuel consumption to provide realistic estimations of ORC energy recovery over a drive cycle. The study revealed a key trade-off between condenser heat rejection and ORC power output that depends on system architecture, operating point and working fluid selection. Hence, the study proposes a revised paradigm for ORC development that focuses on vehicle-level integration from the start of the design process by prioritising fuel economy over ORC power output.