Organic Rankine cycle turbine expander design, development, and 48 V mild hybrid system integration

Commercial vehicle engine efficiencies continue to increase due to friction reduction, combustion refinements, and air path improvements. This drives the cost per efficiency gain upward. As this occurs, waste heat recovery (WHR) becomes appealing as a solution for lower CO₂ emissions, addressing legislative and economic pressures. Distributing this harvested energy through a 48V mild hybrid architecture allows it to be immediately reintroduced to the vehicle powertrain or stored for future use. Likewise, a 48V infrastructure grants additional gains by electrifying accessories and enables the use of electrified turbomachinery such as an eTurbo™, eBooster®, or electric turbocompound system.One solution for exhaust WHR is the organic Rankine cycle (ORC). Ethanol was chosen as the working fluid in this research for its thermodynamic properties, low global warming potential, cost advantages, and low toxicity.Simulation and testing have demonstrated the expansion device is the key contributor to ORC system thermal efficiency and economic viability. For this study, a turbine expander was developed, realizing high thermal efficiency, large expansion ratio, wide operating range, mechanical simplicity, small package volume, and low mass. This design comprises an oil-free bearing system which eliminates the need for lubricant to be suspended in the working fluid, increasing system output and diminishing heat exchanger contamination risk. Also included is a shaft-coupled, high pressure pump which eliminates the cost and complexity of a separate device. A high speed electric machine is built into the expander, which, using a proprietary power electronics controller, distributes recovered energy to a 48VDC electrical bus. This paper details the decisions which took this machine through three generations of hardware development and two years of testing on an engine-coupled ORC system.One of the most difficult decisions in implementing waste heat recovery is selecting the method by which recovered energy is transferred back to the vehicle. This paper discusses the implementation of the above turbine expander/generator into a 48V mild hybrid system which integrates the WHR with a battery that acts as an electrical storage buffer. This stored energy is delivered back to the vehicle drivetrain via a motor/ generator capable of assist and regeneration coupled to the front engine accessory drive (FEAD).