The global market for commercial vehicles is expected to exceed 20 million units per annum by 2015 (Global Industry Analysts Inc. 2011). The Automotive Council's low carbon technology roadmap for commercial vehicles identifies a specific need for technologies to improve thermal efficiency focused on using waste exhaust and coolant heat - particularly for higher duty commercial vehicles - in the period to 2025 and potentially beyond. This project seeks to research and develop a novel free piston expander based waste heat recovery system aimed at the commercial vehicle market. The free piston expander offers efficiency and cost benefits over conventional turbine and screw expanders, impacting positively both on the initial investment cost of the system and potential returns to the end user. Improvements to the business case for waste heat recovery would significantly increase market uptake of the technology in the commercial vehicle market and reduce CO2 emissions from the sector.
The claimed benefits of the free piston expander are realised both through novel manufacturing methods for the Libertine technology and the inherent characteristics of a free piston expander. The Libertine device can be manufactured from extrusions, significantly reducing the manufacturing cost of the device. The device also integrates an electrical machine in the barrel of the expander that improves power conversion efficiency and the controllability of the device. Simulation has indicated that the free piston expander technology gives the opportunity to use high expansion ratios, possibly with some condensation of the working fluid. These features allow a wider selection of working fluids, better matched to the available thermal energy from the vehicle internal combustion engine cooling and exhaust system. The Libertine free piston expander therefore offers potential benefits compared to screw and turbine expanders in matching the heat recovery system to the available thermal energy, improving both power output and conversion efficiency.