A new type of automotive waste heat recovery system has been fired up on a recently commissioned test rig at the University of Brighton. The organic rankine cycle test rig uses a novel linear free piston expander to overcome the technical and economic barriers to using rankine cycle technology for automotive waste heat recovery. Initially configured to simulate heavy-duty truck applications, the 20kWe system can be scaled down to 3kWe for light-duty trucks and passenger cars. It has the potential to become the first commercially viable method to capture heat energy normally wasted in the exhaust, improving the efficiency of all types of vehicles.
The results from rig tests are expected to confirm the system’s potential to convert the high grade heat in the exhaust into electrical power, which can contribute to either powertrain or auxiliary loads. By providing physical test results to validate simulation tools, the data will allow Libertine to model specific customer applications with high confidence.
The test rig and linear free piston expander technology will form a centerpiece at Linear Power 2015, billed as the world’s first technology forum for linear power systems technology researchers and application developers.
“Visitors to Linear Power 2015, held in Brighton in September, will be able to see the system working in the lab, review the results and discuss potential applications,” says Libertine’s CEO, Sam Cockerill. “As part of our continuing relationship with Brighton University, we are co-sponsoring the Linear Power event together with NIDEC SR drives, an industrial partner on the heat recovery demonstration project.”
The project at Brighton, part-funded by the UK government through Innovate UK, uses a pair of Libertine’s linear free-piston expanders in an ethanol rankine cycle to extract energy from a hot gas source which represents the flow of a vehicle’s exhaust under a range of steady-state conditions. A free piston expander system has a number of advantages over turbo-generator systems. The operational flexibility and two-phase flow tolerance permits useful output to be generated during part load and transient conditions, which represent the majority of drive cycle conditions. In addition, the system provides a larger single-stage expansion ratio which suits ethanol/water rankine cycles.
Libertine’s piston geometry, electrical machine design and cylinder construction are ideally suited to rankine cycle applications. By integrating the electrical machine into the cylinder wall, rather than attempting to isolate it with seals, the high friction losses often encountered by free piston engine developers are dramatically reduced. Libertine’s expander also uses a novel transfer valve arrangement to meter the high-pressure working fluid into the expansion chamber without the need for active inlet valve control. This offers a simpler and more efficient solution than alternative inlet valve arrangements.
Initial results from the trials will be published during Q3 this year, with completion of the tests scheduled for the year end.