A prototype 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 from Libertine (earlier post) 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 20 kWe system can be scaled down to 3 kWe for light duty trucks and passenger cars. Libertine expects the results from rig tests 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 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.
Libertine says that 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 technology leverages a combination of piston geometry, electrical machine design and cylinder construction. All three are relatively long and smaller in diameter than those currently used by other free-piston developers. This novel architecture reduces the moving mass of the piston relative to the electrical machine’s force, giving more effective and accurate control of the piston motion.
Libertine says that its piston geometry, electrical machine design and cylinder construction are 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 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.
The test rig and linear free piston expander technology will form a centerpiece at Linear Power 2015 being held at the University of Brighton in September, billed as the first technology forum for linear power systems technology researchers and application developers.
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