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First commercially viable exhaust heat recovery system


kscarbel2
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Transport Engineer / June 17, 2015

Brighton University and engineering innovation firm Libertine are claiming a ‘first’, with the demonstration of a commercially viable exhaust heat recovery system for trucks, running in test at 20kWe (27 horsepower).

The project, which is part-funded by the UK government through Innovate UK, uses a pair of Libertine's linear free-piston expanders.

They run in an ethanol rankine cycle and extract energy from a hot gas source which represents the flow of vehicle exhaust under steady-state conditions.

Libertine chief exec Sam Cockerill (pictured) says the free piston expander system has several advantages over turbo-generator systems.

Its operational flexibility and two-phase flow tolerance permits useful output to be generated during part load and transient conditions, he explains – pointing out that these 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 particularly suited to rankine cycle applications, says Cockerill.

By integrating the electrical machine into the cylinder wall, rather than attempting to isolate it with seals, the friction losses often encountered by free piston engine developers are dramatically reduced, he explains.

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, says Cockerill, 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.

They are expected to confirm the system's potential to convert the high grade heat in vehicle exhausts 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 real vehicle applications with high confidence.

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