2010 INFINIA STIRLING ENGINE

2010 INFINIA STIRLING ENGINE

 SN Y176, 3.5kw at 60 hertz

Beta type free piston Stirling engine

Piston: 100mm dia. x 30mm stroke.

Displacer: 150mm dia. x 15mm stroke

Pressurisation: 55 bar, helium

The movement of displacers and pistons in free piston Stirling engines is controlled solely by spring, inertial and gas pressure forces.  Power is extracted by way of an alternator, the magnets in which oscillate rather than rotating.   Free piston Stirling engines use the beta layout.

They were initially developed by Professor Beale and his ‘Sunpower’ start-up, at Ohio State University from 1974.

Simple in principle but difficult in practice, NASA later put enormous effort into their development for space probes using radioactive isotopes as the heat source. 

A fruit of this development is the ‘contactless seal’, a fit between piston and cylinder so precise that pressure differences can be maintained without significant leakage while there is no actual contact and therefore no wear or requirement for lubrication.  These seals have very low friction and allow 15 years and more of continuous operation without maintenance.  Some of this work was contracted out to Cummins, best known for diesel engines.  Such sealing is temperamental; however.  If contact ever occurs (because of end bumping, g forces, thermal distortion or foreign particles) the engine cannot be operated again.

Using helium pressurised to 55 bar (750 PSI) as its working fluid, this engine has such contactless seals and was mounted at the focal point of a 5m diameter mirror in a solar thermal generator unit providing 3kw of electricity when in direct sunlight.  The narrow end is the engine part; the wider end contains the linear alternator.  24% thermal efficiency is claimed for this engine (by far the most efficient Stirling engine in this collection).

Infinia failed in 2013 when solar cells (photo voltaics) became too cheap to compete with.  Infinia’s designs were taken over by an Israeli company called Qnergy which now produces a version of this engine for various applications.  One of these uses is generating electricity to operate gas pipeline valving by burning gas that would otherwise be dumped to atmosphere.  This is claimed to reduce methane emissions. More recently they have been developing a 1kw domestic combined heat and power unit.

Peter Lynn for the Mahan Heritage Centre, June 2024