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There are
two principal types of Stirling Engine, kinematic and free-piston. All Stirling engines
have two pistons (functionally speaking), one of which shuttles the
working gas between the hot and cold zones and is known as a displacer,
whilst the other is subject to the resulting pressure changes and does
work to drive the engine. In the kinematic engine, these two
pistons are physically connected by a crank mechanism, whereas in the
free-piston engine, there is no physical linkage and the displacer
oscillates resonantly. In theory the LFPSE (Linear Free Piston
Stirling Engine) is much simpler as it contains fewer moving
parts. In practice, the challenges of differential expansion and
linear generator design have so far proved a major obstacle to
commercialisation.
Despite expectations of product
availability as recently as late 2012, the decision by E.ON to abandon a
fully proven product as it no longer fitted their core strategy, may
have been the final nail in the coffin for this technology, particularly
given developments in fuel cell based micro CHP technologies.
For further discussion of the relative
merits of these engine types see section on
papers.
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The WhisperGen micro CHP unit comprises a four cylinder engine which leads to
smooth, low vibration operation, with noise levels similar to a
domestic freezer.
In 2004, the WhisperGen was the first
Stirling engine micro CHP to be made commercially available anywhere in
the world.
In January 2008, WhisperGen announced the
establishment of a joint venture (EHE) with Spanish white goods manufacturer Mondragon CC
to mass produce units for the European market.
E.ON announced their product launch in
November 2012, then just prior to actually doing so, decided to "focus
on core business", whatever that means.
EHE is now in receivership and there seems little
prospect of that technology becoming commercially available again in the
foreseeable future. |
The
Microgen unit, developed by BG Group from a US (Sunpower) design, is a
LFPSE which is intended for wall-mounting; it contains a supplementary
burner which enables it to meet the full heating requirements for even
larger homes.
Following disposal by BG Group in 2007,
development of the Microgen unit was taken over by MEC, a consortium of
gas boiler companies (Viessmann, Baxi, Vaillant, Remeha) and Sunpower.
Each of the boiler companies has developed their own variant of
micro CHP unit incorporating the MEC engine, now being manufactured in China.
The UK variant is manufactured by Baxi, part
of the BDR Thermea Group which also includes Remeha, De Dietrich and
Brötje.
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The
Infinia (formerly known as STC) LFPSE was developed for incorporation in
micro CHP products manufactured by Ariston
(formerly MTS) and Bosch
in Europe as well as Rinnai in Japan.
Rinnai will also produce the LFPSE module for
integration into micro CHP packages by the other partners for the
European market, with a trial of 1000 units planned for 2008-2010.
Although based on a virtually identical core
LFPSE as the MEC derivatives, this unit is more realistically housed
within an integrated floor-mounted unit incorporating a hot water
cylinder.
The Infinia engine previously formed the
basis of the ENATEC micro CHP unit, a joint venture between the Dutch
utility ENECO, ECN and appliance manufacturer ATAG.
In 2013. Infinia was acquired by Qnergy.
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The Disenco unit is a kinematic design with
an electrical output of around 3kWe, significantly higher than the other
products.
The product originated in the Swedish TEM
SCP Stirling engine which was subsequently developed by Sigma
Elektroteknisk AS in Norway, before being taken up by Disenco in the UK.
In January 2008, Disenco announced a
manufacturing partnership with Autocraft to produce the core engine,
with packaging by Malvern boilers and recently announced marketing deals
with Endesa and Centrica.
In early 2010 Disenco was placed in
receivership; the design was been taken over by Inspirit Energy who
expected to trial the unit in 2011.
In 2013, Inspirit still seemed to be facing
funding difficulties and no product availability has been announced.
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Supply only cost (UK)
£contact supplier
Supply only cost (Germany)
€10,000 |
Supply
only cost (Remeha variant)
NL 2010 €10,000* (less €4000
subsidy)
DE 2011 €11,000 (less €1,500 subsidy) |
Supply
only cost
UK 2010 £6-8000 (depending on
FIT)* |
Supply
only cost
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The Cleanergy V161 started life in the
1980's, developed by Kockums in Sweden. Based on previous
developments by Philips, it is from the same stable as the Disenco 3kWe
product.
After several reincarnations, most latterly
as the Solo (Kleinmotoren) product, it has now once again returned to
Western Sweden where it is produced for gas fired CHP and solar power
generation applications.
It is an alpha type Stirling engine meaning
that the working gas shuttles between two cylinders, one containing the
displacer, the other the working piston. Somewhat unusually, it is
not a hermetically sealed package and thus faces the challenge of Helium
leakage from the high working pressure cylinders to atmosphere.
This is overcome partly by means of highly efficient piston seals (which
result in relatively high frictional losses). However, the
significant Helium leakage which still occurs is continuously replaced
from a cylinder of the gas which must be periodically replenished.
The unit is able to modulate power up to
9kWe, achieving an electrical efficiency (LHV) of 25% at nominal power
rating and a flow temperature of 40C. For more realistic flow
temperatures, electrical efficiency falls by 2-3%.
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Formed by Ricor, the Israeli company best
known for its cryogenic Stirling as well as solar conversion
technologies, Qnergy recently (November 2013) acquired the US based
Infinia.
Qnergy are now offering a 5kWe dual-opposed
LFPSE product aimed at commercial applications. This configuration
should provide a more even power output than single piston
configurations, reducing vibration and engine stress leading to enhanced
reliability, although "silent" and "maintenance free" operation remain
challenging.
However, the claimed (target) efficiency of
20% (LHV) is not particularly impressive at that power level and it will
need to demonstrate significant cost, environmental or operational
benefits in terms of maintenance if it is to compete with the wide range
of ICE based products now available.
Qnergy also offer solar Stirling engines
working with Abengoa, and appear to be developing Thermo-Acoustic
Stirling Engines.
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