Stirling Engine
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the big picture
By Jeremy Harrison

 

Stirling Engine | Rankine Engine

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.

 WhisperGen  MEC (Microgen)  Infinia (STC)  Disenco (Inspirit)

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.

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.

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.

No longer available although the website still contains micro CHP information.

Cost and performance data below for historic record purposes only.

For UK sales contact:

Baxi Ecogen

 

For further information see:

Baxi

BDR Thermea

Remeha

Vaillant

Viessmann

For further information see:

Rinnai

Bosch

Ariston

Infinia

For further information:

Inspirit

Electrical output

1kWe

Electrical output

1kWe

Electrical output

1kWe

Electrical output

0.5-3kWe

Thermal output

7kWt (engine) plus 7kWt (burner)

Thermal output

3-24kWt

Thermal output

4-40kWt

Thermal output

12-17.4kWt

Application

Individual family homes

Application

Individual family homes

Application

Individual family homes

Application

Homes & small commercial

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

Installed cost (UK)

2004  £3000 including VAT (initial units subsidised by E.ON)

2010  £6-8000 (depending on FIT)*

Installed cost (Germany)

2010  €14,000

Installed cost

UK  2010  £6-8000

 

Installed cost

Installed cost

Availability:

Initial production runs sold out very quickly.  Mass produced units available in limited numbers 2010 in Germany & Netherlands.

Availability:

2010 limited availability (Baxi)

2010 limited availability from Remeha (NL)

Availability:

2008-2011 trial

2012 limited availability

Availability:

 

       
  Cleanergy V161  Qnergy    

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%.

 

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.

 

   
   

Image: www.stirling-engine.de/

Image: Qnergy    

For further information:

Cleanergy AB

For further information:

Qnergy

   

Electrical output

2-9kWe

Electrical output

5kWe

   

Thermal output

8-25kWt

Thermal output

~24kWt (derived)

   

Application

Commercial CHP

Landfill gas power generation

Biogas power generation

Solar power generation

Application

Commercial

   
Page updated 7th February 2014      
 

the big picture
By Jeremy Harrison

Contact : info@microchap.info

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This site was last updated on 23 February 2014  © Jeremy Harrison