As previously stated there are 3 main technologies used in CHP systems although other technologies are being developed. However, each of these has a number of different variations and this section provides some further information on these alternatives.

• External Combustion CHP

All Stirling engines have a 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. There are two principal types of Stirling Engine, kinematic and free-piston. In the kinematic engine the 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.

Another external combustion system is the Rankine cycle. The Rankine cycle is characterised by the working gas undergoing a phase change (from liquid to gas) which can be utilised to achieve high power densities.  The most familiar Rankine engine is the steam engine in which water is boiled by an external heat source, expands and exerts pressure on a piston or turbine rotor and hence does useful work. New Rankine systems use organic fluids (such as a refrigerant) and operate at temperatures and pressures much closer to conventional heating and refrigeration appliances. This has the significant advantage of allowing the use of conventional, mass produced components and eliminates many of the technical challenges of steam engines.

• Internal Combustion CHP

Internal Combustion Engines (ICE) offer significant advantages over external combustion in many applications, particularly automotive where there is a need for rapid variation in power output, which can be achieved by changing the fuel supply rate.  However, for continuous operation with extended service intervals, as required for domestic heating systems, the challenges facing ICE are formidable; until recently it was considered that the cost of the components required to achieve the appropriate emissions, efficiency and reliability parameters would be excessive.

Early products, based on automotive ICE technology, operated with some success in niche applications, but have been unable to overcome these technical challenges being been beset with high service and operating costs.  However, newer products have been developed to have a for long life with relatively low service intervention and incorporate a range of features aimed at achieving the performance desirable for domestic and small commercial applications.

• Fuel Cell CHP

Commercially viable fuel cells are still some way away and consequently, their use in CHP systems, domestic or otherwise, has yet to be fully developed. Although there are dozens of products under development throughout the world it is unlikely that any of these will be commercially available before 2010. 

The two main types of fuel cells being developed for micro CHP applications are PEM (Proton Exchange Membrane) and SOFC (Solid Oxide Fuel Cells). SOFC units have the advantage of reforming Natural Gas into Hydrogen without the need for a separate reforming process, reducing the cost and complexity of the unit; but have very high operating temperatures and need a continuous heat sink (even when there is no thermal demand). PEM fuel cells, on the other hand, operate at relatively low temperatures and therefore have difficulty providing of hot water at a high enough temperature. This, combined with the higher potential electrical efficiency of SOFC units, seems to favour SOFC in domestic CHP applications.

• Other technologies

There are numerous technologies with potential for micro CHP applications.  Although there is an optimum thermal and electrical output to match the requirements f any given home, it is possible to supplement the thermal output to a greater or lesser extent and to use the resulting electrical output to whatever use is most relevant. For example, the very low power outputs of thermo-ionic devices may not make a substantial contribution to the overall electricity consumption of the home, it may at least cover the parasitic losses (pumps and fans) of a central heating system and make it possible to continue heating even during a power cut.