Information on enabling technologies, which enhance the value of micro CHP, is also included.

The majority of true micro CHP systems are currently based on external combustion technology as their characteristics are best suited to this stationary, constant running application.

External combustion engines separate the combustion process (which is the energy input to the engine) from the working gas, which undergoes pressure fluctuations and hence does useful work.

As the combustion process is used to provide a continuous heat input to the working gas, it is more controllable and generally more efficient, cleaner and quieter than internal combustion engines.  External combustion engines also have the potential for long life and service intervals similar to the annual maintenance of a gas boiler.

Internal combustion engines inject fuel and air into the cylinders where combustion occurs.  The resulting temperature and pressure changes of the fuel/air mixture (which is also the working gas) act on the piston to produce useful work.

As the combustion process is cyclical, rather than continuous, it is more difficult to ensure complete combustion of the fuel, and noise and pollutant emissions tend to be higher than for external combustion engines.

Early mini CHP products were derived from automotive engines converted to run on Natural Gas and suffered from very high service costs and unreliable operation.  Current models, however, have been specifically developed for extended operation between services.

In a fuel cell, the chemical energy within the fuel is converted directly into electricity (with by-products of heat and water) without any mechanical drive or generator.

In theory this can result in high electrical conversion efficiencies and low emissions.  However, numerous additional components are required to condition the fuel and to convert the DC electrical output into AC suitable for domestic installations; their theoretical potential has yet to be realised in any commercially viable domestic product.

Fuel cells have begun to be developed specifically for micro CHP applications and, although large numbers of units are being trialled in Japan it is unlikely that commercially viable products will be available in the UK before 2012, .

There are numerous experimental technologies which may at some future date result in useable products. 

These include thermo-electric technologies which utilise temperature difference acting on metals or semi-conductors to produce electricity and thermo-photovoltaic units which convert the radiant energy emitted by the burner to produce electricity from infra-red sensitive PV cells. There are also novel engine designs such as the MTT pico-turbine illustrated above.

Although these are relatively inefficient and produce little power, there may be applications, for example, in "self-powered boilers" for which such concepts are of value.

Micro CHP has implications not only for the immediate installation, but also for the electricity system to which it is connected.  This section, therefore, also covers related technologies, products and concepts which are either essential  to enable the operation of micro CHP within the national infrastructure, or which enhance the performance or value of micro CHP products themselves.

WhisperGen (UK, Netherlands, Germany)

Honda Ecowill (Japan, USA)

Baxi Dachs (UK, Europe)

EC Power (UK, Europe)