Combined heat and power

‘Micro-CHP’ stands for micro combined heat and power. This technology generates heat and electricity simultaneously, from the same energy source, in individual homes or buildings.

The main output of a micro-CHP system is heat, with some additional electricity generation. A typical domestic system will generate up to 1kW of electricity once warmed up: the amount of electricity generated over a year depends on how long the system is able to run. Any electricity generated and not used can be sold back to the grid.

Domestic micro-CHP systems are currently powered by mains gas or LPG; in the future there may be models powered by oil or bio-liquids. Although gas and LPG are fossil fuels rather than renewable energy sources, the technology is still considered to be a ‘low carbon technology’ because it can be more efficient than just burning a fossil fuel for heat and getting electricity from the national grid. Micro-CHP systems are similar in size and shape to ordinary, domestic boilers and like them can be wall hung or floor standing. The only difference to a standard boiler is that they are able to generate electricity while they are heating water.

There are three main micro-CHP technologies. The difference is the way in which they generate electricity:

• Stirling engine micro-CHP, the electrical output is small relative to the heat output (about 6:1) but this is not necessarily a problem for micro-CHP.
• Internal combustion engine CHP, the most proven technology. These are essentially, and sometimes literally, truck diesel engines modified to run on natural gas or heating oil, connected directly to an electrical generator. Heat is then taken from the engine’s cooling water and exhaust manifold. They can have a higher electrical efficiency than a Stirling engine but are larger and noisier and are not currently available for the domestic market.
• Fuel cell CHP technology, where fuel cells work by taking energy from fuel at a chemical level rather than burning it. The technology is still at developmental stage and not widely available to consumers.