Combined heat and power (CHP) is a way of generating heat and electricity simultaneously. It has been in use in the industrial sector since the energy crisis of the 1970s and has become more popular recently due to the threat of climate change.

While not actually a form of renewable energy, CHP is included here because it is a way of generating some of your own electricity, and can result in a significant fall in your carbon footprint. It has been flagged up by some commentators as the best way to lower domestic carbon emissions.

Micro-CHP (domestic-sized) is different from many of its industrial forebears, as they tend primarily to generate electricity, with heat as a useful bi-product. Micro-CHP is being developed to produce heat (replacing a domestic boiler) and generate electricity as a bi-product.

Research by the Carbon Trust published at the end of 2007 found that micro-CHP is most suited to larger homes with three or more bedrooms, or older houses where it is not currently cost effective to improve insulation (such as those with solid brick walls). This type of home could see carbon emissions reductions of between 5 and 10 per cent.

The micro-CHP systems work best in buildings where they operate for many hours at a time, rather than intermittently. These tend to be ones that need long and consistent heating periods.

You can use Baxi's energy savings calculator here to see how it would work for you.

Micro-CHP works in a similar way to a standard gas boiler. Both floor standing and a wall-mounted varieties are available. There is also potential for biomass fuelled CHP, but domestic versions are not likely to reach the market in the near future.

• A condensing boiler heats your hot water and – through your central heating system – your house.
• The steam produced is captured and used to produce electricity (using a Stirling engine or, potentially in the future, a fuel cell).
• This electricity can be used in the house, or exported to the grid.

Relatively small amounts of electricity are generated – and only when the central heating is on. The aim is to reduce the amount of electricity you draw from the grid (most of which comes from sources high in carbon emissions), not to replace your electricity supply.

For example, the Baxi Ecogen generates a constant 1 watt per hour when the boiler is working at 6w.

Find a CHP installer.

The only wall-hung micro-CHP unit available so far is the Baxi Ecogen. It has recently broadened out its supply network and you can find installers through our directory - or in the MCS installer list for CHP. It costs c. £8 - 10,000 installed.

Baxi calculates that someone replacing a band G boiler with an Ecogen will save £650 per year. This consists of:

• 30% less gas used
• income from the feed-in tariff
• lower electricity bill due to using the electricity generated

The only wall-hung micro-CHP unit currently available is the mains gas version of the Baxi Ecogen. It looks like a traditional boiler, is slightly deeper, and double the weight. This means it must be hung on a load bearing wall on the ground floor, with space to site three meters nearby. There isn't a combi version, so you will also need space for an insulated cylinder. An LPG version for houses off the gas grid is also available.

In the pipeline:

The WhisperGen is expected to be sold by E.on in early in 2011. It is free standing, and roughly the same size as a dishwasher.

Ceres Power aims to launch a fuel cell-based product onto the mass market in 2011. Also expected in 2011 is the wall-mounted Genlec - which claims to be a "low cost" micro-CHP technology comprising an Organic Rankine Cycle module.

Worcester-Bosch is trialling the Greenstar CDi Dual Gen, which uses a sterling engine with condensing boiler technology. It is expected on the market in 2012.

From the blog:

Is domestic CHP dead and buried?

Sources:

Micro CHP

Carbon Trust

Cambridge Consultants