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Generating free electricity while you heat your home sounds like a good deal. Not only could it save you money, but it also might reduce carbon dioxide emissions. Micro combined heat and power (CHP) systems have been doing this for several years, but none have made it to market in the UK 鈥� so far.

After a few false starts, owing to the financial viability of the technology compared with standard boilers, the first crop of micro CHP units will be available on these shores for home use next year. Boiler maker Baxi has signed a deal with British Gas to distribute a micro CHP boiler, the Ecogen, which should be available from June 2009. Hot on their heels, CHP specialist WhisperGen has set up a joint venture with Spanish firm Mondragon Corporation and Meridian Energy with production due to start this January.

There are two main varieties of domestic micro CHP: stirling engines and fuel cells. At present, there are no fuel cell systems on the market, although Ceres Power is planning to launch one with British Gas in 2011. Both the WhisperGen and Baxi systems are stirling engines, although the engines are of slightly different designs and produce somewhat different heat outputs.

Stirling engines produce between 6kWh and 10kWh of heat for every 1kWh of electricity. The more heat they produce, the more electricity they generate, which means lower emissions and more money saved.

A report from the Carbon Trust in 2007 said that stirling engines were suited to larger, older houses with higher and more consistent heat demands. This is because they would need to run for more than an hour continuously for any benefit. In summer, when demand for heating is low, but the need for electricity remains, some models would end up producing excess heat. Others, which switch on and off automatically according to house temperature, would not be on long enough to generate much electricity.

Manufacturers have improved their systems since the trial 鈥� for example, Baxi鈥檚 new system can produce either 6kW of heat for every 1kW of electricity or be turned down to 4kW of heat for 600W of electricity. Bob Knowles, technical manager at Baxi, says his machine is better suited to smaller or better-insulated homes than those the Carbon Trust trialled, because it produces less heat in relation to the amount of electricity it generates. Baxi has so far carried out its own trials in 17 houses, although so far these have been only in three and four-bedroom homes. It is planning a bigger trial for next year and hopes to include a wider variety of house types. Baxi claims householders would save 拢200-400 a year in electricity and that the unit would emit 440kg less carbon dioxide than a high-efficiency condensing boiler.

But the amount of heat produced is still greater than the amount of electricity.

鈥淭he appliance is heat-led 鈥� it generates power when the boiler is on,鈥� Knowles says.

鈥淚t is a boiler with the by-product of generating electricity.鈥�

Micro CHP technology is expected to improve by more closely aligning the amount of heat and electricity produced. The Carbon Trust said that improving electrical efficiency by 3% could nearly double carbon savings.

Stirling engines are heat-led 鈥� they generate power when the boiler is on. It is a boiler with the by-product of generating electricity

Bob Knowles, technical manager, Baxi

Fuel cells are an example of this theory in action: they primarily generate electricity and heat is a by-product. As they produce the same amount of electricity as heat, excess heat is less of a problem, and the output can be altered. Ceres Power鈥檚 system produces a maximum of 1kWh of electricity and heat and a minimum of 300W. It says it is best suited to a three or four-bedroom, well-insulated house. The systems, however, are unlikely to be available for at least three years.

What are the savings?

The Carbon Trust trial, which looked at stirling engine systems rather than fuel cells, found carbon emissions could be cut by 5-10% or between 200kg and 800kg per year compared with a top-rated condensing boiler in a larger, older home with a higher demand for heat. Smaller and better insulated homes, in contrast, might cut their emissions by less than 5% or under 100kg per year, compared with using an A-rated condensing boiler, and might even increase emissions.

The researchers said the boilers would cost about 拢1,500 more than the equivalent condensing boiler and take 20 years to pay back, but manufacturers expected this cost to drop to 拢600 and the payback period to be between seven and 15 years.

Fundamentally, the rate at which people are paid for exported electricity would be key to how much they saved. Householders would save about 拢90 a year if they were paid the full retail price for electricity they exported.

A recent amendment to the Energy Bill proposes a 鈥渇eed-in tariff鈥� that would pay people an above-market rate for power they export to the grid.

The zero-carbon issue

A big future issue for micro CHP firms is the deadline for all new homes to be zero carbon from 2016. These well-insulated homes will need little heat but will require electricity 鈥� which will be problematic for stirling engines. Furthermore, both stirling engine and fuel cell systems use mains gas so they are not completely carbon-free. 鈥淚f micro CHP is not considered zero carbon, the makers will be in trouble from 2016 because they will be limited to retro-fitting,鈥� says Casey Cole, head of sustainable technology at green energy consultant Fontenergy.

But after several false dawns for the CHP industry, Baxi is determined to make its foray into the market work. It will be a while before it can tell, but with the market for residential micro CHP predicted to be 30% of the 1.5m-unit replacement boiler market by 2015, Baxi can remain hopeful.