Combined Heat and Power

Domestic Combined Heat and Power(CHP) installations use a conventional fuel - normally mains gas - to generate electricity and also to supply hot water. While the efficiency of electrical generation does not approach that of a large power station, the fact that the heat is also used means that overall the domestic installation would use the fossil fuel more effectively.

The design aim of a domestic CHP unit is to replace a conventional gas boiler, and to require no special installation skills. The electricity generated reduces the demand from the mains, and may feed electricity back into the mains when local demand is lower. This requires a new electricity meter which records exported electricity separately.

Currently leading contenders for CHP systems are PowerGen, Baxi and Honda.

• Powergen has contracted a small New Zealand company to supply 80,000 WisperGen units over the next 5 years, and offer to buy any surplus power. The WhisperGen can generate 8 kW of heat and about 1.3 kW of electricity. Obviously a suitable house for the WisperGen will be one where the maximum heat loss is around 8 kW. This might apply to a new 5 bedroom house, or an old - less well insulated - 3 bedroom one.
  Such a house might use 18,000 kWh of heat a year, and so the WisperGen would generate about 3,000 kWh of electricity. Powergen will buy any electricity generated and not needed locally currently at a price of 3p a unit. If 2,000 kWh of that generated is used domestically and mains electricity costs 8p a unit this will save £160, and if the remaining 1,000 kWh is exported this will earn £30. The WisperGen costs about £3000. The efficiency is not quoted but from the benefits claimed appears to be about 80%. If so, the WhisperGen would appear to have an advantage over a more efficient condensing boiler giving 90% efficiency both on fuel cost - a saving of around £60 per year - and on greenhouse gas emissions though the saving here is only about 4%. This is probably the only feasible CHP solution for a typical house at present.
• Baxi have a larger unit generating 5.3 KW electrical and 10.4 KW of heat. It is too large for domestic use and costs around £15,000.
• The Honda Micro CHP generates 1 kW of electricity and 3 kW of heat. The efficiency claimed is 85%. 15,000 of these units have been installed in Japan. The amount of heat is small for most UK houses, and so a top up heating unit would be required. CHP units work best when they run for long periods, so this combination of a CHP installation sized to match normal heating requirements with a more rarely used conventional boiler could be sensible. The installed cost is said to be about £5,600, which means that it would not be an economic choice.
• BG (British Gas) has invested considerable sums in developing their MicroGen system, to be available in a range of sizes. This development was halted early in 2007.

A major advantage of such systems could be that they can continue to power essential services such as freezers and a few lights if there is a power cut. This requires specific design, as such systems must be designed not to generate electricity if the mains fails.

PowerGen say their system may cost £600 more than a conventional boiler of the same size.

The electricity from CHP does not come from a renewable resource and so does not qualify for Renewable Obligation Certificates (ROCs). So for the export of electricity to be economically sensible it is necessary to find a supplier prepared to buy the exported electricity. The suppliers considering doing this at present include PowerGen and BG (British Gas). However they link the agreement to buy the power to the use of their selected CHP system, which may not suit the requirement.

The most appropriate comparison for a CHP system would be with the alternative of a modern condensing boiler. With a typical condensing boiler about 88% of the energy in the incoming gas is converted to the energy in the heated water. If the CHP system does not reach a similar efficiency the benefit of generating some of the output as electricity does not compensate for the poorer performance.

CHP installations can be sensible for factories, large offices or whole estates. CHP for domestic installations is less attractive, partly because the technology for domestic use is still in an early stage of development, and partly because the electrical energy generated may not be required. As explained elsewhere it is difficult to be paid for this electricity.

Domestic CHP is potentially a very significant development. If such units were available in a suitable range of sizes, at reasonable prices, with efficiencies similar to that of Condensing Boilers, and with an easy means of being paid for the spare electricity generated, they could become the normal installation for central heating.