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Micro CHP systems can be used to replace a normal gas-fired boiler, but have the additional benefit of producing electricity. They currently come in the form of engine powered generators that also produce heat, or boilers with extra bits to produce electricity. There are a limited number of models currently available for domestic use, with a lot more on the way.
Other micro CHP systems under development include fuel cells (could be very efficient, but are complex), thermo-ionic and thermo-photovoltaic. The fuels cells look like having a real future, especially the Solid Oxide Fuel Cells (SOFC), whereas the other two are a little more remote. It is unlikely that any of these will be commercially available until at least 2010.
The engine types burn fuel to create expansion within liquids or gases, which in turn moves a piston and powers the dynamo or alternator. This can be done through external or internal combustion, with the latter type of engine present in almost all cars. External combustion works by burning the fuel purely to produce heat, which is then used to heat up gases within the cylinder. The process is not explosive, is quiet, efficient and controllable. The only drawback is that it needs to run fairly evenly and give a constant output, which is why it is not used in cars.
CHP produces heat and electricity at the same time, with the heat output usually much higher than the electricity. The external combustion systems will be the most common, so expect a fairly continuous output when running. For this reason it is best to design it to run the hot water and heating system, with the electricity being a bonus. The CHP units tend to have additional burners to provide more hot water at peak demand without having to alter the load on the external combustion engine. They should also be connected to the electricity grid, so that power can be bought in or sold back to the grid depending on demand. The limited number of products available means that your system design options are very limited, so if you buy one you will have to tweak it to get the best results for your household.
The only product currently available that I could find is the Honda Ecowill (internal combustion engine), which has sold lots in Japan but may be a bit expensive for the UK market. There are bigger units available for hotels and community housing, but for single homes they are a bit over the top.
Most of those nearing the market are external combustion types. Whispergen are re-launching in the UK in 2009 and look like being excellent. Disenco seem ready to go but don’t have a distributor yet. The unit is probably larger than necessary for the average house, so you will need to sell electricity to the grid and your house is certainly going to be warm! Enatec is in the development stage, but will be focusing on variable heat production to meet cyclical household demand; electricity production will be fairly low. The Energetix Genlec is a normal boiler that has a reverse-action air conditioning system bolted onto it to produce electricity from heat, rather than the standard air conditioning role of producing cold from electricity. It has a low electricity output and efficiency, but should be comparatively cheap when it comes out in 2008. Otag apparently have a steam-powered unit coming out or already out, but my German isn’t good enough to read their website! Cogen Micro is in development and might have a low cost and moderately efficient system in a couple of years.
CHP does qualify for payments under the Feed In Tariff system, giving you 10p for every kWh of elecricity generated. There were only pilot systems around when the FIT system started in April 2010, so this is probably something for the future.
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