CHP Combined heat and power - where the heat which is produced when electricity is generated is used within a heating system rather than wasted. This can happen at different levels - within a single house, a housing development, a town etc. , or co-generation as it is also known is a technology which has threatened to emerge and change our lives several times but as yet without success (except in a few trial situations).
The idea is that you have some kind of generator in your house which produces electricity. Given that most generators are quite inefficient and mainly produce heat rather than electricity the possibility is that you can use this heat to heat your house and the domestic hot water. Ideally there would be no waste. This would be spectacular, especially if it used a carbon neutral fuel to run on.
There are a few question marks over this technology –
- Will the ratio of electricity / heat match the requirements of the house? And if not, what can be done about this?
- How big and noisy is such a generator?
- How expensive to buy and how costly to maintain
- Will it actually be greener than a modern power station?
There are basically three contenders for generating power on such a small scale
- the internal combustion engine (bit like a car engine connected to a generator)
- the external combustion engine (Stirling engine connected to a generator)
- the fuel cell
The electricity/heat balance
The question over the power / heat ratio compared to the house’s needs is probably the most difficult one because the ratio of needs is constantly varying.
Given that most generators are only about 15% – 30% efficient then the better the house is insulated, the higher the use of electricity compared with heating and the less well matched is the generator –
The percentage of electricity used compared with the total energy used will be something like 35% for a very well insulated modern house compared with maybe 15% for a poorly insulated drafty place. The problem comes that you can always sell electricity back to the grid if you are making too much (see ‘feed-in tariffs‘) but you can’t sell surplus heat. However you are always struggling to make enough electricity. The other problem comes when you need much more electricity than heat at a particular time – maybe on a warm autumn evening. Do you buy it off the grid or do you generate it yourself and store the excess heat in a heat store in case the next few days are cold?
Types of generator
The actual power source for the generator is the next challenge. Early systems in the 1970s and 80s used modified car engines but the problem there is the noise[for the purposes of part E of the Approved Documents] - Noise is unwanted sound., even when heavily insulated. It had to be situated in an outbuilding or garage. Recent developments have focused on Stirling engines which are much more akin to refrigerator technology so they are quiet and long term reliable. However the various commercial field trials over the last few years seem to have stalled although one or two manufacturers such as Baxi with their Ecogen are in the market. Check out the Austrian SPM company who have designed a combined wood pellet stove and Stirling engine.
Finally, fuel cells hold great promise because there is so much effort going into their development. They are small, they have no moving parts and are almost silent, they operate very cleanly and are capable of rapid variation in output. Their efficiency can vary greatly, (15% – 50% in terms of their electrical output and over 80% in terms of their total output) depending on the materials they are made of. At present there are several companies racing to get their products to market with Ceres having recently signed a deal with Calor to produce a bottled gas CHP product. The channel 4 program ‘Home of the Future‘ has featured the BlueGen cogenerator which both heats the house and provides electricity using natural gas. It is being marketed by Ceramic Fuel Cells Limited. Keep watching the market.
In terms of green credentials CHP does at present represent an improvement on most power stations which dump all their excess heat into the atmosphere. However they generally do still use fossil fuels which cause global warming.
The use of CHP will help with the SAPStandard Assessment Procedure - the method used in the building regulations for calculating the energy use of a house. see Part L and SAP calculation when applying for building regulations approval and also in the assessment in the Code for Sustainable HomesCode for Sustainable Homes. A delightful tool for assessing how green a home is. Unfortunately now withdrawn (2015) by this short sighted government.