Ground Source Heat Pumps
Heat pumps (both ‘ground source’ and ‘air source’ are being heavily sold at present. GSHPs work a bit like fridges and freezers by shifting heat from one place to another. With houses they extract heat from the ground which tends to have a constant temperature of about 10°C, increase its temperature a bit (to 40 or 50°C) and move it into the house. The ground extraction can either be via long loops of plastic pipe buried over a meter deep in your garden or, if space is limited, by sinking a number of bore holes in your garden. See Ice Energy’s web site for a good working diagram.
The problem with GSHPs is that they use quite a lot of electricity while operating and the generation of electricity is usually expensive and polluting in the UK context (unlike the hydro electricity in countries such as Canada and Sweden where GSHPs have been pioneered).
If using electricity is the only option for heating then there is a clear choice. -
burn a kilowatt of electricity (say with a fan heater) and get that 1 kilowatt into your house
or
burn a kilowatt of electricity in a ground source heat pump and get 3 or more kilowatts into your house
and the second option wins.
However, if there is a choice of fuels then the decision is not so easy. Burning mains gas directly in a boiler in your house may be just as efficient and cause less pollution. This is because the electricity for a heat pump is mainly made from oil or coal or gas. Now, making and distributing electricity is inherently very wasteful - only about a third of the fuel energy ends up as electricity in your house; two thirds gets wasted in the power station (as heat into the air due to inefficiencies) and also a bit in the national grid (where it leaks out of the wires). This is why a kWh of electricity almost always cost about three times as much as a kWh of gas on your bill!
Burning wood pellets would probably be cheaper, cleaner and more efficient than either GSHP or gas.
The coefficient of performance ( COP )
The COP of a heat pump is the measurement of how much heat it puts out compared with how much electrical energy you need to power it and it tends to range between 3 and 5. In other words, if it is using one kW of electricity to run it, it will give out 3 - 5kW of heat. Each manufacturer states the COP for their heat pumps. This figure is important because if it is over 3 then the heat pump will probably be a better option (ecologically and financially) than burning mains gas. If it is under 3 then it will be creating more pollution at the power station than a gas boiler in your house.
Manufacturers compete to have the highest COPs and they try to measure them under ideal load conditions and with brand new machines. This is how they get figures like 4 or 5. Load conditions vary in practice and the COP is very dependent on how hot you want the output water. A GSHP finds it quite easy to raise the temperature from 10ºC to 30ºC and may achieve a COP of 4.5 at this degree of lift. It works very efficiently and this may be fine for underfloor heating. If, however you want water at 60ºC for radiators and Legionella proof Domestic hot water. The hot water used for washing, washing up, bathing, showers basins etc.”> DHW then the heat pump starts to struggle and becomes much less efficient; maybe the COP drops down to 2.5
The other factor is the age of the equipment. After a few years the COP is likely to drop by a significant amount.
Manufacturers and suppliers have hyped up the ‘efficiency’ and ‘free energy’ side of their advertising to such an extent that the ASA have recently adjudicated against one company, Thormec UK Ltd. for being misleading in their advertising.
The advice is to go very thoroughly into the subject and don’t necessarily believe the ‘ideal’ figures quoted by companies. Try to get a forecast for your own particular situation especially since the installation of GSHP involves high capital cost.
There is an interesting exception to the subject when it comes to Passivhaus design. In this case the heating load is extremely low and the heat is introduced into the incoming ventilation air. This can be done with an electrical resistance heater (since central heating is unnecessary in a Passivhaus) but recent developments are utilising small heat pumps which cater for the DHW as well as the air heating.
The BRE’s Green Book Live web site contains lists of microgeneration companies specialising in GSHPs.
