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Ground Source Heat Pumps: the Basics

by

SelJones

Ground source heat pumps are gaining popularity in the UK. The name is rather long-winded, and the technology is not as intuitive as solar or wind power. It’s hard to believe that the soil in a suburban back garden in winter can hold enough heat to power a central heating system, but, amazingly, it does.

The basicsThe ground, just two metres below the surface, maintains a constant temperature through the seasons. In a ground source heat pump, that fairly modest level of heat is transmitted from the earth into a liquid held in a closed loop of buried plastic pipes. The temperature of the liquid isn’t high enough to provide heating for a building directly, but it’s transmitted along the pipes into the building and the heat is transferred there into another substance, a refrigerant, which is volatile by its nature. The gaseous refrigerant is compressed in a vapour-compression heat pump, which raises its temperature as it condenses. The refrigerant is then used to heat air, which is propelled around the building in air ducts, or to heat water which is piped round the property.In essence, the ground source heat pump acts as a refrigerator in reverse. The back of a fridge holds an arrangement of copper pipes which works to condense and evaporate refrigeration fluid as a vapour-compression heat pump. The appearance of the back of a fridge is quite similar to the layout of outdoor ground source pipes, but in miniature. Ground source heat pumps can double up as air-conditioning systems, though that’s still unusual in UK homes. When reversed like that, the system works to extract heat from the building and release it into the outside soil, in the same way as a fridge.Some properties are more suitable than othersThe pipes buried outside the building are usually positioned horizontally. That means that a fair amount of land is needed to install a ground source heat pump, so not every property is suitable for the installation. An average sized suburban garden might be sufficient, but the calculation isn’t straightforward. The efficiency of the system depends on a number of factors, like soil type and moisture content. The aspect of the land will influence the soil temperature: a south facing garden with an open aspect will be a better source of heat than a shaded north facing one. Soil with a high moisture content will transmit heat more readily.In some situations where the area available isn’t large enough for a horizontal system to be installed, vertical pipes can be used, but they are more expensive to install. In either case, digging machinery must have access to the area. So it’s not a practical proposition to install a ground source system for a small, terraced house.Increasing popularityThe pumps are very popular in areas like the US and Canada where land isn’t as expensive as it is in the UK. In North America, houses and the plots they are built on tend to be large and expensive to heat. The availability of outdoor space and the cost of domestic heating have meant that ground source heat pumps, which represent a very big saving on the cost of heating a property by more conventional means, were adopted early in those parts of the world. But they are now becoming popular in the UK too. Technology is improving all the time, and with heating costs soaring, there is widespread interest in alternative energy sources.Different optionsThe different options among systems using ground source technology can be confusing. The most common type of system is known as the ‘closed loop’ type. In this model, the fluid in the outdoor pipework is water mixed with antifreeze, from which mixture heat is transferred to another medium, refrigeration fluid, within the heat pump cabinet.An alternative, which was in fact the earliest form of the technology, is the ‘direct exchange’ system. In that type, the refrigeration liquid is the only medium used. It’s circulated through the outdoor pipes, then into the building. There is no transfer from water to refrigeration fluid, so on the face of it, it’s a simpler system with less opportunity for energy to be lost. But there are disadvantages. The tubing that’s needed to carry refrigerant is more expensive, for example.In choosing whether to install the technology at all, and which version to use, it’s absolutely essential to find a reliable source of expert advice, because very few lay people are knowledgeable enough to make the decisions involved unaided.Sam Jones

the author recommends to readers wanting more info about the

types of heat pumps

available to visit the comparison website uSwitch.

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Ground Source Heat Pumps: the Basics}