Ground Source Heat Pumps

Green Energy > Ground Source Heat Pumps

The earth’s surface contains a stable source of heat energy, ready for the taking.

Ground source heat pumps (GSHP) use the latent heat energy that’s stored in the ground to produce air and water heating, and air cooling. It might be hard to believe on a frosty morning, but the earth’s surface acts as a huge, stable heat sink for solar radiation. In the UK, ground temperatures remain constant at around 10-12ºC throughout the year. Ground source heat pumps take advantage of this by transferring this heat to buildings in the winter and taking heat out of buildings in the summer.

These systems have been used successfully for years in commercial, residential and institutional buildings in countries such as the US and parts of Europe. While they undoubtedly deliver reductions in CO2 emissions, the actual quantities, and the cost savings, are largely dependent on what kind of conventional energy it is replacing and what kind of distribution system is used.

A ground source heat system has three main components: the ground loop, the heat pump and the heat distribution system. The ground loop is comprised of a series of pipes buried in the ground, either vertically in a bore-hole or in a horizontal trench. The pipes form part of a closed system and are filled with a mix of water and anti-freeze.

Costs and benefits

Efficiencies for ground source heat pump systems are measured by a ratio known as the co-efficient of performance (CoP). Manufacturers claim an average CoP of 3 to 4. This means that for every unit of heat used to drive the system, 3 to 4 units of heat are produced.  However, the actual CoP will vary over the year, likely dipping below 3 at times in winter months.  Useful guidance is provided here.

Initial capital costs can be high, ranging from around £11,000 – £15,000 for a typical 6-8kW system which does not include the distribution system. Government grants may reduce this somewhat. Yearly savings on energy bills can make it worthwhile, but mostly for buildings located off the gas grid. Ground source heat pumps can help to reduce CO2emissions, with amounts varying depending on the efficiency of the system. Using GSHP in conjunction with another renewable, such as photovoltaic cells, to power the compressor and pump, would result in even higher emissions cuts.

Ground source heat pumps have a life expectancy of twenty years and up and typically require little maintenance during their lifespan. Payback times can be between 8 (electrical) and 19 (LPG) years, depending on;

  • Ground conditions
  • The type and size of collector
  • What type of fuel is being replaced and how much it cost
  • The initial financial outlay
  • The price of electricity

Overall they can provide a good, clean, free and stable energy source that reduces carbon emissions. Their effectiveness has been proven in countries such as the US, and while initial capital costs are high, so are the efficiencies. If a property has the space available, a ground source heat pump could be just what it, and the future of the planet, needs.

Just like in a fridge, the heat pump works by evaporating and condensing a refrigerant which moves heat from one place to another. There are three parts within the pump: the evaporator, which absorbs the heat; the compressor, which moves the refrigerant around the pump and compresses it to the necessary temperature; and the condenser, which passes the heat to a hot water tank.

The last part of the system is the heat distribution system. This can take the form of under-floor heating, radiators, or sometimes a hot water system. Ground-source heat pumps are best suited to under-floor heating or low temperature radiators because they can only raise the temperature to around 40-50°C. Therefore, they tend to be better suited to new builds that can plan for this, rather than retro-fitted to work with conventional boilers or immersion heaters that require temperatures of around 60-80°C.

GSHP can be suitable for most building types, but in new builds a developer can work out an overall plan that maximizes efficiencies by including under-floor heating and low temperature radiators, ensuring good insulation, and considering cooling requirements. Ground source heating is particularly suitable for areas not on the gas grid, although rising gas prices may start to make them more financially attractive in grid-served locations.

GSHP also requires a lot of space and ground that’s suitable for digging a trench or bore-hole. The trench size required for a horizontal system is about 1.5-2m deep and about 10m long for every kW of heating load. A bore-hole on the other hand, while needing less land area, is more expensive to drill. The depth of these bore-holes can reach to around 100m which also has the benefit of accessing higher ground temperatures. Also bear in mind that because ground source heat pumps are not in widespread use in the UK and Ireland, trained installers are few and far between.