Ground source heat pumps
In the UK, the earth at a depth of 10 metres and below keeps a constant temperature of around 11-12 ºC throughout the year. Because of the ground’s high thermal mass, it stores heat from the sun during the summer. A ground source heat pump (GSHP) can upgrade this heat from the ground into a building to provide space heating and, in many cases, pre-heating for DHW. For every unit of electricity used to generate the useful heat, 3-4 units of heat are produced.
Whilst not truly a renewable energy technology because of the requirement to input electricity, heat pumps can still have a significant impact in reducing CO2 emissions and are cost competitive against direct electricity, oil and LPG fuelled systems and in some instances mains gas. They can therefore be regarded as a sustainable energy solution.
In the case of GSHP systems there are three important elements to consider:
- Ground loop – comprises lengths of plastic pipe buried in the ground, either in vertical boreholes or horizontal trenches. The pipe is a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.
- Heat pump – these are familiar to us in the form of refrigerators and air conditioners. By vaporising and condensing a refrigerant a heat pump is able to move heat from one place to another. In the case of ground source heat pumps, the evaporator takes heat from the water in the ground loop and the condenser gives up heat to the heating distribution system. A compressor within the heat pump compresses the gaseous refrigerant to increase the temperature to that needed for the distribution circuit.
- Heat distribution system – GSHPs are suited to low temperature heating systems as less energy is needed to upgrade from the source temperature. Under-floor, ducted warm air and low temperature radiator heating systems are therefore particularly suited for heat pumps. Use of conventional radiator systems is possible but needs careful analysis before proceeding, although split systems involving under-floor heating downstairs and radiators upstairs are increasingly popular in new-build properties. Heat pumps can also supply all, or a fraction of DHW and an element of summer cooling, if the installation is designed correctly.
For ground loop based installations there are three main options: borehole, straight horizontal and spiral horizontal, often called ’slinkys’. Each has different characteristics allowing you to choose the most suitable for the site. Horizontal trenches cost significantly less than boreholes, but require greater land area and are less efficient as the ground temperature varies closer to the surface. For a slinky coil, a trench of about 30m length will provide for about 1kW of heating load. Trenches are normally a minimum of 5m apart. Borehole based collectors will be at depths of between 60 – 200m.
Energy is needed to activate the heat pump cycle and to compress the vapour for the production of useful heat. The efficiency of this process is expressed by the ratio between the useful heat delivered and the driving energy used by the compressor. This ratio is called the Coefficient of Performance (CoP). The CoP of the current generation of ground source based heat pumps varies from 2.5 to 5. Since the CoP shows performance at a steady state only, a second parameter is usually used to show the performance of the heat pump over an entire year. It is called the Seasonal Performance Factor (SPF), which is the ratio of annually delivered useful heat over annually used driving energy. When calculating the SPF, it is common to include the annual electricity requirements of auxiliary equipment, such as circulation pumps, fans, etc.