A Heat Pump is a machine working on the Carnot cycle which transfers heat from a heat source to a heat sink using a vapour compression cycle. A refrigerator uses a heat pump to move heat out of a fridge. An industrial heat pump can be used to move heat from an external source into a building. Heat may be obtained from an air source, a ground source or a water source. A heat pump can concentrate such "low grade heat" into higher grade heat that can be used for space heating and domestic hot water.
Ground Source Heat Pump – GSHP
A ground source heat pump extracts heat from the ground – whose temperature will be warmer than the air in winter (and cooler than the air in summer). For this reason they are more efficient than air source heat pumps, especially in the coldest weather when they are most needed. They generate very little noise and should last for many years with minimal servicing.
The most practical way of extracting this energy is through water circulating through pipes in the ground. The pipes for the ground loop are usually laid in horizontal trenches at two metres deep, but vertical boreholes are an alternative, if more expensive, way of achieving similar results where there is not enough land to lay pipes horizontally.
At depths below six metres, the ground temperature does not vary much from the Mean Annual Air Temperature (around 9°C -11°C in the UK depending on location). At a depth below two metres, there is a large store of warmth that can be tapped for heating in the winter. However, this temperature will drop quickly where a heat pump is extracting a lot of heat from a small ground loop – it is therefore very important that the size of the ground loop matches the heating load of the building. The key to achieving this balance is a full thermal modelling exercise.
Advantages of Ground Source Heat Pumps – Pros and Cons
Heat pumps save money. Heat pumps are much cheaper to run than direct electric heating. They are cheaper to run than oil boilers and can be cheaper than running gas boilers. Because heat pumps can be fully automated they demand much less work than biomass boilers.
Heat pumps save carbon emissions. Unlike burning oil, gas, LPG or biomass, a heat pump produces no carbon emissions on site (and no carbon emissions at all, if a renewable source of electricity is used to power them).
Heat pumps save space. There are no fuel storage requirements.
Heat pumps are safe. There is no combustion involved and no emission of potentially dangerous gases. No flues are required.
Heat pumps require less maintenance than combustion based heating systems.
A well designed ground source heat pump system will increase the sale value of your property.
Heat pumps can provide cooling in summer, as well as heating in winter.
Disadvantages of Ground Source Heat Pumps – Pros and Cons
GSHPs are more expensive to install than air source heat pumps because of the need to install a ground heat exchanger. However, this connection to the ground is what enables a GSHP to perform much more efficiently than an ASHP – particularly when the external air temperature is low in winter and you most need heating.
Problems arise with ground source heat pumps if the installation is poorly designed or not matched to the heating needs of the building. The design and installation of an effective ground source system depends on a thorough understanding of the movement of heat in the ground, the local geology and the heating and cooling requirements of your building. To get the potential benefit of a well designed system you will need to be in touch with an expert installer. This increases the cost of the installation, but good design and planning will yield substantial benefits over the life of the system.
Greater benefits can be achieved by moving to a full Interseasonal Heat Transfer system.
Coefficient of Performance – CoP
The Coefficient of Performance is found by dividing the useful heat output by the electrical energy input. A heat pump that produces 4 kilowatts of heat for 1 kilowatt of electricity has a CoP of 4.
Can I expect a CoP of 4 from a GSHP?
Not necessarily. A heat pump may yield a CoP of 4 in test conditions, but it will not always do so in an installation unless the ground loop is well designed and the heat delivery mechanism in the building is also well matched to a GSHP. It is possible to obtain a much higher CoP when a ground source heat pump is integrated with a solar collection system and a ThermalBank in order to achieve Interseasonal Heat Transfer.
Is it true that the CoP of a GSHP degrades as time passes?
As a GSHP draws heat out of the ground, the ground temperature will fall from its natural temperature of 10°C at depth towards 0°C or colder. The heat pump must work harder to produce the same output from a lower heat input: therefore the CoP tends to fall over the heating season (unless the ground loop is large enough and heat is only extracted slowly). This is because heat only moves slowly through the ground. This limitation can be converted into an asset by creating a ThermalBank to store solar heat energy in summer – for use in winter.
Thermal Modelling is critical to good design in any installation of a ground source heat pump. The sizing of the ground loop is key to the successful operation of the heat pump. Before sizing the ground loop correctly it is necessary to understand the qualities of the ground to be used as well as the expected heating loads and cooling loads of the building and these will be affected by the design of the building, its orientation to the sun, the activities in the building and the ventilation strategies. These are complex issues which need to be analysed in a thermal model to ensure that the system will meet the temperatures required in the building throughout the seasons of the year in a well balanced and sustainable way.
See also: Seasonal Thermal Energy Storage