Ground source heat pump

Introduction

In the UK the ground a few metres below our feet keeps a constant temperature of about 11OC throughout the year. This is approximately the average air temperature, derived from solar energy, which it absorbs and holds steady because of its high thermal mass. Ground-source heat pumps (GSHPs) take advantage of this steady temperature. 

Although electricity drives a compressor to upgrade the steady low level heat to a higher usable temperature, and pumps which circulate fluid around a loop of pipe in the ground, heat pumps are considered to generate renewable heat because each unit of electricity can deliver several units of solar-derived heat.

How do GSHPs work?

GSHPs usually work by chilling an antifreeze liquid and pumping it through underground pipes where, being colder than the ground, it absorbs the warmth and heats up a few degrees. This warms a chilled refrigerant in the heat pump enough to evaporate it. The warmed vapour is then compressed, greatly increasing its temperature.

This heat is then transferred, via a heat exchanger, into your home’s heating and hot water systems causing the refrigerant to cool and condense back into a liquid. Its temperature drops again to well below zero when it passes through an expansion valve, ready to start the process again.


There are three important elements to a GSHP system:

  • Ground loop - lengths of pipe either in vertical boreholes, usually 100-250m deep, or in horizontal trenches 1-1.5 metres deep. The pipe is usually a closed circuit through which chilled antifreeze mixture is circulated, absorbing heat from the ground and carrying it to the heat pump.
  • Heat pump - heat pumps are very familiar to us, as fridges and air conditioners are both examples. The heat pump unit has three main components: -
    • Evaporator (like the cold panel inside your fridge) - absorbs the heat from the fluid in the ground loop, causing the refrigerant to evaporate.
    • Compressor (the thing that makes the noise in your fridge) - moves the refrigerant round the heat pump and compresses the gaseous refrigerant up to the high temperature needed by the heat distribution systems.
    • Condenser (the warm pipes on the back of a fridge) – the heat exchanger which transfers the heat, usually to a hot water tank which then feeds the heating and hot water systems.
  • Heat distribution system - consists of radiators or underfloor heating for space heating and often a storage tank for hot water supply. Underfloor heating is more efficient and ideally suited to heat pumps which produce water at lower temperatures than gas, oil or biomass-fired boilers.

What are the options for a GSHP’s heat collector?

Three options are available for the groundloop: borehole, straight horizontal and spiral horizontal (or 'slinky'), or a combination. Each has different characteristics allowing you to choose the most suitable for your property.

Horizontal trenches can cost a lot less than boreholes but require greater land area. For slinky coil, a metre-wide trench of about 10m length can provide for about 1kW of heating load, compared to 15-20m of borehole or 20-30m of trench with two passes of straight pipe, all depending on ground conditions.

Sizing

Correct sizing of the heat pump and the ground loop is crucial to the effective and efficient operation of the system and will depend on your building’s heat requirements and the ground conditions. At the outset it is important to implement all possible energy efficiency measures such as roof, wall and floor insulation and draught-proofing to minimise your heat demand.

When sizing a system it is important to consult a professional installer for expert advice. A heat pump can be designed to meet 100% of space heating requirements but may be sized to provide less, so needing some form of supplementary heating for the very coldest days of the year, either immersion heaters in the heat pump system, or some separate heat source such as a wood-burning stove. Heat pumps will usually heat domestic hot water to 50-60OC so an immersion heater must be programmed to raise this periodically for a while to kill any bacteria.

Is my house suitable?

Heat pumps operate most effectively in very energy-efficient buildings, ideally new builds. Whilst they will provide heat in any situation, their efficiency is very sensitive to the flow temperature demanded of them, which is a function of the size of emitter (radiator or underfloor pipework) and the rate of heat loss.

You should consider the following issues if you are considering a ground source heat pump. An accredited installer will be able to provide more detailed advice regarding suitability.

  • Can you incorporate insulation measures? These measures include wall, floor and loft insulation, as these will reduce your heat demand, making a heat pump more efficient.     
  • The type of heat distribution system. Underfloor heating is better as it works at a lower temperature. GSHPs can be combined with radiators but existing ones will give out less heat than before, so unless you can greatly improve your insulation and draught-proofing, you will need to install some more.
  • Is there space available for a trench or borehole to accommodate a ground loop?
  • Is the ground material suitable for digging a trench or borehole?
  • What fuel is being replaced? If it is electricity, oil or LPG the payback will be more favourable, but currently less so if it is mains gas. This makes heat pumps a particularly good option for off-gas-grid areas.
  • Do you require a backup heating system? If so it will be less cost-effective.
  • Is the system for a new building development? Combining the ground loop installation with other building works can reduce costs.
  • How much heat do you need, and do you have a single, dual or 3-phase power supply? A single phase supply can operate heat pumps up to about 25kW of thermal output.

How much does it cost?

The cost of a professional GSHP installation is dependent on property, location and the type of groundloop installation used, and ranges from about £1,250 to £2,000 per kW of peak heat output, excluding the cost of the heat distribution system (eg. underfloor heating).

Trench systems are cheaper so tend to be at the lower end of this range. The installed cost of a typical 12kW system, for example, would vary between £15,000 and £24,000 plus the cost of the distribution system. The price per kW gets lower as systems get larger.

What are the advantages?

By harnessing an inexhaustible resource, a heat pump uses much less energy than a typical heating system to provide the same benefit. In a well-designed system, up to four units of heat are delivered for each unit of electricity consumed. Because of this, it also has lower CO2 emissions. A good heat pump system (running on mains electricity) causes 70-80% fewer CO2 emissions than an equivalent oil boiler.

Ground source heat pumps are low-maintenance, low noise solutions. The underground elements can be expected to last up to 50 years while the other parts of the system have a reasonable life expectancy of 15-25 years. The actual lifetime will depend on the quality of the unit and its installation. If it is over-sized and therefore starts up much more frequently than it should, the compressor’s lifetime may be significantly shortened.

Ground-source heat pump systems are supported under the Government’s Renewable Heat Incentive (RHI) IF registered before end March 2021. This should return most or all of your capital cost in quarterly payments over 7 years. Interest-free loan may also be available via the Energy Saving Trust.

A well-designed GSHP system in an energy-efficient dwelling will cost less to run than an oil, LPG or direct electric (eg. storage heater) system, and potentially mains gas. Approximate comparative running costs per kWh as at January 2019 were as follows:

 

Per kWh

Heat pump (at 350% efficiency)

5.0p  

Mains Gas

4.8p

Oil

6.0p

Night Storage Electricity

11.5p*

LPG

8.2p

(Source: Sutherland Tables January ‘19)

*Price for radiator use only, not peak tariff backup or hot water heating

For information about the Renewable Heat Incentive scheme (closing end March 2021) see www.ofgem.gov.uk/environmentalprogrammes.

or information on possible loan funding contact Home Energy Scotland on 0808 808 2282 or visit https://www.energysavingtrust.org.uk/scotland/home-energy-scotland