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 (GSHP) 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 closed 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. A heat pump unit has three main components
  • Evaporator (the cold white panel in the back of 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 black pipes on the back of your 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 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 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’s important to consult a professional installer for expert advice. A heat pump can be designed to meet 100% of space heating requirements but often they are sized to provide less, needing some form of supplementary heating for the few coldest days of the year, either immersion heaters in the heat pump system, or some other form of heating such as a wood-burning stove. Heat pumps will usually only partially heat domestic hot water so secondary heating (ie. an immersion heater) must be programmed to top it up.

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 there also a cooling requirement? Heat pumps can cool as well as heat.
  • Is the system for a new building development? Combining the ground loop installation with other building works can reduce costs.
  • Do you want to be 100 per cent renewable? If so, purchase green electricity or install solar PV or some other form of renewable electricity generating system, if it would be feasible in its own right, to power the compressor and pump.

How much does it cost?

The cost of a professional GSHP installation is dependent on property and location and ranges from about £1,200 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 8kW system, for example, would vary between £10,000 and £16,000 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 40% 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.

Ground-source heat pump systems are supported under the Government’s Renewable Heat Incentive, the full domestic phase of which is expected in 2013 as part of the forthcoming ‘Green Deal’, which would eventually pay off your capital cost or even provide some of the cost in the form of a loan recouped through your electricity bills.

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 running costs per kWh might be as follows:

  Per Kwh Per Year
GSHP 4.6 – 6.5p £963 - £1261
Mains Gas 6.1p £1128
Oil 9.0p £1538
Nights Storage Electricity 8.1p £1748
LPG 10.3p £2181

                                   

(Source: Sutherland Tables April ’13)

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

In 2013-14 the RHI pays non-domestic users 4.8p/kWh for each unit of ‘green heat’ generated by accredited non-domestic GSHP systems, and is similar to the Feed-In Tariffs already in place for renewable electricity generators.

For information about the existing non-domestic RHI, the Renewable Heat Premium Payment  and proposed domestic RHI schemes, see www.decc.gov.uk/rhi .