Ground Source Heat Pumps
Ground source heat pumps (GSHPs) use pipes that are buried in the garden to extract heat from the ground. This heat can then be used through radiators, underfloor or warm air heating systems as well as hot water in the home. There are different types of GSHPs; from closed to open loop systems.
Some of the benefits of ground source heat pumps may include:-
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- Reduction of fuel bills
- Providing an income through the governments (RHI).
- Reducing home carbon emissions
- No need for fuel deliveries
- Heating the property as well as the water
- Minimal maintenance
- Indoor components can last over 25 years
- Ground loops could last for 50 to 100 years
A GSHP works on a different principle from traditional gas and oil boilers. It produces heat at lower temperatures over much longer periods, so in winter for instance, it is normally the case that it is kept running constantly in order to heat a home efficiently. This means that the radiators would normally need to be somewhat larger than the standard sized ones and they won’t feel as hot to the touch as those connected to a gas or oil boiler.
There is usually a more involved installation process than that of Air Source Heat Pumps, but generally speaking GSHPs are often more energy efficient. Of course the choice of which system to install is determined by the amount of garden available to the property.
How ground source heat pumps work
Ground source heat pump systems consist of three main parts; The Heat Pump; Underground Heat Exchanger and Distribution System.
Because the Earth absorbs energy released from the sun, underground temperatures remain quite constant throughout the year so the heat pump can be used all year round. In the UK, the temperature of the Earth a few metres down is around 11 to 12 degrees Celsius. The type of ground will affect the level of heat therein, so for example, clay holds more heat than sand.
Once planning, preparation, and getting the necessary permissions are completed, the groundwork and laying the piping system usually takes between 1 and 3 days, depending on the geological conditions and whether it is a horizontal or vertical installation. Once that is done, then the heat pump device itself needs to be installed and connected to the heat distribution infrastructure.
A mixture of water and antifreeze or refrigerant is pumped into the ground via the heat pump, through a loop of pipe, called a ground loop and the antifreeze mixture is continuously warmed up by the constant temperature of the ground. The length of the ground loop depends on the size of the property and the amount of heat required. Longer loops can draw more heat from the ground, but consequently need more space in which to be buried.
The liquid is fed into a heat exchanger and energy absorbed from the ground is transferred into a refrigerant which boils at a low temperature and turns it into gas.
The gas is fed into a compressor and the compressing process makes the temperature of the gas rise. The water is heated in the hydronic distribution system, where water is distributed to either radiators or underfloor heating.
The cooled ground-loop fluid passes back into the ground where it absorbs further energy from the ground in a continuous process when heating is needed.
With sufficient space, the collector loop can be laid horizontally in a trench roughly a metre under the ground. If there is insufficient room to do this, then vertical boreholes can be drilled to between 90 and 160 metres down to extract the heat.
The space needed for a horizontal loop, and the depth you need for a borehole, will depend on several factors such as the type of soils and these matters would be taken into account by the surveyor.
GSHPs do impact on the environment as they need electricity to run, but the heat they extract from the ground, the air, or water is constantly being renewed naturally.
The heat pump itself is installed in the house and some of these installations include a hot water cylinder and can be the size of a large filing cabinet. The smallest pumps can be fitted in an under-sink kitchen cupboard.
Once the pipework is buried, the surface of the ground can return to being what it was in the first place without any signs of the pipes that are buried below.
There are two other criteria when choosing the right ground source heat pumps.
- Direct expansion (DX): Relies on a cyclical process when the refrigerant changes its position back and forth between a gas and a liquid. When the refrigerator absorbs the heat, the compressors start pulling the vapour from suction lines and the process is launched.
- Indirect expansion: Is commonly used in freezer applications with carbon dioxide when changing secondary working media. In other words, during antifreeze solution/water circulation the energy is relocated from or to the refrigerant circuit through ground heat exchange pipework.
Different types of closed loop systems
Horizontal Ground Source Heat Pump
The horizontal array consists of a pipe laid in a serpentine closed loop in a trench that is between 1 and 2 metres deep. This type is usually found in rural areas because of the availability of the land. To install the horizontal system, the area required depends on the heating and cooling loads of the property, the depth at which the loop is going to be buried, the soil and its moisture, the climate, and the efficiency of the heat pump. The average home of 150m2 would need an area of between 300 and 700 m2, so roughly 500m² for a 10kW heat pump in clay soil, and twice that for sandy soil. Pipes can be a straight pipe or in coils, called a ‘slinky’ pipe.
Vertical Ground Source Heat Pump
A Vertical setup, usually referred to as a borehole system will have boreholes drilled into the ground and connected across their tops, in a closed loop. The costs vary depending on several factors and they are undoubtedly the more expensive alternative to horizontally laid pipes; but are often the appropriate choice for suburban homes where space is restricted.
The number and depth of boreholes will be dictated by the composition of the ground, the size of the heat pump and the heat requirement of the property. As an example, a 8kW heat pump is likely to need at least three boreholes 70m to 100m deep (or two slightly deeper boreholes). They do not need a large area of land and if the price is right, a vertical array is a good option.
Closed Loop System for Pond/Lake
Less used than horizontal or vertical systems, a closed pond loop is another option. It requires proximity to a large enough body of water, so an open loop system is usually preferable. This is a more specialist area and requires careful calculation, usually carried out by the heat pump manufacturer. It may be advantageous when poor water quality prevents the use of an open loop.
Installation costs of GSHPs
Installation costs are higher than other traditional heating systems, but that is offset by energy savings and earnings. A ground source heat pump installation can cost anything between £10,000 and £70,000 and the running costs depend on the size of the property and the standard of its insulation.
Assuming around £1,200 per kW capacity a 200m², four-bedroom house, is likely to need an 8kW heat pump. The ground source heat pump is likely to cost around £6,000-£7,000, the balance being the installation cost which can vary significantly with the ground conditions. The cost of borehole digging varies enormously around the country, so the price can vary from reasonable to very expensive.
The price of the whole project also depends on whether any new radiators or a new underfloor heating system is required, therefore these price estimations do not include any sort of wet system upgrade. These numbers are simply given as a general reference and a more accurate figure will be given once a full survey has been completed
GSHP Efficiency
Each kW of electricity consumed by the heat pump produces about 4 kW’s of generated heat in return; this means that there is a 400% cost-effectiveness ratio.
The crucial factor to achieving low running costs and efficiency is a well-insulated house which will therefore need a smaller heat pump, a smaller ground array (or fewer boreholes) and less electricity, reducing capital and running costs. System efficiency therefore starts with minimising the amount of heat required in the house and the amount of electricity needed. Specifying a heat pump is much more involved than choosing a gas boiler. Some may over-specify the size of a gas boiler on a ‘just in case’ basis, this is not good practice when dealing with heat pumps. An over-sized system will be less efficient.
Effect of Cold Weather
The ground below about 1 meter keeps a fairly stable temperature throughout the year, so because the heat pump pipes are at that depth, cold weather is unlikely to significantly affect the efficiency of the heat pump; vertical installations are even less likely to be affected.
CoP and SCoP in relation to Efficiency
Coefficient of Performance (CoP) is found by dividing the useful heat output by the electrical energy input and GSHPs achieve high-performance coefficients with the average between 3 and 4. So a GSHP that transfers 4 kilowatts of heat from the ground for 1 kilowatt of electricity expended, has a CoP of 4. The seasonal coefficient of performance (SCoP) is a more accurate representation of the efficiency during different times of the year.
The CoP is the performance delivered at a specific moment, whereas the SCoP will provide an average for the whole year.
Advantages and Disadvantages of GSHP
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Pros |
Cons |
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One of the most energy efficient technologies for providing water heating |
High setup up costs in comparison with other systems |
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Low maintenance and running costs |
The type of bedrock can affect the amount of energy that can be produced when using a vertical loop |
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Less noise than gas boilers and air source heat pumps |
A horizontal installation requires a reasonable amount of space in order for the pipes to be placed effectively |
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Low environmental impact because they reduce carbon emissions and have an even smaller carbon footprint if electricity is used from a renewable source. |
Unscrupulous, uncertified installers may be tempted to use liquids which are not environmentally friendly and do not comply with the regulations |
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Long lifespan of over 25 years |
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Method of Heat Distribution in the property
Underfloor heating is considered to be the best way to maximise the benefits of a GSHP. However, if for cost or other reasons underfloor heating is not possible, then using the largest radiators available is an excellent option. The SCoP is higher for underfloor heating compared to radiators because good and even heat distribution is much better with underfloor heating. The under- floor system will likely have a lower output temperature than a radiator, meaning a higher coefficient of performance.
Running Costs of a Ground Source Heat Pump
As an example, a four-bedroom house will need around 11,000kWh of heat for space heating and 4,000kWh for domestic hot water. Assuming a SCoP of 4.5, the property will need (11,000 + 4,000)/4.5 = 3,334kWh of electricity to operate.
With totally electricity costs of around 15p/kWh, that would mean an operational cost of £500 per year which compares favourably with the £890 per year to run a gas boiler.
Maintenance
GSHPs usually have a warranty of two to three years and workmanship warranties can last up to 10 years through a variety of national warranty organisations.
They are expected to operate for at least 20 years with minimal but regular maintenance. An annual check by the user and a detailed check by a professional installer every three to five years would normally be sufficient.
Costs & Savings
The amount of savings will depend on which system is currently in use as well as which system is being used as the replacement. Savings will be affected by:-
The heat distribution system:
Underfloor heating can be more efficient than radiators because the water running through the system does not need to be at such a high temperature. If underfloor heating is not possible, then the largest radiators available should be used.
Fuel costs:
GSHPs are powered by electricity which has to be paid for unless it is generated by a renewable energy source such as solar panels. The more expensive the fuel being replaced, the larger will be the savings made. If the old heating system was inefficient, there will certainly be lower running costs with a new heat pump.
Water heating:
The ideal usage for the heat pump is to provide heating for the property; however hot water can also be provided but this could limit the overall efficiency of the system. An alternative that could be considered may be solar water heating to provide hot water in the summer and help maintain the heat pump efficiency.
Use of the controls:
Correct use of the controls will determine how much benefit is gained from the system. The heating may need to be set to come on for longer hours, but the thermostat may need to be set lower to allow the occupants to still feel comfortable. The installer should explain to you how to control the system so it can be used most effectively.
Financial support
It may be possible to receive payments for the heat generated using a heat pump by using the UK Government’s Renewable Heat Incentive (RHI) under which, those with renewable heat-generating technologies are paid back for the heat they produce.
From 1st of April 2019, the rate for domestic ground source heat pumps is 20.87p/kWh, payable for seven years from the date of commissioning.
RHI payments are for the renewable element of the heat produced, so using the figures above, where you need 11,000 + 4,000kWh of heat, less the 3,334kWh of electricity used, then the RHI applies in the following manner:- 11,666kWh at 20.87p/kWh would equal £2,435 per year paid quarterly for seven years; realising nearly £2,000 of profit after the electricity running costs are taken into account.
The Non-Domestic Renewable Heat Incentive aims to compensate the costs of ground source heat pumps installations in commercial applications. The business owners and social housing providers benefit from a quick payback and long-term rate of return. In addition, the UK Government financial support gives an opportunity for retrofit installations and new houses to get modernised.
The third type of grant is the Energy Company Obligation (ECO) which imposes legal obligations on energy suppliers to implement energy efficiency measures. The main focus is on three different areas: Home Heating Cost Reduction Obligation, Community Obligation and Carbon Emissions Reduction Obligation.
Carbon Footprint
Electricity is required to run the heat pump, so if renewable energy is not used, then the system cannot be considered to be carbon neutral. However, when a GSHP is combined with solar panels, it becomes possible to reach a full carbon neutral system.
Points to consider about GSHPs
The first step in assessing the design of ground source heat pumps is to research on the ways of minimising the hot water demand and space heating. It requires accurate energy efficiency measurements, which can be done by obtaining an Energy Performance Certificate (EPC). That information helps to find the heat pumps of the right size that will decrease the energy consumption, heat loss and hot water needs.
The garden does not need to be particularly large, but the ground needs to be suitable for digging either a trench or a borehole and it needs to be able to be accessed by the necessary digging machinery and equipment.
Unlike traditional boilers, ground source heat pumps work best when producing heat at a lower temperature but over a longer period of time. it is therefore important that the property is properly insulated and draught proof in order for the heating system to be at its maximum effectiveness. A home with a high heat demand will need a larger system which will be expensive to buy and install, and more expensive to run.
The time in which a Ground Source Heat Pump system will recoup its initial outlay will be determined by several factors. One of those is to do with which heating system did it replace. If it was an all electric or coal system, then the repay time will be much quicker. Careful consideration has to be made as to whether it would make economic sense to replace an efficient gas boiler system with a GSHP.
The best heat distribution methods in a Ground Source Heat Pumps system are underfloor heating or warm air. If the heart distribution is done via radiators, then the use of large radiators rather than the standard ones is advisable. Remember that the radiators will not be as hot to the touch as with a gas boiler system because the GSHP heating is based on a different concept of longer operation at a lower temperature.
Planning permission
Domestic ground source heat pumps are usually allowed as permitted developments, but it is always best to double check with your local authority to find out whether planning permission is a requirement.