Explainer: Ground source heat pumps

The 2016 Next Steps for UK Heat Policy report revealed that natural gas heated 86% of UK homes, with electricity generating heat for only 8% (see Figure 1). Electrical power has the potential to be lower carbon as more of the supply comes from renewable sources. Natural gas, meanwhile, is a fossil fuel and burning it for heating releases greenhouse gases (GHGs) into the environment. 

Government figures show that fewer than 1% of homes in England are heated using either air or ground source heat pumps. About 5% of dwellings are fitted with photovoltaic panels that feed into the grid and 1% use solar panels directly to heat water. 

If the UK is to reduce its dependency on fossil fuels by 2050, then 24m homes that are reliant on natural gas will have to switch to zero-carbon alternatives. 

The Net-Zero Infrastructure Industry Coalition report The Path to Zero-Carbon Heat sets out three potential routes to achieve net zero in heating:

1. Electrification pathway: the widespread deployment of heat pumps in buildings 
2. Hydrogen pathway: supplying hydrogen directly to the majority of buildings for combustion in hydrogen heating systems 
3. Hybrid pathway: the widespread deployment of hybrid heat pump systems in which a heat pump supplies most of a building’s heating demand, supplemented by a boiler at peak times – initially fuelled by gas but transitioning to biomethane by 2050 

The report concludes that heat pumps will be required for all three pathways. As the hydrogen route relies on hydrogen being supplied through the existing gas network, an alternative (i.e. heat pumps) will be needed for buildings not currently connected to that grid.  

This will require a huge scaling up of the current installation rate. Data from the European Heat Pump Association shows that the UK installed the lowest number of heat pumps per household across Europe in 2021, with 1.48 heat pumps installed per 1,000 households. In Norway the rate is 49.77 per 1,000.  

Heat pumps work by absorbing heat from the environment and transferring it to a fluid, which is compressed to increase its temperature. This heat is then transferred from the compressed fluid into the central heating system, where it can be used for both heating and hot water. 

There are two types: air source heat pumps, which absorb heat from the air; and ground source heat pumps (GSHPs), which absorb heat from the ground. Both options require an electrical power source for day-to-day operation, but if this comes from a renewable source, they do not generate GHGs. 

GSHPs are of particular interest to geotechnical engineers because: 

• Geotechnical factors should be considered when installing GSHPs
• GSHPs can be installed as part of a geotechnical solution, such as piles and tunnel linings

GSHPs make use of the solar heat stored in the ground at shallow depths – usually up to a maximum depth of 500m.

There are two types of GSHP: closed-loop and open-loop. A closed-loop system extracts heat using a network of pipes, known as the ‘ground loop’, installed in the ground either horizontally or vertically, and filled with a mixture of fluids (usually water and antifreeze) that circulates continually around the system. 

This fluid absorbs heat from the ground and carries it to a heat exchanger, where it is transferred to the network of pipes that supply the heating and hot water system. It is a continuous system – the fluid does not leave its pipes and circulates constantly (see Figure 3).

An open-loop system uses water extracted from a warm water source, such as an aquifer or pond, which is carried to the heat pump where the heat is transferred. That water is then either re-injected into the ground or discharged at the surface and is not recirculated. 

Open-loop systems are often cheaper to install and give a higher energy yield, but require a direct connection to groundwater, which usually requires licences and permits. Closed-loop systems are therefore more practical in most domestic and commercial building settings.  

If a horizontal ground loop is being used, this is usually installed at a depth of about 2m below the surface. Heat pump suppliers say that, on average, the surface area of land required for a domestic GSHP installation with a horizontal loop is approximately 2.5 times the square meterage of the house. 

This makes horizontal loops impractical in most urban settings, so vertical ground loops are usually installed in towns and cities. This option involves drilling boreholes, usually to about 100m in depth, with a single 200mm diameter borehole being adequate for a small house. 

Research by University of Surrey research fellow Abubakar Kawuwa Sani indicates that the heat energy exchanged between the infrastructure and the ground depends on several factors:  

1. The number and location of the installed energy loops within the pile (geothermal energy pile)  
2. The thermal conductivity of the concrete pile and the surrounding soil
3. The degree of soil saturation 
4. The soil type and soil grain size 
5. The duration and magnitude of the thermal load applied
6. The spacing between geothermal energy piles when installed in a group

Geology 

When it comes to installing boreholes for vertical GSHP loops, the area’s geology needs to be considered, says David Broom, managing director of GSHP specialist Kensa Contracting. 

“Areas with good thermal conductivity underground are often the easiest to drill,” he says. “Low conductivity might mean an additional borehole for the same load, and that can affect the cost of the infrastructure. It’s an early part of the process to make sure we can serve the load of the building and heating/cooling peak requirements.”

Different geologies typically require different methods of flushing during drilling. Air drills are used for Cornish granite, for example, whereas other areas would require mud flushing. Developers must also check with the Coal Authority whether they need permission to drill.  

The moisture content of the ground can also affect suitability. Generally, fully saturated ground will help the heat transfer: the wetter the ground, the more efficient it is. When considering a site, investigations should indicate the most efficient locations.  

Cost

Cost is a limiting factor for developers and householders thinking of installing GSHPs. Bean Beanland, principal consultant at BBH Energy and director for growth and external affairs at the Heat Pump Federation, says: “From a physics perspective – and a building physics perspective – there is a heat pump solution for everything. Whether it’s financially viable or not is the issue.” 

Currently, installation costs for GSHPs in the UK are high, as they involve excavation or drilling as well as the hardware and pipe network. The Climate Change Committee (CCC) estimates that it costs an average of £26,000 to switch a UK home to low-carbon heating, while high electricity prices make running costs expensive, especially compared with the low cost of gas. However, the electricity-to-gas cost ratio is changing, which may make GSHPs more viable in the near future. 

Large building and infrastructure projects offer the opportunity to install GSHP systems that can provide heating on a district-wide scale. The CCC estimates that, by 2050, about 18% of UK heat will need to come from district heating networks (DHNs), with private and public entities combining resources to share heat and cooling systems. 

Beanland urges those involved in infrastructure projects to think more about broad, shared heat pump arrangements with nearby schemes. “Look over the fence. What are the neighbours doing, either in terms of their heat assets or load, that you can benefit from?” he says. 

Fleur Loveridge, associate professor of geostructures at the University of Leeds, supports this view. “If you are building a new road, railway or other [type of] national infrastructure, such as a tunnel, why shouldn’t it be equipped with heat transfer?” she asks. “What is stopping us from making these solutions? It may be that there aren’t enough district heating networks in the UK. It might be an institutional reason – [stakeholders] may not be ready to add the aspect of selling heat.” 

Beanland believes that the CCC’s estimate could be exceeded once developers realise the benefits of lower-cost energy loops that deliver cooling as well as heating. He points to new business models in which third-party investors own the infrastructure in the ground, meaning developers do not pay for excavation or installation, as this will be an investor’s asset.  

Incorporating heat systems into structural piled foundations is an increasingly common approach to improving energy efficiency and reducing the carbon emissions from new buildings. 

There are two main issues to consider in the design of thermal piles: the required heating and cooling capacity; and ensuring the changes of temperature do not have an adverse effect on the pile itself.  

The thermal behaviour inside the pile will significantly influence its size, the amount of concrete cover to the pipes, and the relative positions and number of pipes within the pile that can cause internal heat transfer.

There are two typical designs for thermal piles (see Figure 4). Either pipes are placed around the circumference of the pile, attached to the steel cage; or the pipes are placed centrally within the pile.

Loveridge says it is logical to use pile excavations to incorporate pipes, although it brings challenges: “Cost factors include additional programme time being added to the construction, depending on the type of foundation and how you get the pipes into the foundations, and whether the activity is onsite or prefabricated.  

“Care is needed at different stages, and pipes must be protected through the process of breaking out any concrete to connect the foundations to any slabs, for example. The pipes must be filled with fluid and kept under pressure and tested regularly, so a named coordinator must ensure the integrity of the system. You need to take care of the infrastructure throughout,” she adds. 

The need for care during piling and construction is underlined by Zoe Baldwin, technical manager at Cementation Skanska. “Keeping the pipes intact and safe during installation, patching them into the pile cages, can be a challenge,” she says. 

“Imagine you have 300 bearing piles and you put loops into all of them. All of those loops have got to make their way to the central heat exchanger. There are definitely logistical challenges to make sure it doesn’t clash with drainage, that pipes don’t get damaged and that the distances are understood. But these are not complex and we now have standard processes.” 

The behaviour of the piles once in use has been studied by researchers at Imperial College London. They looked specifically at the stresses on thermal piles in London Clay undergoing temperature changes as the heat was extracted (or injected if used for cooling). It was found that cooling generated considerable tensile axial stress changes in the pile, indicating that thermo-active piles must be checked for tensile capacity during design.  

Conversely, the research showed that heating caused compressive axial stress changes that may need to be accounted for during the design procedure. However, the researchers also found that the stress changes reduced with time as the surrounding soil reacted to the changes imposed on the pile.

Incorporating heating systems into infrastructure projects such as tunnels can provide a low-carbon option for heating and cooling. Research from the Politecnico di Torino in Italy found that, for deep tunnels, the most promising application was for cooling the internal tunnel air.  

“Thermally active tunnel linings could allow reducing, if not avoiding, these costs with the added value of providing heat at the tunnel portals for potential users,” note the researchers. 

They point out that heat extraction and heat injection through ‘energy tunnels’ are best applied within urban areas, where the exchanged heat can be directly utilised by adjacent buildings and integrated into district heating and cooling systems (see Figure 5). “Geothermal exploitation in urban areas requires city-scale planning, interference evaluation, and regulation,” they add.

The University of Surrey is seeking proof of concept for using different liquids within pipes to store heat. Senior lecturer Liang Cui says: “Using a material that changes phases, going from liquid to solid at about 15C, means the heat can be stored, making the whole process more efficient. The phase-changing material is stored in a closed loop or container – when the liquid runs around it, it can either store or release the energy as required.” 

Elsewhere, the Centre for Infrastructure Materials at the University of Leeds is using its expertise in concrete technology and material durability to ensure optimum performance of concrete thermal heat piles throughout their entire design life, and to look for possible changes in performance as concrete ages. 

Also, in Germany, long-term studies of ground source heating and cooling systems for six different buildings (commercial, institutional and multi-family) have been conducted by research consultancy Steinbeis-Innovationszentrum. Three have borehole heat exchangers, and the others use energy piles. After 10 years of operation, the systems achieved their planned efficiency but required constant control and regulation to avoid faulty operation.  

The Net-Zero Infrastructure Industry Coalition report states: “Heat pump technology is well understood but will require further development to decrease space requirements, improve efficiency and controls (such as integrated solutions for domestic hot water and exhaust air) and integrate with smart controls and phase-changing materials for thermal storage.”  

The installation of GSHPs has significantly reduced carbon emissions at Daisyfield Towers, a development of 183 flats across three high-rise tower blocks in Blackburn, Lancashire. 

The new heating system is part of a scheme by Together Housing Group aimed at tackling fuel poverty, reducing heating bills and combating carbon emissions for residents. The project is expected to save 6,556t of CO₂ over its lifetime, equivalent to taking 1,416 cars off the road for a year.

Key facts

1. Three tower blocks totalling 183 flats 
2. 84 boreholes up to 300m in depth 
3. Total of 16,146m pipe installed
4. Kensa Shoebox heat pumps on shared ground loop array
5. Estimated lifetime carbon savings of 6,556t of CO₂

Specialist contractor Geodrill drilled 84 boreholes across the Daisyfield site, with an aggregate depth of 16,146m. A combination of favourable ground conditions and borehole configuration enabled Geodrill to increase the drilling depths to 300m, in one of the UK’s first trials of GSHPs and medium-depth boreholes. 

Daisyfield is in a densely populated urban area where available land is limited. When installing GSHPs in locations such as this, if boreholes are drilled to a standard depth of 100m-230m then projects can be unfeasible if there is insufficient space to install the required number. 

However, drilling to depths of 300m-400m means that more pipes can be installed in the ground in the same surface area. This allows the loops to absorb extra energy, sustaining a higher heat load for domestic properties or commercial premises in urban or high heat-loss locations. 

“The civils are the same boreholes, but the architecture linking multiple boreholes is the clever bit once we have done the hydraulics and calculations to make it work,” explains David Broom, managing director of the project’s main contractor, Kensa Contracting. “Drilling fewer holes while increasing the depth adds efficiency.” 

Scalability 

There is a significant opportunity to meet government carbon-reduction targets for heating through the increased use of GSHPs, whether shallow or medium depth, or a combination of the two. The use of medium-depth boreholes offers a solution for district heating, which is more common in highly populated areas where space for boreholes is limited. 

The ground array can be utilised for cooling as well as heating, allowing inter-seasonal storage of energy within the ambient temperature pipework. Smart controls can be used to balance peak heat demand.