Ground Source Heat Pump Guidance

Ground source heat pumps
The heat for ground source heat pumps
Collecting the heat
Powering ground source heat pumps
Ground source heat pump application
Planning and regulation
Government funding
Great resources

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Ground Source Heat Pump Guidance

This online Renewable Technology Guide will help to give you an understanding of the technology and application of ground source heat pumps (GSHP) to provide heat for water in domestic and small commercial buildings.

This briefing will give the key outline and link you directly to fuller explanations on the internet (these are not official B&ES endorsed links), and downloadable documents (in case you want to know more about any aspect). Additionally there will be references to key books and pamphlets that can give you better understanding of the subject – many freely available. Much of the underlying heat pump technology is outlined in the accompanying air source heat pump article on this site.

In addition the B&ES TR30 'Guide to Good Practice - Heat Pumps' provides detailed guidance and the association also has a Heat Pump Interest Group that is open to all members. Contact Gareth Keller for more information.

The ground source heat pump

A novel form of short vertical ground loop

Ground Source Heat Pumps can be powered by electricity and gas and can represent an energy efficient alternative to fossil fuel boilers for heating new and existing buildings providing heat for hot water (and air).

When selected and operated properly they can provide more heat energy to the building than is supplied by their powering electricity or gas as they simply move and enhance the temperature of the heat that is freely available in the ground - hence the term 'heat pump'. Appropriate use of an electrically powered heat pump can be cheaper than using an oil fuelled or condensing gas boiler, as well as reducing operational carbon emissions.

The use of the ground as a heat source can restrict the application of GSHP (depending on location) but with increasingly ingenious methods of linking in with the earth their use is expanding in the UK. A key strength (compared to air source heat pumps) is that the temperature (and potential for heat pumping) of the ground will be reasonably stable throughout the whole year allowing extraction of heat both in summer and, importantly, winter.

This guidance will focus on 'ground to water' source heat pumps however the technology is similar to 'ground to air' systems that may be used in ventilation and air-conditioning applications. The refrigerant flow can be reversed to heat as well as cool the internal space and so use the ground as a heat dump rather than a heat source. Their may be advantages in using seasonal heat storage to provide stabilised year round ground temperatures and allow cooling in summer and heating in winter.

Note that GSHPs are different to Geothermal Power Production where high temperature sources of heat are accessed directly from the ground such as in Southampton's scheme. For GSHPs the source of heat is driven by the climate above the ground whereas geothermal uses resources of heat generated from below the earth's surface.

The heat for ground source heat pumps

The temperature of the ground is affected by seasonal changes to just a few metres depth. UK Ground temps Below a couple of metres deep the temperature is unlikely to fall below 5°C during the year as shown in diagram (for southern England). And moving beyond 10m deep it is reasonably constant at around 8.5°C in northern Scotland to 11.5°C in southern England. This will be affected by local ground conditions so, for example, where there is an underground water course the temperatures may vary from this generalised pattern.

The heat is extracted from the ground using 'collectors' or 'ground loops'.

The amount of heat that may be extracted by the collectors will be dependant on the type of ground and the temperature of the ground. As part of the Microgeneration Certification Scheme (the quality assurance scheme required for installations with a thermal load up to 45kW) there are some simple look up tables that provide 'backstop' guidance as to the potential heat available from collectors. For larger installations more detailed surveys and calculations should be undertaken otherwise systems may be oversized and most costly than needed.

As GSHPs are used to draw heat from the ground during the heating season the ground temperature will fall and incorrectly sized or operated systems can create ground freezing and frost heave. If possible it is beneficial to recharge the ground in summer. This would preferably using simple pumped rather than heat pumped systems - for example circulating water around an underfloor heating system and (indirectly) passing that heat into the ground.

Collectors

The are two principle types of collectors - open loop and closed loop - with closed loop (using a piped collector) being the most common. The closed loop is principally broken into horizontal and vertical systems. Whichever system is used it is essential to design collectors for the specific application and location including assessment of operational pumping costs vs the length of pipe systems.

Open loop Water that naturally flows through the ground in aquifers (in spaces between gravel, sandstone, and fractured rock) will tend towards the temperature of the ground through which it is flowing. By drilling holes into the aquifer the water can be diverted (pumped) to provide a source of heat. (The water is normally returned to a separate location once the heat has been extracted). An abstraction licence is likely to be needed to use the water as outlined by the Environment Agency.

To determine feasibility requires a specialist hydrogeological survey to determine the extent and longevity of the available resource. As a consequence of the more complicated survey requirements of this technique and due to greater uncertainties (such as unforeseen geological conditions, air locking and collapse of the boreholes or a falling water table), closed loop systems are more frequently employed. However where there are available 'wells' that can provide ready sources of water open loop systems can be more effectively applied and in some areas (such as London) rising water tables provide a source of water for heat pumps where the water is discharged away from the original water course.

Closed loop heat exchangers usually consist of a sealed loop of high density polyethylene pipe containing a glycol/water fluid which is pumped around the loop. The heat exchanger may be installed vertically or horizontally, the choice of the arrangement will depend on the available land, local soil type and excavation costs. Like open loops they can still suffer from air locks and require protection both during and after they have been installed to prevent blockages as well as consideration of the pipework falls. The amount of heat transfer between the ground and the pipe will depend of the thermal properties of the ground, the material immediately surrounding the pipe, the fluid flowing in the tube and how many hours in the year the heat will be extracted.

Horizontal trenches - Where horizontal trenches are used to extract energy from the ground the depth would normally be between 0.8 and 2 metres - the depth is limited by the practicalities of site excavation. For strip trenches (used commonly with ‘slinky’ type coils) the width of the trench is typically the same as that of a bucket on a digger - the coils being installed either horizontally or vertically in deeper but narrower trenches. The coils are made up on a former (held in loops with plastic 'cable' ties) and then dropped into the trench. Alternatively straight runs of pipe may be used with an increased trench length.

As an example comparison for wet clay ground in SE England a slinky would provide a max heat output of about 40W per trench metre when run for 2400 full load equivalent hours per year and a straight pipe about 12W per metre trench (although the trench for a simple straight pipe would be much narrower).

The pipe would be laid no closer than 0.8m apart and the recommended maximum length (to maintain reasonable pressure drops) for 25mm pipe is 100 metres and for 32mm pipe is 200 metres and so frequently 2 or 4 loops will be used to provide the required heat extraction.

It is recommended that no more than 70kWh per square metre of ground should be drawn annually (unless it is 'recharged' in summer).

Vertical bore hole based ground loops are used where there is limited ground surface available as in more densely populated areas or where there is little soil to excavate. They are likely to be more expensive to install than horizontal systems and require a specialist contractor who will commonly leave the borehole tails for connection by the mechanical contractor.

Bore holes are required to be deep (in the order of 15 to 100 metres) to accommodate the appropriate length (and hence surface area) of pipe to extract the required heat from the ground. The bore holes would normally be no closer than 6 metres apart and take one or two loops.

The HDPE or Pex pipe (having been made into a 'U' tube, filled with water and weighted on the base of the 'U') would be dropped into the borehole and the pipe would be surrounded by a grouting medium (injected at the same time, or soon after, the pipe is installed using a 'tremie' pipe) to ensure good heat transfer between the ground and the pipe. (This may not be needed where the boreholes are permanently flooded with ground water).

As an example for wet clay ground in SE England a borehole U tube would provide a max heat output of about 25W per metre depth when run for 2400 full load equivalent hours per year.

The amount of heat that can be extracted from the closed loop will be determined by the type of ground, the annual operation period, the pipe size, the material surrounding the pipe and the mix of the fluid in the pipe and can be estimated using the MCS tables. BS EN 15450:2007 'Heating systems in buildings – Design of heat pump heating systems' provides more detailed information.

Novel hybrid coiled collectors have been developed including those that can be used with with larger diameter, but shorter, bore holes (as shown here) and those that are integrated into the structural piles of a building. The use of more complex angled boring and underground piping techniques are allowing higher heat capacities from smaller ground surface areas (as in the 2010 Sainsbury's Crayford store).

The collectors will join together in flow and return manifolds and then pass into the heat pump itself to exchange the heat drawn from the ground with the refrigerant passing through the evaporator of the heat pump. It is essential that the underground pipework is 'fusion' welded and particular care should be taken to maintain integrity of all external pipework.

The video below shows drilling a borehole for a 100m U tube collector.

Powering ground source heat pumps

A heat pump is typically based around a vapour compression refrigeration system that uses an evaporator, compressor, condenser and expansion valve linked by a closed pipe circuit in which a refrigerant circulates, transferring heat indirectly from the ground, via the collector loop, to the indoor loads.

The basic details of the heat pump system are discussed in the Air Source Heat Pump article including the measures of performance including COP (coefficient of performance). GSHPs are specific heat pumps designed to operate over an extended range of entering water temperatures as needed to supply the heating and hot water load across a season. They are normally housed in a unit (similar size to a large upright refrigerator) in a dedicated plant area either in the main building or in an external enclosure.

The typical evaporator temperatures are from -5°C to 12°C, whereas the condenser temperature (supplying the heat to the building) typically has a maximum value of around 55°C. (Too low for legionella control with domestic hot water systems so requiring some supplementary heating).

Gas fired and alternative heat pumps are discussed in the ASHP article and a more detailed description of the main heat pump types can be seen here. The common refrigerants used within the heat pump are also discussed in that article.

Ground source heat pump application

The effectiveness of a basic heat pump is measured by the Coefficient of Performance (COP) and this is covered together with SPF (Seasonal Performance Factor) and the Energy Savings Trust's System Efficiency in the Air Source Heat Pump article on this site.

The 2011 report by the Energy Savings Trust surveyed installations and determined that in the UK the ‘mid-range’ ground source system efficiencies (over the whole year) were between 2.3 and 2.5, with the highest figures reaching over 3.0.

Heat pumps work most effectively when supplying heat at lower temperatures. And so heat pumps are best suited for use with low temperature systems such as underfloor heating at 30-45ºC, fan coils at 35-55ºC, and radiators sized to operate at 45-55ºC. Advancements in heat pump technology are providing systems that can successfully heat domestic hot water at 60°C (particularly those using CO₂ as a refrigerant).

Any heat that is not used immediately to heat the building or domestic hot water can be added to a thermal store. The thermal store can be used to reduce unwanted compressor cycling when using systems with relative low water volumes such as low temperature radiators or convectors (underfloor heating has a greater inbuilt 'store').

Where there is substantial domestic hot water consumption the heat pump may be used to pre-heat hot water feed water where, using an indirect cylinder (or thermal store) as an interface, the ground source heat pump can operate in conjunction with a direct gas-fired water heater or a hot water calorifier.

Additionally the use of a indirect heater will provide some buffer storage – this is beneficial as it reduces heat pump cycling. If a buffer tank is employed then particular care must be taken when determining operational requirements to avoid legionella risks.

Noise Unlike air source heat pumps GSHPs are normally 'silent' in the external environment as the active components (pump, compressor) are typically housed internally as part of an integrated unit.

For performance issues of heat pumps supplying loads at different temperatures including monovalent and bivalent applications see the ASHP article.

Microgeneration Installation Standard: MIS 3005 provides detailed approved guidance on design requirements for ground source heat pumps - this is particularly important if government funding is being sought for the project.

Planning and Regulation

In England, Scotland and Wales, domestic ground source heat pumps are generally allowed as permitted developments and in Northern Ireland planning permission should be sought for ground source heat pumps.

Environmental protection

The environment agency publish a "Environmental good practice guide for ground source heating and cooling" that gives detailed guidance on the proper procedures and materials that should be used when extracting heat from the ground.

Building Regulations - Heat pump heating systems for should meet the recommendations of the Non-domestic/Domestic Heating Compliance Guide as a means of demonstrating compliance with the Building Regulations.

For those other than absorption heat pumps and gas-engine heat pumps the COP for for space heating should be 2.2 and a COP of 2.0 for hot water when operating at the rating conditions as defined by BS EN 14511:2011. Absorption heat pumps are required to have a COP of 0.5 and gas-engine heat pumps 1.0 when operating at the standard conditions.

And in terms of seasonal performance factor, for new build an SPF of 3.5 is required and 3.3 for retrofit

Legionella - Approved Code of Practice and Guidance L8 includes hot water storage arrangements and requires that provisions are made to heat the whole water content of the calorifier, including that at the base, to a temperature of 60°C for one hour each day.

Only heat pump packages that are CE marked in compliance with the relevant European Directives may be offered for sale in the EU.

Government Funding

Heat pumps contribute to a reduction of carbon emission and are recognised as a 'renewable energy' technology by the UK Government. In non-domestic installations (ie systems that serve more than a single domestic application or commercial premises) the Renewable Heat Incentive is payable (for eligible installations) that provides income for each kWh of heat produced. (From late 2012 this may also apply to domestic applications). Until March 2012 there are a limited number of domestic installations in Great Britain (that do not have gas as the primary heating fuel) that can attract a government payment of £1250 Renewable Heat Premium Payment.