Heat decarbonisation is a hot topic (pun intended!) due to The Government’s pledge to achieve Net Zero by 2050. According to the Department for Energy approximately 40 % of the UK’s existing energy use comes from heating and ‘heat pumps and heat networks will be the primary means for decarbonising heating over the next decade’. Therefore, Ground Source Heat Pumps (GSHP) are an important part of the decarbonisation transition. Carbon Zero Consulting (An RSK company) have the benefit of over 25 years’ experience working in this field as illustrated in the case studies below.
Heating and cooling
In heating mode, GSHP works by using electricity to amplify a few degrees of (renewable) heat taken from the ground or surface water into a temperature useable within a building or process in the same way as water heated by a fossil fuel gas boiler for space heating and hot water. The efficiency of a GSHP is measured in terms of the Coefficient of Performance (COP). The COP of a boiler is always less than 100%. A heat pump provides efficiencies in excess of 350%, and often much higher where both heating and cooling are supplied.
In cooling mode, GSHP takes heat from buildings and returns it to the ground where it can be stored for future use via the heat pump system.
Types of GSHP
There are two main types of GSHP: open loop and closed loop. Both types have advantages and specific design considerations.
Grantley Hall
Carbon Zero Consulting established from an initial test borehole that a closed-loop borehole system was not viable as very strong artesian pressure was identified within the underlying rock formations of the millstone grit. Drilling of a large number of closed-loop boreholes was judged to be not technically possible. The concept of using groundwater in an open-loop GSHP system was developed in parallel with a third borehole to supply water for the hotel and spa facilities.
Calderdale
Calderdale Council were progressing a far-reaching project to replace oil and gas-fired boilers in a number of their large public buildings. An initial viability assessment identified two listed properties as being candidates for installation of closed loop ground source heat pump (GSHP) renewable heating systems. A test borehole was designed and drilled at each site and a thermal response test (TRT) performed on each to measure the thermal conductivity of the drilled formations. The results of the TRT were then utilised within a closed-loop design model to derive borehole array solutions. The test results and geological appraisals were utilised within design-and- build tenders issued to candidate installers.
Photograph of TRT test kit
Open loop systems extract heat from a flow of water taken from bodies of water such as underground aquifers, lakes, flooded mine workings, rivers, and estuaries; before returning the water to its source (non-consumptive) or, more rarely, discharging the water to another location (consumptive).
Closed loop systems don’t need a direct source of water and instead take heat from the ground or water source via conduction via a ‘closed loop’ of pipe containing a heat transfer liquid. A closed loop system using the ground can use boreholes, pile foundations or shallow trenches.
Applications
Where carbon saving is the main driver, GSHPs can be used to service individual residential homes; but in today’s energy climate the real financial efficiencies occur when applying them to larger district schemes supplying groups of commercial, residential and or public owned buildings. In this way costly underground infrastructure can be shared. The benefits of moving towards district schemes is supported by The Government, who expect that heat networks, which currently provide around 3% of heat, could provide about 20% by 2050 (DesNEZ, April 2024 Heat networks regulation – consumer protection Government response).
Some of the early adopters of district heating schemes in the UK are Government Estates such as universities, schools, local councils, and hospitals, as well as large scale house builders and agricultural clients who use GSHP to heat or cool greenhouses.
Reading Borough Council, Hexagon Theatre
A new aquifer-fed open loop heat pump-is being designed to offer the council an effective route to decarbonising its estate with an initial focus on the Hexagon Theatre. Testing of the aquifer shows that the aquifer is capable of producing enough water to provide much of the heat and coolth required by the theatre. Once complete, the new infrastructure is estimated to save over 500 tonnes of CO₂ per annum.
Multi-megawatt heating for specialist fruit grower, East Yorkshire
Carbon Zero provided a detailed assessment of the viability of a GSHP system and reviewed the potential for either closed-loop or open-loop boreholes. Drilling of a very large number of closed-loop boreholes within the underlying chalk aquifer was judged to be not technically possible as the chalk is highly fractured and difficult to support while drilling. The concept of using an open-loop GSHP system was developed. Altogether, 4 abstraction and 4 injection boreholes were drilled to a nominal depth of 30m within the chalk aquifer. Test-pumping of the 8-borehole system was a highly complex undertaking, but successfully determined that a groundwater flow rate sufficient to provide the peak heating (and cooling if required) was available from the 4-borehole abstraction array.
GSHP can be used in both retrofitted older properties (even listed buildings) and newer, more energy-efficient buildings. As heat pump technology develops, the outlet water temperature which can be achieved can rival that of a traditional gas boiler, which reduces the need to replace radiators and install underfloor heating. There is an impact on GSHP system efficiency, with higher temperature output, however, if there is a good balance of heating and cooling, the system efficiency can be extremely high.
Other potential users for heating and/or cooling include hospitals, warehouses, underground sport pitches, swimming pools, shopping centres. The list is endless.
Infrastructure
With a borehole scheme, the surface footprint of an open-loop borehole system may be as small as a couple of approximately 3m square manholes, which are connected to a plant room containing the heat pumps via underground service pipes in normal service trenches. An array of closed-loop boreholes, once drilled, are buried and not seen again. The depth and area of boreholes or trench required depends on the geology and constraints of the site and the amount of heat (and cool) required. An open loop scheme requires the presence of an underground aquifer and will require only a few boreholes compared to a closed loop scheme, although open-loop boreholes are of a very different design and cost.
For an above ground water-source scheme, a heat collector, or a means to abstract and return water, will need to be installed in the target surface water feature.
Surface water river bed mounted abstraction/discharge system
Feasibility Study
The best solution will be determined by carrying out a feasibility study which looks at the ground conditions and amount of land available compared to the energy demands of the project. The design can be further refined by thermal and/or geological modelling following field tests which measure and refine estimates of the amount of renewable heat available.
Innovative ways of incorporating GSHP into existing schemes include inserting closed loop heat collectors within building structural piles or other existing infrastructure such as tunnels. This is best considered during the project planning and feasibility phase; so that opportunities are not lost. The amount of heat energy that can be obtained from thermal piles is limited because pile depths are generally very shallow (circa 20m compared to a closed loop borehole which could be 200 or 300m deep).
Regulation
Open loop schemes are currently regulated under existing abstraction and permitting laws, which can need professional help to manage the process from inception through to issue of the formal licence and/or permit. However, in most cases, the process results in a licence being granted, which gives the user a protected right to abstract water for heating and cooling. There are plans to modify these regulations and streamline the process, this is due to come into force in the next 2 years.
Until recently closed loop schemes were not regulated in England. However, under modifications to the Environmental Permitting Regulations, some restrictions are now in place in areas considered to be particularly sensitive, such as large schemes adjacent to important wetlands.
Regulatory requirements can be identified at the feasibility stage.
The future
There are clearly a lot of opportunities related to GSHP for geotechnical and geoenvironmental specialists in the future. For example, the geological data collected during ground investigations can be used to contribute to feasibility studies and test open or closed loop boreholes could be constructed as part of a detailed ground investigation. The sooner building decarbonisation is considered within a project lifetime, the more efficient it will be to incorporate its inclusion into construction projects.
Perhaps your next site will be suitable to include closed loop borehole collectors into building piles to provide some of the heat/cool solution or you could tap into a district GSHP scheme to reduce your project’s carbon footprint.
Article provided by Dr Anna Hitchmough, Carbon Zero Consulting (An RSK Company)