Assessment of BGS data for ground source heat pump installations in the UK

This report describes an assessment of data held by the British Geological Survey (BGS) in relation to the identification of barriers to the development of the ground source heat pump (GSHP) market in the UK. It forms part of the BGS Partnership Programme where projects are co-funded by industry and BGS. In this case the work was done as part of a project funded by Building Research Establishment (BRE) and the NIFES Consulting Group (Agreement F10/9/1027/7490) as part of the Future Practice R&D call from the UK Government Energy Efficiency Best Practise programme.
Ground source heat pumps (GSHP) are one of the most energy efficient environmentally clean and cost effective space conditioning systems available. A heat pump is a device that takes up heat at a certain temperature and releases it at a higher temperature. Ground coupled heat pumps work by utilising the heat stored in groundwater or the ground. At a relatively shallow depth (15m), the ground temperature will be approximately equal (±0.1°C) to the average mean air temperature throughout the year. In winter the ground temperature is generally higher than the air temperature, and in summer lower than the air temperature thereby providing capability for both heating and cooling modes. Generally thermal energy is transferred from the earth to fluid in a closed loop and is upgraded by passing through a water source heat pump. Typically the closed loop is configured in a vertical borehole or horizontal loop system in a shallow trench (1-2m). In the UK borehole depths are usually in the range 50-100m, in other EU countries depths of up to 150m are common.
GSHP are common in Europe and North America but there are only about 300 systems in UK homes and a few larger systems in commercial buildings. One of the factors identified as a barrier to uptake is the expense and uncertainty associated with installation of the external ground loop. Typically this can be 30-60% of the installation cost. This contributes to the capital cost premium on GSHP and can be a market barrier. Reduction in design uncertainty may help to reduce this cost.
Geological parameters impact directly on many aspects of the uncertainty: site temperature, performance prognosis, drilling costs. In addition the variety of the UK geology means that site-specific solutions need to be identified. The near-surface temperature profile includes components due to heat conduction, fluid flow, antecedent climate and the effects of diurnal and seasonal change in solar radiation. The British Geological Survey (BGS) has a unique spatial geothermal database of observed equilibrium sub-surface temperature. These data indicate that some areas have stable ground temperatures of 15°C at depths of 100m whereas other sites have temperatures of only 7°C. The GSHP performance for a 100m borehole is therefore likely to vary significantly with location. Existing models of shallow (0-100m) temperatures in the UK are inadequate and relevant properties are not readily available.
BGS holds a large amount of geoscience data with potential use to the GSHP industry. The main classes of data relate to geological formations, attributes of those formations especially thermal properties and state of the formations especially temperature and water saturation. Currently these data are not easily accessible to GSHP users and installers. The existing BGS GeoReports facility provides detailed geological reports and water borehole prognosis reports, both of which are useful for GSHP but key information on thermal state and properties is not provided. Residential GSHP installations may not require detailed geological assessments but confirmation of temperature and thermal properties together with some indication of ground conditions would be useful. Current use of the BGS Georeports facility by GSHP installers is limited.
Key points relative to the design process for GSHP in the UK
• Observed stable ground temperatures at 100m depth vary between 7-11 °C in Scotland and 11-15 °C in Southwest England
• The range of local mean annual air temperatures is strongly affected by elevation, site and aspect
• Thermal conductivity of UK rocks varies from about 1.5 - 3.5 W m-1 K-1
• Site-specific GSHP design for depths of up to 200m requires
-temperature, thermal conductivity and hydrogeology
-nature and thickness of drift or made ground in relation to drilling costs