Thermal conductivity and diffusivity estimations for shallow geothermal systems

Horizontal closed-loop ground collectors for ground source heat pumps are located within the soil and the top of the underlying superficial deposits. Estimating thermal properties for this zone is difficult as it is heterogeneous and is subject to seasonal water content variations. Soil thermal diffusivity values have been calculated at 56 sites using temperature data from UK Met Office weather stations. The technique utilizes the decrease in amplitude and increase in phase shift with depth of a transmitted heat pulse in the ground, the magnitudes of which are determined by thermal diffusivity. The weather stations are located throughout Great Britain and incorporate different soil types. The apparent thermal diffusivities derived from seasonal temperature cycles spanning several years generate seasonally averaged site-specific estimates that can be considered alongside diffusivity values determined in the laboratory or obtained by point measurements using field needle probes. Associated thermal conductivities have been estimated from the thermal diffusivities from knowledge of soil texture. Median thermal conductivities for the sand, loam and clay soil types have been estimated as 1.56, 1.15 and 1.81 W m−1 K−1 respectively with corresponding thermal diffusivities of 0.9961 × 10−6, 0.7173 × 10−6 and 1.0295 × 10−6 m2 s−1 respectively. Shallow ground source heat collector loops often comprise straight pipes or coiled pipes (commonly referred to as slinkiesTM) that are laid horizontally along the base of a trench, or coiled pipes that are inserted vertically in a slit trench (Banks 2012). The suggested depth of the trenches varies, but GSHPA (2014) recommended 0.8–1.5 m below ground level and Banks (2012) indicated 1.2–2 m. These trenches are therefore located within the soil and the top of the underlying superficial deposits. This unconsolidated geological material is often referred to as the parent material of the soil and is a geological deposit over and within which a soil develops (Lawley 2008). Soils can be categorized as sand, silt, clay and loam (or combinations of these) where a loam is composed of approximately equal amounts of sand, silt and clay. The length of the collector loop depends on many factors, but the ground's thermal properties (thermal conductivity and thermal diffusivity) will need to be either estimated or measured (e.g. IGSHPA 1996; VDI 2001; Preene & Powrie 2009; Banks 2012; Curtis et al. 2013; GSHPA 2014) to ensure adequate sizing of the loop. There is a paucity of data on soil thermal properties required for the sizing of horizontal collector loops that is compounded by their seasonal dependence. A field method for estimating soil thermal properties has been given by IGSHPA (1989). Many quoted measured soil thermal properties are based on laboratory measurements (e.g. Clarke et al. 2008). These often use bulk soil samples that are bagged in the field, in which case the in situ consolidation is lost and is re-created in the laboratory. However, this will alter the bulk density, which is an important parameter in determining the thermal properties (e.g. Kersten 1949). Alternatively, field samples can be taken with a corer that incorporates a liner to preserve the natural texture and moisture, before transfer to the laboratory for thermal properties testing. For borehole-based vertical systems, a thermal response test can be performed to measure in situ bulk thermal conductivity (e.g. Banks et al. 2013), but there is at present no equivalent for horizontal systems. Thermal conductivities at a point on the ground can be measured with a needle probe (Campbell et al. 1991; Bristow et al. 1993; Bilskie et al. 1998). Field probes are mounted on a long handle so that they can be inserted into the base of auger holes to over 1 m depth. The probe generates a constant heat output and is a transient technique that monitors the increase of temperature with time. The determined thermal conductivity is representative of only a small cylindrical volume around the probe and errors can result from the contact between the probe and the soil. King et al. (2012) have indicated that a minimum of 12–16 measurements should be taken at a site with a field probe to produce a representative geometric mean thermal conductivity. However, such values are still valid for only a particular point in time, as near-surface thermal properties are affected by the seasonal variation in soil moisture. As an example of this variation, Gonzalez et al. (2012) quoted a 37% increase in soil thermal conductivity at 0.75 m depth and a 23% increase at 1 m depth between dry summer and wet winter conditions for a loamy sand (average composition clay 2.4%, silt 33.2%, sand 64.4%) that developed over a superficial deposit of sand and gravel. Apparent thermal diffusivity can be determined from soil temperature measurements and has been widely reported (e.g. Kappelmeyer & Haenel 1974; Adams et al. 1976; Horton et al. 1983; Verhoef et al. 1996; Gao et al. 2009). The technique utilizes the decrease in amplitude and delay in temperature change (phase shift) with depth of a transmitted heat signal applied to the ground surface, the magnitudes of which are determined by thermal diffusivity. If the heat signal is periodic (i.e. the diurnal or seasonal temperature cycle) and it is assumed that the heat transfer is governed by the 1D heat conduction equation, six different methods for calculating thermal diffusivity can be defined (Horton et al. 1983). Adams et al. (1976) and Horton et al. (1983) found that some of these methods gave erratic results. This may be partly due to using temperature measurements from the upper 10 cm of the soil, a zone where heat transfer is unlikely to be purely by conduction, and to too few temperature measurements, which do not adequately describe the periodic signal. This paper explores the calculation of soil thermal properties by utilizing the database of British meteorological soil temperature measurements taken at multiple depths to a maximum depth of 1 m. Thermal diffusivity is calculated directly from the depth-distributed soil temperatures and thermal conductivity is estimated from the diffusivity measurements with the addition of assumed parameters based on soil texture. The soil temperature measurements are widely dispersed covering many soil types and occupy the depth range of a horizontal ground collector loop. The calculated thermal properties are annual averages rather than a single seasonal value taken at a point in time. Although specifically incorporating British datasets the results and conclusions are applicable to shallow ground source heat in general

Determination of thermal properties for horizontal Ground Collector Loops

Horizontal closed loop ground collectors for ground source heat pumps are located within the soil and the top of the underlying, unconsolidated geology. Estimating thermal properties for this zone is difficult as it is heterogeneous and is subject to seasonal water content variations. Field measurements taken with needle probe instruments only provide data for the small annulus around the needle probe and are a snapshot in time, highly dependent on the state of saturation. Alternatively, apparent thermal diffusivity can be determined from soil temperature measurements. The technique utilises the decrease in amplitude and increase in phase shift with depth of a transmitted heat pulse in the ground, the magnitudes of which are determined by thermal diffusivity. Soil temperature data from 65 United Kingdom Meteorological Office weather stations have been used to calculate soil thermal diffusivity values. These are located throughout the UK, including different soil types and occupying the depth range of a horizontal loop ground collector. The apparent thermal diffusivities derived from seasonal temperature cycles spanning several years results in seasonally averaged, site specific estimates that are more representative of the ground conditions than diffusivity values determined in the laboratory or obtained by point measurements using field needle probes. Associated thermal conductivities have been estimated from the thermal diffusivities from knowledge of soil texture. These determinations have been compared against other thermal property estimation schemes and provide a data set that can be used for assessing and calibrating modelled data sets

UK shallow ground temperatures for ground coupled heat exchangers

Accurate estimations of shallow ground temperatures are required when sizing the horizontal closed loops and air supply culverts of ground coupled heating and cooling systems. These collector loops and culverts are within the zone affected by the seasonal swing in temperatures. Soil temperatures from 106 Met Office weather stations, located across the UK, have been analysed from which mean annual, seasonal minimum and maximum, and daily minimum and maximum temperatures have been calculated. Mean annual temperatures at 1 m depth, reduced to sea level, range from 12.7°C in southern England to 8.8°C in northern Scotland, with corresponding seasonal ranges in temperature of 10.3°C and 7.9°C respectively. An average urban heat island (UHI) effect at 1 m depth of 0.55°C has been observed at localities adjacent to urban green spaces, from which it can be assumed that the UHI effect will be greater in densely developed city and town centres. A linear relation has been derived for the mean annual temperature at any non-urban UK locality, at 1 m depth. The seasonal temperature cycle has been extrapolated accurately to several metres depth with site-specific thermal properties derived from the soil temperature measurements

Initial geological considerations before installing ground source heat pump systems

The performance of an open- or closed-loop ground source heat pump system depends on local geological conditions. It is important that these are determined as accurately as possible when designing a system, to maximize efficiency and minimize installation costs. Factors that need to be considered are surface temperature, subsurface temperatures down to 100–200 m, thermal conductivities and diffusivities of the soil and rock layers, groundwater levels and flows, and aquifer properties. In addition, rock strength is a critical factor in determining the excavation or drilling method required at a site and the associated costs. The key to determining all of these factors is an accurate conceptual site-scale model of the ground conditions (soils, geology, thermogeology, engineering geology and hydrogeology). The British Geological Survey has used the modern digital geological mapping of the UK as a base onto which appropriate attributes can be assigned. As a result it is possible to generate regional maps of surface and subsurface temperatures, rock strength and depth to water. This information can be used by designers, planners and installers of ground source heat pump systems. The use of appropriate geological factors will assist in creating a system that meets the heating or cooling load of the building without unnecessary overengineering

Assessment of the resource base for engineered geothermal systems in Great Britain

An assessment of the engineered geothermal system (EGS) resource base that might be available for the generation of electricity for Great Britain has been undertaken by adopting a globally self-consistent protocol that if universally adopted, would allow estimates of EGS made for different countries and regions to be comparable. Maximum estimated temperatures at depths of 5 and 7 km are greater than 200 and 300 °C respectively, a considerable increase over previous estimates. The total heat in place in the basement, to a depth of 9.5 km that is theoretically available for EGS is 357,197 EJ. If it were possible to develop just 2% of this resource, this would be equivalent to 1242 times the final UK energy consumption in 2015. The theoretical and technical potential power has been calculated from the available heat in place. The total technical potential power, to a depth of 6.5 km, is 222,393 MWe and represents just 0.4% of the theoretical potential power. Current EGS exploitation is more likely to be restricted to a depths of around 4.5 km and reservoir temperatures greater than 175 °C. In which case technical potential power is mainly restricted to regions of high heat producing granites and represents a total technical potential power of 2280 MWe. However, improvements in drilling technology are expected to enable economic drilling to depths of 7 km or greater that will enable EGS exploitation in all regions of Great Britain

The Cornubian geothermal province: heat production and flow in SW England: estimates from boreholes and airborne gamma-ray measurements

The Cornubian granite batholith provides one of the main high heat production and flow provinces within the UK. An extensive programme of borehole measurements was undertaken in the 1980’s to characterise the geothermal resource. Here we revisit the published data on heat flow and heat production from 34 boreholes and revise the published heat flow values in accord with modern palaeoclimate knowledge. This leads to a more rigorous (and increased) set of estimated temperatures at depth across the granite outcrops. Predicted temperatures at a depth of 5 km, largely exceed 200 °C and are 6-11% higher than previously estimated values. We also reconsider the borehole heat production values in conjunction with new heat generation information from a recent regional scale airborne geophysical survey. The radiometric (gamma-ray) data provide detailed (~70 m along-line) ground concentration estimates of the heat-producing radioelements. These are then combined to estimate heat production in the near-surface. The airborne estimates are subject to attenuation by the soil profile. Here we demonstrate and then adopt an assumption that the observations of the soil-bedrock medium undergo a flux attenuation by a factor of about 2 compared to the response of the underlying material. The revised estimates are then correlated with their equivalent deeper borehole estimates. Linear regression is then used to correct the shallow airborne estimates to values that are consistent with the deeper borehole determinations. The procedure provides a detailed and extensive mapping of heat production at both on- and off-granite locations across SW England. The Dartmoor and Land’s End granite offer the greatest spatial geothermal potential in terms of their intrinsic radionuclide concentrations and associated heat production. District-scale heat production is studied using the airborne data acquired uniformly across conurbations. The analysis identifies the towns of Camborne, Penzance, St Austell, Redruth and St Ives as having high values (> 4 μW.m-3) within their urban perimeters

How hot are the Cairngorms?

Heat flow measured over the East Grampians batholith in the 1980s was found to be unexpectedly low and at odds with high radiogenic heat production within the outcropping granites and a very large volume of granite predicted from an interpretation of gravity data. Past climate variations perturb temperature gradients in the shallow subsurface leading to erroneous estimates of heat flow. A reconstruction of the surface temperature history during the last glacial cycle has enabled a rigorous palaeoclimate correction to be applied to the heat flow that shows an increase of 25% over previously reported values; revised to 86 ± 7 mW m−2. An interpretation of recent mapping reveals that the surface exposures of the East Grampians granites are the roof zones of a highly evolved magma system. Rock composition, therefore, is likely to become more mafic with depth and the heat production will decrease with depth. This petrological model can be reconciled with the gravity data if the shape of the batholith is tabular with deep-seated feeder conduits. The increased heat flow value leads to revised predictions of subsurface temperatures of 129°C at 5 km depth and 176°C at 7 km depth, increases of 40% and 49%, respectively, compared to previous estimates. These temperatures are at the lower end of those currently required for power generation with Engineered Geothermal Systems, but could potentially be exploited as a direct heat use resource in the Cairngorm region by targeting permeable fractures with deep boreholes

A modelling study of the variation of thermal conductivity of the English Chalk

Thermal conductivity is required when designing ground heating and cooling schemes, electrical cable conduits and tunnel ventilation. In England these infrastructures are often emplaced within the Chalk. To improve knowledge on chalk thermal conductivity, over the few scattered measured values, estimates have been made from multi-component mixture models based on the mineral composition, porosity and the structure of the Chalk. The range in mid values for the thermal conductivities is 1.78–2.57 W m−1 K−1 where the lowest values are for the Upper Chalk. Variations in porosity are the main factor for the variation in thermal conductivity. The effect of fracturing is to reduce the bulk thermal conductivity, but the reduction is small for fractures that are saturated. For an averagely fractured chalk with 60% fracture saturation, the reduction in thermal conductivity is around 22% for a thermal conductivity of 2.15 W m−1 K−1. In the near-surface zone, where fracture apertures will be at their greatest and unsaturated conditions may prevail for part of the year, the seasonal variation in thermal conductivity may be significant for infrastructure design

Future energy

Energy resources have been a major focus for BGS over our 175 year history. In the past, our geologists searched for coal to keep the UK supplied with energy crucial for economic development. Coal mining subsequently declined and by the 1980s we were studying abandoned mines to try and resolve problems of subsidence, flooding as the dewatering pumps were switched off, and contaminated water discharging into rivers. More recently we have returned to our geological maps and archives of coal mine plans with a new energy source in mind — geothermal energy