Jon Busby

Initial geological considerations before installing ground source heat pump systems

Abstract 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.

An evaluation of combined geophysical and geotechnical methods to characterize beach thickness

Beaches provide sediment stores and have an important role in the development of the coastline in response to climate change. Quantification of beach thickness and volume is required to assess coastal sediment transport budgets. Therefore, portable, rapid, non-invasive techniques are required to evaluate thickness where environmental sensitivities exclude invasive methods. Site methods and data are described for a toolbox of electrical, electromagnetic, seismic and mechanical based techniques that were evaluated at a coastal site at Easington, Yorkshire. Geophysical and geotechnical properties are shown to be dependent upon moisture content, porosity and lithology of the beach and the morphology of the beach–platform interface. Thickness interpretation, using an inexpensive geographic information system to integrate data, allowed these controls and relationships to be understood. Guidelines for efficient site practices, based upon this case history including procedures and techniques, are presented using a systematic approach. Field results indicated that a mixed sand and gravel beach is highly variable and cannot be represented in models as a homogeneous layer of variable thickness overlying a bedrock half-space.

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.

A GIS for the planning of electrical earthing

When creating an electrical earth for a transformer with vertically driven earthing rods, problems can arise either because the ground is too hard or because the ground is too resistive to achieve the required earthing resistance. To assist in the planning of earthing installations a geographic information system (GIS) layer has been created. In its simplest form it consists of a colour coded map that indicates the most likely earthing installation: a single vertically driven rod (indicated by dark green); multiple vertically driven rods (indicated by light green); a horizontal trench, where a rod installation is unlikely (indicated by yellow); for difficult ground, a specialist installation (i.e. drilling; indicated by red). However, the GIS can be interrogated to provide site-specific information such as site conditions, likely depth of installation and quantity of earthing materials required. The GIS was created from a spatial model constructed from soil, superficial and bedrock geology that has been attributed with engineering strength and resistivity values. Calculations of expected earthing rod resistance, rod or trench length, and all possible combinations of ground conditions have been compared with the ‘likely’ conditions required for each of the four proposed installation scenarios to generate the GIS layer. The analysis has been applied to the electrical network distribution regions of Western Power Distribution, in the English Midlands, and UK Power Networks, which covers East Anglia, London and the SE of England. Because the spatial model that underlies the GIS has been constructed from national databases the analyses can be extended to other regions of the UK.

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.

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.

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.

Developing a near surface electrical resistivity model of Great Britain

A model of the near surface electrical resistivity of Great Britain is presented. The first geological unit beneath the base of soil layer is attributed with electrical resistivity. Geotechnical data are used to derive modeled resistivity distributions for each of the lithostratigraphic units described in the near surface geological model of Great Britain. Resistivities are calculated using an effective medium methodology and these values are tested against electrical resistivity sounding data and apparent resistivity estimates from the airborne electromagnetic surveys. The central moments of the statistical distributions are coupled to the geological map and the resulting resistivity distribution shows a general increase in resistivity from SE England to NW Scotland. This pattern reflects the age of the bedrock geology of Great Britain. The methodology described in this paper can be applied to other regions where suitable geotechnical information and geological mapping is available.