The ground and water heat resource - principles
Sustainable heat extraction from varying types of soil, rock and water.
The thermal energy that is stored in the ground and in bodies of water in the UK is not “hot”, but nevertheless contains heat that – with the use of a heat pump – has the potential to warm buildings, swimming pools and industrial processes. By extracting energy from a large volume of ground or water, we cool it by a marginal amount. However, so long as the system is designed correctly, the energy that is extracted is naturally replenished over time and a balance is achieved in which the ground or water temperature remains stable from one year to the next.
This module explains the most important factors in determining how much heat can be extracted from a given area of land while still achieving that balance. It may not allow you to design a system yourself, but it should at least help you better understand the results from specialist survey and design work.
The Ground Heat Resource
The two main factors of an early-stage estimate of the amount of heat that can be sustainably extracted from an area of land are the temperature and thermal conductivity of the ground. When considering a horizontal system, it is the temperature and conductivity of the soil around 1 metre below the surface determine the maximum amount of heat that can be extracted per metre of trench length. The maximum heat extraction from vertical systems depends on the temperatures and conductivities along the length of the borehole. Both values are likely to vary as the borehole passes through different geological layers.
Temperature
Both soil temperatures and deeper underground (15 – 200 metres) temperatures vary across the UK. The soil temperature at 1 metre of depth varies with the seasons, with some shorter-term fluctuations driven by hot or cool spells. The annual average soil temperature is generally approximately equal to the annual mean air temperature in any given location. Urban heat island effects can disturb the pattern of southerly locations and lower altitudes being warmer. Regional average soil temperatures range from 8.5°C in north-east Scotland to 11.5°C in the Thames Valley. When translated into a measure of the ground heat resource, this variation means that around 1/3 more heat can be extracted from soil in the Thames Valley than in north-east Scotland (all other factors being equal).
The ground/bedrock temperatures at the greater depths accessed by borehole heat collectors also tend to be higher in the south and at lower elevations. At these depths, there are small variations from the heat that is conducted upwards from deeper layers in the Earth’s crust; however, the influence of this true ‘geothermal’ heat is normally small compared to the influence of latitude and elevation. The ground temperature at 15 metres depth is approximately constant throughout the year and roughly equal to both the annual mean air temperature and the annual mean soil temperature at 1 metre-depth. Below 15 metres, the temperature increases by around 0.026°C per metre.
Figure 1 shows an example temperature profile along the length of a 170-metre-deep borehole.
Conductivity
Thermal conductivity is measured in units of watts-per-metre-Kelvin (W/mK), also known as the ‘k Value’. If a material has a k value of 1, it means 1m3 of that material will transfer heat at a rate of 1 watt for every degree of temperature difference between the opposite faces. The higher the value is, the more heat the material will transfer.
Thermal conductivity is governed by the types of material and how much they are saturated with water. However, predicting the thermal conductivity of the ground in a particular location is far from straightforward. The types of soils and rocks that lie beneath the surface vary greatly across the UK, and the 3D data needed to understand the ground composition for up to 200 metres below a particular location is not freely available.
Soils have a wide range of conductivities, with the degree of water saturation playing a very important part. Superficial deposits (on top of the bedrock) also vary widely. Going deeper into the zones reached by a borehole heat exchanger, the most common rock types have conductivities that vary by a factor of two or three. Including less common rock types widens the range to around 1 W/mK to over 6 W/mK.
Fortunately, the ground heat resource is not directly proportional to the ground conductivity, and making a conservative assumption before obtaining more accurate information can be acceptable.
Other Factors
Beyond the ground temperature and thermal conductivity, there are other factors that combine to determine the maximum rate of heat extraction, including thermal diffusivity and parameters relating to groundwater (groundwater levels, flow rates and flow directions). A full ground heat exchanger design will take these factors into account, but they can be ignored for early-stage explorations of closed-loop ground source heat opportunities.
Further characteristics of a particular location may affect the feasibility and cost of installing horizontal or vertical ground heat collectors. These include the presence and properties of the water table and underground aquifers, unusual chemical characteristics of the materials in the ground and the strength and consistency of the rock itself. In some locations, contaminated land could prevent borehole drilling. Sometimes these issues can be overcome by technical solutions; in other situations, ground source heat simply will not be appropriate. However, “show-stopper” conditions only affect a minority of potential sites, and the vast majority of parks and green spaces will be able to host ground heat infrastructure without major design challenges.
The Water Heat Resource
For water source heat systems, the amount of heat that can be sustainably extracted is mainly governed by the water temperature and either the volume of water (for a static body of water) or the flow rate (for a flowing body of water). There will be limits to the rate at which heat can be extracted based on the need to avoid freezing in the coldest months of winter, and potentially also to maintain water temperatures within boundaries set by ecology or other human uses of the water.
For non-flowing bodies of water - lakes and reservoirs – the amount of mixing of the water is also important: stratification can lead to different zones of a lake or reservoir having very different temperatures. The maximum heat extraction will also be constrained by the rate at which the energy in the water body is naturally replenished. Modelling of this replenishment process is complicated by the interactions of climate (affecting evaporation, precipitation and conductive/convective heat loss), ground temperatures and conductivities, shading and water cover, inflowing and outflowing water flows and mixing within the water body itself.
Designers of water source heat pump systems use software packages to simulate the impact of heat extraction on water bodies, optimising designs within set boundaries. However, where the proposed amount of heat extraction is small relative to the thermal capacity of the water body, such simulation is not required for an early-stage assessment of a water source heat opportunity. For a guide to assessing this condition, see module 5.2 - estimating the heat resource.