Estimating the heat resource
How to obtain an estimate of the ground heat resource density for use in opportunity searches and assessments.
The resource density (how much energy can be extracted per square metre of green space) is the basis from which we can determine how much heat load can be met from a particular area of green space, or how much area is required to meet a particular heat load.
In any given location, there will be two different figures for the resource density for horizontal and vertical systems respectively.
Horizontal schemes resource density
The three dominant factors that determine the resource density for a horizontal scheme are
the soil temperature,
soil thermal conductivity and the
heating system’s capacity factor (the ratio between average heat output and peak heat output).
However, all that is needed for an early-stage assessment of opportunities is an estimated value for each parameter in the area of interest. The annual mean soil temperature is generally approximately equal to the annual mean air temperature in any given location, and data on air temperatures is much more widely available than soil temperatures so it is recommended to use the air temperature to assess opportunities at an early stage. The urban heat island effect may be more pronounced in soil than in air temperature, so using the air temperature will produce a conservative estimate when assessing ground source heat opportunities in city parks.
Other data sources
Annual average air temperature data for the UK is available from the Met Office and other providers.
If a soil conductivity value to be applied across a wide area is required – for example to generate an average resource density for matching resources to heat demands across a portfolio of sites – then 1.2 W/mK is a sensible (and typically conservative) value to use.
If a soil conductivity estimate is required for a specific site and the soil group (clay/silt/sand/loam) is known, then the following values can be used:
| Soil group | Thermal conductivity [W/mK] |
|---|---|
| Clay or silt | 1.7 |
| Sand | 1.4 |
| Loam | 1 |
Heating system capacity factor
For guidance on calculating or estimating the heating system capacity factor, see module 5.1. If the capacity factor is not known – or a resource density is required for a wide-ranging opportunity search - a good assumption is 20%.
Calculating the resource density
The three parameters – ground temperature, soil conductivity and heating system capacity factor – can be entered into the Resource Density Estimation Tool to generate an estimate of the maximum heat extraction per unit area of land, and the total length of trenching that will be required for the heat collector (not including the trenching required for the pipes that run between the heat collector and the building or buildings).
Instructions for using the Resource Density Estimation Tool are found in the Introduction sheet within the tool.
Vertical schemes resource density
Similarly to horizontal schemes, the three dominant factors for a vertical scheme are the temperature and thermal conductivity (this time of the ground between the surface and the deepest point of the borehole) and the heating system’s capacity factor.
Ground temperature
Information may be available that gives a clue as to underground temperatures in a particular location – for example, scientific papers or borehole records.1 However, in most instances no information will be available and the best estimate for the borehole average temperature is made by taking the annual mean air temperature (which will be approximately equal to the ground temperature at 15 metres depth) and applying the geothermal gradient between 15 metres and the mid-point of the borehole.
Average temperature of borehole = Annual mean air temperature + 0.026 * (0.5 * borehole depth – 15)
Ground thermal conductivity
The types of soils and rocks that lie beneath the surface vary greatly across the UK, and the three-dimensional data that would be required in order to understand the ground composition for up to 200 metres below a particular location is not freely available. Therefore, unless specific geological information is available, a suitable assumption for the ground thermal conductivity that will be conservative in most areas is 2.5 W/mK. If specific geological information is required, this can be obtained (for a charge) from the British Geological Survey.
Heating system capacity factor
For guidance on calculating or estimating the heating system capacity factor, see module 5.1. If the capacity factor is not known – or a resource density is required for a wide-ranging opportunity search - a good assumption is 20%.
Calculating the resource density: land available > 3000m2 and/or heat load >200kW
The three parameters – ground temperature, ground conductivity and heating system capacity factor – can be entered into the Resource Density Estimation Tool to generate an estimate of the maximum heat extraction per unit area of land, and the total number of boreholes that will be required for the heat collector.
Instructions for using the Resource Density Estimation Tool are found in the Introduction sheet within the tool.
Calculating the resource density: land available < 3000m2 and/or heat load <200kW
For smaller schemes, the possibility to locate the boreholes near the edge of the plot means that more boreholes can be installed than is suggested by the Resource Density Estimation Tool.
The following adjustment factors can be applied to the outputs from the Resource Density Estimation Tool to determine the heat available from smaller plots of land, or the land required for schemes with smaller heat outputs.
| Plot size | Adjustment factor | Heat load | Adjustment factor |
|---|---|---|---|
| 200 to 400 m2 | 2 | Up to 40 kW | 2 |
| 400 to 1,000 m2 | 1.7 | 40 to 100 kW | 1.7 |
| 1,000 to 2,000 m2 | 1.5 | 100 to 175 kW | 1.5 |
| 2,000 to 3,000 m2 | 1.25 | 175 to 200 kW | 1.25 |
Alternatively, if sufficiently detailed maps or drawings of an area of suitable land <3000m23 are available, possible locations for boreholes can be experimented with to establish a more robust number. Until detail design is undertaken, the spacing of the boreholes should be no smaller than 10-12 metres.
Footnotes:
https://www.bgs.ac.uk/data/boreholescans/home.html
Provided that there are no other ground source heat schemes in the immediate vicinity which could affect the proposed installation.