Sustainable use of Urban Groundwater with Ground Source Heat Pumps
Heating and cooling buildings can account for up to half UK energy use and a similar proportion of carbon emissions. To meet our targets for carbon emissions reductions and to deliver net zero homes, it is essential that the heating of buildings is decarbonised. As the carbon intensity of the electricity grid reduces this makes use of ground source heat pump (GSHP) technology increasingly attractive (Figure 1). GSHP systems operate in conjunction with ground heat exchangers (GHEs) which transfer heat to or from the geological units beneath the building (Figure 2). Where flowing groundwater is present, the effective thermal properties of the ground are improved, leading to higher efficiency GHE and hence GSHP systems. However, the increased ability for heat dissipation comes with an increased volume of the sub-surface which is effected by the resulting thermal pollution. This brings a risk of raising aquifer temperatures and hence making future GHSP systems less sustainable.
Figure 1 The carbon density of grid electricity in the UK over the last 15 years and future predictions. The trends are compared against the carbon cost of heating with gas and with heat pump systems.
Figure 2 A borehole heat exchanger influence by groundwater flow, and connected to a domestic property. After Pedchenko (2019).
Figure 3 Increased extent of groundwater temperature change around a borehole heat exchanger due to the processes of advection and dispersion which are present in aquifers. Non aquifers, by contrast, only transfer heat by conduction (after Pedchenko, 2019).
As more GSHP systems are installed in congested urban areas, the risks of over exploitation of the aquifer will increase, with the potential for thermal interference between systems (Figure 4), hence reducing the energy availability in the long term. Open loop GSHP systems which extract water directly from aquifers require licensing, thus offering some control of the aquifer exploitation. However, closed loop GSHP systems where the GHE comprises a closed plastic pipe loop installed in the ground are not regulated in any way. While the extent of the downstream thermal pollution will be less with closed loop systems compared with open loop systems, the extent of the induced temperature changes in the aquifer can still span 10’s or even 100’s of metres in some cases.
Figure 4 Thermal interactions of the vertical borehole heat exchangers installed in an aquifer in an urban environment. After Rivera et al, 2015.
This project will look at the long term impact of closed loop GSHP systems installed in aquifers beneath urban areas, and how they may interact with other systems to effect the thermal resource available. Different types of aquifer will be investigated, from classic Darcy flow to aquifers dominated by fracture flow and with high dispersivity. The project will lead to recommendations about aquifer characterisation for design, GSHP system density, operating conditions, and regulatory requirements for different hydrogeological conditions.
Pedchenko (2019) Investigating heat transport by groundwater in fractured aquifers for ground energy applications, PhD Thesis submitted for examination to the University of Southampton.
Rivera et al (2015) Analytical simulation of groundwater flow and land surface effects on thermal plumes of borehole heat exchangers, Applied Energy, Volume 146, 2015, Pages 421-433, ISSN 0306-2619, https://doi.org/10.1016/j.apenergy.2015.02.035.