TableĀ 1.

Summary of DGSW technology variants

NotationIllustrationDescription
wDGSWFigure 1aOpen-loop design in which hot water is produced, flows through a heat exchanger, and is then discharged into the environment at about the rejection temperature of the heat exchanger. Corresponds to the Southampton project in its original form. Requires permeable bedrock; in the UK requires regulatory approval for the discharge, which will limit future applicability.
hwDGSWFigure 1bOpen-loop design in which hot water is produced and flows through a heat exchanger, before being cooled further to near-ambient temperature using a heat pump, then discharged into the environment. Corresponds to the Southampton project in its present, modified, form. Requires permeable bedrock; in the UK requires regulatory approval for the discharge, which will limit future applicability. In the USA, shallow versions of this design are known as open-loop groundwater heat pump systems.
cDGSWFigure 2aClosed-loop design in which water circulates through a borehole, passing through a heat exchanger at the surface, re-entering the borehole at about the rejection temperature of the heat exchanger. Subsurface heat flow to and from the borehole is by conduction only, so the design imposes no constraints on bedrock permeability. However, the reinjection above ambient temperature means that some of the heat produced contributes to heating the bedrock at shallow depths, limiting the usefulness of this design (and favouring the hcDGSW variant, discussed below, instead). I am not aware of any deep geothermal project that uses this variant, although it has featured in desk studies (e.g. by Law et al. 2015).
hcDGSWFigure 2bClosed-loop design in which water circulates through a borehole, passing through a heat exchanger then a heat pump at the surface, re-entering the borehole near ambient temperature. The surface heat exchanger is bypassed if the produced water is below its reject temperature. Subsurface heat flow to and from the borehole is by conduction only, so the design imposes no constraints on bedrock permeability. I am not aware of any deep geothermal project that uses this variant, which is investigated in detail in the present study given its future potential. Excluding the surface heat exchanger, this design is equivalent to an upscaled (deep geothermal) version of what is known in the UK as a ground source heat pump system and in the USA as a closed-loop ground coupled heat pump system.
dDGSWFigure 3aOpen-loop design in which water circulates through a borehole, passing through a heat exchanger at the surface, re-entering the borehole at about the rejection temperature of the heat exchanger, supplemented by flow bled from groundwater then discharged into the environment. Requires permeable bedrock; in the UK requires regulatory approval for the discharge, which will limit future applicability. I am not aware of any deep geothermal project that uses this variant, although it has featured in desk studies (e.g. by Law et al. 2015).
hdDGSWFigure 3bOpen-loop design in which water circulates through a borehole, passing through a heat exchanger then a heat pump at the surface, re-entering the borehole near ambient temperature, supplemented by flow bled from groundwater then discharged into the environment. The surface heat exchanger is bypassed if the produced water is below its reject temperature. Requires permeable bedrock; in the UK requires regulatory approval for the discharge, which will limit future applicability. I am not aware of any deep geothermal project that uses this variant, although it has featured in desk studies (e.g. by GEL et al. 2016). Excluding the surface heat exchanger, this design is equivalent to an upscaled (deep geothermal) version of what is known in the USA as a standing column well groundwater heat pump system.

Details summarized here are discussed at length in the text. US terminology is from Deng et al. (2005).