Assessment and Minimization of Potential Environmental Impacts of Ground Source Heat Pump (GSHP) Systems

Abstract

Ground source heat pumps (GSHPs) gained increasing interest owing to benefits such as low heating and cooling costs, reduction of greenhouse gas emissions, and no pollutant emissions on site. However, GSHPs may have various possible interactions with underground and groundwater, which, despite the extremely rare occurrence of relevant damages, has raised concerns on their sustainability. Possible criticalities for their installation are (hydro)geological features (artesian aquifers, swelling or soluble layers, landslide-prone areas), human activities (mines, quarries, landfills, contaminated sites), and groundwater quality. Thermal alterations due to the operation of GSHPs may have an impact on groundwater chemistry and on the efficiency of neighboring installations. So far, scientific studies excluded appraisable geochemical alterations within typical ranges of GSHPs (±6 K on the initial groundwater temperature); such alterations, however, may occur for aquifer thermal energy storage over 40 • C. Thermal interferences among neighboring installations may be severe in urban areas with a high plant density, thus highlighting the need for their proper management. These issues are presented here and framed from a groundwater quality protection perspective, providing the basis for a discussion on critical aspects to be tackled in the planning, authorization, installation, and operation phase. GSHPs turn out to be safe and sustainable if care is taken in such phases, and the best available techniques are adopted.

The Effect of Shank-Space on the Thermal Performance of Shallow Vertical U-Tube Ground Heat Exchangers

“…Most of the research carried out in this area has focused on vertical boreholes with typical depths of 50 m and over, as commercially these are the ones that are used primarily to ensure that enough contact area to reject or absorb the heat is available. However, reaching such depths is not always possible, and other design limitations such as the requirement for limited interaction with the water table [30,31] or indeed the need to reduce drilling costs [32] may require that a system makes use of shallower heat exchangers (up to 50 m depth).…”

How Can We Make Pump and Treat Systems More Energetically Sustainable?

“…The choice between discharge in surface water bodies, wastewater collection systems, or re-injection depends on the water quality standard reached after treatment, and on national legislation. Reinjection into the subsurface requires an evaluation of groundwater thermal alteration, which can influence groundwater quality as well [13].…”

Can Borehole Heat Exchangers Trigger Cross-Contamination between Aquifers?

“…Closed-loop geothermal systems equipped with borehole heat exchangers (BHEs) are the most diffused type of GSHP thanks to the lower maintenance requirements and the possibility to install them even in the absence of an aquifer. BHEs are composed of one or two U-pipes installed in a borehole with a depth usually ranging between 50 and 200 m. At such depths, BHEs generally intercept different aquifers and, if incorrectly grouted during the completion, they tend to become preferential pathways through the less permeable layers (aquitards), thus making deep aquifer more vulnerable to contamination [10]. This concern led to restriction and bans of BHE installation in different protection areas of some European countries, as reported in a recent report by Prestor et al [11].…”

“…The ongoing research on geothermal grouts is focused on increasing the thermal conductivity, e.g., with the addition of graphite flakes [22][23][24], and reducing the hydraulic conductivity, generally with the addition of bentonite. Figure 1 from Casasso and Sethi [10] compares the experimental results of Allan and Philippacopoulos [25], Park et al [26] and Indacoechea-Vega et al [27], who tested grout specimens in different conditions. Hydraulic conductivities range between 10 −12 m/s, far below the conductivities of aquitards and of nearly all aquicludes, and 10 −6 m/s, i.e., a value typical of fine sand.…”

“…Hydraulic conductivity ranges of geothermal grouts from laboratory tests and of typical aquifer, aquitard and aquiclude lithologies. Source: Casasso and Sethi[10].…”