Thermal Impact Assessment of Groundwater Heat Pumps (GWHPs): Rigorous vs. Simplified Models

Abstract

Groundwater Heat Pumps (GWHPs) are increasingly adopted for air conditioning in urban areas, thus reducing CO 2 emissions, and this growth needs to be managed to ensure the sustainability of the thermal alteration of aquifers. However, few studies have addressed the propagation of thermal plumes from open-loop geothermal systems from a long-term perspective. We provide a comprehensive sensitivity analysis, performed with numerical finite-element simulations, to assess how the size of the thermally affected zone is driven by hydrodynamic and thermal subsurface properties, the vadose zone and aquifer thickness, and plant setup. In particular, we focus the analysis on the length and width of thermal plumes, and on their time evolution. Numerical simulations are compared with two simplified methods, namely (i) replacing the time-varying thermal load with its yearly average and (ii) analytical formulae for advective heat transport in the aquifer. The former proves acceptable for the assessment of plume length, while the latter can be used to estimate the width of the thermally affected zone. The results highlight the strong influence of groundwater velocity on the plume size and, especially for its long-term evolution, of ground thermal properties and of subsurface geometrical parameters.

Challenge of Using Groundwater for Buildings Air Conditioning in Subtropical Areas

“…Such systems have stringent requirements related to the medium for removing indoor heat. Piga et al, Sedaghat et al, Todorov et al, and Al-Madhlom et al studied the use of TES for different areas [9][10][11][12][13]. Song et al used an indoor sandbox test to simulate different operation modes of pumping and recharging well and found the results were well [14].…”

Environmental Performance of Innovative Ground-Source Heat Pumps with PCM Energy Storage

“…Previous studies on GSHP systems [24,25] have shown that final COP depends on system configuration and local site conditions (e.g., the BHEs setup, the climate, the ground properties, and the HP nominal power), as well as the transient operative conditions of the system's duty-cycle: the ground's undisturbed temperature, the temperature difference between the surface and the underground, and the temperature of the water leaving the heat pump and the BHEs. These temperature fluctuations lower the system COP.…”

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

“…Urban heat islands and global warming trends also impact groundwater temperatures, as highlighted by recent studies such as [14,15]. Shallow geothermal installations upstream of the P&T may impact the groundwater temperature, propagating the so-called thermal plumes through advection, dispersion and, to a lesser extent, conduction [16,17]. A further possible alteration is represented by thermal recycling, which occurs when part of the reinjected water returns to the abstraction well [18,19].…”

“…While the geothermal exploitation of P&T systems is still at an early stage, the mining sector offers several examples of pumped groundwater thermal use, especially for building and district heating systems, which reduces the overall environmental costs of such operations. Farr et al (2016) [8] reported that, so far, 16 GWHP systems have been installed in flooded mines in Europe and USA, with the first applications dating back to the early 1980s. System capacities range between 18 and 700 kW, and depths range between 65 and 600 m below ground level.…”