Thermally Affected Zone (TAZ) Assessment in Open-Loop Low-Enthalpy Groundwater Heat Pump Systems (GWHPs): Potential of Analytical Solutions

Abstract

Thermal perturbation produced in the subsurface by open-loop groundwater heat pump systems (GWHPs) must be predicted and constantly controlled, especially in the shallow aquifers of more densely urbanized areas, in order to guarantee plants’ long-term sustainable use and to avoid adverse effects on adjacent geothermal systems. Transient conditions in the flow dynamic can be successfully modelled by means of numerical modelling tools. However, for small plants in suitable hydrogeological systems, an alternative tool for predicting the thermally affected zone (TAZ) around the injection well can be found in analytical solutions for steady advective transport in a shallow aquifer. The validity of using steady analytical solutions to predict the TAZ development at the end of two different cooling seasons (2010 and 2016) was tested in the Politecnico di Torino GWHP system (NW Italy). When fixing the constant thermal difference (ΔT) between the injection and abstraction wells at 5°C, results revealed that a rather reliable assessment of the TAZ of Politecnico di Torino GWHPs, in Turin shallow aquifer, can be performed by plotting the cumulative distribution function of the injected discharge rate (Q) and setting 63% as a steady value.

City-scale heating and cooling with aquifer thermal energy storage (ATES)

“…For this purpose, we use the so-called thermally affected zone (TAZ) around the ATES wells in each box model after 30 years of ATES operation (Lo Russo et al 2012). In the literature, the TAZ is commonly defined as the area where the absolute value of the temperature increase or decrease caused by ATES or GWHP systems exceeds 1 K, i.e., by the ± 1 K-plumes (Gizzi et al 2020;Lo Russo et al 2012;Piga et al 2017). However, this limit for adverse thermal interferences lacks scientific justification and seems to be chosen almost arbitrarily (Pophillat et al 2020a).…”

City-scale heating and cooling with Aquifer Thermal Energy Storage (ATES)

“…In the literature, the TAZ is commonly defined as the area where the absolute value of the temperature increase or decrease caused by ATES or GWHP 265 systems exceeds 1 K, i.e. by the ± 1 K-plumes (Gizzi et al, 2020;Lo Russo et al, 2012;Piga et al, 2017).…”

A Review of Groundwater Heat Pump Systems in the Italian Framework: Technological Potential and Environmental Limits

“…A submersible pump is installed above the top of the well-screen. As [44] reports, the performance of GWHPs depends strongly on the thermal loads required for the building's heating and cooling, the heat pump design characteristics (e.g., compressor efficiency and heat exchanger configuration), and aquifer characteristics (e.g., the undisturbed temperature of the aquifer, thermal and hydraulic conductivity of geological formations). Large flow rates, of the order of tens or even hundreds of L/s, can be subtracted to supply a large quantity of thermal power (MW).…”

“…The ordinary functioning conditions of open-loop GWHPs are time-variable. As described in [44], in modern plants, installed technologies are continuously able to modify the operating conditions of the specific components, matching the real energy demand of the building. Cycles in outdoor air temperature on a daily and weekly scale usually induce dynamic functioning in the systems using the heat pump, abstraction wells, and reinjection wells.…”