Nested Shallow Geothermal Systems

Abstract

The long-term sustainability of shallow geothermal systems in dense urbanized areas can be potentially compromised by the existence of thermal interfaces. Thermal interferences between systems have to be avoided to prevent the loss of system performance. Nevertheless, in this work we provide evidence of a positive feedback from thermal interferences in certain controlled situations. Two real groundwater heat pump systems were investigated using real exploitation data sets to estimate the thermal energy demand bias and, by extrapolation, to assess the nature of thermal interferences between the systems. To do that, thermal interferences were modelled by means of a calibrated and validated 3D city-scale numerical model reproducing groundwater flow and heat transport. Results obtained showed a 39% (522 MWh·yr−1) energy imbalance towards cooling for one of the systems, which generated a hot thermal plume towards the downgradient and second system investigated. The nested system in the hot thermal plume only used groundwater for heating, thus establishing a positive symbiotic relationship between them. Considering the energy balance of both systems together, a reduced 9% imbalance was found, hence ensuring the long-term sustainability and renewability of the shallow geothermal resource exploited. The nested geothermal systems described illustrate the possibilities of a new management strategy in shallow geothermal energy governance.

Thermal modeling of a Swiss urban aquifer and implications for geothermal heat pump systems

“…In this specific case, an open-loop system would find optimal conditions for an increased efficiency, since groundwater temperature would show peaks of 17°C in fall-winter, when heating needs are greater, and values of 7°C in spring-summer when cooling needs are dominant. Compared to an undisturbed groundwater temperature of 13.6°C (MAAT), this would mean an improved coefficient of performance (COP) in winter and an improved energy efficiency ratio (EER) in summer, which represents a synergic effect that could be profitably exploited (García-Gil et al 2020b). For large energy demands, this would represent significant energy and economic savings; it must be however considered that the installation of large GWHP systems with significant pumping rates could contribute to altering the natural exfiltration velocity and therefore the arrival time of the hot/cold thermal plume.…”

The Geobattery Concept: A Geothermal Circular Heat Network for the Sustainable Development of Near Surface Low Enthalpy Geothermal Energy to Decarbonise Heating

“…Commonly, sustainability assessments of BHEs consider the engineering performance of the BHE and demonstrate steady borehole wall temperatures and consistent heat pump coefficient of performance values (COP) (Signorelli et al, 2005;Rybach and Eugster, 2010;Casasso and Sethi, 2014;Walch et al, 2021). BHE's do not extract water and therefore rely on conduction as the heat transport mechanism, but groundwater flow can have a significant impact on subsurface temperatures (Casasso and Sethi, 2014;Rivera et al, 2015;García-Gil et al, 2020;Abesser et al, 2021).…”

“…There is increasing evidence from both modelling and field studies to suggest that rapid development of the shallow geothermal resource via closely spaced BHEs could lead to thermal interferences and reductions in subsurface temperatures that cause decreases in heat pump efficiencies in heat-demand dominated schemes (Vienken et al, 2015;Casasso and Sethi, 2019;Meng et al, 2019;Vienken et al, 2019;Abesser et al, 2021). Even "local" systems need to be considered and monitored at the district and/or city scale to enable effective management of the subsurface thermal regimes (Epting et al, 2017;Mueller et al, 2018;Bayer et al, 2019;García-Gil et al, 2020). Intriguingly, García-Gil et al (2020) report an example in Zaragoza, Spain, in which the groundwater flux had the positive (and accidental) effect of transferring rejected heat from a cooling-dominated BHE system down gradient to a heat-only BHE extraction system.…”

“…Even "local" systems need to be considered and monitored at the district and/or city scale to enable effective management of the subsurface thermal regimes (Epting et al, 2017;Mueller et al, 2018;Bayer et al, 2019;García-Gil et al, 2020). Intriguingly, García-Gil et al (2020) report an example in Zaragoza, Spain, in which the groundwater flux had the positive (and accidental) effect of transferring rejected heat from a cooling-dominated BHE system down gradient to a heat-only BHE extraction system. The authors term this a "nested BHE system" and it raises the possibility that, with careful planning, design and management, groundwater flux could be harnessed to transfer stored heat to BHEs further downstream within a district-scale scheme.…”

“…These benefits do, however, raise the possibility of a novel approach that builds on the observations of García-Gil et al (2020) and transforms the liability of groundwater flux for thermal energy storage into a key component of a subsurface energy transfer and storage system that we term a "geobattery." At the centre of the concept is the creation of a geothermal circular heat network harnessing recyclable heat to recharge shallow geothermal resources via a transmissive subsurface pathway such as legacy mine workings.…”