Nested Shallow Geothermal Systems

Alejandro García-Gil,Miguel Mejías Moreno,Eduardo Garrido Schneider,José Ángel Sánchez Navarro,Jesús Mateo Lázaro,Miguel Ángel Marazuela,Corinna Abesser

doi:10.3390/su12125152

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.

Highlights

Anthropogenic climate change is a major public concern worldwide [1]
The present work identified and examined for the first time a real case of nesting between shallow geothermal systems. This nesting process shows how a shallow geothermal system is able to absorb subsurface heat to satisfy its heating demand from an already existing hot thermal plume generated by an upgradient cooling-biased system. It is an example of a symbiotic relationship engendered by positive thermal interferences between two shallow geothermal systems
This newly identified relationship between shallow geothermal systems provides a novel vision of controlled thermal interferences between systems, which would be useful as a management measure in shallow geothermal energy resource governance

Summary

Introduction

Anthropogenic climate change is a major public concern worldwide [1]. An economy with net-zero greenhouse gas emissions is the objective of the European Green Deal in line with the EU’s commitment to global climate action under the Paris Agreement. The study of thermal interferences between closed systems has questioned the long-term sustainability of the exploitation of SGE energy in urban areas with high system density This is due to the fact that it takes a long period of time to reach the steady state of the exploitation regimes. And opposed to the latter, it was demonstrated [16] that a distance of 6 m between BHEs, as recommended by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) [17], may be insufficient to avoid thermal interferences, especially when the systems present strong imbalances in the heating–cooling thermal loads throughout the year. To prevent or even eliminate thermal interferences, other authors have proposed the interconnection of facilities using ultra-low temperature district heating and cooling networks (DHCs) [19]

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