Subsoil characterization in the urban area for the evaluation of the shallow geothermal potential
Vaiva Cypaite, Víctor Camps, Ariadna Conesa, Georgina Arnó, and Ignasi Herms (ICGC), 2 July 2021
Between 2019 and 2021 in the urban area of Girona extensive field studies were carried out and a wide range of data was collected which significantly improved understanding about the subsoil thermal properties and geological setting. Back in 2019 a shallow geothermal monitoring network was installed in Girona which consists of primary and secondary monitoring networks. For the installation of the primary monitoring network 11 boreholes were drilled between February 2019 and January 2020. Each well is 100 m deep and is instrumented with strings of temperature sensors that measure temperature at different depths. Additionally, 6 of the 11 wells act as open piezometers with an additional single water level and temperature sensor. All data is transmitted automatically through a telemetry system. The remaining 5 boreholes are equipped with simple U geothermal probes which allowed the realization of Thermal Response Tests (TRT). The secondary monitoring network consists of 11 pre‑existing piezometers and water wells with a single water level and temperature sensor at a given depth installed in each monitoring point. Periodically, vertical temperature profiles are taken in the secondary monitoring network.
Lithological data obtained from the 11 newly drilled boreholes of the primary monitoring network were integrated into a 3D geological model of Girona. This allowed updating the model and adding more precision to it. Once the geological model was finished, thickness grids on a raster maps of 28 lithological sections were transferred into GIS environment where estimation of the shallow geothermal potential took place.
In order to assess the shallow geothermal energy potential, data obtained from field measurements were included as well. Thermal conductivity and thermal capacity of the ground were obtained from TRT and measurements within primary and secondary networks provided information about groundwater temperature distribution under the Girona urban area. The shallow geothermal potential was estimated applying the G.POT method (Casasso & Sethi, 2016). It ranges in the Girona urban area between 10 and 17 MWh/year for one 100 m deep borehole heat exchanger (BHE). Furthermore, data obtained from field measurements not only allowed the estimation of the potential for shallow geothermal energy, but also demonstrated the existence of likely subsurface urban heat island (SUHI) effect in the city of Girona city. The information obtained in the development of the project will facilitate the government, municipalities and end-users the promotion and implementation of shallow geothermal energy on an urban scale.
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