title: Advection and dispersion heat transport mechanisms in the quantification of shallow geothermal resources and associated environmental impacts.
abstract: Borehole Heat Exchangers (BHEs) are increasingly being used to exploit shallow geothermal energy. This paper presents a new methodology to provide a response to the need for a regional quantification of the geothermal potential that can be extracted by BHEs and the associated environmental impacts. A set of analytical solutions facilitates accurate calculation of the heat exchange of BHEs with the ground and its environmental impacts. For the first time, advection and dispersion heat transport mechanisms and the temporal evolution from the start of operation of the BHE are taken into account in the regional estimation of shallow geothermal resources. This methodology is integrated in a GIS environment, which facilitates the management of input and output data at a regional scale. An example of the methodology's application is presented for Barcelona, in Spain. As a result of the application, it is possible to show the strengths and improvements of this methodology in the development of potential maps of low temperature geothermal energy as well as maps of environmental impacts. The minimum and maximum energy potential values for the study site are 50 and 1800 W/m(2) for a drilled depth of 100 m, proportionally to Darcy velocity. Regarding to thermal impacts, the higher the groundwater velocity and the energy potential, the higher the size of the thermal plume after 6 months of exploitation, whose length ranges from 10 to 27 m long. A sensitivity analysis was carried out in the calculation of heat exchange rate and its impacts for different scenarios and for a wide range of Darcy velocities. The results of this analysis lead to the conclusion that the consideration of dispersion effects and temporal evolution of the exploitation prevent significant differences up to a factor 2.5 in the heat exchange rate accuracy and up to several orders of magnitude in the impacts generated.
authors.affiliation: Institute of Environmental Assessment and Water Research (IDÆA), CSIC, Jordi Girona 18, 08034 Barcelona, Spain; Department of Geotechnical Engineering and Geosciences, Universitat Politècnica de Catalunya (UPC), Jordi Girona 1-3, 08034 Barcelona, Spain; Associated Unit: Hydrogeology Group (UPC-CSIC), Spain. Electronic address: mar.alcaraz@idaea.csic.es.
authors.forename: Mar
authors.initials: M
authors.lastname: Alcaraz
authors.affiliation: Department of Earth Sciences, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain; Associated Unit: Hydrogeology Group (UPC-CSIC), Spain.
authors.forename: Alejandro
authors.initials: A
authors.lastname: García-Gil
authors.affiliation: Institute of Environmental Assessment and Water Research (IDÆA), CSIC, Jordi Girona 18, 08034 Barcelona, Spain; Associated Unit: Hydrogeology Group (UPC-CSIC), Spain.
authors.forename: Enric
authors.initials: E
authors.lastname: Vázquez-Suñé
authors.affiliation: Department of Geotechnical Engineering and Geosciences, Universitat Politècnica de Catalunya (UPC), Jordi Girona 1-3, 08034 Barcelona, Spain; Associated Unit: Hydrogeology Group (UPC-CSIC), Spain.
authors.forename: Violeta
authors.initials: V
authors.lastname: Velasco
keywords: Borehole heat exchanger; Environmental thermal impact; GIS; Shallow geothermal potential; Thermal advection; Thermal dispersion