Sustainability evaluation of a medium scale GSHP system in a layeredalluvial setting using 3D modeling suite

In this study the effects on the thermal field of a medium scale GSHP system located in Alessandria(Italy) and installed in highly heterogeneous alluvials are considered. The system was monitored over ayear and data showed a progressive loss of efficiency. Simulation results obtained from a 3D numericalmodel show that the analyzed GSHP system has an inadequate design even for a short/medium periodoperation. A strong probe interference phenomenon was observed, due to the particular layout of theprobe field and to the high energy request for building conditioning. The efficiency loss is also amplifiedby the presence of alluvial deposits with poor thermal properties. Adopting a homogeneous distributionof the thermal properties of the subsurface results in an underestimation of the thermal alteration of 25%while the increase of probe distance by 55% produces a reduction of thermal alteration of 45%. In thiscase study the homogeneous subsurface assumption leads to excessive simplification of the observedstrong heterogeneity and it underrates thermal impact on the soil, especially in layers with poor thermalproperties.© 2015 Elsevier Ltd. All rights reserved.1. IntroductionGeothermal energy can play a valuable role in reducing theamount of greenhouse gas emissions caused by the combustionof fossil fuels. A very interesting aspect of this type of energy,based on the heat radiated by the sun and the heat flux from theinterior of the Earth to the surface, is its suitability to produce dif-ferent kind of energies depending on the working temperatures.Geothermal systems that exploit large thermal reservoirs withhigh working temperature produce vaporized water that is usedto produce electrical energy, while low temperature systems areexploited for heating/cooling of buildings. Low enthalpy systemsdo not require geothermal reservoirs with high porosity, permeable rocks or high temperature gradients (geothermal anomalies)as it happens for medium and high enthalpy systems, since theyexchange heat with the shallow ground at a low temperature.The application of low-enthalpy geothermal technologies in several European countries has been very profitable during the lastdecades and has seen a significant growth in the last years.