Influence of Ground-Source Heat Pumps on Groundwater

Abstract

Differently to the deep, high enthalpy geothermal resources exploitation, the widespread use of low grade, shallow geothermal systems for heating and/or cooling has not come along with a proportional development of conceptual and numerical understanding of the impacts of these systems on groundwater. Environmental agencies? requirements often neglect the particularity of the hydrogeological systems and consequently, adverse situations such as increase of temperature of water may occur in aquifers but also in the heat pump extraction wells themselves. This latter should be of particular concern yet reduces the long-term efficiency of these systems. A state-of-the-art on the existing regulation at different countries where groundwater geothermal systems are implemented is briefly presented, putting special attention on some of the key regulated issues that concern the resources. The paper presents a two-phase methodological approach, followed to fulfil the Catalan Water Agency requirements. First, analytical expressions are proposed and the direct equations are solved to obtain a preliminary assessment of the thermal impact. Second, we simulate explicitly the impact of groundwater heat pumps with numerical methods that can take the particularities of the aquifer into account. Codes that can handle the coupled heat and flow problem are preferred, but in fact, most codes that solve for groundwater flow and solute transport can be used in the methodology. The methodology is applied to a study case where interferences with groundwater wells and underground constructions like concrete walls and tunnels are analyzed. In general terms, the study shows the feasibility of the system and that the thermal plume is mostly controlled by the amount of water involved and the difference in temperatures between these waters.