Geothermics 43 (2012) 66–74 Contents lists available at SciVerse ScienceDirect Geothermics journal homepage: www.elsevier.com/locate/geothermics Development of the thermally affected zone (TAZ) around a groundwater heat pump (GWHP) system: A sensitivity analysis Stefano Lo Russo a,∗ , Glenda Taddia a , Vittorio Verda b a Department of Environment, Land and Infrastructure Engineering (DIATI), Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy b Department of Energy (DENERG), Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy article info Article history: Received 25 November 2010 Accepted 2 February 2012 Available online 28 March 2012 Keywords: Groundwater heat pumps Thermally affected zone FEFLOW Sensitivity analysis Advective heat flow abstract Open-loop groundwater heat pumps (GWHPs) are considered one of the most energy efficient and envi- ronmentally friendly air-conditioning systems for temperate zones. A fundamental aspect in GWHP plant design is early evaluation of the thermally affected zone (TAZ) that develops around the injection well. This is particularly important to avoid interference with previously existing groundwater uses (wells) and subsurface underground structures. Numerical modelling is useful for delineating temperature anoma- lies. We carry out numerical simulations and a sensitivity analysis for the subsurface parameters affecting the TAZ. Using the simulation results we obtain a relative hierarchy of significance for the parameters with respect to the final result and then apply this analysis to an actual site. The results of the analysis indicate that the hydrodynamic parameters correlated with groundwater flow such as the hydraulic conductivity and the gradient are highly important, particularly those relating to the advective heat flow component. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Geothermal energy usage has experienced continuous and rapid development within the last several decades. The use of this energy source has become attractive due to inherent savings of fossil fuels and relatively low CO 2 emissions (Blum et al., 2010; Lund et al., 2011; Bertani, 2012). Geothermal heat pumps are expected to reduce significantly the primary energy required for heating and cooling. Criteria of the groundwater use as a hydrogeothermal energy resource in heat pumps are complex, and they deal with aspects of incoming temperatures and groundwater quantities. The use of aquifers as storage of the thermal energy (ATES), the heat propagation after warm water injection and the consequent environmental impact have been intensively studied and modelled (Sauty et al., 1982; Molz et al., 1983; Xue et al., 1990; Molson et al., 1992; Palmer et al., 1992; Nam and Ooka, 2010). These researches highlighted the potential of aquifers to supply the heating and cooling needs for buildings and encouraged the adaptation of the heating system to this new thermal energy source, especially for new constructions (Milenic et al., 2010). Among the available heat pump technologies, groundwater heat pumps have potential advantages in terms of energy efficiency and environmental impact, but their performance depends strongly on ∗ Corresponding author. Tel.: +39 011 564 7648; fax: +39 011 564 7699. E-mail address: stefano.lorusso@polito.it (S. Lo Russo). the heating and cooling load, the heat pump design characteristics (compressor efficiency, heat exchanger configuration), the control strategy and the characteristics of the aquifer (groundwater tem- perature, transmissivity, etc.). Groundwater heat pump (GWHP) systems are open-loop sys- tems that draw water from a well, pass it through a heat exchanger, and discharge the water into an injection well or nearby river. The relatively stable temperature of groundwater yields a higher performance efficiency and offers greater energy savings than air- source heat pump (ASHP) systems especially in temperate climate conditions (Florides and Kalogirou, 2007; Milenic et al., 2010). Depending on the use mode (heating or cooling), energy may be extracted or injected. During the process, the ambient aquifer tem- perature is disturbed and cold or warm plumes develop. These disturbances are reduced by lateral conductive heat transport and convection due to moving water (Hecht-Mendez et al., 2010). The Peclet number for energy transport relates the transport of energy by bulk fluid motion to the energy transport by conduction, that is, it is the ratio between heat convection and heat conduction (Domenico and Schwartz, 1990): Pe = ql f c f  m with  m = n f + (1 - n) s (1) in which q is the volumetric flow rate per unit volume of aquifer [m/s], l is the characteristic length [m],  f is the density of the fluid (water) [kg/m 3 ], c f is the specific heat capacity of the fluid (water) [J/(kg K)],  m is the effective thermal conductivity of the porous media [W/(m K)], n is the porosity,  f is the water thermal 0375-6505/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.geothermics.2012.02.001