Modeling and Design of Borehole Heat Exchangers for Ground-Source Heat Pump Applications

Geothermal Heat Pumps, or Ground Source Heat Pumps (GSHPs), are systems combining a heat pump with a ground heat exchanger, which is called, in the closed loop vertical configuration, borehole heat exchanger (BHE). These systems use the earth as a heat source when operating in heating mode, with a fluid (usually water or a water-antifreeze-mixture) as the media transferring the heat from the earth to the evaporator of the heat pump. In cooling mode, they use the earth as a heat sink. In either winter or summer operation, the favourable ground temperature allows very high coefficient of performance to be achieved, provided that the BHE is properly designed. Furthermore, as the BHE strongly affects the overall plant cost, the correct sizing of the ground heat exchanger is a compulsory task in order to achieve the financial sustainability of the plant. Despite the possibility of employing geothermal heat pumps is assessed since more than 30 years, market penetration of this technology is matter of the last decade. The increasing demand of sustainable energy systems with respect to CO2 emissions allowed this technology to be applied in large scale in the USA and in some European countries like Germany, Switzerland, Austria, Sweden, Denmark, Norway and France. In Italy the applications of the GSHP technology are very few, due to a lack of knowledge of the modeling and design criteria to be applied with respect to the typical energy loads in our country. In this paper a review of existing methods to model and design the BHE is presented with particular emphasis on the transient modelisation at different time scales (hours, weeks, years). Finally an algorithm for ground probe simulation and GSHP performance evaluation is presented with respect to the g-function approach introduced by Eskilson.