COMPARISON OF BOREHOLE HEAT EXCHANGERS RESPONSE BASED ON DIFFERENT HOURLY LOAD MODELS

Borehole heat exchangers (BHE) are the most frequently adopted solution for ground coupled heat pump applications. In most installations, BHEs also represent the most important cost item, and a careful design analysis is needed to either assure long time performance or reduce the payback period, both parameters related to overall BHE length. The most efficient way, from a computational point of view, to predict the temperature evolution in time and space of a ground volume in contact with a system of BHE, is the recursive calculation of a basic thermal response factor, evaluated at different time steps and for given different heat pulses representing the building energy demand. Hourly load simulations, along multiyear periods, are considered the most reliable approach for simulating the thermal interactions between the ground and a system of BHEs and thus simulating the ground coupled heat pump (GCHP) behaviour during the expected lifetime of the whole system. Among the literature models, the DST one is often used as the reference analysis tool. The DST model is based on a description of the ground/BHE system in terms of interacting cylindrical volumes, arranged in a regular geometry. In this paper, the DST solution, in terms of hourly temperatures of the heat carrier fluid, is compared with the correspondent results obtained by implementing the MLAA approach of Bernier et Al. into a model able to employ suitable g-functions generated starting from the Finite Line Source solution of Lamarche and Beauchamp. The study is aimed at comparing the predicted temperature values by the DST and MLAA models with reference to different BHE configurations, having the same number of ground heat exchangers but different geometrical distribution (e.g. square configurations vs in-line configurations)