Influence of different ground thermal properties in a borehole heat exchanger's performance using the B2G dynamic model

Abstract

Ground source heat pump (GSHP) systems for heating and cooling represent an efficient alternative to conventional air source heat pump systems, always provided that they present an efficient design and operation. In this context, the development of energy optimization strategies becomes essential, with the aid of integrated dynamic models of the system, specially the ground source heat exchanger. In previous works, a single U borehole heat exchanger (BHE) dynamic model, called Borehole-to-Ground (B2G), was developed and experimentally validated. The B2G model is based on the thermal network approach, combined with a vertical discretization of the borehole. However, the thermal properties of the surrounding ground were modelled as an average, constant with depth. For homogeneous type of soils, this assumption might be acceptable but, when considering heterogeneous type of soils, modelling the presence of different layers with different materials could provide more accurate results. In this work, the B2G model has been adapted in order to account for the effect of a heterogeneous ground profile on the fluid temperature evolution along the borehole depth. Experimental data corresponding to a Thermal Response Test (TRT) performed in a real BHE existing at the Universitat Politècnica de València, were used to validate this new feature of the B2G model. Finally, it is concluded that the model can serve as a ground thermal properties estimation tool.