Modeling the Heat Regime of Thermal Boreholes with Pore Moisture Freezing/Melting

Abstract

Modern computer programs modeling heat transfer near the boreholes of geothermal or ground source heat pumps (GSHP) ignore freezing and melting of pore moisture. This is so because most GSHP are deployed in regions with moderate climate, where ground temperatures are generally positive. To ensure efficient exploitation of GSHP under Russia's climatic conditions, we have to allow the heat carrier temperature, i.e., the ground temperature, to drop below zero. During the heating season, pore moisture freezes and melts releasing and absorbing the latent heat, which may affect the heat regime in the neighborhood of the thermal borehole. To assess the effect of latent heat on the borehole heat regime, we need a sufficiently simple and "fast" algorithm that can be used in engineering calculations during GSHP design. In this article, we present a pore moisture freezing/melting algorithm developed by the Insolar group of companies. The algorithm utilizes the concept of equivalent or "effective" ground thermal conductivity. Results of computer experiments are reported, confirming the essential dependence of the GSHP efficiency on ground-moisture phase transitions.