Heat transfer in the ground with a horizontal heat exchanger installed - Long-term thermal effects

Heating and cooling of buildings using heat pumps coupled with ground heat exchangers (GHE) causes changes in the ground temperature. Long-term temperature changes, regardless of cyclical changes, can occur in the ground depending on the relation between the extracted and supplied heat from/to the ground when a horizontal GHE coupled with a heat pump is installed in the ground. Determining the size and direction of these changes is the main objective of this work. A mathematical model of the heat transfer process was developed and temporal changes of the ground temperature, temperature profiles in the ground and heat fluxes in various conditions of heat extraction and supplying were determined. It was found, that changes in annually averaged temperature of the ground in the initial period of exchanger operation concern only the subsurface layer. In the following years, these temperatures in this layer do not change further. The longer the operation time of the exchanger, the annually averaged temperature changes expand their range to larger ground depths. The concept of sol-air-evaporation temperature is used in the model. This allowed the description of heat transfer between the ground and the environment with a simple equation analogous to the heat transfer equation.