Experimental heating and cooling curves of the ground for temporary energy storage applications

The building sector represents around 36% of global energy consumption and 39% of atmospheric emissions. Renewable energies require energy storage systems to adjust the gap between energy generation and demand. In this work, an exploratory study of heating and cooling curves of the ground with the aim of evaluating its suitability for temporary energy storage has been performed. Experiments were conducted at three different year periods by injecting heat into a circuit of vertical pipes buried in the ground and then leaving the system evolve on its own, whilst registering the temperature evolution of probes at 5 different depths. The analysis of the dimensionless experimental data revealed a certain asymmetry in the behavior during heat injection and heat dissipation process. During heat injection, the main driving mechanism is the heat power source coming from the water flowing through the buried pipes; whereas during heat dissipation, it is the particular characteristics of the ground. The influence of the water table level was determining, separating an upper and lower zone with a slower and faster response respectively. It was also possible to detect the presence of an underground water stream, that led to convection effects and the removal of heat due to mass transfer, as well as to obtain estimates of the apparent thermal diffusivity of each ground layer. The influence of the year period and atmospheric conditions was especially noticed at the upper ground layers. Accumulated rain beneath the ground and the relative amount of water carried by the underground stream also affected ground thermal properties. Finally, guidelines for the design and placement of heat storage systems based on the results of the study are provided.