Numerical simulation of the ground thermal storage of solar heat through an energy wall system

The heating and cooling sector makes up around half of the European Union's total gross final energy consumption (Eurostat, 2024). For this reason, technological advances are required in this regard. Energy geostructures have the double role of providing structural stability and exchange heat by installing absorber pipes in the structural parts (Barla et al., 2019). The underground is used as a heat tank from which to collect heat in winter and where to store it in summer. Their performance during the heating season could be improved if the heat obtained by solar collectors is previously stored in the ground. In this study, a 3D FE thermo-hydraulic model has been built to reproduce the prototype of a shallow energy wall developed at the Politecnico di Torino, named Geothermskin. First, monitoring data (heat carrier fluid and ground temperatures) collected during previous experimental campaigns (Baralis & Barla, 2021) were used to calibrate the soil geothermal properties. After the validation of the numerical model, various operating scenarios were simulated to assess the improvements of the energy wall's performance, achievable when heat storage is performed through the geostructure by combining different summer and winter operation modes. During summer, three scenarios were simulated: no operation of the energy wall; cooling operation through the energy wall (pipe inlet fluid temperature of 28°C); heat storage operation through the energy wall (pipe inlet fluid temperature of 60°C). For winter, the simulated scenarios were four, all including a 13 h heat extraction during daytime, plus, at night, no operation or ground heat storage mode (for four hours) at the different temperature of 25°C or 35°C or 55°C to examine both low and high temperature storage. The different summer and winter combinations allowed to explore the possibility of storing heat also during the winter season. Despite the shallow depth of the wall, the results obtained demonstrate that summer cooling or storage is advantageous for the winter production. When the energy geostructure is inactive during the winter nights, the amount of geothermal energy exploited at the end of winter increases up to more than 15%, if the energy wall was used for cooling in the previous summer, and up to more than 50%, if the energy wall was used for heat storage at 60°C in the previous summer, with respect to the situation when the thermal wall is not activated during summer. Moreover, if night storage operation is activated, the geothermal energy extracted at the end of winter can increase up to 100% more. As expected, the major improvements are achieved when the winter nightly inlet temperature of the fluid is the highest (i.e. 55°C).