In European countries seasonal thermal energy storage is an emergent task due to availability of solar energy in summer and thermal energy demand in winter. In this study the performance of an uninsulated buried storage tank is analyzed. Summer temperatures reached 45 °C in the storage tank and 22 °C in the soil, 1 m from the tank shell. Wintertime temperature of the storage tank dropped to 8 °C, near the freezing limit of the heat pump, and soil cooled down to 9 °C. While in wintertime heat transfer from earth to water was the limiting factor, a summertime temperature difference of more than 20 °C allowed enough energy transport to charge the soil storage system. An analytical model showed that more than 50% of the solar energy stored could be recovered by this application.