This study presents the experimental and numerical simulation based characterization of a new modular solid-liquid sensible heat storage system. The field-scale storage prototype was constructed in the shallow subsurface and consists of 25 coupled 1.5 m³ storage units, each equipped with a helical heat exchanger in a cement based water saturated matrix. A charging and passive cooling experiment was performed over a period of 3 months, with a maximum storage temperature of 60°C and distributed temperature monitoring of the system. A detailed 3D finite element model of the storage system was developed and parameterized in order to analyze the governing heat transfer processes and quantitatively characterize the storage behavior. Experimentally observed and simulated storage temperatures show a good agreement, with differences of less than 2.7 K, which proves the appropriateness of the model approach. Average loading rates of 14.6 kW during the first 2 days and 4.3 kW during the following 10 days of heat charging correspond to a used storage capacity of 660 kWh and 1310 kWh after 2 and 12 days, respectively. During passive cooling the storage temperature was reduced to approximately 30°C within 30 days, which corresponds to a heat loss rate of 1.4 kW during that time and demonstrates the necessity for proper thermal insulation of subsurface heat storages.
Keywords sensible heat storage, subsurface, field scale experiment, storage characterization, numerical modeling