Assesment for optimal underground seasonal thermal energy storage.

Abstract

[EN]An optimal design for seasonal underground energy storage systems is presented. This study includes the possible use of natural structures at a depth of 100 to 500 m depth. For safety reasons the storage fluid considered is water at an initial temperature of 90 °C. A finite element method simulation using collected data on the thermal properties of the soil was performed. As a practical example of this methodology, an analysis with data collected in the region of Avila, Spain is made. A temperature-depth map using data measured in the zone was generated. A 3D model of the underground material composition was obtained by electromagnetic field diffusion techniques carried out from the surface. This allows for an analysis of available solutions in energy storage strategies, tailored to the specific conditions on the site with a sufficient degree of precision for a first evaluation without the need for deep excavations. This study shows alternating regions of sands and clays, with natural structures for potential use within a depth of 500 m. Thermal properties of water depending on temperature and pressure are considered. A variety of size configurations shows that, in a cylindrical geometry, a storage system with a radius beyond 2 m does not offer significant benefits in energy stored per mass unit. The benefits of a clay envelope are noticeable, compared with the scenario of a cavity surrounded by sand and followed by clay even after 6 months of storage. According to the underground temperature and the energy needed to transport the storage fluid, it is shown that the thermal performance does not significantly improve between 50 m and 100 m of depth. However, between 100 m and 200 m a noticeable improvement is achieved, and from there down to 500 m the improvement is negligible. Several materials for containing the storage fluid and for thermal isolation are analyzed. For periods beyond 14 days, the thermal properties of thermoplastics are relevant, as found in the case of the Acrylonitrile-Styrene-Acrylate which exhibited the best performance in the simulation. In the best configuration, it is possible to see that by storing water at 90 °C (obtaining 138.78 kJ/kg from an exchange with a typical system at ambient temperature in the months of January–February) compared to the case where the water is stored at the temperature of the underground, that is 25 °C (obtaining 77.08 kJ/kg), it is possible to store 1.8 times more energy per kg of store water without affecting the surrounding medium. Finally, the efficiency of the storage system is calculated from the thermal energy that can be potentially recovered according to the input energy needed to raise the temperature of the fluid from an ambient temperature, up to the initial storage temperature of 90 °C. Due to the thermal properties of clay in the subsoil, previous efficiencies ( ) reported in aquifer energy thermal energy storages can be obtained with relatively small storages without continuous energy inlets as is the case of the majority of seasonal thermal energy arranges, with a potential to recover 70% of the inlet thermal energy under optimum conditions of the storage