Heat losses in water pit thermal energy storage systems in the presence of groundwater

Water pit thermal energy storage (PTES) systems have proven a cheap and efficient storage solution for solar district heating systems. This is partly due to their low cost, deriving from low material usage as the ground is used as the weight-bearing structure. Further savings are obtained by the absence of insulation toward the ground, although this makes the ground-storage thermal interaction more pronounced than other storage technologies. However, it remains unclear how the different operation strategies affect the ground temperature and heat losses, especially in cases where groundwater is present. A simulation model was created using ANSYS and validated using operation data from the PTES in Marstal to investigate this. Using the validated model, it was found that the presence of groundwater could increase heat losses up to 60%, while the heat losses were unaffected when the groundwater table was more than 13 m below the storage bottom. The groundwater temperature could be maintained below 20 °C for a seasonal PTES if the groundwater table was at a depth of 25 m. However, maintaining this temperature for a short-term PTES operation was not feasible. Generally, the soil temperature was unaffected outside of a 100 m radius around the storage regardless of the operation type or the soil's thermal properties.