Assessment of seasonal Borehole Thermal Energy Storage in the seawater intrusion region of a carbonate aquifer

In coastal areas the seawater intrusion region underlying freshwater aquifers represents a low quality but wide and deep geo resource. Seasonal thermal energy storage and recovery is an important component of district heating and cooling system to manage renewable energy fluctuations, such as solar irradiance or waste heat from industrial processes, and the corresponding mismatch of thermal energy demand and supply. A numerical tool to evaluate the performance of seasonal Borehole Thermal Energy Storage (BTES) system to store and recover solar energy in the seawater intrusion region, underlying the shallow freshwater aquifer and with thermally and hydraulically insulated upper borehole section is developed and applied to the coastal carbonate aquifer of the metropolitan area of Bari (Italy). The hourly thermal demand of the University Sport Centre of Bari is used as benchmark. The design and performance of the BTES system is strongly dependent on the geological and hydrogeological context as well as on the environmental and operational conditions. The aquifer characterization suggests to locate BTES zone at a depth higher than 100 m from the freshwater – saltwater interface where carbonate unit appears less fractured and karstified showing a value of bulk permeability less than 10−12 m2 and the groundwater flow is slow (∼10−3 md−1). Rayleigh number criterion is used as constraint to determine the maximum heat storage temperature (∼70°C) in order to preserve the lateral thermal stratification and the thermal impact on the shallow freshwater resource. A novel mathematical and computational model is developed to help the design of BTES system and to evaluate its efficiency. The results indicate that the thermal losses within thermally insulated zone influence the effective thermal recovery factor which, according to the baseline scenario, is equal to 47% after five years of operation. The heat energy production of the solar heating system, covers the heat demand with percentage range of 79–117%. The location of BTES in deep seawater region attenuates the decreases of efficiency due to the groundwater flow which became significant at specific discharge around 10−2 md−1. Changing the operation schedule with a shorter heating storage period increases the thermal recovery factor of the BTES of 11.19% after five years of operation, but at the same time the trend of the heat energy production shows a surplus during the midseason and deficit during the winter and summer season respect to the thermal demand. Great care must be taken on the maximum heat storage temperature. A low heat storage temperature ensures a wider safety margin with regards to the thermal stratification and the thermal impact on the shallow aquifer. Anyway, a decrease of 10°C on the maximum heat storage temperature produces a deficit of the heat production respect to the thermal demand in the range of 15%–21%.