Borehole thermal energy storage (BTES) is an option to provide large-scale seasonal storage of cold and heat in natural underground sites. Boreholes are used to transfer heat from the working fluid to the ground in charging phase and vice versa in discharging.

The design of boreholes influences system efficiency, installation costs and the charging time required to reach the design temperature in the ground. The latter can result too long but may be reduced significantly through the selection of an optimal design and a storage temperature that maximizes the efficiency. Nevertheless high performances are usually accompanied by high purchase and installation costs of the borehole exchangers. The increase in efficiency and the decrease of investment costs are two conflicting objectives that must be considered in the design stage to select the best configuration.

This work is focused on the optimal configuration of BTES in which the boreholes are used to charge the ground to the design temperature and to supply the thermal energy demand during the operation. Several designs are explored at two different levels of temperature in the storage.

A novel design strategy, based on energy, exergy and thermoeconomic analysis, is proposed to select the optimal configuration that guarantees a balance between expenditure on capital costs and exergy efficiency. This constitutes a novel approach which ensures high performances of BTES systems for long periods of operation, which is an interesting area of research that is currently not sufficiently explored.

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