This master's thesis delves into a system design configuration aimed at efficiently storing surplus heat generated during the summer from Härnösand CHP plants, boilers, and solar collectors for subsequent retrieval and utilization in the winter. The thesis focuses on three main objectives: identifying suitable Seasonal Thermal Energy Storage (STES) options, developing an initial dynamic model for STES, and optimizing STES properties while minimizing heat loss. The author conducts a comprehensive review of various STES technologies, including TTES, PTES, ATES, and BTES. The TOPSIS method is employed to ascertain BTES as the most viable option for the proposed system.

The storage system is simulated and modeled using Carnot Simulink MATLAB, facilitating the exploration of diverse design parameters. To mitigate heat loss, a greenhouse is constructed over the BTES, and its operational efficiency is evaluated. The optimal borehole depth is determined to be 50 meters with a spacing of 3 meters between boreholes, resulting in an energy efficiency of approximately 17% over a four-year operational period.