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This thesis is focusing on optimizing a new system combining underground thermal energy storage with district heating. The system is operated by storing heat during summer time to lower the heat demand during winter. By analyzing the heat transfer model of the new system, the effects from mass flow rate of the circulating fluid on the heat transfer rate, the exiting fluid temperature and the ground temperature are significant. After economic analysis, it is possible to draw a conclusion that with a lower mass flow rate during summer time and a higher mass flow rate during winter time, the heat utilization can be maximized thus the economic profits can be maximized as long as the ground mean temperature after a whole year is higher than the initial ground temperature. For Turku case, the minimum payback years is 3.54 years with one borehole operated for the whole year, while in summer, the mass flow rate is 0.05 kg/s and in winter, the fluid is circulated with a mass flow rate of 0.5 kg/s and the corresponding COP of the connected heat pump ranges from 2.8 to 3.