More than 50% of the overall energy demand in Germany is due to heating and cooling purposes. Therefore, groundbreaking tech-niques are needed to save energy and reduce greenhouse gas emissions especially in this low exergy sector. The combination of differ-ent renewable energy sources – solar thermal and geothermal – with already existing district heating systems fed by combined heat and power stations (CHP) is a promising new approach. In summer, excess solar thermal energy is available, while in winter when thermal energy is needed for heating systems its quantity is usually not sufficient. There are different options to cope with the seasonal offset of thermal energy supply and demand. Besides conventional storage tanks at the surface, thermal storage in shallow aquifers and shallow borehole thermal energy storages (BTES), geothermal heat storage in moderate depths is an innovative and yet barely tested concept. In difference to shallow heat storage sys-tems, the proposed approach upgrades the naturally available geothermal energy in the subsurface by means of external heat input. This is done in summer when no space heating is required or at times when surplus energy from nearby sources is available. In winter when other sources of energy are not sufficiently and cheaply available, the thermal energy from the geothermal storage is used for heating purposes. The focus of the presented study is coupled simulation of an environmentally friendly and energy efficient redesigning of a more than 50 years old office and laboratory building. By storing excess heat from solar panels or thermal power stations of up to 110 °C in summer, a medium deep borehole thermal energy storage can be operated on temperature levels above 45 °C. Storage depths of 500 m to 1,500 m below surface avoid conflicts with groundwater use. Groundwater flow is decreasing with depth, making conduction the dominant heat transport process. Feasibility and design criteria of such a system have to based on coupled simulations of both the above ground facilities (heating, ventilation and air conditioning, HVAC) and the subsurface thermal behavior. The coupled geother-mal-solarthermal modelling approach is presented and discussed. A BTES system as well as an energy efficient building design will help to use sustainable energy sources for the next period of the building's lifetime.

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