Geocooling potential of borehole heat exchangers' systems applied to low energy office buildings

The potentiality of geocooling (i.e. free cooling) with borehole heat exchangers is analysed for low energy office buildings. The borehole heat exchanger field is coupled to a heat pump in winter and to the cold distribution system in summer through a flat plate heat exchanger. The cooling requirement satisfied by a direct heat transfer into the ground through the borehole heat exchangers is so-called geocooling. A dynamic system model has been developed to simulate the building, the emission of thermal energy through thermally activated building systems, the technical installation including the borehole heat exchanger field and the interconnected thermal interactions. Thermal comfort requirements determine the building energy needs and the size of the ground coupled system. A methodology is presented for best system design. Building design, system technical feasibility and limits of the ground coupled system are discussed and result from the analysis of the numerous system simulations. Geocooling potential depends on the quality of the building design and its heat emission. The importance of the ground thermal conductivity and the ground recharge ratio on the system design are highlighted. Simple design sizing keys are proposed for a fast pre-sizing of a borehole field.