Design and operation of Borehole Thermal Energy Storage systems

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In a Borehole Thermal Energy Storage (BTES) system, the goal is to increase the ground temperature above its normal value and to store energy over long periods. BTES systems have not been studied extensively and scientific developments are required to better design and operate these systems. The long-term objective of the research program is to develop knowledge and models to better understand the performance of BTES systems so that the engineering community can use them extensively to fully utilise the potential of renewable energy. Heat transfer coefficients used to predict borehole heat transfer are typically calculated based on standard correlations. However, for reduced flow conditions, current natural and mixed-convection correlations are inadequate for boreholes. It is proposed to develop a heat flux sensor that will enable the determination of local heat transfer coefficients inside borehole tubes. Tests will be performed in a small-scale borehole test facility to develop general heat transfer coefficient correlations applicable to boreholes. More advanced BTES systems have been proposed where electric heat pumps, operating at night when greenhouse gas emissions from the grid are the lowest, are used to transfer heat from an outer ring of boreholes to the central core boreholes. In these systems, injection/collection of heat will be performed on individual boreholes to rapidly increase (or decrease) the storage temperature in certain regions of the BTES. This will provide flexibility for buildings to operate on direct space heating from the central core or cooling from the outer ring. A new load aggregation scheme will be develop to handle temporal superposition of the thermal history of each borehole to improve computational efficiency. There is an industry need to develop guidelines and tools to asses the design and operation of BTES. First, engineers need to know the maximum heat transfer rate that can be expected from a BTES. It is proposed to represent borehole heat transfer using the concept of variable heat exchanger efficiency as a function of the BTES state-of-charge. The second design problem facing engineers is the determination of the optimum BTES operating temperature for maximum energy savings and minimum greenhouse gas emissions. This optimum is a compromise between a high enough temperature to provide space heating directly and a low enough temperature to limit BTES heat losses and the resulting temperature lift imposed on the heat pump. Thus, parametric studies will be needed to find the best set of parameters for particular applications.  As we advanced towards community planning where storage resources are shared for energy flexibility and resilience, BTES systems will play a larger role. The projects proposed here will enhance our understanding of the thermal behaviour of BTES and provide rigorous calculation methods to better design these systems.