Effect of active grout irrigation in ground heat exchangers with helical-shaped pipe

Improving the efficiency of ground source heat pumps during their operation is a key aspect to foster their diffusion, and grout irrigation strategies could be valuable solutions to implement for this purpose. However, this strategy is still largely unexplored and needs thorough study. In this work, it is investigated whether the active irrigation of the grout surrounding helical borehole heat exchangers can improve the heat pump seasonal efficiency, and under which conditions this benefit is not offset by the consumption of the irrigation system itself. The effect of variable water content on the properties of a sandy grout was taken into account through empirical correlations reported in the literature and implemented in an open-source CApacity-Resistance Model, named pyCaRM, in order to be updated at each time step. Three main strategies, adopted only in the summer season, were compared against a baseline case over a twenty-year simulation, including the electricity consumption of the auxiliary water supply system. Results showed that, in the best-case scenario, the heat pump seasonal efficiency during cooling mode can improve by about 5.3%, dropping to 4.5% if considering the auxiliary system, at the end of the 20 th year, and that 96 m 3 /borehole/year were necessary to reach these values. As for the energy efficiency in heating mode, its relative reduction is about 2%. The total volume pumped and its source should therefore be taken into account when assessing the strategy, and the different strategies should be implemented according to the building load characteristics to achieve the highest rate of improvement.