Impact of grout thermal conductivity on the long-term efficiency of the ground-source heat pump system

Abstract

Quality cementation of a borehole heat exchanger is often considered to be important parameter when analysing long-term heat pump efficiency. Implementing low conductivity grout leads to an increase of borehole thermal resistance and poor heat transfer. This paper analyses one real retrofit project which comprises of 16 borehole heat exchangers with thermal enhanced grout completion. Novel approach of testing the borehole heat exchanger was implemented, so called Steady-State Thermal Response Step Testing, which incorporate series of power steps to determine borehole capacity at steady-state heat transfer conditions and thermogeological properties of ground. Paper objective was to quantify what is real benefit by implementing more expensive thermal enhanced grout, compared to traditional bentonite, silica-sand grouts. Simulation of borehole temperatures and coefficient of performance (COP) changes over time was carried out with Ground Loop Design(GLD), taking into account four different grout mixes. Resultsshown that in thermogeological environment with mediocre thermal conductivity, distinction in COP was modest for each of the four analysed grouting material during 30 years. Comparing investment costs with savings in electricity for each grout mix case, conclusion is that there is no real benefit of implementing enhanced grout in mediocre thermal conductivity environment such as marly-clays.