Performance Characteristics and Modelling of Cementitious Grouts for Geothermal Heat Pumps

Abstract

The objectives of this work are to develop, characterize and model the behaviour of thermally conductive cementitious grouts for use with geothermal heat pump systems. The grouts have been tested for thermal, hydraulic, mechanical and other properties. The mean thermal conductivity of a grout formulation with 2.13 parts of sand to one part of cement by mass was 2.42 W/m.K. This is up to three times higher than bentonite grout and has a significant impact on lowering the overall thermal resistance. The mean coefficient of permeability for the same grout was 1.6 x 10-10 cm/s. When polyethylene pipes representing a U-loop heat exchanger were included in the system the mean coefficient of permeability increased to 1.9 x 10-7 cm/s due to imperfect interfacial bonding. Infiltration tests on grouted U-loops were conducted and the effect of circulating fluid temperature was measured to determine the efficacy of the grout for sealing boreholes. The infiltration rate remained of the order of 10-7 cm/s when fluid temperature was varied from 3 to 35oC demonstrating that the grout sealing performance was not significantly impacted. Finite element analysis was employed to evaluate the temperature distribution and the corresponding stresses and deformations developed within a grouted borehole as well as in the surrounding formation. The models developed incorporate all major components of the system (pipe/grout/formation). Heat transfer analysis was performed to evaluate the temperature distributions for the cooling and heating modes of operation. This task was followed by a thermal stress analysis.