Thermal enhancement of PFA-based grout for geothermal heat exchangers

As part of a programme of finding new uses for industrial wastes, a trial of PFA (pulverized fuel ash) as a thermal grout for borehole heat exchangers has been tested experimentally. Several mixes of PFA-based grouts were developed by blending PFA with different solid materials by weight in different mix proportions with a constant percentage of cement. The materials used in these mixes are: fine sand, coarse sand, ground glass, and fluorspar. The thermal conductivity of seven different groups of grouts has been measured at dry and saturated conditions. A new thermal cell that utilizes the steady state technique developed by Newcastle University was used for these measurements. The results show poor enhancement of thermal conductivity using fine sand or medium ground glass, with maximum value of 1.15 W/m K at saturation. Results obtained using coarse or mixed ground glass gave a maximum value of 1.39 W/m K. The highest thermal conductivity values were achieved using fluorspar or coarse sand where the thermal conductivity reached 2.88 and 2.47 W/m K respectively at 20% of PFA. It was also observed that the combination of fluorspar with coarse ground glass can offer relatively high thermal conductivity at both dry and saturated conditions. Moreover, this combination of materials comprises a practical amount of low-cost material (PFA and ground glass).