Improving the Performance of Geothermal Heat Pumps Through Borehole Grout Materials

Abstract

The performance of Geothermal Heat Pump (GHP) systems depends on the efficiency of the heat transfer process in the ground heat exchangers. Much of the interest by the geothermal industry has focused on how to reduce the cost and increase the efficiency of GHPs. A critical issue that needed to be addressed in response to such interest is the material used to fill the boreholes in vertical loop systems. The boreholes in GHP installations were traditionally filled with bentonite grout. Such grout is a relatively poor thermal conductor and also prone to severe cracking and shrinkage under drying conditions. Water table fluctuation has an adverse effect on the ability of the ground heat exchanger to perform its function in boreholes filled with such materials. Long-term effects with shrinking grouts can be assessed by considering that the heat exchanger loop within the borehole has a reduced contact with the surrounding formation. An improved cement-sand grout material (Mix 111) was developed in the laboratory to address shortcomings in conventional grouts and subsequently was subjected to field validation tests in different geologic environments. This paper describes the grout properties, field performance and commercial use.