Rock Thermal Conductivity as a Key Parameter for Geothermal Modelling
Ground source heat pumps are one important class of geothermal energy system. These heat pumps reject heat into the earth or into the groundwater in the summer, and pump heat from the earth or groundwater into a building during the winter. They consist of an indoor heat pump and an underground system of tubing, which may be either open or closed-loop. They typically have higher efficiencies than other varieties of heat pump.
The efficiency of a geothermal system is dependent upon the efficiency of heat transfer into the heat carrier, which in turn is dependent upon the heat transfer properties of the surroundings, particularly the bedrock. Rock thermal conductivity, thus, is a key performance parameter for the design and construction of geothermal energy systems.
Figure 1. Three of the granite samples analyzed. a) Fresh granite, b) granitic saprolite, c) metamorphized granite.
As part of the ongoing VIGOR project
, an interdisciplinary team comprised of researchers from the Institute of Geosciences and Earth Resources
, the Institute for the Dynamics of Environmental Processes
, and the University of Padua
used the C-Therm TCi
to analyze the thermal conductivity of ten granite samples (three of which are shown in Figure 1) under both dry and water-wetted conditions. The samples were dried in an oven for several days to obtain dry thermal conductivity, then soaked in water for several days to obtain water-wet thermal conductivity – this was to better understand the effect of ground water level on bedrock thermal conductivity and therefore on geothermal energy feasibility.