Geothermal heating and cooling systems are becoming a more popular option for homeowners looking to save on their household energy bills. Depending on what resource you look at, these systems run at between 300-600% efficiency and rely on the steady temperature of the ground to draw heat into a home in winter, and absorb excess heat in the summer (Figure 1). This is accomplished through a system of underground pipes, which are filled with a water and glycol mixture to facilitate this heat transfer. A grout mixture surrounds the pipes to ensure excellent thermal contact between the pipes and the soil, allowing for efficient energy exchange.
Figure 1. Illustration depicting how a geothermal heating and cooling system relies on the steady ground temperature and a network of embedded pipes filled with liquid to provide heat during cool months, and disperse heat during warm months. 1
As a result of its placement in the system, the thermal conductivity of the grout is an important factor in the efficiency of the overall system. Ideally, the grout should have a high thermal conductivity to allow for good heat transfer. A variety of factors can affect the thermal conductivity of this material, however in this application the presence of groundwater can have a drastic impact. As the grout is a porous mixture, imbibition of groundwater through capillaries can result in high water contents. Campanale et al. (2013) from the University of Padova, Italy, used the Hot Disk Transient Plane Source Thermal Conductivity System to test the thermal conductivity and thermal diffusivity of two different grout mixtures, with the goal of quantifying the influence of density and water content on the thermal properties of the material.
Thermal Conductivity Testing of Geothermal Grout
Grout samples were composed of a mixture containing either Portland cement, or a combination of bentonite, concrete, and quartz sand (grouting material- GM). Tests were accomplished using the double sided Hot Disk sensor, which is composed of nickel foil surrounded by a Kapton film coating. The sensor is sandwiched between two identical sample pieces, and then acts as both the heat source and temperature sensor during the test (Figure 2). Thermtest also offers a single sided sensor, which performs the same measurements but requires only a single sample piece. Initial thermal conductivity testing was performed on each sample at room temperature prior to water exposure. The samples were then exposed to water in the form of contact with water soaked cotton wool, which allowed a natural imbibition process to occur. Thermal conductivity testing was performed on the freshly soaked samples.
Figure 2. Illustration of the placement of the Hot Disk double sided sensor during thermal conductivity testing.
Results obtained by Campanale et al. (2013) illustrated the large influence that water content exerts over the thermal conductivity of the samples. Prior to water infiltration, the sample composed of Portland cement had the higher thermal conductivity of the two, (0.51 W/mK compared to 0.38 W/mK) (Figure 3). Campanale et al. (2013)attributed this to the higher density of the Portland sample. However, after water imbibition, the sample containing GM had the higher thermal conductivity, (1.210 W/mK compared to 0.949 W/mk) (Figure 3). In this case, the increased number of interstitial spaces within the sample allowed for higher water uptake, which subsequently increased the thermal conductivity. As water has a much higher thermal conductivity than air, the sample that was able to absorb more water had the higher performance. Campanale et al. (2013) also made the interesting discovery that the water content varied throughout the sample, as water evaporated from the edges more quickly than from the rest of the material. This indicated that the thermal conductivity could vary as well.
Figure 3. Graph from Campanale et al. (2013) depicting the results of thermal conductivity testing of geothermal grout using the Hot Disk TPS. Thermal conductivity increased with water content.
Campanale et al. (2013) determined that in order to maximize the efficiency of geothermal heating and cooling systems, the thermal conductivity of the grout should be optimized through tweaking the porosity, composition and waterproofing of the materials used. This paper is an excellent example of how the Hot Disk TPS is a fantastic tool for providing accurate thermal conductivity measurements that can be applied to improving and developing renewable energy systems.