34 MARCH 2021 WorldWide Drilling Resource® The Importance of Thermal Grout Conductivity Adapted from Information by GeoPro, Inc. When it comes to geothermal heat pumps, thermal grout is the only major component in the vertical ground heat exchanger, (GHEX) the system designer can control. Mother Nature controls the undisturbed soil temperature, its ability to transfer heat, and to a certain extent, the drilling depths in a given region. Loopfield designers are able to control bore spacing, pipe diameter, overall field configuration, and thermal grout conductivity (TGC), and TGC plays the biggest role in determining the overall amount of bore required. Grout acts as a bridge between the pipe and the earth in the GHEX. It is the only thing standing in the way of heat transfer between the two, so it makes sense to reduce the thermal resistance of the grout as much as possible. The GHEX system is typically modeled as a u-bend placed inside a grouted column, completely surrounded by native soil. The reason bore performance is so sensitive to TGC is the heat flux being transferred between the GHEX and the earth is highest at the part of the system with the smallest amount of surface area (i.e. - just on the outer surface of the u-bend pipe). The material in direct contact with the outer surface (grout) will greatly affect the performance of the system. Using higher TGC values will reduce the overall thermal resistance in the GHEX. This means the same amount of heat can be delivered with less bore. In this example, there is a notable reduction in thermal resistance from the first jump in thermal grout conductivity values (from 0.40 to 0.88), but notice how the reduction trails off. For example, increasing thermal grout conductivity from 0.40 to 0.88 reduces overall resistance by 22.2%, while increasing again from 0.88 to 1.20 only reduces overall thermal resistance by 6.4%. Remember, there is no way to control the soil’s thermal resistance. As the TGC is increased, soil resistance will eventually become the bottleneck restricting heat transfer in the system. In this example, grout resistance goes from about 41% of the total (with 0.40 grout) to nearly 15% (with 1.60 grout). Across the same range, soil resistance varies from 49.3% to 71.0% of the total. TGC not only affects the resistance to heat transfer in the GHEX, it also directly affects design lengths (the amount of bore required to deliver a certain level of performance) and ultimately, the drilling, pipe, and labor costs to install the system. In the end, using thermal grout can reduce overall borehole design lengths and lower the up-front cost of the system. However, bigger isn’t always better when it comes to TGC; the key is to balance the physics and economics of each project to select the correct grout for each system. GEO
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