High solids bentonite grouts are becoming increasingly popular in geothermal projects because they provide an excellent annular seal and improve thermal conductivity in geothermal heating and cooling. There are several versions of this type of grout on the market today. Several have been formulated so silica sand can be added to give even higher thermal conductivity.
A geothermal system requires a series of boreholes in which pipes are installed vertically beneath the earth's surface, creating a loop system through which fluid is circulated. Heat generated underground is captured and absorbed in the fluid and transported to a system of heat exchangers to provide heat to a building. In the warmer seasons, heat from the home or building is transferred to the earth to cool the home or building.
In boreholes drilled for the system, it is extremely important that the annular space be filled with a grout material that will give maximum aid in transfer of heat and also provide a low-permeability seal. High solids bentonite grouts are a great choice for geothermal projects. They provide the following advantages:
* A degree of thermal conductivity before sand is added.
* Bond to the loops and the borehole wall.
* A complete seal top to bottom.
* A low permeability seal.
* Easy mixing, placement, and clean-up.
* No heat of hydration.
As indicated above, bentonite manufacturers are recommending addition of silica sand to the grout mixture. This greatly improves thermal conductivity of the annular seal. Sand additions may range from a minimal amount to 600 lbs. per bag of bentonite grout. As the amount of sand is increased, thermal conductivity will improve.
Below are some specifications of a typical sand for this application:
1. Silicon Dioxide: 99%
2. Grain Shape: Rounded
3. Grain Mesh Size: # 40 30-36%
# 50 40-50%
# 70 15-20%
It is very important to follow the grout manufacturer's specifications to ensure the grout performs as intended. The chosen grout should at least be equal to thermal conductivity of soil surrounding the borehole. The grout should also allow the contractor to adjust the mixture to attain the thermal conductivity required for the project.
The bentonite, sand, and water are typically mixed in a paddle-type mixer and pumped through a tremie pipe into the annular space of the borehole. The annulus should be filled from bottom to top, displacing any water in the borehole. A positive displacement pump is often recommended, and the tremie pipe should be 1" to 1-1/4" I.D.
Tables showing thermal conductivity of various grout mixtures can be obtained from grout manufacturers. Information on solids content, grout weight, permeability, and batch yields are also available. If you have any questions regarding geothermal grouting products, please contact me through the National Driller or contact a bentonite manufacturer or distributor.