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The use of graphite in thermally-conductive grouting applications has exploded in recent years with more and more drillers abandoning the use of sand either completely or partially. Some of this greater interest has been due to economics and some to labor reduction or worker safety issues. In this article, I will discuss the benefits and some of the potential trade-offs between using graphite versus sand in geothermal heat-loop grouting.
OVERVIEW: GRAPHITE AND CARBONS
For those who may not be familiar with graphite or its use in ground-source heat pumps, here is a quick primer. We most commonly find graphite in standard pencils. The "lead" is actually graphite mixed with clay to harden it. The pencil mark is composed of thin layers of graphite laid down on the paper when we press down. It may at first seem strange that such a material can be used to increase thermal conductivity in grouts. However, crystalline-carbons conduct both heat and electricity and, of these, graphite is the softest type and is economical
Graphite is a form of crystalline carbon in which the individual atoms are arranged in sheets, rather than in a three-dimensional lattice (like diamond) or in hollow tubes and spheres (like fullerenes). The sheets look something like honey-combs with six individual atoms arranged in rings that join together in the x-y axis. Each graphite carbon atom is joined to three other carbon atoms.
Pure graphite has no bonding between layers. The layers can slip easily past one another. In contrast, coal is a form of carbon in which the carbon atoms are not arranged into any set pattern. This lack of arrangement is often referred to as being amorphous. Coal also contains significant amounts of non-carbon impurities such as sulfur, nitrogen and hydrogen, whereas graphite, diamond and fullerenes are much more pure.
Graphite can be mined or produced synthetically using extreme heat. For the purposes of geothermal heat loops, both sources work exceptionally well for conducting energy, but not all are economically sensible. The purity of synthetic graphite can approach 99.9 percent or even higher for some pyrolytic or vapor-deposited forms. This level of purity is expensive and not necessarily advantageous for our purposes. Even an 80-percent pure graphite will still conduct heat far better than...