Graphite has a thermal conductivity of approximately 170 W/(m·K). Ordinary concrete 1.5 to 2.1 W/(m·K), cement screed even less. Thermally conductive concrete is produced by replacing part of the aggregate with graphite granules, or by adding fine graphite to the binder. According to published tests, replacing one tenth of the aggregate with graphite granules roughly doubles the thermal conductivity of the concrete while maintaining compressive strength at the level of a standard structural class.
Unlike electrically conductive concrete, there is no percolation threshold here – thermal conductivity increases steadily with the graphite content. What matters is the volume and size of the grains, not their fineness.
A power cable heats up in operation and dissipates the heat through the bedding and encasement into the surrounding soil. The thermal conductivity of the encasement therefore directly determines the current rating (ampacity) of the route – the better the concrete dissipates heat, the more current the cable may carry at the same permissible conductor temperature. Thermally conductive concrete makes it possible to increase the rating of an existing route or to use a smaller conductor cross-section on a new one.
The thermal conductivity of the screed determines the output of underfloor heating. A graphite screed responds faster, works with a lower flow temperature and can be laid in a thinner layer. Graphite powder or granulate with a grain size of up to about 1 mm is used for screeds so that the mechanical properties of the layer are not changed.
Where the concrete itself is the heat-exchange surface – thermally activated building systems (TABS), energy piles, foundation slabs with heat exchangers – graphite improves the heat transfer between the pipes and the mass of the structure.
Concrete is an inexpensive storage medium, and its limitation is precisely its low thermal conductivity. Graphite increases both the thermal conductivity and the volumetric heat capacity and is used in high-temperature thermal storage – from solar power plant storage to power-to-heat storage of surplus electricity.
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