Thermal conductivity | materials feature
can graphite be used to successfully increase the thermal conductivity of pE copolymer pipes for cost-effective geothermal
applications?Raffaele Gilardi andDaniele Bonacchi of
Timcal graphite & carbon report on some promising test results
for geothermal pipes
Thermally conductive compounds
metal replacement with plastics and composites is a clear trend and thermal conductivity is a key parameter that is playing an increasingly important role. Among the different applications of thermally conductive polymers, heat exchangers for cooling or heating systems are of particular interest [1]
. For example,
polyethylene (pE) based pipes have almost completely replaced copper and stainless steel pipes in geothermal applications thanks to their better flexibility, resistance to corrosion, ease of installation and costs. However, standard pE pipes used in heat exchangers
were originally intended for hot and cold water distribution and have low thermal conductivity (λ~0.4 w/(m.K)) that represents a limit to their potential. in order to build more efficient geothermal systems,
pipe materials with better thermal conductivity are required, while keeping good flexibility and mechanical properties. Such pipes with increased thermal conduc- tivity will reduce the thermal resistance of the borehole and decrease the length needed for geothermal heat exchangers, therefore lowering system costs.
www.compoundingworld.com one way to increase thermal conductivity of plastic
pipes is to use compounds containing thermally conductive fillers. Traditional thermally conductive fillers include metals, such as copper or aluminium, or nitrides, such as boron nitride and aluminium nitride. However, due to the high density of metals and high costs of nitrides, other solutions have been pursued. carbon materials represent a valid lightweight
solution for thermally conductive compounds especially when electrical insulation is not a requisite. Among carbon-based materials, expanded graphite is one of the most advanced solutions combining good properties with competitive costs [2]
. one drawback of expanded graphite is its inherent
extremely low bulk density. This makes it very difficult to feed into a polymer melt using common feeding/ mixing technologies and lowers the throughput of twin- screw compounding units. in order to overcome these problems, Timcal has developed a new range of Timrex c-Therm expanded graphite with higher, carefully adjusted bulk density [3]
. February 2012 | compounding world 15
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