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Geothermal heating systems have been available on the market for several decades. Among the most important properties of the materials used in geothermal heating systems are thermal conductivity, corrosion resistance, heat stability, tensile strength and price. High density polyethylene is the most common choice for geothermal heat source pipes, because of its flexibility, low maintenance costs, ease of processing, long life in the soil and reliability. One of the important properties for better heat exchange is thermal conductivity of materials. Plastic materials exhibit rather low thermal conductivity which can be enhanced by the incorporation of particles with higher thermal conductivity, mainly metal powders, carbon additives and mineral fillers. This research is focused on the enhancement of thermal conductivity of high density polyethylene by adding two types of fillers, copper and silica. Design of experiments was implemented for the investigation of mentioned fillers on the thermal conductivity of polyethylene, which was measured by an optimized hot wire method. The results showed that the addition of 10% by volume of copper raised the thermal conductivity of polyethylene by almost 100%. The tensile modulus and stress at yield increased by 18.5%, whereas the stress at yield decreased by 18%. Although silica did not have a significant impact on the thermal conductivity of polyethylene, it additionally increased the tensile modulus of polyethylene and copper mixture, while simultaneously maintaining the same level of stress at yield and reducing elongation.
polyethylene; copper; silica; thermal conductivity; tensile properties; geothermal heating
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