Thermally Conductive High-Density Polyethylene Composites for Ground Heat Exchangers

Abstract: Today, the energy used for the heating and cooling of offices, shopping centres, schools, residential areas, andother buildings is as large as 50% of the total energy consumption in the world. The climate change, globalwarming, and environmental issues have forced high requirements for energy efficiency and clean energyproduction in buildings. The general targets in all sectors are toward nearly zero energy buildings (nZEB) andthe next step solution is positive energy buildings (PEB). To achieve a net zero carbon footprint and evenpositive energy buildings, sustainable energy production systems that mainly rely on renewable energycombined with energy efficiency and integration technologies must be implemented.Geothermal energy has a unique position among the renewable energy resources. Near-surface or shallowground maintains almost a constant temperature during the seasons, and therefore ground can be used as aheat source during the winter and as a heat sink during the summer. A ground source heat pump system(GSHP) consists of a heat pump, which is connected to the delivery system and a ground heat exchanger(GHE).The majority of the ground source heat pump systems in Europe use vertical borehole heat exchangers (BHE).The key problem here is obtaining the largest possible heat flow exchanged with the rock mass. The energydemand and ground properties are the most parameters that influence the performance of a GSHP. Theamount of heat taken from the rock mass depends on the thermal conductivity of the surroundings and theborehole thermal resistance. Numerous attempts have been made to reduce the thermal resistance of theborehole in different ways but until recently, little attention has been paid to the enhancement of propertiesof the material used to produce GHEs.In light of the above considerations, a novel thermoplastic composite, to have lower thermal resistance in theGSHP systems that affects the maximum heat flow exchanged between the heat carrier and the rock mass,has been introduced. This includes the development of a state-of-the art research and developmentinfrastructure to support the use of new materials and test methods for the BHEs. High density polyethylene(HDPE) composites reinforced with inorganic fillers and their properties were investigated. The compositeswere prepared by melt blending and injection moulding. In the context of this study, the thermo-physical,mechanical, and morphological properties of these composites were studied; thereafter, using a numericalmodel simulation, the effectiveness of the new material was evaluated. The very positive finding was that theaddition of talc particulates, not only improved the thermal conductivity and thermal diffusivity of thecomposites, but it also simultaneously increased the properties of the composites regarding stiffness andimpact resistance, which are important parameters in the deep GSHP system.Thermal conductivity, thermal diffusivity, and density values of the composites increased almost linearly, butthe increase in moisture absorption in the long-term showed non-linear behaviour under the chosenexperimental conditions. The maximum thermal conductivity was up to 70% higher than for the unfilled HDPEat a talc concentration of 35 wt.-%. The numerical simulation showed that the enhancement of the thermalconductivity of the material can significantly reduce the overall borehole thermal resistance.Melt rheological investigation of the HDPE-talc blends at constant shear stress and constant shear rateindicated that the melt obeyed the power-law model and shear thinning behaviour.The results have also illustrated that the presence of talc has considerable effect on the lifetime expectancy ofthe product. It was presented that the thermal stability was enhanced, while the oxidation induction timedecreased in cooperation with the talc. Furthermore, the temperature of the α relaxation shifted towardhigher temperature and finally, the strain hardening modulus for the HDPE/talc composites was assessed andcompared to the neat HDPE as a measure of environmental stress crack resistance.