In pursuit of a replacement for conventional high-density polyethylene tubes in ground source heat pumps from their composites: A comparative study

Ground-source heat pumps, as the most environmentally friendly and energy-efficient air conditioning technology, suffer from a great required length of ground heat exchanger, partly arising from the low thermal conductivity of high-density polyethylene tubes commonly used in ground heat exchangers. In an attempt to find a replacement with an acceptable thermal conductivity for high-density polyethylene tubes, in this study, first, a comprehensive comparative study on fillers commonly used in thermally conductive polymer composites and resulting high-density polyethylene composites was conducted. Then, based on the advantages and disadvantages presented, an appropriate composite was selected and applied in the modeling of a case study to demonstrate the effect of thermal conductivity of the tube on the borehole length of the ground heat exchanger. The findings indicated that low-temperature in situ expandable graphite was a suitable filler to add to high-density polyethylene polymer, resulting in a composite with acceptable thermal and rheological properties. Investigating the effect of thermal conductivity of the tube on the borehole length revealed some intriguing findings. First, using a composite with a thermal conductivity of approximately 1.4 Wm−1 K−1, for instance, the affordable high-density polyethylene composite filled with 10 wt% of low-temperature in situ expandable graphite, the length of the ground heat exchanger reduced by a notable amount of 10 %, which is more than 68 % of the maximum potential for reducing borehole length could be achieved by improving thermal properties of the tube. Furthermore, using polymer composites with thermal conductivity in the range of 2 Wm−1 K−1 could obviate the need for using metal tubes, which are used even nowadays in certain cases. However, due to the lack of results of some specific test, such as Hydrostatic Design Basis, the mechanical properties of the newly introduced composite require further investigation.