# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanomaterials
#### 2.2.2 Experimental Work
NG and NAO were added into the FSPCM to improve the thermal conductivity using myristic acid (MA) as a solid-liquid phase change material (PCM) and the HDPE. The FSPCM can be a solution to overcome the defect of storing and releasing latent thermal energy. A field emission scanning electron microscope (FESEM) showed that the mass fraction of the MA in the MA/HDPE composite was less than 70%. The thermal conductivity of the FSPCM can increase from 0.2038 W(mK)\({}^{-1}\) to 0.3972 W(mK)\({}^{-1}\) (NAO) and 0.4503 W(mK)\({}^{-1}\) (NG) at30 \({}^{\circ}\)C when the mass fraction of nanoaddiitives is 12%. Additionally, at 60 \({}^{\circ}\)C the value can increase from 0.1918 W(mK)\({}^{-1}\) to 0.3471 W(mK)\({}^{-1}\) (NAO) and 0.3923 W(mK)\({}^{-1}\) (NG). An important result was found in [56]. The reinforcement effect depends on the shape and the dimension of the carbon-based nanofillers. They mixed HDPE with graphene nanosheets (GNs), and HDPE with multi-walled carbon nanotubes (MWCNTs) nanocomposites at filler loadings (0.5, 1 and 3 wt%). The material, graphene synthesis, nanocomposites preparators, characterization techniques of material and methods, and the structural, morphological, rheological, mechanical, and thermal properties of the mixtures were reported. In this last property, the result was the HDPE nanocomposite containing 0.5 wt.% GNs shows an increase of 24 \({}^{\circ}\)C in 5% (weight loss of 5% for neat HDPE) compared to neat HDPE. The HDPE/MWCNTs exhibits a smaller increase of 3 \({}^{\circ}\)C at the same content, concluding that the addition of nanofillers leads to a notable improvement in the thermal stability of HDPE. In [57] is patented a pipe with enhanced thermal conductivity for geothermal applications (GreenGeopipe). The thermal properties of HDPE material can be enhanced using different types of thermally conductive fillers, for example, metallic oxide, non-oxides, and graphite. The latter is a material that has shown promise in improving the thermal properties of HDPE material. Different studies showed the graphite is a good material applied to HDPE material. For instance, [58] experimentally analyzes the thermal and electrical conductivity, mechanical properties, elongation at break, and stress at break of HDPE/graphite and LDPE/graphite composites. The percolation concentration (defined as the increase in electrical conductivity with a particular filler concentration) exhibits a non-linear increase of 11 vol% for both compounds with increasing graphite content and thermal conductivity comparison of filled HDPE vs. filled LDPE was higher due to the higher degree of crystallinity of HDPE. In the pioneering study [59], the authors used GEOperforms HDPE pipes to test the thermal performance of the installed wells and the experimental results of the thermal response tests showed that the equivalent thermal resistance of the well was 17% lower. Simulations consider a vertical pipe thermal conductivity of 0.41 W/m/\({}^{\circ}\)C (conventional HDPE) and 0.71 W/m/\({}^{\circ}\)C (GEOperform) and the results showed a reduction in borehole lengths of around 10% with respect to a conventional HDPE pipe. In addition, the heat dissipated by the GEOperforms test with a single well was 25% higher (2300 W) compared to the HDPE pipe. In [60], through differential scanning calorimetry (DSC) and thermomechanical analyzer (TMA) tests, the authors investigated the thermal and mechanical properties of the HDPE-CaCO\({}_{3}\) nanocomposite mixture to improve the poor thermal properties of HDPE polymer and polypropylene (PP). The principal composition, 10% CaCO\({}_{3}\) nanocomposite, was added to HDPE. The results of the tests showed that the addition of nanosized calcium carbonate to HDPE increased the heat capacity, sensible heat and crystallinity index. The dimensional stability of HDPE increases as the nanometer size of calcium carbonate leads to a lower thermal expansion and compressibility coefficient than plain HDPE.