# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanomaterials
#### 2.2.2 Experimental Work  
Experimental work is one of the least explored types of research due to the cost of test benches for GSHP. In [48], the mixed fabrication of HDPE and graphite nanoplates at different filler concentrations is studied, analyzing the thermal, mechanical, and electrical properties of nanocomposites. The addition of Graphene Nanoplatelets (GNPs) has a significant effect on the thermal conductivity of HDPE, as mentioned in [49; 50; 51; 52]. These works used fillers, such as graphite and carbon nanotubes, due to their ability to add thermal and electrical properties to HDPE composites by increasing Young's modulus. The best result was at 40 wt% of GNPs with HDPE, the \(\kappa\) of composite increases at 1.32 W/mK and Young's modulus reached 1330 MPa, while the pure HDPE values were 0.36 W/mK with 821 MPa. The GNPs greatly reduced the electrical resistivity of \(1.6~{}\times~{}10^{14}\) pure HDPE to \(3.0\times 10^{4}\) for mixed \(40 GNP\)-HDPE. The work in [53] prepared nanocomposites and hybrid nanocomposites based on high-density polyethylene (HDPE) as a matrix reinforced with an alternate and possible combination of \(0D\) nanodiamonds (NDs), \(1D\) Multi-Walled Carbon Nanotubes (MWCNTs) and \(2D\) GNPs. This work evaluated the nanomechanical behavior of HDPE by the addition of nanofillers. The hybrid filled with ND and GNP exhibited superior surface properties. Exceptional toughness, a high elastic modulus, high fracture toughness, a low coefficient of friction, and excellent thermal conductivity can be achieved using ND. The HDPE/ND/GNP combination would improve the quasi-static and dynamic properties. Experimental results of the form-stable phase change materials (FSPCM) are presented in [54]. Due to high thermal energy storage capacity and thermal conductivity, the FSPCM was manufactured by adding expanded graphite (EG) to stearyl alcohol (SAL) and HDPE mixtures. In the composites, HDPE was used to prevent SAL leakage and EG was not only a support material like HDPE, but also a promoter of thermal conductivity. The thermal conductivity of FSPCM underwent a 3% EG increase up to 0.6698 W(mK)\({}^{-1}\), while the thermal conductivity of FSPCM without EG was only 0.1966 W(mK)\({}^{-1}\). Finally, the thermal properties, chemical stability, and the microstructure of the FSPCM were presented. The addition of EG could provide a considerable thermal energy storage capacity and a high thermal conductivity for latent heat storage. A similar work is found in [55], where the thermal properties of phase change materials (PCMs) using mixes with cetyl alcohol (CtA), HDPE, and carbon fiber (CF) are measured. The experiment was divided into groups of 8 mixtures with temperatures around 5-100 \({}^{\circ}\)C. The TC with 5 wt% CF + HDPE + CtA were 0.3220 W(mK)\({}^{-1}\) and 0.4719 W(mK)\({}^{-1}\) (liquid and solid states) compared with HDPE, 0.1834 W(mK)\({}^{-1}\) liquid and 0.2023 W(mK)\({}^{-1}\) solid state. In [51], the researchers carry out a series of studies improving the thermal conductivity of the form-stable phase change materials (FSPCM) using palmitic acid (PA) and high-density polyethylene (HDPE) modified by graphene nanoplatelets (GNP). The authors conducted a miscellaneous analysis of the different compositions: FT-IR, XRD, microstructure, thermal energy store properties, thermal reliability, leakage and thermal conductivity. They concluded that the PA is dispersed uniformly into the network structure of the HDPE and the PA-HDPE composite is attached to the broad surface of the GNP. The thermal conductivity of FSPCM increased to 0.8219 W(mK)\({}^{-1}\), which is almost 2.5 times that of pure FSPCM when the GNP mass fraction is 4%. As a variant of the previous work, the authors presented an improved the thermal conductivity of FSPCM with two types of nanopowders with a high thermal conductivity [52], using Nano-Al\({}_{2}\)O\({}_{3}\) (NAO) and nanographite (NG) as nanoparticles.