# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanomaterials
#### 2.2.1 Theoretical and Numerical Simulations
A review is given in [10] on the enhancement of the thermal properties of individual components affecting the heat exchanger between the heat carrier fluid and the ground applied in geothermal energy piles. Some geometrical modifications and the application of nanofluids as a heat carrier fluid are presented. Additionally, modifications of the pipe material (the heat extracted/rejected in the ground using HDPE pipes) and the concrete mixture are carried out to reduce the total pile thermal resistance. This was achieved using a geometrical optimization in different pipe configurations. Thus, reducing the number of pipes and their arrangement improves the thermal behavior of the GHE. The authors mentioned that Versaprofiles (an HDPE pipe manufacturer for geothermal applications) developed an HDPE pipe for borehole heat exchangers, known as the GEOperforms, with a thermal conductivity 75% higher than the conventional HDPE pipe. In a similar work, several types of heat pipes classified into three groups, sintered, groove and mesh types, were presented, [41]. The authors reviewed various types of heat pipes for different applications such as ground source heat pumps, thermal diodes, rotating heat pipes, etc. They included a review of nanotechnology applications in heat pipes using nanoparticles and introduce a new term, \"nanobubbles\". In this case, no research work has been conducted so far on the applications of nanobubbles to improve heat pipes, only theoretical research, as a recent study phenomenon. As a conclusion, they claim that the technology and the science using nanoparticles in heat pipes is still in the initial stages. In [42], the performances of different configurations of vertical coaxial exchangers are modeled. These configurations are tested using a thermally improved HDPE pipe, applying carbon nanoparticles to the outside. This enhances the thermal conductivity of 0.7 W/m-K, corresponding to a 75% higher than ordinary HDPE. The GLHEPro(r) and the Earth Energy Design (EEDe) programs were used for the simulation. The thermal resistance of the well decreased and the thermal mass of the water contained in the coaxial exchange increased. GHE coaxial configurations helped reduce the total well length by up to 23%. The review [43] introduced a new interdisciplinary theory: nanothermodynamics. This is an extension of the classic thermodynamics theory to the nanometer scale. It serves as a bridge between macroscopic and nanoscopic systems. The focus and emphasis are placed on the use of nanothermodynamics models to find the size-dependent thermal stability, magnetic properties, photoelectric behaviors, thermoelectric phenomena, mechanical properties, electrical properties, volume expansion coefficient, mass density, and energies of nanomaterials. It is desirable to predict the macroscopic, mesoscopic, and nanoscopic properties of materials. This work presents a complete comparison of different materialsevaluating the thermal stability, enthalpies, magnetic and mechanical properties, thermal and electrical conductivity, diffusivity, photoelectric behaviors, relativity permittivity, and other parameters. The thermal conductivity of \(CNTs\) and their polymer nanocomposites offers new possibilities to improve TC in several applications, including power electronics, electric motors, and generators, heat exchangers, among others; this is reviewed in [44]. Some papers using CNT-HDPE were reported. The crystallinity of the polymer strongly affects its TC, which roughly varies from 0.2 W/mK, for amorphous polymers such as polymethyl-methacrylate (PMMA) or polystyrene (PS), to 0.5 W/mK, for highly crystalline polymers such as HDPE. The use of the different nanoparticles in HDPE filled with 7% expanded graphite of nanometric size has a TC of 1.59 W/mK, double that of the microcomposites which have a size of 0.78 W/mK at the same volumetric content. The conclusion was that the unusually high thermal conductivity makes CNT the most promising candidate material for thermoconductive composites.