# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanofluids
#### 2.1.1 Theoretical and Numerical Simulations
This work simulates important data, such as the effects of soil temperature, the Reynolds number, the effect of the size, type, the concentration of the nanoparticles, the suspension stability of composites, gravity, the effects of a pulsed flow, the borehole geometry and the turbulent eddy diffusivity. The recent report in [16] shows that the heat transfer characteristics of current fluids are greatly improved by suspending nanosized solid particles with diameters of less than 100 nm. Recent research of nanofluids has analyzed the convective heat transfer rate, thermal achievement rate, viscosity, surface tension, friction factor, environmental impact, thermo-physical properties, the effect of fluid temperature, inlet velocity, the use of a surfactant to achieve a better stability of nanofluids, particle size, and the volume concentration effects, for instance, [17; 18; 19]. Several types of nanoparticles have been widely studied by researchers. Thus, the suspension of small amounts of nanoparticles of oxides (Al\({}_{2}\)O\({}_{3}\), CuO, TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), etc. ), metals (Cu and Ag), and carbon nanotubes (CNTs) in traditional base fluids (water, ethylene glycol, propylene glycol, engine oil, etc.) have increased thermal conductivity [16]. On the other hand, some works study the relevance of the physical properties of a nanofluid. The work [20] presents an approach using nanofluids in geothermal energy applications as working fluids to extract more energy from reservoirs and for space heating/cooling and industrial applications. In this research, a sensitivity analysis was performed to demonstrate the importance of the fluid viscosity and the heat capacity in geothermal energy production. The potential to apply nanofluids as working fluids in abandoned oil wells converted into double-pipe heat exchangers is studied, taking advantage of the significant improvement in the heat transfer of nanofluids. This research remarks on the importance of the thermophysical properties of nanofluids; for example, the viscosity and specific heat capacity and the fluid circulation rate improve the performance and the cost-efficient production of geothermal energy. A study of the effects of the Al\({}_{2}\)O\({}_{3}\)-water nanofluid, as the heat transfer fluid, to reduce the length of a vertical GHE in a GSHP is presented in [21]. The authors used an innovative nanofluid which was engineered by dispersing solid nanoparticles in conventional heat transfer fluids. They evaluated the impact of the optimized thermophysical properties of nanofluids, such as the thermal conductivity and viscosity, which play prominent roles in convection heat transfer, both of which are optimized by using the Multi-Objective Flower Pollination Algorithm (MOFPA). The bore length computation with an Al\({}_{2}\)O\({}_{3}\)-water mix reduced the borehole length by 1.3% more than when just using water. Another important result revealed that the use of tubes and grout reduces the bore length due to their thermal resistances. Some applications of nanofrigerants and nanolubricants, mainly in air conditioning and heat pumps, were realized in [22]. The physical-thermal properties of suspended nanoparticles in refrigerants and lubricating oils of refrigeration systems were reviewed, classifying them into six topics: studies related to the Al\({}_{2}\)O\({}_{3}\), CuO, TiO\({}_{2}\), CNT, and Cu nanoparticles and studies related to other nanoparticles (\(R134a\)/polyester mixtures with SiO\({}_{2}\), CuO, POE and other \(R410a\) mixtures with NiFe\({}_{2}\)O\({}_{4}\)). The solubility of the mineral-based nanof refrigeration oil (MNRO) in different fluids, \(R134a\), \(R407C\), \(R410a\), and \(R425a\) was experimentally investigated. The conclusion of this work is that nanofrigerants have a much higher and stronger temperature dependence on thermal conductivity at very low particle concentrations than conventional refrigerants. Additionally, the effect of carbon nanotubes (CNTs) enhances heat transfer.