# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanofluids
#### 2.1.1 Theoretical and Numerical Simulations
In [7] is presented an exhaustive review of theoretical and experimental works related to the use of nanotechnology in renewable energy systems (solar, hydrogen, wind, biomass, geothermal and tidal energies). This review includes works for all renewable energies, with the use of nanofluids, nanomaterials, and nanoparticles. In the case of geothermal energy, different applications with nanotechnology were addressed, with two of them being highlighted. The first to be addressed was the use of nanofluids to cool fluids inside pipes exposed to high temperatures, using nanofluids to cool sensors and electronic devices in drilling machines and GHE depending on temperature (low, medium or high), and district heating applications using networks piped hot water to heat many buildings in entire communities. Second, the authors noted that nanofluids can be used as a working fluid to extract energy from the ground and process it in a power plant system or produce large amounts of working energy. Similarly, the paper [23] summarizes works with the use of suspended nanoparticles to enhance the heat transfer characteristics into the heat pipes. The researchers found new opportunities with the use of nanofluids, e.g., to improve the thermal efficiency and reduce the thermal resistance of the heat pipe. This work summaries experimental and theoretical studies of some of the preparation methods, processes and heat transfer characteristics of nanofluids. The thermal performance of the nanofluid heat pipe was superior to that of the conventional working fluid, usually water. A preliminary evaluation of the potential of nanofluids that guarantee the vertical temperature of the heat carrier in the borehole was performed in [24], using the mathematical model in [25]. Some assumptions were imposed on the model, for example, the variations of temperature with depth, the heat conduction in the vertical direction within the pipe wall, the grout, the borehole thermal resistance between the pipe and the borehole wall, and the ground. The objective was to know which nanoparticle leads to the best performance in the borehole heat exchanger, showing the properties and comparative cost of various nanoparticles. The best thermal performance was found with Cu, followed by graphite, SiO\({}_{2}\), and Ag. On the other hand, Al\({}_{2}\)O\({}_{3}\) and CuO were the worst choices. The mixes tested using CuO, Al\({}_{2}\)O\({}_{3}\), Ag, SiO\({}_{2}\), Al, graphite and Cu with a volumetric concentration of 1% allowed reductions in the borehole length to 0.14, 0.19, 0.33, 0.36, 0.36, 0.52 and 2.33%, respectively. Therefore, the cost of GHE increases using nanofluids depending on the shape and size of the particles, plus the energy consumption of the circulation pump due to the increased pressure drop. However, this cost is marginal compared to the costs of nanoparticles due to the low value of the mass flow rate. The results of numerical simulations for the application of the CuO-water and Al\({}_{2}\)O\({}_{3}\)-water nanofluids, as the working fluids of a geothermal GHE, were reported in [26]. The simulations were compared with literature data. The Reynolds Averaged Navier-Stokes (RANS) equations with the shear stress transport (SST) k-\(\omega\) turbulence model were numerically solved to represent the flow and the physical properties of the nanofluids using the available correlations. Fluent software and the SIMPLEC algorithms were used for the coupling between pressure and velocity, and the finite volume method on collocated cells was applied to discretize the RANS equations. In the same research, some studies of the influences of natural factors in the analytical method, such as groundwater flow on heat pump design, was presented to obtain the temperature distribution along the heat exchanger. Additionally, the modeling and optimization of a novel combined cooling, heating, and power (CCHP) cycle driven by geothermal and solar energies using the CuO-water nanofluid are presented.