# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanofluids
#### 2.1.1 Theoretical and Numerical Simulations
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Authors** & **Nanofluids** & **Applications** & **Results and Remarks** & **Model or Method** \\ \hline \multirow{6}{*}{[PERSON] and [PERSON] [26]} & \multirow{6}{*}{AlgO3/water, CuO/water} & \multirow{6}{*}{Applications of the CuO water and \(Al_{2}\)O3\({}_{3}-\)water} & Numerical simulation using Fluent software, finite volume method, and the SIMPLEC algorithms. & \\  & & & & The CuO-water nanofluid gives higher extracted heat than the & \\  & & & fluids of a GHE & alumina-water nanofluid and it has higher coefficients of convection heat transfer & \\ \hline \multirow{6}{*}{[PERSON] et al. [27]} & \multirow{6}{*}{MgO-water} & Numerical solution to optimize MgO-water nanofluids to reduce the heat transfer coefficient. & The NSGA-II algorithm has been used to reduce the cost and increase the heat transfer coefficient. & Non-dominated sorting genetic algorithm II (NSGA-II) \\ \cline{1-1}  & & & The optimization has been able to reduce the costs up to 38\%. & \\ \hline \multirow{6}{*}{[PERSON] et al. [20]} & \multirow{6}{*}{AlgO3, AlgO3-EG} & Nanofluids as working fluids to extract more energy from reservoirs and improve the exploitation of geothermal resources. & The viscosity and specific heat capacity of nanofluids in the operation of geothermal wells are very important for geothermal applications, to improve the exploitation of reservoirs. & \\ \hline \multirow{6}{*}{[PERSON] et al. [28]} & \multirow{6}{*}{CuO-_EG_} & Heat transfer in rotating heat pipes. & The heat transfer depends on the nanofluid mass, nanoparticle size and the concentration. & Non-linear differential equations with a new methodology based on particle swarm optimization (PSO). \\ \hline \multirow{6}{*}{[PERSON] et al. [21]} & \multirow{6}{*}{AlgO3-water} & \multirow{6}{*}{Effects of the nanofluids to reduce the bore length of vertical GSHP.} & Using Al2O3/water nanofluid instead of water reduced 1.3\% the bore length of a vertical GHE. & \\  & & & However, the results of the application of nanofluids are not entirely satisfactory, since the grout had the greatest potential to decrease the length of the perforation. & \\ \hline \multirow{6}{*}{[PERSON] et al. [22]} & \multirow{6}{*}{Cu, Al2O3, CuO, TiO2, and CNTs} & Studies and applications on nanonrefriggerants and nanonubricants, mainly for air conditioning and heat pumps & The nanofrefriggerants reduced the energy consumption. They have a higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional refrigerants & Heat transfer models through thermal and rheological properties \\ \cline{1-1}  & & & Geothermal energy has various applications, such as district heating networks using piped hot water to heat many buildings in entire communities. More than 72 countries have reported the direct use of geothermal energy & \\ \hline \multirow{6}{*}{[PERSON] et al. [23]} & \multirow{6}{*}{AlgO3/water, CuO/water, Al2O3, CuO, TiO2, ZnO, Fe2} & Some experimental methods and theoretical studies in the preparation of nanofluids for thermal conductivity & The thermal performance of the nanofluid heat pipe was superior to that of the conventional working fluid, mainly water. & Heat transfer models \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & & Numerical simulation of the unsteady flow of a non-Newtonian Casson nanofluid to investigate the slip condition and solar thermal transport in terms of convection & Boundary layer equations \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of theoretical and numerical studies on heat exchanger pipes using nanofluids (Continuation).