# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 4 Conclusions  
Each paper reviewed in this article benefits the field through simulations or experiments, using nanofluids or nanomaterials. However, the results are usually reported through percentage estimates, without mentioning the actual costs. For future work, contributing to this field of knowledge may be of great value in solving current heat pump technology challenges outlined in this review paper. This will allow the widespread use of the systems on a larger scale and could lead to accessible applications of GSHPs in countries that are developing technology in this area. Therefore, further works should focus on finding the effect of the particle shape (temperature, size, aspect ratio, and weight concentration) on thermal conductivity of nanomaterials. Additionally, tests of different concentrations or various types of nanoparticles should be conducted. In addition, investigations of different shape configurations, such as U-shaped or double-shaped tubes and slinky configurations, among others, should be conducted. The structures of nanoparticles need to be investigated further. Models and correlations that consider all effective factors are required to mathematically evaluate the effects of key parameters on the thermal conductivity of pipes. More cost comparisons of real systems considering every aspect involving nanofluids or nanomaterials need to be carried out to make the field of heat pump usage more attractive for real engineering implementations. \begin{table}
\begin{tabular}{l l} \hline \hline
**Particle Material** & **Theoretical and Numerical** & **Experimental** \\ \hline Ag & [13,14,16,23,24,37] & [33] \\ Cu & [13,14,16,22,24,37] & - \\ Al & [13,23,24,37] & - \\ Graphite & [13] & [34] \\ Al\({}_{2}\)O\({}_{3}\) & [7,13–16,20,22–24] & [35,52] \\ CuO & [13,16,22–24] & [34] \\ Al\({}_{2}\)O\({}_{3}\)-water & [13,21,23,26] & - \\ CuO-water & [13,23,26] & - \\ MgO-water & [27] & - \\ SiO\({}_{2}\) & [13,14,16,23,24] & [34] \\ Au & [14,23] & - \\ TiO\({}_{2}\) & [14,16,22,23] & [34,35] \\ ZnO & [14,23] & - \\ Fe\({}_{2}\)O\({}_{3}\) & [14,16,23] & [34,35] \\ Fe\({}_{2}\)O\({}_{4}\) & [15] & - \\ MgO & [14] & [33] \\ Al\({}_{2}\)O\({}_{3}\)-EG & [20] & - \\ CuO-EG & [28] & - \\ CNTs & [22,37,44,45] & [34,53,61,62] \\ Al\({}_{2}\)O\({}_{3}\)-EG & [23] & [11] \\ ZnO-EG & [23] & - \\ MWCNT & - & [33,53,56,62] \\ DWCNT & - & [33] \\ SiC & - & [34] \\ Titanium & - & [36] \\ LTEG & [37] & - \\ HDPE-TE & [4,42] & - \\ HDPE-Al wires & [38] & - \\ HDPE-EG-SAL & - & [54] \\ HDPE-PA & - & [51] \\ HDPE-MA & - & [52] \\ HDPE-graphite & - & [58,63] \\ CNT-metallic & [44] & - \\ CNT-ceramic & [44] & - \\ GNPs & - & [48,51,53] \\ EG & - & [48] \\ GN & - & [56,57,59,62] \\ _CACO\({}_{3}\)_ & - & [60] \\ graphite-Ks & - & [63] \\ graphite-EG & - & [63] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of parameters and analysis of heat exchangers. Finally, the trends of the GSHP according to the current revision are summarized in the following points:  
* The most common theoretical GHE models consider the general Navier-Stokes equations. However, when considering more system components, such as GSHPs, researchers need to propose novel dynamic models based on thermal resistances to simplify the simulation of GHE systems. In this context, more research is needed to integrate control systems in numerical simulations with the aim of improving GHE efficiency and minimizing operating costs. * While most successful applications of nanoparticles are those involving CuO, Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), and Ag [4; 7; 13; 24], additional experiments are encouraged to compare their performances. Concerning nanomaterials, HDPE-TE seems to be the most successful for GHE applications. * Until now, most research studies have involved laboratory tests, while long-term experiments for everyday applications are required to validate the real benefits of new nanomaterials and nanoparticles in GHE applications. Likewise, the study of the durability of the new nanomaterials will help to validate the cost-efficiency ratio.