# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 3 Application of 2D Nanofluids
### Geothermal Heat Recovery
Although the best overall performance was given by graphite nanofluids, the 2D nanofluids used also exhibited an improved performance. The main conclusions were as follows:  
* The temperature of the working fluid at the outlet increased in heating mode, while the required pipe length decreased when using the nanofluids compared with the water alone. * The outlet temperature difference was reduced with the flow rate enhancement of the operating nanofluids. * The effect of the increase in the GHE efficiency using the nanofluid decreased with the increasing flow rate of the operating fluid and the decrease in the difference between the soil and inlet fluid temperature values. The efficiency of the system was improved with the increases in the pipe and borehole radii and total depth. * The reduction in the fluid temperature at the outlet was obtained with MWCNTs and Gr nanoparticles of approximately 65% with an increase in the flow rate of 0.4 L/s. * The reduction in the pipe length was obtained with MWCNTs of approximately 32.9% with an increase of 0.4 L/s in the flow rate. The homologous value for the Gr nanoparticles was approximately 37.1% with the same enhancement in the flow rate. * The maximum outlet fluid temperature difference and GHE length reduction were 11.4% and 53.4%, respectively, using MWCNTs and when the pipe diameter was changed from 20 to 50 mm. The homologous values for the Gr nanoparticles were 12.06% and 50.2% with the same pipe diameter enhancement. * An augmentation of 14% in the outlet temperature and a reduction of approximately 20% in the pipe length were achieved when the borehole size was increased from 70 to 110 mm for the MWCNTs. The homologous values for the Gr nanoparticles were 18.9% and approximately 20% with the same increase in the borehole size. * The maximum increase in the fluid temperature at the outlet and pipe length reduction were obtained with the MWCNTs and Gr nanoparticles at approximately 68% and 61.7%, respectively, with the increase in the difference between the temperature of the inlet and that of the soil (16 \({}^{\circ}\)C) from 7 \({}^{\circ}\)C to 15 \({}^{\circ}\)C.  
More numerical simulations in both laminar and turbulent regimes of 2D nanofluids flowing in geothermal heat exchangers are required in the future. Further studies on the innovative use of 2D nanofluids and hybrid formulations in modern geothermal heat exchangers, such as the finned conical helical type, should be addressed. Additionally, efforts should be made to optimize the use of 2D nanofluids with appropriate flow rates and specifications. Considering the flow rate, some geothermal energy extraction systems working with nanofluids have demonstrated that an increased flow rate provoked a returning flow in a geothermal heat exchanger with a lower temperature and an enhanced ground heat extraction. Special emphasis should be attributed to the sedimentation of the nanoparticles in the borehole heat exchangers, since it can be verified in the published literature works that after many hours of continuous operation, the accumulation of the nanostructures completely blocked the bottom of the borehole in cases where the nanofluids circulated with relatively low velocities. Numerical simulations on the subject are also recommended.