# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 3 Application of 2D Nanofluids
### Geothermal Heat Recovery  
Two-dimensional nanofluids are suitable for use in heat exchangers in geothermal systems. They possess great potential for use in geothermal borehole heat exchangers or ground heat exchangers as circuit heat transfer fluids. In recent decades, the need for energy consumption is increasing continuously. A major part of the energy requirement is satisfied through the use of traditional resources of energy, such as gas and coal, resulting in elevated resource spending rates. Nearly one quarter of the total energy is applied in responding to worldwide heating/cooling needs of the large-scale industrial and commercial sectors by conventional equipment, which requires a large amount of improved energy, mainly originating from power plants working with fossil fuels, releasing large number of discharges into the environment. The ever-increasing concern of pollution has persuaded researchers to shift their focus to the design and implementation of eco-friendly energy systems with minimized greenhouse gasses emissions to satisfy heating/cooling global demands by employing a smaller electrical energy input quantity. Consequently, there is an urgent need to substitute the polluting energy resources with renewable energy resources, and the sustainable energy ground source heat pump system (GSHPS) is an alternative solution. The GSHPS is an environmentally benevolent system capable of (partially) meeting universal heating/cooling requirements. The system has gained popularity in house heating and cooling, since these applications directly contact the ground energy resource that remainsalmost constant in the in-depth temperature profile beyond 15 meters of depth. The performance of the GSHPS depends on a heat interaction within the ground by a ground heat exchanger (GHE), also known as a geothermal heat exchanger. The heat transfer efficiency of the GSHPS is higher because of the heat rejection and the extraction is carried out under an almost constant ground temperature. Traditional energy systems are conceived to work under continuously changing ambient temperatures to fulfill space conditioning loads. The vertical borehole GHE is commonly employed in GSHPS. The elevated installation cost of the GSHPS compared with conventional air conditioning systems hinders their global implementation. A decrease in the depth of the borehole with negligible alterations in the output of the GHE is required for the economic viability of the system. As a result, many researchers have searched for the most adequate mode to improve the efficiency of the GHE. Faizal et al. [84] found that an enhanced thermal conductive filler and material can also be mixed with concrete for the heat transfer to reach considerable amelioration. Only small improvements were observed in the efficiency of the GHE with different methods. This may be justified as the operation of the GHE was within a small temperature difference between the working thermal fluid and the underground soil temperature values. Nanofluids may be considered as suitable working fluids to enhance the heat transfer performance of a GHE for a better use of the ground available energy. The mathematical modeling developed by Tarodiya et al. [158] was applied to different nanoparticles, including MWCNTs and Gr, with varying concentrations dispersed in water to study the influence of the nanofluids in the ground source heat pump system. The most relevant findings of the modeling and numeric simulation were the increase in the outlet temperature difference of the heat transfer fluid, and the decreases in the length of the ground heat exchanger, pressure drop of the system, and vertical temperature profile in a single U-tube vertical closed-loop ground heat exchanger. The impact of the concentration of the nanoparticles, imposed flow rate, and temperature gradient between the soil and the inlet operating fluid, pipe and borehole radius, and overall depth on the ground heat exchanger efficiency was also determined.