# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanofluids
#### 2.1.2 Experimental Work  
An experimental study was presented in [11] using Al\({}_{2}\)O\({}_{3}\)/ethylene glycol-water (EGwater) nanofluids applied to a spiral and U-type GHEs. The GSHP system was used to heat a 21 m\({}^{2}\) room located in Sivas Cumhuriyet University Campus, Turkey, where normally the weather is cold. The authors addressed the improvement of thermal conductivity only in Al\({}_{2}\)O\({}_{3}\) nanofluids, but these studies do not provide test results related to the use of nanofluids in the specific system of GHEs or GSHPs. Other studies show that nanofluids have better heat transfer properties than base fluids under constant heat flow. Their effects have been investigated in many areas, such as radiators, heat exchangers and electronic devices. Researchers have described the experimental setup, including the heat load calculation, the trench depth and GHE design based on the ASHRAE book, using a 25% EG ratio of the fluid to prevent freezing. The experiments showed that a concentration of 0.1% nanofluid in the U-type heat exchanger increased the performance by 19% compared to glycol-water and by 17.7% compared to the nanofluid with a concentration of 0.2%. The nanofluid with a concentration of 0.1% increased the heat transfer rate by up to 2% in a U-type GHE and 3.2% in a spiral GHE according to the unit pipe length heat exchange rates. The thermal performances and economic efficiencies of different nanofluids (Ag, MgO, \(MWCNT\) and \(DWCNT\)), comparing the price, the efficiencies and the benefices, to improve the heat transfer are addressed in [33]. The experiments compared the relative thermal conductivity and the relative viscosity of \(MWCNT\)-water, \(DWCNT\)-water, Ag-water and MgO-water. The \(MWCNT\)-water nanofluid showed higher relative thermal conductivity (0.2 vol.%) and relative viscosity than the other nanofluids. Some factors influenced the thermal conductivity of nanofluids, e.g., the method of nanofluid production (single and two stage), the type of surfactant, sonication rate, the device of characteristic measurement, nanoparticle size and shape. The thermal efficiency of carbon nanofluids is slightly better than that of oxide nanofluids, although their price is several times higher. For this reason, the authors state that the use of nanofluids for heat transfer is not profitable and can only be used in high-tech and high-profit industries. The existence of two different methods (one- and two-step methods) useful for the preparation of nanofluids is explained in [34]. The authors noted that thermal conductivity and viscosity are important parameters to study the potential for heat transfer enhancement. They presented experimental and theoretical studies affecting thermal conductivity, for example, particle type, charge, size, and shape, including environmental parameters such as the base fluid, concentration, temperature, and dwell time. In summary, the material type has a great effect on the thermal conductivity of nanofluids such as graphene, \(CNTs\), Au, Ag, etc., which are more conductive than the TiO\({}_{2}\), \(SiC\), and SiO\({}_{2}\) nanofluids. However, it appears that the type of material has little effect on the viscosity of nanofluids. Most of the results reveal that the viscosity and thermal conductivity increase as the particle load increases [34]. [PERSON] [35] patented a nanofluid to increase heat transfer, introducing nanoparticles into the GHE (propylene glycol or a heat transfer oil). The inventor improved the thermal conductivity by 40% or more, achieved with only a 0.10% nanoparticle concentration. These nanofluids are quite different from conventional two-phase flow mixtures because it is recommended to use a new parameter for nanofluids, the so-called Mouromtseff number (Mo), which is a function of the viscosity (\(\mu\)), the conductivity thermal (\(\kappa\)), density (\(\rho\)) and specific heat (Cp). The improved heat transfer reduced the installation cost because the circulation loop can be smaller and the pumping cost is also less expensive.