# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 3 Application of 2D Nanofluids
### PV/T Systems  
Two-dimensional nanofluids can be included in the novel class of heat transfer fluids in the most recent PV/T systems, owing to their extremely high TC and heat transfer rates. It has also been reported that PV/T systems using 2D nanofluids, such as Gr, CNTs, or hybrid formulations, in conjunction with the use of phase change materials, can improve the overall efficiency of existing PV/T systems by 20% or more compared with the same systems operating with traditional heat transfer fluids. The uniform dispersion of the 2D nanostructures in the base fluids is vital for achieving an enhanced solar radiation absorption and to achieve a very high TC. In addition, the hybrid 2D nanofluids employing, for instance, Gr and its derivatives and CNTs, are great contributors to the high thermal and electrical energy output by effectively controlling the solar radiation that is transmitted to the PV module. Of all the two-dimensional nanomaterials, the Gr is very suited to play the role of transparent conducting electrode and Schottky barrier junction layer for PV cells because it is lightweight, ultrathin, and flexible. Two-dimensional nanomaterials are progressing and emerging as PV cells and on-chip integrated configurations of energy storage systems and equipment with energy harvesting purposes. Moreover, 2D nanomaterials can respond to the needs of novel PV systems, entailing less expenditure and high efficiency rates. These nanomaterials are welcome in synthesis procedures of nanofluids to dissipate enhanced heat charges from the solar panel and in evoking more attention from the scientific community than other types of nanoparticles. For this reason, some researchers are studying different Gr hybrid nanofluids, prepared using different methods dispersed into different fluids such as oil, ethylene glycol, and water. Figure 13 schematically illustrates the typical PV/T configurations of PV/T systems operating with nanofluids as coolants. Aravind and Ramaprabhu [134] manufactured Gr-wrapped MWCNTs dispersed in water nanofluids using a solution-free, eco-friendly method. The obtained results revealed an enhancement of 10.5% in the TC for the Gr-MWCNT, which was higher than the TC increase of only 9.2% when using the 0.04% vol. Gr nanofluid. The TC increase was explained based on the production of strongly bonded clusters that determine the interfacial thermal resistance because of the ameliorated heat transfer capability between them. Yarmand et al. [135] prepared Gr nanoplatelet-silver hybrid nanofluids using a facile chemical procedure. The GO was synthesized from graphite flakes using the Hummer method and was then chemically reduced. The fundamental steps of the GO preparation method are presented in the diagram of Figure 14. The results indicated a 32.7% increase in the Nusselt number and a 1.08-fold friction factor enhancement with a pumping penalty  
Figure 13: Configurations of typical PV/T systems with nanofluids: (**a**) as a coolant; (**b**) as a coolant and spectral filter with double-pass channel; (**c**) as a coolant and spectral filter with separate channels. compared with the water itself for a concentration of 0.1% wt. and a Reynolds number of 17,500. Zubir et al. [136] prepared rGO and produced hybrid complexes with other carbon sources, including MWCNTs, carbon nanofibers, and Gr nanoplatelet aqueous nanofluids to achieve an improved heat transfer capability. The obtained results exposed a heat transfer increase when employing the hybrid nanomaterials in comparison to the rGO alone. The results also showed an increase of 63% in the HTC and a 144% increase in the Nusselt number. In the study performed by Wahab et al. [137], an effective approach to use active and passive cooling mediums to decrease the temperature of the PV surface was followed. This reduction is relevant given that the rise in the ambient temperature has a negative impact on the heat transfer behavior of the PV cells.