## 4 Application of Nanofluid in PTC  
By increasing use of nanofluids, many investigators have been interested in the application of nanofluids in solar collectors. There are many researchers that conducted reviews on the application of nanofluids in solar collectors. [PERSON] et al. [34] conducted a review on the progressive application of nanofluid in solar collectors, in which thermophysical properties of nanofluids and performance evaluation of different types of solar collectors using nanofluid were discussed. [PERSON] et al. [35] also summarized the application of nanofluids in different solar collector systems. More specifically, the thermophysical properties, different types of nanofluids, and the history of the solar energy and its diverse applications were discussed. [PERSON] et al. [36] reviewed different methods used to enhance heat transfer in PTCs, specifically the use of nanofluids in heat transfer enhancement. [PERSON] et al. [37; 38; 39] provided a review on nanofluids application in solar collectors, mostly concerning their applications on thermal efficiency enhancement. However, there is no comprehensive literature review on nanofluids application in PTCs. There are many ways by which the studies concerning the application of nanofluids in PTC can be classified:  
The first classification can be done on types of nanoparticles used in studies. The nanoparticle can be categorized into two main sections ([PERSON] et al. [39]):  
1. Metallic nanoparticles such as Fe, Cu, Zn, Al, Au. 2. Non-metallic nanoparticles such as SiO\({}_{2}\), TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), ZnO, CeO\({}_{2}\), multiwall carbon nanotubes (MWCNT), carbon nanotubes (CNT)  
As it can be seen from Figure 4, Al\({}_{2}\)O\({}_{3}\) is the most applicable nanoparticle among researchers since 41% of studies use it in the studies, followed by CuO, TiO\({}_{2}\), etc. Both metallic and non-metallic nanoparticles can be dispersed into the base fluid at a different volumetric concentration ranging from 0.01-6% [37; 40; 41] or weight percentage ranging from 0.1-20 wt % [42]. Additionally, the other way to study nanofluids is to talk about different types of base fluids in which the nanoparticles are dispersed in. There are mostly four significant types of base fluid used so far:  
1. Water
2. Oils (such as Synthetic oils, thermal oils, Therminol-VP-1, Dowtherm-A, etc.) 3. Ethylene glycol (EG)
4. Molten salts (such as 60% NaNO\({}_{3}\)-40% KNO\({}_{3}\))  
Another way to categorize the studies is to classify them by type of studies:  
1. Experimental studies
2. Numerical investigations  
Nanoparticles used in the thermal enhancement of PTC have different diameter sizes which range from 4-100 nm [43; 44]. As stated earlier, thermophysical properties of nanofluids are of high importance. There are numerous studies trying to review the equations stating these parameters in terms of temperature, nanoparticle size, concentration, etc. A comprehensive review of these thermophysical properties is given in [34; 35]. The main reason encouraging investigators to use nanofluids (instead of base fluids) are their enhanced thermal properties. Thermal efficiency and convective heat transfer are two main factors that are required to be enhanced to give a better performance in PTC. Also, nanofluids have a higher thermal conductivity, density, and viscosity while lower specific heat capacity is comparing to the base fluid. It should be noted that higher viscosity is actually a drawback in the nanofluid application as it increases friction factor and hence causes pressure drop along the receiver tube of the PTC which finally increases the pumping demand of the nanofluid in PTC [39]. Thermal enhancement of PTCs has been in the canon of interest recently. The enhancement can be done by two methods: geometrical modification and application of nanofluids instead of common base fluids in PTC [39]. Figure 4: Share of different nanoparticles on the studies concerning PTC enhancement using nanofluids.