## 4 Application of Nanofluid in PTC
### Experimental Studies  
Among different studies about enhancing PTC efficiency by nanofluids, 30% of research studies have been carried out experimentally. The effects of different nanofluids with different nanomaterials percentage have been investigated to find the efficient one. It was reported by most of them that the application of nanofluid in PTC leads to higher thermal efficiency comparing to the base fluid. In this regards, [PERSON] et al. [41] set up an experimental setting and conducted a study about the effect of two different nanofluids of Al\({}_{2}\)O\({}_{3}\)/water and CuO/water at 0.01 and 0.05 vol. % on PTC thermal performance and compared the results with water as working fluid. The results indicated that CuO/water nanofluid could cause the highest thermal enhancement; followed by Al\({}_{2}\)O\({}_{3}\)/water nanofluid and water, itself. More specifically, they found that using 0.05 vol. % of CuO and Al\({}_{2}\)O\({}_{3}\) have the maximum overall thermal efficiency of 15.25% at a maximum flow rate. Whereas this value for Al\({}_{2}\)O\({}_{3}\)/ water nanofluid and water itself was 12.39% and 10.58%, respectively. This means using CuO/water nanofluid led to approximately 44% increase in efficiency, compared with water. Another experimental study carried out on the effect of MWCNT/mineral oil (with 0.2 and 0.3 wt %) on the performance of PTC by [PERSON] et al. [45]. Authors claimed that using 0.3% wt % of MWCNT enhances the thermal efficiency of the PTC between 5-7%. [PERSON] et al. [46] examined Al\({}_{2}\)O\({}_{3}\)/water nanofluid to investigate its effect on the thermal efficiency of PTC. They used up to 0.06 vol. % of Al\({}_{2}\)O\({}_{3}\). They reported a maximum efficiency enhancement due to the nanofluid application of 28.95%. [PERSON] et al. [40] studied two nanofluids, CuO/water and CuO/ethylene glycol to experimentally investigate their effect on PTC performance at different volumetric concentrations (0.01, 0.05, and 0.1 vol. %) and flow rates. They found that the maximum thermal efficiency can be attained by application of different nanofluids are 11.01%, 8.03%, 7.46%, and 6.04% for CuO/water, CuO/ ethylene glycol, water, and ethylene glycol at 160 L/h. This means that there was an approximately 82% thermal efficiency increase when using CuO/water nanofluid comparing with ethylene glycol. An experimental investigation was conducted by [PERSON] et al. [47] on the effect of 1% volumetric concentration of Al\({}_{2}\)O\({}_{3}\) in Al\({}_{2}\)O\({}_{3}\)/water nanofluid in the thermal characteristic enhancement of PTC. The results showed that the application of nanofluid could enhance the heat transfer coefficient and thermal efficiency by 32% and 7%, respectively, comparing to the base fluid (water). They also reported an approximately maximum increase in Nusselt number of 32%. An interesting experimental study was done by [PERSON] et al. [42], where they examined many different nanoparticles (Fe\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), TiO\({}_{2}\), ZnO, Al\({}_{2}\)O\({}_{3}\), and Au) with different weight fraction ranging from 0.1-35% with water as the base fluid. They showed that using these different nanofluids had almost no effect on the thermal efficiency of the PTC as well as no significant change in pressure drop. The only relatively significant enhancement was observed when the convection heat transfer increased by approximately 50% when Au/water nanofluid with 0.01% volumetric concentration of Au was applied with increasing the temperature (from 40-80 \({}^{\circ}\)C). An experimental and numerical study was conducted to study the effect of CuO/water nanofluid on thermal efficiency and temperature distribution in direct absorption parabolic trough collectors for a different range of volume fractions by [PERSON] et al. [48]. The numerical and experimental results were in good agreement with each other. The results were shown that the thermal efficiency of the collector increases by 18% to 52%, when using 0.002-0.008 vol. % CuO nanoparticles, compared to the base fluid case.