**4.2. Concentration**

Researchers have found contradictory results when it comes to concentration enhancement of nanofluids. Manikandan and Rajan [39] harnessed sand for the cooling of PV/T system in order to enhance the efficiency. They tested 0.5, 1 and 2 vol% concentration and the collection efficiency ratio for these concentrations was found to be 3.6%, 11.2% and 26.9% whereas the solar collection efficiency increased by 9% and 16.5% for 0.5% and 2% respectively. Sardarabadi and Fard [40] also examined that increasing the mass fraction of nanoparticles from 0.05 to 10 wt%, the thermal performance of the system increased by four times. Wei An. [24] examined the effect of nanofluid concentration in spectral splitting filter based PV/T system. They observed that increasing the concentration of the nanofluid increased the nanofluid temperature and system's electrical efficiency, but the thermal efficiency gets decreased in this way.

The maximum overall efficiency of the system was found to be 75.93% and 80.58% when the ferrofluid concentration was increased from 1 wt% to 3 wt% respectively [37]. Khanjari et al. [41] observed that increasing volumetric concentration of the nanoparticle (from 1–5%) increased the heat transfer coefficient and thus the overall efficiency (from 1.33% to 11.54% for silver and 0.72% to 4.26% for alumina). Radwan et al. [20] observed efficiency escalation with increasing concentration. But some researchers witnessed contradictory results. Karami and Rahimi [34] examined that increasing concentration of nanoparticles reduces the efficiency because of agglomeration or clustering of the suspended particles. Abd-Allah, [42] found best results at 0.1 wt% amongst (0.01, 0.1, 0.5 wt%).

Cieslinski et al. [43] found no impact of nanoparticle concentration on the performance of the PV/T system. They observed that 1 wt% of *A l* <sup>2</sup> *O*<sup>3</sup> /*water* rather decreased the thermal efficiency compared to the distilled water and 3 wt% and 3 wt% did not change the thermal efficiency as compared to the distilled water thermal efficiency. Whereas, the overall efficiency of the system reached up to 80%.

In order to obtain best results, there is always a need to determine the optimum concentration of nanoparticles in base fluid instead of using high volume fraction of nanofluid [43, 44]. However, instead of increasing the concentration of the same kind of nanoparticle, blending a different kind of nanoparticles can help improve the efficiency of PV module in a more efficient way [45].
