ABSTRACT In order to increase module efficiency in photovoltaic systems, research on passive fin cooling or active fluid cooling applications continues. It was thought that a collective cooling, which is a combination of both applications, could be more effective on the increasing electrical efficiency and so the increase of the total efficiency of the PV/T system by increasing the thermal gain. In this study, the effects of the amounts of nanoparticles on the electrical and thermal performance of the Al2O3 nanofluid in a newly designed, manufactured system which is named collective cooling system with internal direct fins were investigated in detail. Nanofluid coolants were prepared with the use of Al2O3 nanoparticles at a mass ratio of 0.2%, 0.4% and 0.6% and they were used in the system at constant mass flow rates. The highest panel temperature drop was observed as 17.3 °C and corresponding power increase rate was observed as 6.11% in the panel with 0.4% Al2O3-water nanofluid at 12:00. The averages of daily power increase rates for 0.2%, 0.4%, 0.6% of Al2O3-water nanofluid cooling compared to the uncooled panel were 3.862%, 3.286%, 3.238% and 2.693% for the water-cooled panel, respectively. Compared to the water-cooled panel, the Al2O3-water nanofluid panel has average thermal efficiency increases of 39.77%, 28.92% and 15.92%, respectively, for 0.2%, 0.4%, and 0.6%.