In this paper, the effects of ferrofluids as a coolant on the overall efficiency of a PVT (photovoltaic thermal unit) system are experimentally investigated. The fluids considered in the experiment are distilled water and a ferrofluid (Fe3O4-water) with 1% and 3% concentrations by weight (wt%). The experiments were performed in indoor conditions under two constant solar radiations (1100 W/m(2) and 600 W/m(2)) using a solar simulator. Due to the unique characteristic behavior of ferrofluids, their theological and thermophysical properties can be changed under an external magnetic field. The ferrofluids in this study were placed under constant and alternating magnetic fields in the cooling section in order to investigate the effect of both types of magnetic fields on the overall efficiency of a PVT system. The results show that by using a 3 wt% ferrofluid, the overall efficiency of the system improved by 45% and when an alternating magnetic field with 50 Hz frequency was applied, the overall efficiency increased to about 50% compared to that of the distilled water as coolant fluid. The overall exergy output of the system with and without ferrofluids, was also compared with that of the PV system with no collector. It was observed that by adding a thermal collector to a PV system and using a 3 wt% ferrofluid under an alternating magnetic field, the total exergy can be increased as high as 48W. (C) 2015 Elsevier Ltd. All rights reserved.