In photovoltaic (PV) solar systems, electricity can be produced from part of the sunlight and the remaining energy raises the temperature. The aim of the current investigation is to suggest a novel design for the heat transfer tube's (HTT) cross section for overcoming this challenge in photovoltaic/thermal (PVT) systems. In this numerical simulation, the range of fluid inlet velocity (Vin) was set between 0.01 and 0.18 m/s. The value of ambient temperature (Tamb) and fluid inlet temperature (Tin) is 30 °C, and solar radiation (G) has been adjusted to 800 W/m2. A comparison was made between the circular tube and the 8-lobed tube with the same hydraulic diameter, and the better performance of the 8-lobed tube was proved. To meet the increased efficiency of the PVT system, copper fins with four different arrangements were employed. The highest efficiency occurred when the fins were placed all around the inside of the HTT. To create heat transfer fluid (HTF), non-toxic graphene nanoplatelets (GNP) have been mixed with water involving two weight fractions (0.025 % and 0.1 %). Examining the effect of changing the fluid inlet velocity (Vin), it was concluded that the system's electrical performance is increased by 5.8 %, with rise of Vin, in the best case. By using the GNP nanoplatelets (0.1 %), at Re = 1611, the maximum exergy, electrical, and thermal performances of the unit were 15.32 %, 14.05 %, and 55.22 %, respectively, in the best case. In the best case, more carbon dioxide (about 7.1 Tons) can be reduced and the carbon credit is 5.5 % higher than that of base case. In addition, the payback period is less than 2 years in the best case with 18700$ profit in the 10th year.
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