## 1 Introduction  
Nanofluids have excellent thermal conductivity [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11] and have been applied as the working fluids in traditional heat pipe applications to enhance thermal performance. \"In the presence of an external magnetic field, the thermal conductivity of magnetite nanofluids increases with the increase in the volumetric concentration of magnetic particles, which is associated with the increase in the magnetic field strength\" [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Currently, the efficiency of commercial solar PV solar panels is up to 15%, and most incoming solar radiation energy is either absorbed or reflected, and significant excess heat is dissipated and wasted. \"To improve the efficiency of solar PVs, the novel concept of a photovoltaic-thermal solar panel hybrid system using nanofluids has been developed [25; 26; 27; 28; 29; 30; 31]. Regarding the PV-thermal solar hybrid system\", references [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31] reported that \"the solar irradiance is converted into electrical energy in the PV's cell; the excess thermal energy generated due to the intrinsic conversion efficiency limitation of the cell is dissipated into the water-based nanofluids. This, in turn, reduces the cell temperature and enhances the conversion efficiency of the cell that increases the integrated photovoltaic-thermal efficiency of the hybrid system\". This subject has been covered extensively in the literature. Readers interested in the subject can consult references [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. \"The work of reference [11] presented an experimental study of heat transfer and flow of Fe\({}_{3}\)O\({}_{4}\)-distilled water at field concentrations. As reported in reference [11], all tests were carried out with a Reynolds number range of (2900). The results show that, as the nanofluid concentration and magnetic intensity increase, the Nusselt number increases. The maximum enhancements in the Nusselt number using magnetic nanofluid were 5.4%, 26.4%, and 42.7% for volume concentrations of 0.3%, 0.6%, and 0.9%, at 0.3 tesla, respectively, and 43.9%, 44.3%, and 46% at a volume concentrati4 on of 0.9%. The Nusselt number decreased with increases in the concentration and the magnetic intensity, and decreased with an increase in the Reynolds number\". [PERSON] [12] investigated the heat transfer coefficient of the heat pipe and its use in solar energy applications. \"His experimental research focused on the heat transfer performance of heat pipes by introducing magnetic fields and nanofluids. The experimental results showed that, without a magnetic field, the heat transfer rate of the heat pipe with magnetic fluid was 13.9% higher than that of a conventional heat pipe. Under a DC magnetic field, the heat transfer rate of the magnetic fluid heat pipe was improved by an average of 10.6%, and that of a water heat pipe was improved by 6%. The outcomes of his studies proved that the heat transfer performance can be enhanced by the use of a magnetic field and magnetic fluids\". \"Using a numerical investigation, reference [13] studied the effect of a magnetic field on the nanofluid flow inside a sinusoidal two-tube heat exchanger. The study focused on the influence of variable magnetic fields on the heat transfer of heat exchangers while the mixture was a single phase. In this heat exchanger, the inner tube was sinusoidal and the outer tube was considered to be smooth. The magnetic field was established orthogonal to the sinusoidal tube. The basis fluid was water with 4 vol.% nanoparticles (Fe\({}_{3}\)O\({}_{4}\)). Ferrofluid flowed in the internal tube (sinusoidal tube) as a hot fluid, and the air flowed in a counter-current in the external tube as the cold fluid. The finite volume method and the SIMPLE algorithm were used to calculate the pressure-velocity relationship. The numerical results were validated with experimentally measured data, and showed good agreement with the measurements.