Thermo-Hydraulic Analysis of Flat-Plate Solar Collectors (FPSCs) Using Fe3O4-Water Ferrofluid in the Presence and Absence of an External Magnetic Field
Authors:   , Shafiq Mehak 
Published: April 2024

Traditional heat transfer fluids, because of their low thermal conductivity, are the primary reason for flat-plate solar collectors' low thermal performance. Enhancing collector performance is feasible by replacing these fluids with nanofluids possessing better thermophysical properties. Despite substantial research on improving thermal efficiency using various nanofluids, the potential of magnetic nanofluids or ferrofluids which are responsive to external magnetic fields remains largely unexplored. Furthermore, there is a notable gap in understanding the hydraulic and economic aspects of ferrofluid-based flat-plate solar collectors (FPSCs). This research presents a comprehensive numerical investigation of the thermal and hydraulic performance of a FPSC utilising Fe3O4-water ferrofluid, both with and without the influence of an external magnetic field. It also compares the performance of a ferrofluid-based FPSC to that of a traditional water-based FPSC. Considering various parameters such as inlet temperature (293–333 K), mass flow rate (0.0027–0.05 kg/s), nanoparticle volume fraction (0.1–2%), magnetic flux density (0.02–0.1 T), solar radiation (100–1000 W/m2), ambient temperature (278–313 K), and wind speed (1–10 m/s), a combined approach using MATLAB and computational fluid dynamics (CFD) in ANSYS® Fluent was applied to investigate the heat losses, convection heat transfer, energy and exergy efficiencies, entropy generation, friction factor, pressure drop, and needed pumping power of the FPSC. An economic analysis was also conducted to evaluate the costs and benefits of the ferrofluid-based FPSC in the absence and presence of an external magnetic field. Without a magnetic field, employing a 2% volume fraction of Fe3O4 led to significant improvements in convection heat transfer coefficient, energy, and exergy efficiencies. However, this improvement was accompanied by increased friction factor and pressure drop. Through the performance index criterion, the optimum volume fraction of Fe3O4 was found to be 0.5%, resulting in a 12.90% increase in convection heat transfer coefficient, and a maximum improvement of 2.84% and 4.60% in energy and exergy efficiency, respectively, compared to a water-based collector. This came with a 3.60% increase in friction factor and a 1.98% increase in the pressure drop, raising the pumping power requirement by only 0.39%. Introducing a non-uniform transverse magnetic field further enhanced performance, with a 2.50%, 1.28%, and 1.74% improvement in convection heat transfer coefficient, energy, and exergy efficiency, respectively, using the optimum 0.5% volume fraction of Fe3O4. Compared to a water-based collector, employing 0.5% Fe3O4 led to a maximum 0.21% reduction in entropy generation, both in the absence and presence of a magnetic field. The application of the magnetic field to the ferrofluid-based FPSC improved the thermal performance mainly at low flow rates, low temperatures, and low volume fractions (of less than or equal to 0.5% Fe3O4). High volume fractions above 0.5% Fe3O4 in the presence of magnetic field at a very low flow rate of 0.0027 kg/s deteriorated the collector’s thermal performance. Additionally, the positive effects of the magnetic field diminished with increased mass flow rate and fluid inlet temperature. Economically, while Fe3O4-water ferrofluid can enhance FPSC’s performance, the total costs associated with ferrofluid and a magnetic field outweigh the benefits, rendering it economically unviable, particularly at volume fractions exceeding 0.5%. This economic consideration highlights the challenges of implementing ferrofluids in FPSCs on a large scale; emphasising the need for further research to optimise the cost-effectiveness ferrofluid-based FPSCs for practical applications.