Experimental study of the effect of magnesium oxide hybrid nanofluid and multi-walled carbon nanotubes on increasing the solar radiation absorption efficiency

The solar radiation absorption efficiency is one of the challenges for engineers in solar energy systems. Using the hybrid nanofluids, which are obtained by dispersing two or more types of nanoparticles in the base fluid, can be effective in better absorption of solar energy. In this study, the effect of type, concentration, and mixing percentage of nanoparticles on optical properties and solar thermal utilization efficiency has been tested. First, magnesium oxide hybrid nanofluids and multi-walled carbon nanotubes with water-based fluid were fabricated by a two-step method in volumetric concentrations of 0.01%Vol, 0.02% Vol, and 0.04%Vol, and then the optical properties of individually and hybrid nanofluids were measured by spectrophotometer and have been compared by existing relationships. Then, to obtain the solar radiation absorption efficiency, a solar simulator has been used. The results show that increasing the concentration of nanofluid, improves the optical properties and solar radiation absorption efficiency. Magnesium oxide nanofluid, multi-walled carbon nanotubes and hybrid nanofluid have the values of extinction coefficient about 38, 40 and 50 times higher than pure water, respectively. The hybrid nanofluid absorbs more than 90% of the sun's energy at the highest concentration (0.04% Vol) and at a penetration depth of 0.3 cm. Solar energy absorption increases by increasing the concentration, hybridizing the nanofluid, and increasing the penetration distance.