Abstract:
Direct absorption collectors (DASC) with nanofluids demonstrate superior thermal performance
relative to conventional flat plate collectors. The volumetric absorption of heat by both the nanoparticles
and the liquid in DASC becomes more efficient in case the extinction coefficient of the nanofluid is optimal.
When the metal nanoparticles are used, the collector gains extra performance due to enhanced thermal
conductivity and an excess heating originating from surface plasmon resonance. Even more, the magnetic
nanoparticles may increase the collector efficiency, converting under the influence of an external magnetic
field. In this contribution, we foremost demonstrate experimentally that a lab-scale DASC with an aqueous
iron oxide nanofluid (184 nm) is up to 25% more efficient than the same collector with a carbon black
nanofluid (51 nm) at equivalent particle concentration. We further develop a multiphase CFD-model for
the solar heating of a magnetic nanofluid with 8nm MnZn ferrite particles. Enhancing the most standard
approach to model nanofluids as single-phase liquids with altered thermal properties, we extended the
two-fluid Eulerian-Eulerian method to account for such details of the process as: Brownian dispersion and
sedimentation of nanoparticles, inter-phase slip, in-phase volumetric absorption of thermal radiation and
magnetic forces, acting on nanoparticles. The model was validated against two independent experimental
benchmarks, demonstrating discrepancies below 10%. As it followed from our simulations, the magnetic
convection increases DASC efficiency up to 30% in a moderate magnetic field of 113 mT.
