Application of nanoparticles for solar-driven steam generation

Abstract

Nanoparticles suspended in the base fluid intensify the light-absorbing properties of the medium within wavelength range from 0,2 to 2,5 μm significantly. Also, they can serve as vapor bubble nucleation sites when the base fluid is boiling. Such suspensions are widely used to design the direct absorption solar collectors, in which the nanofluid is possessed both the working fluid and solar energy absorber. However, there is a lack of theoretical models that are capable to evaluate the steam rated capacity of direct absorption solar collector. Thus, the aim of the present paper is an experimental and theoretical study of the boiling of nanofluid exposed to thermal radiation. The experimental study has been carried out on a laboratory-scale unit with a solar radiation simulator. The experimental unit is capable to operate at a continuous steady-state mode with recirculation of condensed fluid. Two types of nanoparticles have been tested: multiwall carbon nanotubes with two sizes of 49 nm and 72 nm and 110 nm iron oxide particles Fe3O4. Distilled and salt water has been used as the base fluid for all types of particles. Mass fraction of particles is varied from 0,5 to 5 %. The reference experiment has been carried out for salt water in a blackened receiver. The theoretical model assumes that boiling is localized in a narrow region adjacent to the irradiated surface. An experimental study of the steam rated capacity for various types of particles has been carried out. An analytical expression has been proposed to estimate the steam rated capacity during the boiling of a nanofluid exposed to thermal radiation. Comparison of the experimental data for the distilled water-based and saltwater-based nanofluids has been carried out. It is found that the addition of sea salt does not affect the steam generation significantly. The maximum difference is 12 %. The highest steam rated capacity is achieved when using carbon nanotubes of mass fraction of 5 %. The steam rated capacity is increased by 23 % compared with the reference experiment. The theoretical model reproduces the experimental data with an average deviation of 7 %. Application of nanoparticles in direct absorption solar collectors allows us to increase the steam rated capacity compared with the boiling of the base liquid in a blackened receiver. The theoretical model is capable to estimate to a high precision the steam rated capacity in case of boiling of suspension exposed to thermal radiation. The results of this study may be of interest during the development of solar power plants with a steam turbine cycle.