Why self-cleaning is important for solar thermal receivers?

Abstract

Summary Surface cleaning remains essential for the sustainable operation of high performance solar thermal receivers. Cleaning of optical surfaces, such as solar troughs and absorbers, requires energy intensive efforts because of the large surface area involvement such as those observed in solar farms. In addition, self‐cleaning of such surfaces becomes demanding because of lowering the cleaning costs, reducing the waste of resources, such as clean water, and minimizing the complication of the mechanical systems incorporated. Self‐cleaning of surfaces is associated with the low adhesion between the surface and the foreign particles; in which case, these particles can be removed easily from the surfaces in a cost‐effective way. The surface energy and contact area of the surface are two main important parameters influencing the particle adhesion on the surfaces. In this case, reducing the surface energy and forming micro/nano size pillars on the surface through texturing lower the particle adhesion on the surfaces significantly. In solar thermal energy harvesting applications, metallic or composite materials are used and texturing the surface remains challenging in terms of cost and precision of operation when conventional texturing methods are used. One of the methods to create surface texture consisting of micro/nano pillars is to use the laser beam ablation. This results in hierarchical distribution of surface texture with desired pillar heights1. In addition, laser surface texturing offers significant advantages over the conventional techniques. Some of these advantages include fast processing, precision of operation, and low cost. Although the laser processing involves with high temperature processing and thermally induced stresses remain important, the defects sites can be minimized via controlling the process parameters during the texturing. Introducing the assisting gas on the texturing surface enables to generate compounds such as oxide or nitride species, which lower the surface energy considerably. Consequently, investigation of laser texturing of solar energy materials while incorporating the assisting gas becomes essential. In the present perspective, the laser surface texturing of solar energy materials for thermal power applications is presented together with challenges and future perspectives. Specifically, the followings will be presented: (1) the texture characteristics of laser treated metallic and ceramic surfaces; (2) wetting state of the textured surface, and optical properties of textured surface in terms of absorption of the solar irradiation.

Harnessing Reversible Wetting Transition to Sweep Contaminated Superhydrophobic Surfaces

“…As a result, there are low adhesion forces between the particles and superhydrophobic surfaces, and the particles can be cleaned effectively . The methods employed to sweep particles on contaminated superhydrophobic surfaces include mechanical removal of the settled particles with brushing, water jet splashing, air jet blowing, and ultrasonic oscillation . Surfactants are also frequently used; however, they have negative effects on the environment.…”

Solar energy harvesting and a water droplet cleaning of micropost arrays surfaces

“…Texturing of PDMS wafers via generating micropost arrays on the surface further improves the hydrophobicity and enhances the self‐cleaning characteristics of surface. Although PDMS replication of textured surfaces, such as laser texturing and micropost arrays, has been studied previously, the self‐cleaning characteristics of the surface via droplet dynamics are left for future study. In addition, PDMS‐replicated micropost arrays have good optical transmittance and are easy to manufacture.…”