A Perfect Solar Energy Absorber

Researchers have developed a technique that can be used to collect sunlight to heat etched metal surfaces like the one featured here, which can then power an electrical generator for solar power. (Source: S. A. Jalil et al.)

The University of Rochester research lab that recently used lasers to create unsinkable metallic structures has now demonstrated how the same tech­nology could be used to create highly efficient solar power generators. Now, the lab of Chunlei Guo, professor of optics, describes using powerful femto-second laser pulses to etch metal surfaces with nanoscale structures that selec­tively absorb light only at the solar wave­lengths, but not elsewhere.

A regular metal surface is shiny and highly reflective. Years ago, the Guo lab developed a black metal tech­nology that turned shiny metals pitch black. “But to make a perfect solar absorber,” Guo says, “We need more than a black metal and the result is this selective absorber.” This surface not only enhances the energy absorption from sunlight, but also reduces heat dissipation at other wave­lengths, in effect, “making a perfect metallic solar absorber for the first time,” Guo says. “We also demons­trate solar energy harnessing with a thermal electric generator device. This will be useful for any thermal solar energy absorber or har­vesting device, particularly in places with abundant sunlight,” he adds.

The researchers experi­mented with aluminum, copper, steel, and tungsten, and found that tungsten, commonly used as a thermal solar absorber, had the highest solar absorption effi­ciency when treated with the new nano­scale structures. This improved the efficiency of thermal electrical generation by 130 percent compared to untreated tungsten. The lab has also used the femto-second laser etching technology to create super­hydrophobic and super­hydrophilic metals. In November 2019, for example, Guo’s lab reported creating metallic structures that do not sink no matter how often they are forced into water or how much it is damaged or punctured.

The new study, however, expands upon the lab’s initial work with femto-second laser etched black metal. Prior to creating the water attracting and repellent metals, Guo and his assistant, Anatoliy Vorobyev, demons­trated the use of femto-second laser pulses to turn almost any metal pitch black. The surface structures created on the metal were incredibly effective at capturing incoming radiation, such as light. But they captured light over a broad range of wave­lengths.

Subsequently, his team used a similar process to change the color of a range of metals to various colors, such as blue, golden, and gray, in addition to the black already achieved. The appli­cations could include making color filters and optical spectral devices, a car factory using a single laser to produce cars of different colors; etching a full-color photograph of a family into the refri­gerator door; or proposing with a gold engage­ment ring that matches the color of your fiancee’s blue eyes.

The lab also used the initial black and colored metal technique to create a unique array of nano- and micro-scale structures on the surface of a regular tungsten filament, enabling a light bulb to glow more brightly at the same energy usage. “We fired the laser beam right through the glass of the bulb and altered a patch on the filament. When we lit the bulb, we could actually see this one patch was clearly brighter than the rest of the filament,” Guo said. (Source: CAS)

Reference: S. A. Jalil et al.: Spectral absorption control of femtosecond laser-treated metals and application in solar-thermal devices, Light: Sci. & Appl. 9, 14 (2020); DOI: 10.1038/s41377-020-0242-y

Link: The Institute of Optics, University of Rochester, Rochester, USA

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