Energy Focus: Ceramic nanoneedle arrays increase sunlight absorptance in concentrated solar power plants
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Ceramic nanoneedle arrays increase sunlight absorptance in concentrated solar power plants
5 µm
92
99.3%
Co3O4
HfO2-coated Co3O4
CuCo2O4
separated by 170 nm produced the highest absorption. The sharpness of the tips of the nanoneedles was also important to achieving good antireflective properties. This is an issue when the temperature rises above 650ºC due to grain growth and diffusion. To maintain the sharpness of the tips, despite the high-temperature treatment, the researchers deposited 3–5 nm of a glassy SiO2 or Hf O2 on the ceramic needles. As predicted, the sharpness of the needle was maintained and the solar absorptance improved. Of the two ceramics and coating compositions investigated, the SiO2-coated CuCo2O4 nanoneedle arrays displayed the highest solar absorptance of 99.5%. During the operation of a CSP plant, the receiver is subjected to temperature cycles for a long time. The researchers found that even after 100 h at 800ºC, the SiO2-coated CuCo2O4 nanoneedle arrays still absorbed 99.3% of the sunlight. To apply this strategy as a coating for the long metallic tubes of the receiver, Chen and his team plan to develop a more scalable process and avoid the atomic layer deposition process of the protective coating that is both costly and not easily
800ºC 100 h
Unannealed
96.7%
800ºC 100 h
650ºC 100 h
94.8%
Unannealed
97.7%
650ºC 100 h
94
99.5%
98.2%
98
96
1 µm
99.3%
99.0%
Unannealed
d
1 µm
Unannealed
concentrating solar power (CSP) plant consists of a ~150 m-high power tower surrounded by solar-tracking mirrors called heliostats. The heliostats reflect the sunlight in the direction of a receiver located at the top of the tower. This receiver is made of 10–20-m-long tubes of nickel or aluminum alloys, into which water or molten salts are flowed. When the sun shines, the light reflected by all the heliostats heats the receiver to above 650ºC, producing steam or a flow of the molten salt, generating power. One approach to enhance the power generation of a CSP plant is to increase light absorption at the receiver tubes. Typically, 4–5% of the energy from the heliostats is lost by radiation due to heating of the tubes. Increasing the absorption, that is, decreasing the reflectance, is particularly challenging because traditional antireflective coatings made of organic composites cannot withstand high temperatures. To address this issue, researchers from the University of California, San Diego, led by Renkun Chen, have developed nanoneedle arrays made of ceramics that increase the solar absorptance. The results from this development were published in APL Materials (doi:10.1063/1.5084086). The researchers used a hydrothermal process to grow CuCo2O4 or Co3O4 on a substrate made of the same material as the receiver tubes, a nickel-chromium-tungsten-molybdenum alloy (Haynes 230) with excellent high-temperature strength and oxidation resistance. During this process, they added urea as a cationic surfactant to drive the preferential formation of oriented nanoneedles instead of spher
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