Ultra-black nanoneedles absorb 99.5% of light for future solar power towers

Researchers from the Materials Thermophysical Properties Group at the University of the Basque Country (EHU) used state-of-the-art equipment to analyze the ability of ultra-black copper cobaltate nanoneedles to effectively absorb solar energy. They showed that the new nanoneedles have excellent thermal and optical properties and are particularly suitable for energy absorption. This will pave the way for concentrated solar power generation in the renewable energy sector.

Testing was conducted in a specialized laboratory capable of conducting high temperature research. The results were published in the journal Solar Energy Materials and Solar Cells.

The renewable energy of the future is concentrated solar power, which can easily be used for thermal energy storage. Despite the fact that it has historically been more expensive and complex than solar power, the technology has made great strides in recent years, and concentrated solar power plants are becoming popular in more and more countries as a resource for a sustainable future.

“We are researching ultra-black materials for use in the solar tower,” explained Iñigo González de Arrieta, a researcher in the Thermophysical Properties of Materials Group. In this type of plant, solar energy is directed by mirrors onto an energy-absorbing tower.

“The more effective absorption materials we can achieve, the more competitive the system will be and the more this type of energy opportunity will open up,” said Dr. González de Arrieta. Using state-of-the-art equipment created in the EHU laboratory itself, “we perform thermo-optical analysis to measure the absorption properties of the samples that arrive at us. Internationally, there are very few laboratories dedicated to carrying out high-temperature research.”

EHU researchers conducted thermo-optical characterization of cobaltate copper nanoneedles, patented by the University of California, San Diego. “We found that these cobaltate copper nanoneedles performed better than previously used carbon nanotubes, and even better when coated with zinc oxide,” said Dr. González de Arrieta.

Aiming for 100% light absorption rate

Hundreds of mirrors placed around the solar tower focus the sun’s rays on one point. The objective is to achieve maximum absorption in the tower. So the absorbent material had to be super black. The blackest materials currently available on the market are composed of vertically aligned carbon nanotubes.

Although these materials have good optical indices and very good light-trapping geometries, “carbon nanotubes are not stable in the presence of high temperatures and high humidity, so they have to be coated with more resistant materials, which reduces optimization,” Dr. González de Arrieta explained. “Carbon nanotubes absorb about 99% of light, but they cannot be used in solar power towers.”

So he felt that copper cobaltate nanoneedles represented a major step forward. “Nanoneedles are more stable at high temperatures, and furthermore, nanoneedles coated with zinc oxide have greater absorption than nanotubes that have been used to date. The material currently used in photovoltaic towers (black silicon) absorbs 95% of the light, which is a significant amount. However, copper cobaltate nanoneedles absorb 99% of the light, and nanoneedles coated with zinc oxide absorb even more light.” 99.5%,” explained Dr. González de Arrieta.

Dr. Renkun Chen from the University of California, San Diego, in collaboration with the U.S. Department of Energy, is beginning to use cobaltate copper nanoneedles coated with doped zinc oxide in solar power towers, “but given the uncertain situation in the U.S., we don’t know how that will turn out,” the researcher explained.

Solar tower power plants are located in Andalusia and several deserts around the world. In Spain, only 5% of the energy is supplied by this technology. Researchers have revealed that “promoting this renewable energy” is critical. “Renewable energy has many advantages because it is completely clean and can be used even when the sun is not shining.”

The sun’s heat absorbed by the tower is stored as thermal energy. “The sun’s heat is used to melt certain salts, among other things. Molten salts retain heat very well, making it much easier to feed that thermal energy back into the power grid.”

Dr. González de Arrieta emphasized the need to continue developing and characterizing new coatings with improved optical properties for use on photovoltaic towers. He added that in the future, the researchers may continue to explore the possibility of coating the nanoneedles with materials that improve their conductivity.