‘Black-hole’ material falls short of a breakthrough
A material developed at the University of California promises to convert 90% of captured light into heat. But how much it can improve on current technology remains to be seen.
By Jason Deign
Sources consulted by CSP Today say a new absorber material developed in the US falls short of being a breakthrough for the industry.
Dr Luis Crespo, president of the European Solar Thermal Electricity Association, believes a silicon boride-coated nano-shell substance unveiled in October does not offer a very significant improvement over existing technologies, despite being able to convert 90% of light into heat.
Functional materials engineering researchers at the University of California, San Diego (UCSD), Jacobs School of Engineering created the material as part of the work being funded by the US Department of Energy’s SunShot programme.
The substance has been described in two articles in the journal Nano Energy. Besides boasting a high solar-to-thermal conversion efficiency, the material can withstand temperatures of more than 700ºC and could last for several years before having to be replaced.
In tests, the material has withstood 750ºC for around 1,000 hours without significant impact on its optical and mechanical adhesion properties.
It features what the UCSD says is a “’multi-scale’ surface created by using particles of many sizes, ranging from 10 nanometres to 10 micrometres.”
Professor Sungho Jin, director of the UCSD Materials Science & Engineering Program, says: “We are aiming for at least several years of CSP sunlight absorber usage at 750ºC before repainting or repair.”
The UCSD team believes this could make the material attractive to the CSP industry on the basis that plant owners currently have to shut down power tower plants for several days at a time on an annual basis to remove, repaint and cure existing absorber coatings.
Repainting receivers
Crespo, however, disputes the idea that current solar absorber materials need to be replaced on an almost yearly basis after operating at high temperatures. With existing technology “you will only need to repaint receivers every five to 10 years,” he says.
Additionally, he points out, current coatings yield an overall receiver efficiency that is already in the region of 85%, so the 90% level achieved with the nano-particle substance, while significant, is still not a massive step forward.
“This may be an improvement but it is not a breakthrough,” he says.
Ultimately, believes Crespo, the value that the new substance can offer the industry will really depend on “how much it costs and how long it lasts.”
On the economics of the substance, says Jin: “We have been developing several different types of solar absorber coatings for the past three years, so the cost may vary depending on what materials are selected and how they are manufactured.
“Generally speaking, these nano-powders are not that difficult to produce and the raw materials involved are not that expensive, so we think that they are relatively inexpensive.”
Pointing out that some of the new materials being used are ceramic-based and therefore potentially easy to manufacture in quantity, he continues: “The exact cost would be dependent on the scale of manufacturing, market size and how much unique characteristics we want.
“As we plan to continue this effort for the next few years for further improvements in properties, manufacturability [and] long-term reliability, we will have a better idea on the cost aspects later, but overall these materials will not be very expensive.”
Higher temperatures
That is just as well, since the material only really seems suited to the absorbers for power towers, which typically operate at higher temperatures than other types of CSP plant.
While the popularity of power towers is on the rise, the technology is just one of several types of plant used in CSP. And it still lags behind parabolic trough in terms of megawatts deployed.
Furthermore, if the substance only needs to be applied once every decade or so then the demand for it is likely to remain relatively low, unless it can be used for other applications as well.
The UCSD team is working on similar technologies for other types of plant, although it is unclear whether the manufacturing processes for them would be similar enough to create economies of scale and drive down costs.
“For parallel trough-type CSP applications with lower sunlight concentration factors, we have other variations of nano-material coating to reduce thermal emission loss, which are also actively being developed at UC San Diego,” says Jin.
Another potential problem for commercialisation is that unless the new material offers a significant benefit over existing coatings then operators may shun it because of higher perceived risk.
For now, Jin and his team, which includes Professor Zhaowei Liu of the UCSD Department of Electrical and Computer Engineering and Mechanical Engineering Professor Renkun Chen, are looking to take the nano-material concept to the next level, through commercial partnerships.
“We have many companies and research and development groups showing interest in these solar absorber coatings,” Jin comments.
“We are discussing further development and potential commercialisation routes with a few CSP-related companies and other solar absorber device companies.”