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High-Tech Mirrors and Superheated Fluid Make Solar Power More Efficient and Cost Competitive

Concentrated solar power, or CSP, is generated by mirrors, called heliostats, that focus sunlight on a receiver containing a heat transfer fluid that absorbs the energy, which is then used to produce steam to spin electric turbines.

The University of Arizona College of Engineering will lead a $5.5 million, 5-year research project, funded by the U.S. Department of Energy, to develop more affordable and efficient concentrated solar power systems.

Photo courtesy of DOE/NREL (credit: Dennis Schroeder).

Abengoa is erecting more than 3,200 mirrored parabolic troughs at its Solana plant near Gila Bend, Ariz. When at full operation, the CSP plant will serve more than 70,000 homes.

The award was made as part of the Department of Energy's SunShot Initiative, in an effort to make solar energy cost-competitive with other energy sources by 2020. About 80 percent of the funding will go to the multidisciplinary UA Engineering research team, which will conduct the research in partnership with Arizona State University Poly and Georgia Tech.

The research team will be led by energy expert Peiwen "Perry" Li, an associate professor in the UA department of aerospace and mechanical engineering. Li's co-researchers in this department are professor Cho Lik Chan and assistant professor Qing Hao.

The project will also involve several researchers from various engineering disciplines within the UA College of Engineering's School of Sustainable Engineered Systems, or SSES.

The research program will investigate the composition, properties and costs of new molten-salt-based CSP heat transfer fluids, which must absorb, transport and store solar energy, and generate electrical power efficiently and cost-effectively.

Photovoltaic systems such as solar panels can convert up to 15 percent of the sun's energy into electricity, but that conversion efficiency can jump to 45 percent in CSP systems operating at more than 1,200 degrees Fahrenheit.

Photovoltaic solar energy is produced by direct conversion of sunlight into electricity, whereas CSP systems convert light into thermal energy, which is further converted into electrical energy. These different systems share a common problem: the sun doesn't shine at night.

To overcome this nocturnal drop in power generation capability, an objective of this research is to develop molten-salt-based CSP heat transfer fluids with low melting points and low corrosivity that can be heated to about 2,400 degrees Fahrenheit. Temperatures thus have much further to fall before the transfer fluid cools and solidifies. Insulating the fluid storage tanks and circulation system will enable the stored heat to generate steam, and electrical power, throughout the night.

The salts used in current CSP plants are nitrates, which can operate at a maximum of about 1,000 degrees Fahrenheit before they become unstable, Li said. "This is not efficient enough, and this research has a requirement to find a salt that reaches about 1,500 degrees," he said. "But if we can stretch to 2,400 degrees, that will be super."

Li, Chan and Hao, and investigators from Georgia Tech, will study properties of molten salts such as thermal conductivity and viscosity, but there is much more to this complex project than researching the properties of various salts, or combinations of salts, said professor Pierre Deymier, SSES director and head of the materials science and engineering department. Low cost and sustainability are critical to the success of CSP technology.

With these criteria in mind, Deymier said an early candidate for study will be plain old table salt, sodium chloride. A cornerstone of civilization since Neolithic times, we eat it and preserve food with it; we've used it as currency, in religious rituals, and fought wars over it. And now we could use it to produce sustainable energy.

"We need to look at existing salts that already have a very high boiling point," Li said. "Then we take this basic candidate and see if we can fine-tune its properties by changing the composition or adding other compounds to push the boiling point higher."

Deymier agreed and quantified the need to use common elements. "The current objective for this project is a molten salt that costs less than a dollar per kilo," he said. "When you think about the thousands of tons to be used, we're talking about millions and millions of dollars just for loading power plants."

This is where another SSES department fits into the research program. Moe Momayez, associate department head and associate professor in mining and geological engineering, will research the logistics and economics of large-scale extraction and processing required to supply CSP plants with massive quantities of sustainable minerals.

"The whole project is driven by cost," Deymier said. "That's how you achieve sustainability. You don't want to reinvent everything -- you improve dramatically on existing technologies." Li added that we will need millions of tons of salt for this technology to succeed. "It must be cheap and we must have large reserves," he said.

Deymier and departmental colleagues, professor Pierre Lucas and assistant professor Krishna Muralidharan, will be responsible for screening various compositions of salts and confirming their chemical composition and behavior, and tweaking the various salt mixtures in response to feedback from the thermal and flow testing conducted by Li's subgroup.

Anyone who's lived near the coast, or driven their car on the beach, knows just how quickly damp, salty air eats away metal, so it will come as no surprise that superheated molten salt has a highly corrosive effect on the pipes and tanks used to circulate and store CSP transfer fluids.

Don Gervasio, a research professor in another SSES department, chemical and environmental engineering, will work with ASU Poly engineers to determine the corrosiveness of the various salts investigated by the project team.

Concentrated solar power is a sustainable technology in its infancy. The United States has just one commercial solar tower power plant, the 5 megawatt Sierra Sun Tower built and operated by eSolar in Lancaster, Calif. The world's first commercial solar tower power plant is the 11 megawatt Plantar Solar 10 near Seville in Spain, which came online in 2008 and is operated by Abengoa Solar.

Spain is a world leader in CSP and Spanish operator Abengoa is building one of the world's biggest CSP plant near Gila Bend, Ariz. The 1,900-acre, 280 megawatt Solana Generating Station, due to be completed in 2013, will produce enough power for 70,000 homes. Arizona Public Service has contracted to buy all the power generated by Solana to meet the Arizona Corporation Commission's mandate that 15 percent of the electricity provided by Arizona's utilities should be from renewable sources.

SunShot was inspired by President Kennedy's Moon Shot program, and the DOE is funding a similar project at UCLA, in partnership with UC Berkeley and Yale, to investigate the potential of liquid metals as high-temperature heat transfer fluids.

Such is the promise of this emerging technology that some industry groups think CSP plants could generate as much as 25 percent of global electricity needs by 2050. Solar concentration technology would also create hundreds of thousands of jobs and keep millions of tons of carbon dioxide out of our atmosphere.

Related Information
NREL News Feature: "Thermal Storage Gets More Solar on the Grid"
UA College of Engineering School of Sustainable Engineered Systems
UA Department of Aerospace and Mechanical Engineering

Steve Delgado | Newswise Science News
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