Researchers have made significant strides in new energy generation technologies. Yet, before renewable sources can make a significant contribution to our energy supply, similar strides will be needed in energy storage, making it the new holy grail.
“When it comes to renewable energy sources, there can be a mismatch between when power is available and when it’s needed,” said Tim Lieuwen, director of Georgia Tech’s Strategic Energy Institute (SEI) and professor at the George W. Woodruff School of Mechanical Engineering. He points to grid faults caused by temporary loss of wind and solar power during the day. “In contrast to conventional power plants where you can turn power on, off, up or down, you can’t dispatch solar or wind — storage is a key enabler for significant penetration of these non-dispatchable sources,” Lieuwen said.
Different challenges exist in the transportation sector, which accounts for about 30 percent of U.S. energy usage. Although there are a number of electric vehicles on the market, their limited range and high cost are obstacles to widespread adoption, which has researchers pursuing scalable ways to increase the power and energy density of electrochemical devices. “Storage is one of the critical issues required for electric vehicles to gain traction,” Lieuwen said.
At the SEI, Lieuwen coordinates energy work across campus. Georgia Tech stands out from many research universities for its systems analysis and ability to tackle large-scale energy challenges, Lieuwen observed. “We not only have deep domain expertise but also people who can think about plugging innovations into a bigger system. Having these people work side by side creates real synergy.”
Georgia Tech is participating in a number of high-profile projects sponsored by the Department of Energy (DOE), including its Advanced Research Projects Agency-Energy (ARPA-E).
SOME LIKE IT HOT
Among ARPA-E awardees is Asegun Henry, an assistant professor in the Woodruff School. He is developing technology for a new type of concentrated solar power (CSP) plant that could increase efficiency by more than 50 percent over current facilities.
Unlike photovoltaic plants that directly convert sunlight into electricity and have no means of storage, CSP facilities transform sunlight into thermal energy. The thermal energy can then be stored in molten salt for later use, when it’s discharged through a heat exchanger to create steam to run a turbine.
Current CSP power plants cost about twice as much to operate as fossil fuel plants. The greatest inefficiencies occur at the turbine, where 60 percent of captured energy is lost, Henry explained. “You can make the engines more efficient by operating them at higher temperatures — about 1,500 degrees Celsius. But to do this, you need different infrastructure.”
Key to this infrastructure could be liquid metals, such as tin. Unlike salt, which will vaporize and decompose at high temperatures, metal can remain in a liquid state over a much greater range of temperatures — about 230 to 2,600 degrees Celsius. Yet, there’s also a drawback. When liquid metal comes into contact with other metals, it reacts immediately and corrodes. So, new materials are required for the facility’s pipes, valves, storage bins, and pumps.
“Our answer is to use ceramics,” Henry said. “There are a number that are commercially available that are not corroded by the liquid metals we’re interested in.”
Moving to higher temperatures also requires a new design for the solar receiver, which sits on top of a tall tower surrounded by a field of heliostats, collecting the sun’s rays. Without a new receiver design, the efficiency of the entire system would drop dramatically. In response, the researchers have created an optical cavity receiver that traps the light.
The researchers are now testing their design, using a small-scale prototype. “If we’re successful, this could be a real game changer and help make solar energy cost-competitive with fossil fuels,” Henry said.
LOOKING ON THE BRIGHT SIDE
“The potential of solar energy is amazing,” said Peter Loutzenhiser, an assistant professor in the George W. Woodruff School of Mechanical Engineering, who is also focused on concentrated solar technologies. “Sunlight is the most abundant energy source on earth. Transforming sunlight and storing it in a long-term medium is the way of the future.”
Loutzenhiser’s research team is collaborating on the design of a new thermochemical storage system: a three-year, $3.5 million project funded by the DOE’s SunShot Initiative and led by Sandia National Laboratories.
Key to this storage system are perovskite materials, a type of metal oxide. Known as “the new black” in solar thermochemical circles, perovskites are prized for their electronic conductivity and oxygen exchange kinetics. In this application, the perovskites will enable CSP plants to operate at higher temperatures, resulting in more efficient cycles, Loutzenhiser said.
He explained how the system would work: Concentrated solar energy from a heliostat field would heat the perovskites to about 1,000 degrees Celsius, where it’s possible to drive a chemical reaction and extract oxygen. This would result in reduced metal oxides, which would be stored in highly insulated tanks. To tap the energy later, the reduced metal oxides would be introduced into a stream of pressurized air that recovers the high thermal heat along with chemical heat. The resulting stream of hot pressurized air would run through a turbine generator to produce electricity.
In this first year of the project, Sandia is developing materials while Loutzenhiser’s team is designing a solar thermochemical reactor to measure reaction properties and determine the performance of the perovskite materials. Then they will design and test a reactor that enables the perovskites to efficiently trap solar radiation. “Ideally, we want to capture more than 80 percent of the solar heat into our medium,” Loutzenhiser said, noting this would translate into far higher efficiencies than current CSP plants.
To test the technology, Loutzenhiser’s team uses a high-flux solar simulator, one of only three in the United States. The simulator consists of seven xenon arc lamps (each being 6 kilowatts) that enable the researchers to test their technology under repeatable conditions. “Instead of waiting for the sun to come up or hoping a cloud doesn’t pass by, we can run it 24 hours a day,” Loutzenhiser said, adding that the simulator can melt holes in ½-inch-thick steel plates in less than a minute.
In other projects, Loutzenhiser is investigating ways to drive chemical reactions that result in the production of synthetic fuels. For example, his team has developed a hybrid solar/autothermal process that takes materials with high carbon content, such as sorghum or coal, and introduces water and concentrated solar power to transform them into synthesis gas. When sunlight is unavailable, pure oxygen is also introduced to burn a portion of the materials for heat, resulting in a continuous supply of synthesis gas. This synthesis gas, a mixture of hydrogen, carbon monoxide, and carbon dioxide, can be converted into liquid hydrocarbons like gasoline and jet fuel, using existing chemical processes.
PUTTING WASTE HEAT TO WORK
In other materials innovations, Samuel Graham, a professor in the School of Mechanical Engineering, is developing composite materials to capture and store waste heat generated by electric motors and electronics.
In a project with Oak Ridge National Laboratory, Graham’s research team has developed a high thermal conductivity and high thermal storage capacity material by integrating phase change materials with expanded graphite nanoplatelet foams.
“Whenever a material goes through a phase change, there is a large amount of heat that can be stored by the chemical rearrangement of the material’s structure,” Graham explained. Most phase-change materials have low thermal conductivities, causing heat to flow in and out very slowly. Graham’s team has been able to create a novel nanocomposite by combining graphite flakes and organic phase-change materials with high thermal conductivities. The result is an expanded graphite foam composite with thermal conductivity an order of magnitude higher than is possible by simply mixing the materials together — and the ability to retain up to 90 percent of the thermal storage capacity of the phase-change material.
The goal of the project is to integrate the materials into heat exchangers to reduce the energy consumption in household appliances by providing hot water and hot air to dishwashers and clothes dryers. But the material also has other applications, Graham said, such as providing thermal management of electronics used in hybrid electric vehicles.
In addition to its high thermal conductivity and storage capacity, the expanded graphite composite can be easily scaled in manufacturing. “In contrast to aluminum foams, our expanded graphite foam can be easily machined, formed into shapes, and is far less expensive to produce,” Graham said.
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Written by: TJ Becker, freelance writer
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