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). 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 Georgia Tech’s George W. Woodruff School of Mechanical Engineering. 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.
NEW BREED OF FUEL CELL
While raising temperatures is a key objective for Loutzenhiser and Henry, cooling things down is the aim of other energy researchers.
Meilin Liu, a Regents Professor in the Georgia Tech School of Materials Science and Engineering and co-director of the Center for Innovative Fuel Cell and Battery Technologies, is developing a new breed of fuel cell — one that operates at intermediate temperatures.
Currently, there are two leading fuel cell technologies: solid oxide fuel cells (SOFCs) and polymer electrolyte membrane (PEM) fuel cells. Used in stationary applications, SOFCs operate at temperatures of 700 degrees Celsius or higher, while the PEM fuel cells that power electric vehicles operate at low temperatures of about 80 degrees Celsius. Both technologies have challenges, ranging from water and heat management to operational life — plus they’re expensive.
With funding from ARPA-E, Liu is developing fuel cells that will be powered by methane, a clean and abundant fuel, and operate within a range of 300 to 500 degrees Celsius. “At these temperatures, we can overcome difficulties associated with SOFC and PEM technologies and achieve higher power density, longer life, and lower costs,” Liu said.
The intermediate temperature fuel cells require new materials for electrodes and electrolytes, and Liu’s approach is to introduce nanostructured materials that will dramatically enhance the ionic transport along the interfaces.
The goal is to create fuel cells for distributed power, which would serve as the primary power supply to individual houses. If successful, Liu envisions larger systems that could replace conventional fossil fuel power plants. “These fuel cells could without the pollutants,” he said.
Liu is also working on two other ARPA-E projects:
Bi-functional electrochemical systems. In collaboration with the University of California-Los Angeles, Liu’s research group is creating a new class of materials that can be used as electrodes and simultaneously store chemical fuels like hydrogen or methane. The system would operate like a fuel cell, but if fuel weren’t available, then the battery would kick in to provide electricity. “The idea is to prevent any disruption of energy, which can happen with conventional fuel cells,” said Liu.
Graphene-based supercapacitors. Liu and C.P. Wong, another Regents Professor in the School of Materials Science and Engineering, are working on a graphene-based supercapacitor that would offer significantly increased energy density while maintaining high power and long operational life.
Another wunderkind in materials science circles, graphene is a two-dimensional material that conducts electricity better than copper and is 100 times stronger than steel, but much lighter. “Yet graphene has a tendency to stack together and form graphite,” Liu said. “And with a supercapacitor, you want to be able to use all the surface area.”
To get around this problem, Wong and Liu are placing molecular spacers and incorporating metal compounds between the graphene sheets, creating a 3-D porous structure. About 18 months into the project, the researchers have already demonstrated a capacity of 1,000 faradays per gram from the material — quadruple the energy density of current supercapacitors that use activated carbon particles.
In automotive applications, Georgia Tech researchers are striving to make batteries and fuel cells more reliable and less expensive.
“The reason we don’t see greater penetration of electric vehicles is largely driven by cost and life issues,” said Tom Fuller, a professor in the Georgia Tech School of Chemical & Biomolecular Engineering and co-director of the Center for Innovative Fuel Cell and Battery Technologies.
Fuels cells have garnered much of the limelight in recent years; however, Fuller believes the ultimate solution for electric vehicles blends fuel cell and battery technologies. “Fuel cells eliminate the range problem,” he said. “Yet you need some means of energy storage to recover energy from braking and for operating the engine at its most efficient point.”
In battery research, Fuller’s group is working with advanced materials, such as silicon and tin, which can store more lithium than electrodes made from carbon and metal oxide materials — potentially 10 times more. They have also been investigating different binders to better hold materials and make improvements in cycle life.
On the fuel cell side, Fuller is focused on understanding causes of platinum degradation to make better use of the expensive material. “People have been trying to find a substitute for platinum for decades, but it hasn’t happened,” said Fuller. “I believe it’s more practical to work with what we have at the moment.” He points to catalytic converters, where the amount of precious metals required has been reduced over the past couple of decades by orders of magnitude. “What used to be a very expensive amount of platinum or precious metal in your exhaust system is now pretty reasonable.”
Fuller is also faculty lead on EcoCar3, an advanced vehicle technology competition, sponsored by the DOE and General Motors. In the four-year competition, student teams from 16 different universities will each be given a new Chevrolet Camaro to convert into a hybrid-electric car.
At the end of each year, students will be evaluated on various criteria, such as reducing emissions and petroleum use while maintaining performance. The first phase of the competition is slated for May when mechanical, electrical, and overall systems design will be judged.
Georgia Tech has been involved in two previous DOE competitions for advanced vehicles. “It’s a great opportunity for students to get involved with renewables in a real-world way,” said Fuller. “And the fact that the designated model for EcoCar3 is a Camaro sent everyone over the edge.”
At Professor Gleb Yushin’s laboratory in the Georgia Tech School of Materials Science and Engineering, researchers are addressing energy density challenges in electrochemical devices for a wide variety of applications. His research group is one of the few groups in the country that works with multifunctional batteries, research that is sponsored by the Air Force Office of Scientific Research. Recent innovations include carbon nanotube-based nonwoven fabrics that are coated with different types of ceramic materials and conductive polymers for lithium-ion storage.
“This lightweight, flexible material can store energy but also bear a very high mechanical load,” said Yushin. “In fact, we’ve demonstrated specific strength higher than titanium, copper, and even structural steel.”
Nanostructure is the secret sauce behind these multifunctional batteries, Yushin explained: “There are materials that can be brittle like glass or silicon, but if you make them in very small dimensions, they become much tougher. We’ve been able to gain precise control over the nanostructure and surface chemistry.” The battery material could be produced on an industrial scale and used to both construct and power unmanned aerial vehicles, high-performance ground vehicles, or smart textiles such as apparel with computing capabilities.
Other projects from Yushin’s team include:
Rechargeable alkaline batteries. Working with researchers at Princeton University, Yushin’s team is developing a novel chemistry to make alkaline batteries cycle like lithium-ion batteries. “Because the technology is water-based, it’s not flammable, so it’s much safer to use,” Yushin said. “The alkaline batteries have less energy than lithium batteries but could be recharged faster and be cheaper to make.”
Highly stable sulfur batteries. Lightweight sulfur-based batteries are being used in military unmanned aerial vehicles, but their cycle life is too short for consumer applications. Yushin’s group has developed composite lithium sulfide-based cathodes with a unique microstructure and core-shell morphology that allows batteries to maintain high-rate performance for more than 1,000 cycles.
Yushin has also co-founded a startup company, Sila Nanotechnologies, to commercialize materials for next-generation lithium-ion batteries. Launched in 2011, the company spent its first three years in the Advanced Technology Development Center, Georgia Tech’s business incubator, and then relocated last fall to Alameda, California, where it has a 31,000-square-foot facility and 25 employees. Backed by Silicon Valley venture capital firms, Sila has already won industrial customers and is growing quickly, Yushin said.
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.
New materials and technologies aren’t the only ways to address energy storage challenges. Another approach is power management, points out Valerie Thomas, a professor who has a dual appointment in Georgia Tech’s Stewart School of Industrial & Systems Engineering and School of Public Policy.
In a recent study, Thomas and Deepak Divan, professor in the School of Electrical and Computer Engineering, looked at how a high adoption rate for electric vehicles would affect the cost of various sources of electricity. Among their findings: If you could control when vehicles are charged, so it could be done when most cost-effective for grid operators, the cost of electricity for the entire power system would be reduced — including for renewables.
Power management is nothing new, Thomas said, pointing to demand-response programs where utilities pay customers to reduce power usage during hours when energy consumption is the highest. “It’s something I think needs more emphasis,” she said. “Energy challenges are typically viewed from the supply side; not to say we don’t want a better battery, but there are some very interesting opportunities on the demand side — changing how we use energy and how the system is managed.”
At the same time, major advances in energy storage, especially for small-scale renewables, have the potential to dramatically change the power game, Thomas said. “For example, if it became easier to produce and store electricity on an individual basis, then we might not need the grid anymore.”
Added Thomas: “These are really interesting times. Significant advances in energy storage could alter our entire way of managing and delivering electricity — resulting in less vulnerability to power outages and real environmental pluses.”
T.J. Becker is a freelance writer based in Michigan. She writes about business and technology issues.