PROBLEM: What happens to renewable power when the wind doesn't blow and the sun doesn't shine?
One of the most critical scientific challenges confronting American energy security is the elusive ability to integrate plentiful, yet intermittent, power sources, such as wind and solar, into the electrical grid. The Department of Energy estimates that by 2020, wind and solar power could account for more than 20 percent of the nation's generation capacity. Equal to America's existing inventory of nuclear power, the goal will only become a reality if researchers can develop new energy storage technologies capable of capturing excess energy at peak generation times and storing it for use when generation from these renewable sources is low or non-existent.
Tom King, Director of ORNL's Energy Efficiency and Electricity Technologies Program, notes that the electrical grid differs currently in one significant way from other energy distribution systems, such as oil and natural gas, in that electricity cannot be stored cost effectively. "At present, generation must be balanced with consumption," King says. "In simple terms, if I turn on the light, more power needs to be generated. If I turn off the air conditioner, less power needs to be generated. America has operated for more than a century with the ultimate just-in-time process."
The only large-scale power storage strategy currently in use is "pumped hydro," a system that every night uses excess energy from hydro-electric dams to pump water uphill to reservoirs when demand is low. The next day, when demand rises, the stored water is released and routed through turbines to generate electricity. "The Tennessee Valley Authority has a pumped hydro system that can generate 1500MW of power," King says. "That's like having an extra nuclear power plant."
While pumped hydro is an option only where hydroelectric dams already exist, similar levels of electrical energy storage can be achieved with the use of massive batteries. "Having that much storage at multiple points on the grid would fundamentally transform the way electricity is delivered," says King. "If every substation had a low-cost storage battery, the country could have a more reliable electrical system."
Widespread use of storage batteries would enable the grid to adapt to changing demands by stockpiling power for later use. This buffer would also enable the power generation system to increase and decrease output less frequently. Since electricity generation is often fueled by coal or natural gas, the result would be an added benefit of lower emissions.
As is often the case, the primary obstacle to widespread implementation of battery technology on the grid is cost. The few large-scale battery systems currently operating generally use nickel-cadmium or sodium-sulfur batteries. These systems perform well, but their cost is still above the threshold required to make the batteries attractive to utilities for use with wind or solar farms.
ORNL materials researcher Claus Daniel agrees that solving the issue of cost will be necessary before batteries are widely used for both grid and transportation applications. "Put simply, we need to develop batteries that last longer and cost less," Daniel says. "Funded mostly by the Department of Energy's Industrial and Vehicle Technologies Program, we are working with battery manufacturers to do both of these things by developing new low-cost manufacturing processes and better quality control practices for battery components. This capability is enhanced by our ability at ORNL to use computer simulation to narrow down the parameter space that defines the best solution to a processing challenge. The relationships our staff enjoys with automotive and industrial companies are key to our current success."
ORNL's extensive experience with process development and materials characterization enables the laboratory to help industrial partners scale new processes for industrial production. "Some of the battery manufacturers have new materials that show tremendous performance on the bench scale," says Daniel, "but it can be difficult to scale to mass production of these materials and systems. We can use our experience to help them overcome these problems."
Some of the most innovative research in this field is focused on improving battery reliability by understanding—at a microscopic level—how batteries charge and discharge. "One of the factors that limits the lifetime of batteries is mechanical degradation of the electrodes," says Daniel. "For example, when we charge a lithium-ion battery, lithium ions enter the anode (the negative terminal of the battery), expanding it by 10 percent, in the case of carbon anodes. When the battery discharges, the ions leave and the anode shrinks." Repeated expansion and contraction can cause cracks in the anode that eventually lead to degradation and capacity fade. One of Daniel's Ph.D. students is developing an in-situ characterization technology to investigate this degradation and understand it better.
To understand this and other degradation mechanisms, ORNL materials researchers Karren More and Niels DeJonge have designed a system that enables a working battery to be examined in an electron microscope. DeJonge had previously developed a way use the microscope to image biological samples in a liquid environment. "We are using that same idea to image the operation of a small liquid battery cell at very high resolution," Daniel says. "We are hoping to see exactly how battery materials interact with the liquid electrolyte, how they degrade, and how dendrites—irregular deposits on the electrodes that can cause short circuits in batteries—are formed."
"This research should help us understand what goes on at the material interfaces and how battery components break down," says King. "If so, researchers can develop ways to extend the life of the materials and create more durable and cost-efficient batteries." King and his colleagues plan to extend this line of inquiry using ORNL's unique neutron scattering capabilities. "The goal is to extend the life of the systems, in effect, to get more charges and discharges," says King. "In the same process, a second goal is to determine if we can reduce the cost of the systems by using lower-cost materials."
Researchers are also looking at ways to increase the power-density of battery systems—storing more energy in a smaller battery. This effort has important implications for both grid storage batteries and for batteries used in electric vehicles. King believes the development of high-power, high-energy-density batteries on the vehicle side will feed into the electricity delivery side—and vice versa.
Encouraged by the American Recovery and Reinvestment Act, King anticipates a wave of grid-related storage research. He expects millions, if not billions, of dollars of research and development.
If the funding becomes available, there are several areas to which King would like to give more attention, including modeling and simulation of battery systems and research and development related to low-cost manufacturing processes. King hopes to incorporate some of this research, with demonstration projects, into ORNL's Sustainable Campus Initiative. Preliminary plans include solar-covered parking areas for plug-in electric vehicles that are linked to innovative energy storage devices. King believes such a project would provide an opportunity to study user behaviors, charging and discharging of the vehicles, and impacts to the electric distribution system. He wants to gather data on when vehicles charge and how fast they charge—with an eye toward reducing vehicle charging rates on hot afternoons to divert power to building cooling systems. In other words, he hopes to learn how to control the use and generation of electricity.
"The challenges are imposing, but the opportunities are huge to make genuine progress and make a lasting contribution for the country."
Web site provided by Oak Ridge National Laboratory's Communications and External Relations