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March 15, 2024

Energy Reservoirs

Written By: Jason Daley

We’re capturing the potential in a portfolio of new, more sustainable storage technologies.

Every year, renewable resources like wind and solar become a larger slice of the energy mix in the United States and across the world. But for those technologies to have maximum impact, we will need better ways to store their intermittently generated energy—so that regardless of whether you’re microwaving a midnight snack or tuning into the Super Bowl on a gloomy Sunday afternoon, you’ll have power anytime you want it.

Energy storage, however, comes in many sizes, shapes and forms. It will take a combination of high-tech batteries, pumped hydropower, electrochemically derived fuels and even some molten salt to make sure renewables continue to provide a stable, affordable electricity supply that can grow along with our energy future.

“While deployment of those generation sources will continue to grow, the technical challenge has shifted to how to provide the energy when it is needed. Hence, energy storage,” explains Craig Turchi, program lead for the concentrating solar power program and the thermal energy science and technologies group manager at the National Renewable Energy Laboratory, in Golden, Colorado. “While most folks immediately think about batteries—and those play a major role—their modular nature make them best suited for smaller deployments and shorter durations. Much research is now on how to provide long-duration energy storage, greater than 10 hours, for the electric grid.”

Breaking up with lithium

Mike Wagner, an assistant professor of mechanical engineering at UW-Madison who works on energy system modeling and energy storage optimization, says it’s likely that energy storage will develop in two phases. Currently, wind and solar power are expanding at a rapid rate and are expected to produce about one-third of the world’s electricity by 2030. For the most part, these projects will be paired with large lithium-ion batteries (the same technology found in most electric cars and laptops), which will allow them to store energy for a few hours and feed it to the grid during periods of high demand.

But as wind and solar reach their peak, Wagner says other technologies will likely reach maturity, allowing for different types of storage to replace lithium-ion batteries, which have several drawbacks. Not only can lithium-ion batteries overheat, for example, but lithium also is difficult to source and its supply chain is geopolitically complicated. More concerning, the batteries do not scale well, meaning energy costs could add up quickly if they are used for long-term storage.

That’s why researchers, including several at UW-Madison, are looking into a whole host of alternatives.

Big batteries that do more with less

Fang Liu, an assistant professor in materials science and engineering, for instance, is working to make sodium-sulfur batteries a viable technology. “This battery variation replaces the lithium, nickel and cobalt in lithium-ion batteries with abundant and cheaper sodium and sulfur,” she says. “They are energy-dense and could be used for both electric vehicles and power grids.”

Eric Kazyak, an assistant professor of mechanical engineering, is also investigating batteries beyond lithium-ion, including sodium-ion batteries, which are safer and cheaper than the current generation of lithium-ion. Matt Gebbie, the Conway Assistant Professor in chemical and biological engineering, is working on new types of electrolytes, or the charge-carrying part of batteries. His new “ionic liquids” (think: electrolytes) could enable safer, more powerful batteries that rely on cheap, plentiful multivalent ions (elements with more than one possible charge) like magnesium, calcium, zinc and aluminum instead of lithium.

Another alternative to grid-scale lithium-ion batteries is called a redox flow battery, in which the anode (negative electrode) and cathode (positive electrode) are in liquid form. This enables manufacturers to scale the batteries up cheaply and easily by simply making the tanks bigger. Dawei Feng, Y. Austin Chang Assistant Professor in materials science and engineering, and Patrick Sullivan (PhD MS&E ’22) are commercializing an organic redox flow battery through their spinoff company Flux XII.

A current relationship with chemistry

Another emerging option is converting and storing energy chemically, in tandem with renewable sources. Massive amounts of heat and pressure are required to break petroleum into hydrocarbon fuels and chemicals—meaning they create emissions during the production process and also add carbon to the atmosphere when combusted. However, with electrochemistry—chemical reactions created by electricity and a catalyst—it’s possible to produce these fuels and other chemicals using carbon dioxide captured from the atmosphere or industrial processes, along with electricity derived from renewable sources. The advantage, of course, is that these energy-dense fuels have a long “shelf life” and can be used to produce electricity or to power transportation.

UW-Madison researchers, including Marcel Schreier, the Richard H. Soit Assistant Professor in chemical and biological engineering, are working on methods to interconvert electrical and chemical energy. “We need to even out the immense fluctuations in energy over weeks, months and even seasonally; you need to take some energy from summer into winter,” says Schreier. “Batteries are not going to do that. You need something else—and that can be some form of chemical storage.”

Similarly, other researchers are looking to convert renewable energy into liquid hydrogen, which also can be stored and used as transportation fuel or to power fuel cells. Luca Mastropasqua, an assistant professor of mechanical engineering, is developing electrochemical devices that can produce hydrogen with an eye to using the fuel to power industrial processes like steel and concrete production—both of which are big contributors to carbon dioxide emissions.

Taking the heat (and returning the favor)

While batteries are good for eking a little more life out of your unplugged laptop or contributing a few extra hours of energy to the inevitable evening electricity spike, eventually power systems will need to add technologies that can provide steady, cheap electricity over the course of the day, or even many days.

One potential solution is thermal storage—variations of which have reached the commercial stage in sites across the globe. In general, thermal storage means using excess energy to heat up a large mass, and later converting that heat energy to electricity as needed. In some of the most sophisticated setups, concentrated solar power is used to melt salt, which is then used to produce steam to generate electricity at non-peak hours. In other versions, tanks of water, beds of particles like sand, or even a large concrete block are heated up instead of salt. Several of our mechanical engineering researchers, including Wagner, Assistant Professor Allison Mahvi, Consolidated Papers Associate Professor Mark Anderson, William A. and Irene Ouweneel-Bascom Professor Greg Nellis, and Professor Doug Reindl, are involved in research to realize and improve these processes.

Future prospects, on demand

Wagner says that the energy storage landscape looks promising, and most of these and other storage technologies are well on the road to becoming viable. Instead of competing with one another, he thinks most of these approaches will find niches where they make the most sense—whether that’s storing energy for the grid, use in transportation, as on-site energy for industrial processes, or something else.

Wagner points out that it took about 120 years for coal technology to fully mature during the Industrial Revolution—yet many emerging storage technologies have gone from an idea to a market-ready solution in a fraction of that time. “With energy storage, researchers are now solving problems that are engineering problems, not fundamental problems,” he says. “Energy storage technologies are maturing; researchers are learning how to make cost-effective systems without exotic minerals. I’m optimistic that the current technology options we have available can get us a big step on the way to decarbonization.”