The cost of renewable energy has been steadily decreasing, but the big question has always been how to store it for future use.
“In California alone, millions of dollars’ worth of renewable energy is being lost because it can’t be stored,” says Alfred Spormann, professor of chemical engineering and of civil and environmental engineering.
Now, thanks to a collaboration with Stanford chemical engineer Thomas Jaramillo, Spormann may be close to a solution. They are developing a practical way to use microbes to convert surplus renewable electrical energy into methane, which could be burned at those times when solar and wind power aren’t available, or during times of peak demand.
If they are successful, it will be after years of experiments designed to turn a microorganism called Methanococcus maripaludis into a renewable energy storage system. In nature, these microbes ingest hydrogen and carbon dioxide and exude methane. Spormann has been trying to feed these microbes surplus electricity from wind or solar sources, and then collect the methane they produce. Prior experiments had worked, but the process used in his earlier experiments made methane too slowly to be feasible on an industrial scale.
Spormann knew he needed to accelerate the microbial conversion process, so he teamed up with Jaramillo, who develops processes that use electricity to drive chemical reactions. In this case, Jaramillo ran electricity through metal electrodes placed in water to split H2O into hydrogen and oxygen gases. The oxygen bubbled off harmlessly. The released hydrogen atoms carried electrons to the microbes, where the arrival of that energy prompted the microbes to pluck carbon dioxide from the atmosphere and form methane. Because the methane doesn’t dissolve in the water, it’s easy to capture and store. The critical novelty was using electrodes to free many hydrogen atoms for methane conversion. Using electricity to split water and microbes to make methane provided the speed of methane synthesis and storage that had been previously lacking.
The idea of a hybrid process combining electrochemistry and biology isn’t inherently new. What had made it difficult in the past, Spormann says, was that most electrochemical reactions usually take place under conditions that kill the microbes. The microbial activity can also muck up the electrodes. By experimenting with different metals and settings, the Stanford researchers were able to get useful methane production rates using electrodes that didn’t hurt the microbes. Even better, the electrode metals that worked best, such as nickel-molybdenum, are cheap and abundant, suggesting that the process could be cost-effective at utility scale.
Spormann acknowledges that it may seem counterintuitive to store surplus wind and solar power in the form of methane, a key component in natural gas and usually considered a greenhouse agent. But while burning methane releases carbon dioxide, Spormann said the methane produced in their system recycles carbon dioxide already in the atmosphere.
The results have already attracted outside interest. With funding from the Department of Energy, the researchers are now working with Lawrence Livermore National Laboratory and Southern California Gas on more efficient designs. Because the microbes must absorb electrons from a metal cathode, one strategy is to develop cathodes that have a huge surface area in a small volume of space to provide more opportunities to promote the conversion of atmospheric carbon dioxide into methane.
Spormann said the process has another benefit when considered in the context of the overall electrical grid. Utilities currently use natural gas-powered turbines to provide extra electricity at times of peak demand. Natural gas turbines are favored for this role because they can be turned on an off like the burners on a stove. Today the natural gas they burn is pumped from underground fossil fuel sources and therefore eventually releases more carbon into the atmosphere. In contrast, this process will store surplus solar energy in the form of natural gas that is carbon-neutral, and then reuse it in a way that takes advantage of the existing infrastructure for generating electrical power on demand.
“We can store clean energy in a form that feeds right into the current electric grid,” Spormann said.
Collaborating on the research were postdoctoral research fellow Frauke Kracke and life science researcher Joerg Stefan Deutzmann in Spormann’s lab, and postdoctoral research fellow Andrew Wong in Jaramillo’s lab. This work was supported by Stanford’s Global Climate and Energy Project and by SLAC National Accelerator Laboratory.