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The Quest for Solar Batteries, the Holy Grail of Clean Energy

Storing the sun is a tall order, but probably the only one that can help us move forward with solar energy.

The sun beams down enough energy to supply all the electricity the Earth needs, but for all our advances with renewable energy, one of the most frustrating barriers to a zero-emissions world is the lack of a great solar battery.

Solar panels are hailed as an easy, affordable step toward reducing cities' reliance on fossil fuels for electricity, but at the moment most solar panels can only produce electricity on-demand. They can store energy, but not for long enough to keep the lights on during cloudy days and long winter nights, said Nate Lewis, head of a solar fuels initiative at the California Institute of Technology.

Even though scientists and engineers are building cheaper and more efficient solar panels, one major bottleneck is the ability to store that energy for times when the sun is not shining, said University of Wisconsin, Madison, professor Song Jin, whose team is trying to develop a better battery.

Lewis said the best battery developed so far stores 200 watt hours per liter, roughly enough energy to power a water heater for maybe two weeks, whereas one liter of gasoline has about 12,000 watt hours (enough to power that same water heater for two-and-a-half years). In terms of potential, it's like comparing a tricycle to a motorcycle—you're not even in the same ballpark.

"In fact, the limit for many forms of renewable energy that are intermittent (solar, wind, etc.) is that the electrical grid will become unstable," if cities rely on them too much, Song told Motherboard. So making this technology more powerful is as important as getting solar panels into more areas.

This solar-charged device directly transfers energy from sunlight into a liquid battery and stores it in the container. Image: David Tenenbaum/University of Wisconsin

But to reduce the current greenhouse gas emissions that are causing climate change, researcher are looking to close this energy storage gap. Some labs are using sunlight to separate water into oxygen and hydrogen, in the hopes of burning the hydrogen as a clean energy source. Other labs are using reverse photosynthesis, where specially designed chlorophyll reacts with sunlight to create biofuels.

Lithium-ion batteries are the most common batteries used for most purposes (think the batteries in your flashlights and TV remotes), but researchers are working on different batteries for solar storage, Lewis said.

Redox flow batteries are the more common choice for solar panel electricity storage, he said/ They work by flowing reactants from two holding tanks into a central area where it reacts, and then the resulting products from the reaction is deposited into different containers. It's "a twist" on regular batteries because the whole thing is fairly large and the products and reactants aren't all stored in the same way as a lithium-ion battery.

Jin's team is working on making a better redox flow battery by using liquid electrolytes, which he said are able to store more energy. He essentially combined a solar cell with a large-capacity battery that transfers energy directly to the battery, rather than having to create electricity first before storing it in the battery in the form of energy.

But Lewis isn't sure that even better batteries will do the trick—at least, not batteries that look the way we expect, square and small and connected with wires. He wondered: What if solar power were stored another way?

One possible answer: storing solar energy in the form of fuels. He and his team at the Lewis Research Group are working on ways to create fuel that could be used in cars, boats and electricity generating stations as an alternative to gasoline or coal. His goal is to make a fuel powered from the sun that has as many watt hours as gas.

"We want to keep all our options open, and we're voting with our feet for what could be the ultimate answer if we can pull it off technically," he said. "If we can do that, we solve the storage problem and we solve the transportation problem."

He declined to elaborate on his process—his lab hasn't released its product yet—but he said he's not the only scientist working on this.

When it comes to powering homes, cars and businesses, the sun is humanity's best ticket to a zero-emissions world. But significant research and development still needs to be done if the world is going to be able to make use of that ample resource.

"It's probably inevitable we're going to use the most intense source known to mankind—the sun," Lewis said. "Let's not wait. Let's do it now."

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