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Banks of scorching hot batteries filled with molten metals may be the long-sought silver bullet to make large-scale adoption of wind and solar energy a practical, purely green reality. Such a storage solution is needed because, as we know, the wind doesn’t always blow and the sun doesn’t always shine where and when it’s needed. “Right now, if you run a solar farm or a wind farm and you want to deliver electricity when the wind isn’t blowing or the sun isn’t shining, the cheapest way is to get a gas-fired peaking unit,” Donald Sadoway, a materials chemist at the Massachusetts Institute of Technology, told me Monday.
Gas-fired peaking units are mini power plants that can be turned on and off quickly to meet demand for electricity when other sources are unavailable or maxed out. “Those things are cheap to buy, they are cheap to run, and the price of natural gas has been falling recently in the United States. So the way people have been looking at (shoring up) renewables is to turn to natural gas,” Sadoway explained. “And that’s fine. It’s not illegal. There’s nothing immoral about it,” he added, “but it is not 100 percent green at this point.” For the industry to adopt the greener battery technology, the cost of the battery has to be as cheap, efficient, and reliable as state-of-the art natural gas-fired peaking plants.
Sadoway and his colleagues are hard at work on a liquid battery they believe will meet these criteria. On Monday, they described their progress in the Journal of the American Chemical Society.
The battery is a cocktail of metals that naturally settle into distinct layers because of their different densities, similar to a “black and tan” pint served at British pubs, where dark stout rests on top of denser pale ale. Batteries need three layers — positive and negative poles and a membrane between the charges. In the case of the liquid battery, molten metals on the top and bottom serve as the positive and negative poles and a layer of molten salt serves as the membrane. “The principle of the battery is an alloying reaction,” Sadoway explained. Alloys are metals made via the combination of two or more metallic elements. In the liquid battery, the top layer is magnesium and the bottom layer is antimony. “The driving force for current is the desire of magnesium to enter the antimony and form an alloy,” Sadoway said. “In order to alloy, the magnesium has to first get across the molten salt and in order to do so, the magnesium has to lose two electrons and become a magnesium ion.” Those two electrons are what escape to the wires to power our gadgets and appliances.
“When the magnesium ions get to the interface with the antimony, they acquire two electrons which have been pulled out of the external circuit and then that makes neutral magnesium which then alloys with the antimony,” said Sadoway. To charge the battery, the process is reversed. Sadoway and his colleagues have tweaked the recipe of this liquid cocktail for several years and gradually scaled up the size of the batteries.
The initial tests consisted of a battery about the size of a shot glass; then they went to a battery the size of a hockey puck and, now, the team reports a six-inch-wide version that has 200 times the storage capacity of the original.
Keeping them hot
To keep the metals in a liquid state requires a battery operating temperature of 700 degrees Celsius (1,292 degrees Fahrenheit). This heat comes at an energy cost — “we have to lose some of the energy we are storing in order to keep the battery at temperature,” Sadoway explained. Tests show about 75 percent efficiency — that is, for every 100 units of electricity put in the battery, 75 units come out. The rest is spent keeping the battery hot and lost due to inefficiencies in power electronics and converting back and forth between AC and DC. A loss of 25 percent is actually quite reasonable, according to Sadoway. As long as more than a 25 percent spread between the price of electricity when the battery is charged and discharged, a utility can recoup its investment cost and make a buck.
For example, a utility could charge up its battery in the middle of the night when the wind is blowing and rates are low and then sell it back to the grid in the afternoon when rates are high. “In certain markets like California, there can be day-night price swings that can be not so many percent, but so many X,” Sadoway noted. “In a market like that, this thing would do just fine.”
Battery vs battery
According to Sadoway, who has started a company, Liquid Metal Battery Corp. to scale up and sell these batteries, the liquid approach is potentially better than competing technologies such as lithium ion batteries, which require the expansion and contraction of solid parts in order to work. All this swelling and contracting amounts to wear and tear, which is often why the lithium ion batteries in laptops, for example, go kaput after a few years. “Those kinds of failure mode are absent in this battery because it is all liquid and liquid can accommodate volume changes,” Sadoway noted. Lab tests, he said, show that the lifetime of the battery isn’t limited by its capacity to hold a charge so much as by the lifetime of materials used to encase and insulate it.
Current materials, he said, may begin to corrode after 10 to 15 years sufficiently to change the chemistry of the battery or permit the battery “to eat its way out of the case.” Another advantage to the liquid technology, he added, is the abundance of the raw materials used to build it. Magnesium and antimony are abundant in the United States and low cost. As well, assembly of the battery is straightforward. Due to density differences, for example, the layers self assemble. “No clean rooms, no fancy nano-tech, nothing like that,” Sadoway said.
Daniel Kammen directs the Renewable and Appropriate Energy Laboratory at the University of California at Berkeley. He said the biggest challenge for the liquid battery is the high operating temperature. “Even if the waste heat can be harvested for an added benefit, systems operating at over 1,000 degrees are going to be a challenge for long-term maintenance,” he told me in an email exchange on Tuesday.
Sadoway’s startup up is focused on scaling up the battery technology with the best chemistry that comes out of his lab at MIT. While the lab has reached a six-inch diameter cell, the company has cells that are 16-inches in diameter, he said. The idea is to take these cells, stack them about 20 high, and link the stacks together in rows about 20 deep that that fit inside a 40-foot shipping container. This shipping-container-sized battery would provide about 2 megawatt hours of juice. By the end of 2014, the goal is “to have something that can be readily shipped to a potential customer for testing,” Sadoway said.
While the utility companies may be interested in the batteries as an alternative to gas-fired peaker plants to make their solar and wind farms viable, the batteries also could ease transmission line congestion. This could be particularly useful in tech-heavy regions such as the Bay Area, where the energy demands of server farms are steadily climbing, noted Sadoway. On certain days of the year, for example during a heat wave in the middle of the summer, “you can’t get enough electricity through the lines. The transmission lines are running at full capacity,” he said.
Instead of building additional transmission capacity — bringing more wires into the city, which is usually controversial and requires a drawn-out permitting process — companies could plop a battery in the basement of their buildings. “From midnight to 5 a.m., when the lines aren’t congested [and rates are low] you could be shipping electricity into the center of the city and storing it in the basement of these buildings,” Sadoway explained. “Then, in the middle of the day, you just take it right out of the basement into the servers.”
Kammen, the University of California energy professor, said this is “exciting stuff and a welcome area of long-overdue innovation.”