Sun and wind are not constant. So, if we're going to harness energy from them, we need a way to store the power for use when it's needed. Batteries are the way to do it, but a big question in the power industry is how to build a battery that's efficient enough and stores enough power to be useful.
At the Massachusetts Institute of Technology, professor of materials chemistry Donald Sadoway and David Bradwell are investigating batteries made from molten metal.
The molten battery design uses liquids of different densities (think of how oil and water separate) to work as the anode, cathode and electrolyte. Magnesium, for example, is used for the anode on top, in the middle there is a mixture of salt and magnesium chloride that serves as the electrolyte and at the bottom, there's antimony, which acts at the cathode. It a battery parfait, really, that's about the size of a shot glass. The whole thing is heated to 700 degrees Celsius, or about 1,292 degrees Fahrenheit.
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At that temperature, the magnesium atoms lose two electrons, becoming magnesium ions that sink through the electrolyte to the antimony at the bottom. The electrons are captured and sent along a circuit to the outside, where they'll provide power.
Recharging is done the same way as ordinary batteries: hook up a source of current to the positive electrode and the magnesium migrates out of the antimony and back to the top of the cell.
Such high-temperature electrochemistry is already done in the aluminum smelting industry, so scaling the size of these batteries up for use by a power utility is doable. Sadoway's team think they can scale up their version to a six-inch battery.
The big challenge will be keeping the batteries hot enough and reducing the temperature at which they operate, so as to make them efficient enough for industrial use. Corrosion is also something that needs to be addressed, since one component is molten salts.
Then there's costs. One reason the team chose magnesium is that it is cheap and abundant. Any large-scale version can't depend on anything too exotic, or elements that need special handling.
That said, Sadoway and Bradwell have formed a company to commercialize the technology, so there may come a day when giant battery cells full of liquid metal are helping deliver power to the grid.