Currently, sodium sulfur batteries operate at very high temperatures - above 300 degrees C (572 degrees F), and the components in them are corrosive. This isn't the sort of thing that you would want in your home, and, for efficiency, they work best at a much larger size; they aren't really at a home-scale size. On the other hand, there are some advantages to sodium sulfur batteries. They use very common and inexpensive materials, which makes them attractive. And the high energy density means that a small battery is all that is needed for a large amount of energy storage.
The Ceramatec battery separates the sulfur and sodium from each other with a thin ceramic membrane which allows electricity to be stored while operating at a much lower temperature. Ceramatec envisions a refrigerator-sized unit that would remain below 98 degrees C (208 degrees F), the melting point of sodium. Keeping the sodium solid makes for a much safer battery. The battery could store 20 kWh worth of energy, either from local, sustainable sources such as wind or solar, or from off-peak recharging from the grid, much like a plug-in hybrid car recharges when the grid demand is low.
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