In what can't possibly fail to be a symbolic gesture, old coal mines in Germany are being repurposed into giant storage tanks for wind energy.
When I picture storing wind energy in a coal mine, I picture rows upon rows of mason jars stuffed to the brim with wind, sitting there in the darkness just waiting to be cracked open and harvested by a big white wind turbine.
This is not at all how it works.
Instead, the energy of the wind itself is stored in what's called a closed loop pumped storage system inside the mine, which relies on water to act as a rechargeable battery of sorts. When it's blustery out, wind turbines around the mine are used to pump water up into reservoirs in the mine's top levels. Then, to generate electricity, the water is allowed to run down to the bottom of the mine, spinning turbine generators as it goes, rather like a conventional dam. More wind energy is then used to pump the water back up to the top, and the cycle begins again.
The nice thing about this system is that you can pump a bunch of water up at night and then let it slowly drain down during the day, "time-shifting" your energy production to when you need it rather than when it just happens to be extra windy out. This helps to mitigate one of the big disadvantages of this kind of renewable energy: namely, that it's not always windy when you need it to be.
Building a test plant in Germany is estimated to cost $290 million over the next three to five years, and it would generate enough reliable power to light up a city the size of Tacoma, Washington for a day using the water battery alone.
Via Clean Technica