European engineers have now found a way to combine wind with hydropower to allow for the continuous transmission of electricity even when the wind stops blowing. The idea is that water will act as a giant battery to discharge and modulate output during periods when the atmospheric air is calm.

The German firm Max Boegl Wind AG approached GE Renewable Energy to collaborate on the first wind farm with an integrated hydropower plant in March 2017. The four-turbine pilot project is to be connected to the grid by the end of 2017, and the hydropower plant is expected to be operational by the end of 2018.

The base of the wind turbines will double as a reservoir holding 1.6 million gallons of water.These will be the world’s tallest wind turbines, standing 584 feet high when the blade is pointing straight up. The base of the turbines—the bottom 131 feet—will double as a reservoir holding 1.6 million gallons of water, and the whole turbine will sit in another reservoir holding an additional 9 million gallons.

The setup essentially works as a hydro pump station. When electricity is needed, water flowing downhill from the reservoirs will power the hydro plant. When the energy supply is high, the hydro plant will pump the water back up the hill to the reservoirs and act as a battery. The hydro plant will benefit from this arbitrage, making power when the market price is high and using power when the price is low, while complementing the wind farm. The combination of the two power sources, which will work in parallel to each other, will ensure that electricity is always flowing from the plant.

For it to work, the wind turbines must sit at the top of a hill, and there must be room in the valley for a man-made lake—in this case 600 feet below the wind farm—that will store the water when it is not being used by the turbines.

The Swabian-Franconian Forest, in Germany, proved to be an ideal location for the set-up. Built into the hills will be a hydroelectric plant capable of producing 16 megawatts (MW) of power, while the wind farm on its own will produce 13.6 MW. The team behind the farms says the innovative design will provide a very fast increase in output, allowing the hydro plant to react immediately to fluctuations in the German energy market.

If it proves successful, the first-of-its-kind project could pave the way for future wind-hydro combinations. As the reservoirs can also use saltwater, the plants can be built close to the sea.

“It’s a bit risky, and it can’t work everywhere,” says Cliff Harris, general manager for onshore wind at GE Renewable Energy. “But the plant will run for several decades, and we expect the benefits will be felt over that time.”