Five miles east of the German port city of Hamburg lies Tiefstack, a massive coal power plant that local officials want to shut down. While replacing its output will require a patchwork of smaller solutions, one piece is warming water with so-called waste heat from steel mills and aluminum foundries and storing it almost a mile underground. “The idea has been around since the thermos,” says Kirsten Fust, chief engineer at Energiewerke Hamburg, the local power utility.
The experimental technology, called aquifer thermal energy storage, pumps hot water 1,300 meters below the surface, then brings it back up for use in municipal systems known as district heating when it’s needed. If all goes as planned, by next year the technology will be able to provide heat to more than 13,000 households.
The city has drilled an initial hole for the project at a cost of just over €10 million ($10.7 million), and geologists are testing the stone. If it meets their expectations they’ll drill a second hole, allowing water to circulate through the system. In the summer, the groundwater in the aquifer—which is about 45C (113F)—will be pumped to the surface through one borehole, heated to 90C as it circulates through nearby millworks, then pumped back down into the porous sandstone via a second borehole. The rock structure is expected to hold more than 100 million liters (26 million gallons) of water, maintaining a temperature of about 70C until the cold season, when it can be returned to the surface and fed into the district heating network.