The problem with wind and solar power production is that the wind sometimes blows and the sun sometimes shines when the energy isn’t needed. Batteries are needed to store excesses until they are needed.
One form of storage, which has earned Switzerland the nickname of the battery of Europe, is hydro. Excess power is used to pump water up to a dam so it can be released later when power is needed. This is great, however there are limits to how much water can be stored. Plus building dams is expensive and leaves a big mark on the landscape.
Another idea is to use excess electricity to pump air into a pressurised container. The air can then be released, turn a turbine on its way out and turn the rotation back into electricity.
Now imagine the pressurised container is a disused tunnel. This is exactly what a Swiss company called Alacaes is testing in a tunnel once used in the construction of the world’s longest train tunnel which runs under the Gotthard pass between northern and southern Switzerland.
The project, costing around CHF 4 million, is 40% funded by the Swiss government and involves experts from ETHZ, a technical university in Zurich.
The idea is not new and has been around since the 19th century. What is new is the size and efficiency of the Swiss project. Currently there are two large compressed air batteries in operation, one in Germany dating from 1978 and one in the US, constructed in 1991. Both use natural underground salt caves. The air inside them is mixed with natural gas and burnt as it is released. At the US plant, a mix of one third natural gas and compressed air yields the same amount of electricity as pure natural gas, essentially magnifying the yield from the gas.
Compressing air generates a lot of heat. The energy efficiency of an air battery is largely down to how much of this heat can be captured and reused. The Swiss team think they have made a big leap forward on this part. They think their technology will return 72% of the energy put into it. If 100 KWh are put in, 72 KWh will come out. This compares to around 50% for other plants. Hydro batteries typically yield 80%.
The tunnel has two 5 metre thick concrete plugs 120 metres apart. One contains a very strong steel door. Within the walls is a thermal energy storage (TES) unit. As the air is compressed on its way in the TES extracts and stores the heat. When the cold air is released it passes through the TES which releases heat into the air before it flows through the turbine that generates the electricity. The TES is Alacaes’ magic box.
Talking to RTS, Giw Zanganeh, a directer of Alacaes who is originally from Iran, said that the potential of this technology is enormous. A plant like this could power a town the size of Lugano (71,500) for 12 to 24 hours.
Just don’t forget to close the door.