ÑDz©ÌåÓý¹ÙÍøÊ×Ò³

A 2-billion-Swiss franc (�2.05 billion/$2.10 billion) project could help stabilise Europe's increasingly expensive electricity as it shifts to renewable energy. The water battery, Nant de Drance, located between two reservoirs in a cave 600 m (nearly 2,000 ft) underground in the Swiss canton of Valais, is being described as a big step in Europe's transition to green energy. Europe will need to develop 200 GW of energy storage capacity by 2030 � more than fourfold its current capacity, the European Association for Storage of Energy estimates. The project, which took 14 years to finish, is made up of 17 km (10.5 miles) of underground tunnels, housing six huge turbines powered by water cascading down a steel pipe in a cavern the length of two football fields. At the peak of construction, 650 workers were on-site, working to excavate some 1.5 million cubic meters of mountain rock at an altitude of 2,000 m. The project involved raising the water level of one of the two reservoirs, the upper one (Vieux-Emosson) by 21.5 meters to double its capacity. It now holds as much water as 6,500 Olympic-sized swimming pools (25 million cubic meters of water). Crucially to its success, Nant de Drance uses variable speed pump turbines. This means the power-pumped hydropower plant can switch from pumping at full power to running the turbine at full power within five minutes. The volume of water passing through the turbines, 360 cu m a second, corresponds to the flow of the Rhone River in Geneva in summer.