The big problem of the energy transition is storing natural energy from the wind or sun efficiently. The Swiss company Energy Vault has now presented a possible solution: huge metal towers from which concrete blocks are suspended on steel ropes. When there is an excess of wind energy, the blocks are raised; when they are allowed to fall again, their drop energy is used to generate electricity.

The towers are 33 floors high, cover the area of a football field and have six crane arms from which up to 5,000 weights can hang. Their movements are controlled by algorithms to make the loading and unloading movements as efficient as possible. One tower has a capacity of 35 megawatt hours, allowing it to deliver an output of 4 megawatts. The energy efficiency of the system should be around 90%. Energy Vault wants to install the towers close to wind and solar parks in particular and use their connections to feed the electricity from the storage into the grid.

A small demonstration system is on display at the main office of Energy Vault in Lugano to illustrate the principle. The first 35 MWh system is to be built in India in 2019 and will be able to store the daily requirements of 35,000 four-person households.