Pumpspeicherwerke, or pumped storage power plants, are a type of hydroelectric energy storage system. They work by using excess electricity to pump water from a lower reservoir to an upper one during times of low demand. This stored water can then be released to generate electricity when demand is high, helping to balance the energy supply. These facilities are crucial for integrating renewable energy sources like wind and solar power into the grid. By storing energy when production exceeds demand and releasing it when needed, Pumpspeicherwerke enhance grid stability and support the transition to a more sustainable energy system.