Pumping plants store energy in the form of gravitational potential energy of water, lifting water from a lower reservoir to a higher one. During periods of high demand, the water stored in the upper reservoir is released by turbines to a lower reservoir to produce electricity, while in periods of low demand the water is pumped back to the upper reservoir and stored again. Although in general these are surface reserves (packaging, deposits, etc.), the sea or underground caverns can also be used as a lower reserve.
The technical peculiarity that occurs with respect to conventional hydroelectric plants is the use of reversible turbopump groups (although they can have two different independent units) that allow alternately turbined or pumped water between two reservoirs located at different levels.
From a hydraulic point of view, the storages are classified in closed or open circuit systems, or what is the same, pure accumulation plants, or mixed. In the case of closed circuits, the storage hydraulic circuit is not continuously connected to a natural channel – only for the initial filling, and to recover losses due to evaporation or infiltration -; while in the case of open circuits, the connection is permanent.
Learn all about the past, present and future of pumped energy storage in this article.