Hydroelectric power plants, which convert hydraulic energy into electricity, are a major source of renewable energy. There are various types of hydropower plants: run-of-river, reservoir, storage or pumped storage. The basic operating principle is similar for all of them: water flows through a turbine to generate electricity. However, unlike run-of-river or reservoir power plants, pumped storage plants enable us to store and schedule hydroelectric power generation, while also playing a crucial role in stabilizing the power grid. Storage or pumped storage power plants Storage hydropower plants, also called pumped storage plants, are facilities that produce electricity by storing water in an upper reservoir, then releasing it and running it through turbines at a lower level, thus generating electricity. Their name is derived from the pumping system that allows them to store the gravitational potential energy of water by pumping it from the lower basin to the upper basin during periods of low energy demand. How hydroelectric pumped storage works During hours of low energy demand, water from the lower reservoir is pumped up to the upper reservoir, so that the system is ready to generate power during peak demand. In this phase, the alternator acts as a synchronous motor and the turbine acts as a pump, bringing the water back up. During periods of high energy demand, water from the upper basin is released back into the lower basin. While in transit, the water flows through a turbine, converting mechanical energy into electricity. Generally, these plants use reversible turbines and generators, which can function either as pumps (moving water to the upper reservoir) or as generators (producing electricity). Advantages of pumped storage hydropower Pumped storage plants offer numerous advantages, including: High energy efficiency Energy storage Flexibility and stabilization of the electrical grid Reduction of CO₂ emissions Energy resilience High energy efficiency Due to the fact that water is reused in a continuous cycle, the efficiency of a pumped storage plant is around 70%-80%. This means that for every 10 kWh consumed for pumping, about 7-8 kWh is generated during the production phase. In addition, these power plants can come on line within minutes, providing backup power extremely effectively, balancing fluctuations in generation from intermittent sources and improving grid reliability. Energy storage One of the main advantages in the context of renewable energy is the ability to store water for use on demand. Reversing the flow and storing water in an upper reservoir creates an energy storage system that’s ready to be used whenever it’s needed. Pumped storage systems are the most common form of energy storage in the grid; they’re particularly useful for optimizing generation from variable renewable sources. Flexibility and stabilization of the electrical grid Pumped storage hydropower plants help stabilize the grid by balancing supply and demand, storing water and releasing it during peak demand. Due to their ability to respond quickly, they significantly improve grid flexibility. Reduction of CO₂ emissions By optimizing the use of energy sources, reducing dependence on fossil fuels and maximizing power generation from renewable sources, pumped storage power plants help reduce CO₂ emissions. Energy Resilience In the event of a power outage, a pumped storage plant can reactivate the grid by harnessing the energy produced by sending "emergency" water – which is kept in the upper reservoir for this very purpose – through the turbines. Types of plants Pumped storage hydropower plants fall into two categories: Pure (or closed-loop) pumped storage: in this type of plant, naturally flowing sources of water into the upper reservoir contribute less than 5% of the volume of water that passes through the turbines annually. Mixed (or open-loop) pumped storage: here, the upper reservoir is partially fed by naturally flowing sources of water that contribute more than 5% percent of the volume of water that passes through the turbines annually. Pumping in these plants is referred to as "voluntary pumped storage." Applications and the situation in Italy Internationally, the largest pumped storage hydropower plant is Fengning in China, with a capacity of 3.6 GW and a storage capacity of 40 GWh, surpassing the Bath County plant in Virginia (USA), with 3 GW of power and 24 GWh of capacity. According to Terna’s latest data (2023), there are 22 pumped storage plants in Italy, 14 of which are located in the North, with a maximum absorption capacity of about 6.5 GW and 7.6 GW in production, and a total storage capacity of 53 GWh. In terms of location, 84% of the storage capacity is concentrated in the largest plants, including three in the North (Edolo, Entracque and San Fiorano) and two in the South (Presenzano and Anapo). The role of pumped storage power plants in the future of energy Pumped storage hydropower plants play a key role in the future of energy, contributing to grid stabilization, renewable energy storage and reduced dependence on fossil fuels. Together with BESS systems, renewable energy storage in pumped storage power plants will be a strategic ally for a resilient, secure and sustainable energy system.
