Pumped Hydro Storage — Traditional large-scale storage using water reservoirs.

Pumped hydro storage is the most widely deployed form of Advanced Energy Storage Systems (AESS) in the world. It is a mature, reliable, and large-scale technology that has been used for decades to help manage the power grid. While it doesn't have the high energy density of modern lithium-ion batteries, its sheer scale and long operational life make it an essential part of the energy mix.

How It Works
The concept behind pumped hydro storage is both simple and brilliant. It involves two reservoirs at different elevations. When there is a surplus of electricity on the grid—for example, from a nearby nuclear plant during the middle of the night—the excess power is used to pump water from the lower reservoir to the upper one. The water in the upper reservoir represents stored potential energy. When there is a need for electricity on the grid, the water is released from the upper reservoir and flows through a turbine, which generates electricity as it returns to the lower reservoir.

Advantages and Limitations
A key advantage of pumped hydro storage is its ability to provide long-duration energy storage. It can store a massive amount of energy for many hours or even days. It is also an incredibly reliable and long-lasting technology. However, it is geographically limited, as it requires a specific terrain with two reservoirs at different elevations. The initial investment is also significant, and the environmental impact of building new reservoirs can be a major hurdle.