Short note: Pumped storage hydropower

A pumped storage hydropower (PSH) plant stores energy as the gravitational potential energy of water, by moving water between two reservoirs at different elevations. It is the largest and most mature form of grid-scale energy storage in the world (it still holds most of the world's stored grid energy, although batteries are growing fast).

Working

• Off-peak / surplus time: cheap or surplus electricity (solar, wind, base-load plants) runs a reversible pump-turbine, pumping water from the lower reservoir to the upper reservoir.
• Peak demand: water from the upper reservoir is released through the turbine, generating electricity, and returns to the lower reservoir.

   [Upper reservoir]
         |  penstock
   [Pump-Turbine / Motor-Generator]
         |
   [Lower reservoir]
 Pump mode: grid -> water up
 Generate mode: water down -> grid

Stored energy E = ρ g V H, and the energy recovered is about η ρ g V H. Round-trip efficiency is about 70–85%.

Advantages

• Large capacity and long duration (hours to days); long life (50–100 years).
• Fast response, black start capability, and grid stability (frequency and voltage control).
• Helps integrate solar and wind and offers a use for surplus monsoon electricity.

Disadvantages

• Needs suitable topography and water; high capital cost and long construction.
• Environmental impacts on land and water; energy loss of about 15–30% per cycle.

Nepal context

Nepal has surplus electricity in monsoon and a deficit in winter, so storage and pumped storage are options being studied for the future (for example proposals considered by NEA and private developers). Existing storage is limited, with Kulekhani being the only large reservoir plant.