Pumped storage plant and feasibility in Nepal

A pumped storage plant is a hydro plant with two reservoirs at different levels. It uses cheap surplus electricity in off-peak hours to pump water from the lower reservoir to the upper one, and releases the same water back through the turbine to generate power during peak hours. It works like a large rechargeable battery for the grid.

  UPPER RESERVOIR (head pond)
  ~~~~~~~~~~~~~~~~~~~~~~~
        |  penstock / pressure shaft
        |  (water flows down when
        |   generating, up when pumping)
        v ^
  +---------------------+      +-----------+
  | reversible          |------| motor-    |---- grid
  | pump-turbine        |      | generator |  (supply
  +---------------------+      +-----------+   or draw)
        | ^
        v |  tailrace
  ~~~~~~~~~~~~~~~~~~~~~~~
  LOWER RESERVOIR (tail pond)

Main parts: upper reservoir, lower reservoir (or river/tail pond), intake, penstock or pressure shaft, power house with a reversible pump-turbine (usually Francis type) coupled to a reversible synchronous motor-generator, and tailrace.

Operation:

1. Generating mode (peak hours): Water from the upper reservoir flows down the penstock, drives the machine as a turbine, and the machine works as a generator feeding the grid. The water is collected in the lower reservoir.
2. Pumping mode (off-peak hours, e.g. night): Surplus power from base-load plants (thermal, nuclear or run-of-river hydro) drives the machine as a motor. The machine rotates in the reverse direction and works as a pump, lifting water from the lower reservoir back to the upper reservoir.
3. The cycle repeats daily (or weekly/seasonally).

Energy relation: Energy stored $E = \rho g V H$. Overall cycle efficiency

$$\eta_{cycle} = \frac{E_{generated}}{E_{pumping}} \approx 0.70 \text{ to } 0.80$$

So about 1.3 kWh of off-peak energy is used to get 1 kWh at peak. It is still economic because peak energy is far more valuable than off-peak energy.

Feasibility in Nepal's power system

Pumped storage is technically feasible and is becoming economically attractive in Nepal, but it should be planned for the near future rather than treated as an immediate priority over storage projects. Reasons:

Points in favour

• Topography: Nepal's hills give high heads over short distances, so small reservoirs can store large energy.
• Run-of-river dominance: Most of Nepal's plants are run-of-river (ROR). They cannot follow the daily peak; the evening peak is hard to meet, while there is surplus at night.
• Wet-season spill: In the monsoon, ROR plants produce more than domestic demand. Off-peak surplus energy that is now spilled or sold cheaply can be used for pumping.
• Cheap import / solar: Off-peak energy can be imported from India at lower rates, and growing daytime solar can also be stored.
• Grid support: Quick start and reserve capacity improve frequency control of a small, weak grid.
• Existing reservoir sites such as Kulekhani (the only storage scheme of NEA) have been studied for adding pumped storage.

Points against (in the current scenario)

• In the dry season Nepal is itself energy deficit; there is little surplus energy to pump with, which is exactly when peaking power is most needed.
• High capital cost, long construction time and difficult access in the hills.
• Cycle losses of 20–30 %; it is economic only if the peak tariff is well above the off-peak price.
• Sediment-laden Himalayan rivers cause abrasion of pump-turbines and siltation of reservoirs.
• Seasonal storage projects (reservoir type) may give more benefit per rupee in a deficit system.

Conclusion: A daily-cycle pumped storage scheme is feasible in Nepal, especially using wet-season and night surplus, cross-border trade and solar. With proper tariff (time-of-day and seasonal pricing) and site selection, it can solve the peaking problem of an ROR-dominated system.