Operation of pumped storage hydro plant

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.

Advantages:

• Supplies peak load quickly (start-up in a few minutes); also gives spinning reserve and frequency support.
• Lets base-load thermal plants run at a steady, efficient load; raises the system load factor.
• Very little water is consumed; the same water is used again and again.
• Long life and low running cost.

Limitations: high capital cost, need for suitable topography with two reservoir sites, and a net energy loss of 20–30 %.