Pumped storage to reduce peak generation loss

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.

Scheme to reduce peak generation loss of a steam plant

A steam power plant is efficient only at a steady load near its rating. If it has to follow the daily load curve, it runs at part load at night (poor heat rate) and must be pushed above its economic loading, or backed up by costly peaking units, in the evening. Frequent load swings also cause thermal stress and higher fuel use per kWh.

  Load
  (MW)        peak met by pumped
   ^          storage (generating)
   |          ______
   |         /######\
   |--------/########\------  steam plant
   |  ......            ....   output kept
   |  :pump:            :pump: constant
   |  :mode:            :mode:
   +--------------------------> time (h)
   0        12         18    24
   .... = valley filled by pumping load

Scheme (coordinated operation):

1. The steam plant runs at a constant, near-rated load for all 24 hours, where its efficiency is highest.
2. During off-peak hours (night), the extra steam-plant output is absorbed by the pumped storage plant working in the pumping mode. This "fills the valley" of the load curve.
3. During peak hours, the pumped storage plant works as a turbine-generator and supplies the peak. This "shaves the peak".
4. The pumped storage plant also takes sudden load changes and acts as spinning reserve, so the steam units are not forced to ramp quickly.

Benefits:

• Higher average efficiency and lower fuel cost of the steam plant; fewer start-stops.
• Load factor of the steam plant rises; the installed capacity needed for peaking falls.
• Even with a cycle efficiency of 70–80 %, the saving is positive because cheap off-peak energy replaces expensive peak generation (gas turbines, diesel, or the steam plant at its worst heat rate).