Francis turbine in pumped storage plant

In a pumped storage plant the Francis turbine is used as a reversible pump-turbine. One machine works as a turbine when water flows down and as a centrifugal pump when it is rotated in the opposite direction. It is coupled to a reversible synchronous machine that works as a generator or as a motor.

 Upper reservoir
   |   generating: water down
   v ^ pumping: water up
 +-----------------+   +------------------+
 | Francis pump-   |===| motor-generator  |== grid
 | turbine (spiral |   | (reverses for    |
 | casing, runner, |   |  pumping)        |
 | wicket gates)   |   +------------------+
 +-----------------+
   | ^ draft tube
 Lower reservoir

Why a Francis machine is used

• The Francis runner is a mixed/radial flow reaction runner, so its shape is very close to that of a centrifugal pump impeller. When driven backwards it pumps water efficiently.
• It covers the head range of most pumped storage sites (about 30 m to 700 m).
• One machine replaces a separate pump and turbine, so the power house, valves and civil works are smaller and cheaper.
• Wicket gates control flow in turbine mode and are set for best efficiency in pump mode.
• Mode change is quick (a few minutes), so it gives fast peaking and spinning reserve.

Operation

1. Turbine mode (peak hours): Water from the upper reservoir enters the spiral casing, passes through the wicket gates and runner, and leaves through the draft tube. The machine generates power.
2. Pump mode (off-peak): The machine is started as a motor (by a starting motor, back-to-back start or static frequency converter) in the reverse direction with the runner dewatered by compressed air, then water is admitted and pumped up.
3. Efficiency is slightly less than a separate pump and turbine (about 90 % in each mode, 70–80 % for the cycle).

Feasibility in Nepal

It is feasible in Nepal, mainly for the reasons below, though high cost and dry-season energy deficit are the main obstacles.

• For: high heads in hilly terrain; run-of-river plants give night and monsoon surplus that can be stored; time-of-day peak in the evening; possibility of cheap off-peak import from India and solar surplus; better frequency and reserve support for the grid; sites near existing reservoirs like Kulekhani have been studied.
• Against: large investment and long construction; in winter there is little surplus to pump with; 20–30 % energy loss; heavy sediment in rivers wears Francis runners; reservoir projects may be a higher priority.

Overall, with time-of-day tariffs and power trade, a pumped storage plant using Francis pump-turbines is a sound option for meeting Nepal's peak demand.