Energy & Power Principle Animation:Pumped Storage Plant Lab

Load at 4 a.m. can be a third lower than at 8 p.m. When the evening peak exceeds what conventional units can supply there is a shortfall; when the night valley drops below their minimum output, wind power has to be curtailed. The pumped storage plant is still idle.

The pump-turbine and motor-generator form one reversible unit. To generate, water flows from the spiral case through the guide vanes into the runner, which spins forward. To pump, the unit stops, a frequency converter drives it up to reverse synchronous speed, and the runner flings water back into the spiral case and up the mountain. The switch takes about ten minutes.

Whenever load falls below the fill line, the units reverse and pump, turning the grid’s surplus—including night-time wind—into water in the upper reservoir. Fixed-speed units can only switch whole units on at full power, so the curve steps; variable-speed units can follow the load smoothly.

Whenever load rises above the shaving line, the units run forward and reach full output within minutes. Water runs from the upper to the lower reservoir and is nearly used up as the evening peak passes, then pumped back up overnight for the next day.

Four units of pumping energy give back about three—round-trip efficiency is around 77%, with most losses in the pump and turbine. In return, conventional units run steadily all day, no extra plant is needed for a few peak hours, and night-time wind no longer has to be curtailed.