 When energy demand is high, water from the upper reservoir is released down to the underground power station via a single long headrace tunnel which splits into two tunnels before each separate into two long penstocks.Each penstock feeds a single reversible 400 MW Francis turbine-generator with water before it is released into a long tailrace tunnel which discharges into the lower reservoir.When energy demand is low and therefore inexpensive, the turbines reverse into pumps and send water from the lower reservoir back to the upper reservoir.