The reservoir is filled during the day using solar energy to power pumps that draw water from reservoir below dam. At night electricity can be generated using water pumped up during day. Many facilities use the same turbines that generate electricity at night to pump water to upper reservoir by reversing turbine rotation. I included document about Hydro Electricity in Links below.
Pumped Storage Online Research
Read section on Oceans Themal Energy Conversion ( OTEC )