The machines in pumped storage stations are reversible pump/turbines. In other words, they function as conventional hydro turbines in the generating mode but can reverse direction to pump the water back again. In the pump mode the generator becomes a huge motor which absorbs electricity from the network in order to drive the machine in the pump direction.
In a pumped storage scheme, the power station is located between an upper and lower dam. In generating mode, water runs from the upper dam through the station's turbines and into the lower dam where it is stored. During periods when there is sufficient electricity available, the machines are put into pump mode to pump water from the lower dam back into the upper dam where it is stored until the station needs to generate again. Pumped storage schemes are nett consumers of electricity.
The flexibility and speedy reaction of these machines allow them to service the network in many ways, and they operate around the clock. When not generating to meet the morning and evening peaks on the system, they are used to regulate the voltage.
Eskom, and South Africa, has two pumped storage schemes, both operated in conjunction with the Department of Water Affairs and Forestry as part of water transfer schemes. Palmiet Pumped Storage Scheme in the Western Cape can be used to pump additional water from the Palmiet River via the Rockview Dam to the Steenbras Dam to supplement the Cape Town water supply. Drakensberg Pumped Storage Scheme in the KwaZulu Natal Drakensberg is able to transfer 20m3 of water per second from the Kilburn Dam to the Sterkfontein Dam from where it flows into the Vaal River to augment water supply to Gauteng.
- Water is a renewable source of energy.
- The process of electricity generation has no emissions.
- Pumped storage schemes allows energy to be stored in the form of the water in their upper and lower dams.
- SA is a dry country with few sites suitable for pumped storage schemes.
- The construction of dams in rivers always has an impact on the environment.