As midnight approaches and Carolinians retire for the evening, energy consumption trails off sharply across Duke Energy’s electric generating system. Out of sight and mind, the utility’s nuclear generating fleet operates at full power night and day, with fossil units added to the system as needed to ensure customers have an uninterrupted electricity supply.
Demand for electricity varies, with the highest or peak demand occurring in the morning or afternoon, and lowest demand at night and on weekends. Utilities operate large nuclear and fossil units around the clock as “baseload” generation to meet the majority of demand, adding or removing smaller generation units as consumption rises and falls.
Baseload units operate best at full capacity 24 hours a day, which can create a generation surplus on the system during low-demand periods (nights and weekends). For almost 40 years, Duke Energy has used pumped-storage hydroelectric technology to capture this surplus generation for use during heavy or peak demand and to help reduce customer energy bills.
Conventional hydroelectric stations use water from a river or lake to spin turbine generators to produce electricity primarily for peak demand periods. Duke Energy has used hydroelectric generation for more than 100 years.
Pumped-storage hydroelectric stations have the unique ability to operate in either a generation or pumping mode. In the generation mode, water held in an upper reservoir flows by gravity through a turbine generator producing electricity before flowing into a lower reservoir just like a conventional hydroelectric station.
During pumping operations, excess electricity available during low-demand periods is used to spin the turbine generator in reverse. Water from the lower reservoir is pumped back to the upper reservoir, allowing the water to be recycled or used over and over again to produce electricity.
Bound by the laws of physics, the pumping mode uses more electricity than can be produced during the generation phase. What makes pumped storage useful is the ability to take surplus electricity during low demand periods to pump water uphill, making it available for use during peak demand periods.
In addition to giving a utility the flexibility to meet customer demand as economically as possible, pumped storage can respond to increases in electricity demand or voltage fluctuations within minutes.
Designed to operate at full capacity, the availability of Duke Energy’s low-cost nuclear generation during nights and weekends make it an ideal source of electricity for pumping operations at Jocassee and Bad Creek. Located in the Keowee-Toxaway River Basin in South Carolina, these two stations represent one of the largest concentrations of pumped-storage generation in the country, with a combined maximum generating capacity in excess of 2,000 megawatts (MW), enough power for approximately 1.6 million homes.
When used in a well-planned generating scheme, pumped storage plays an important role and can be one of the most economical forms of generation available.