The Richard B. Russell Dam enables a unique capability within the Savannah River Basin as a provider of “pumpback” hydropower. Pumpback turbines are reversible, so they allow us to re-use water stored in the reservoir system multiple times. This capability is especially helpful during hot, dry summers when we can provide more peak-demand electricity with the same amount of water again and again.
How does pumpback work?
We typically operate the pumpback units seven days a week but only during the night. By pumping back at night-time, we maximize the benefit to taxpayers because night-time electricity costs about a third less than day-time electricity (due to decreased power demand).
Starting about one hour after sunset, we switch the designated generators into pumpback mode. Once they reverse direction, these units pull water from below the Russell Dam (in the upper portion of Thurmond Lake) and pump it back upstream into Russell Lake. Then, the next day during peak power demands, we generate electricity by releasing that water back down into Thurmond Lake.
How does pumpback at Russell affect Hartwell and Thurmond?
On weekends, we don’t typically generate power at Hartwell and Russell. This is because the Southeastern Power Administration (the agency that markets our hydropower to private power companies) requires less hydropower on weekends. SEPA’s customers typically meet their weekend needs with other power sources, such as nuclear and fossil fuels.
Unlike Hartwell and Russell, Thurmond always has a minimum flow requirement focused on meeting downstream water supply needs. That means on weekends, Thurmond is typically the only dam releasing water (and generating power).
As a result, Thurmond Lake usually declines on weekends, because water is released from the Thurmond Dam to meet the minimum downstream requirements, and at the same time, water is transferred from Thurmond back into Russell via pumpback. The temporary weekend drawdown at Thurmond due solely to pumpback at Russell can vary between 3.5 to as much as 13 inches. The animation below demonstrates this process:
For example, at the current release rate of 4,000 cubic feet per second (cfs), Thurmond declines about .24 feet on weekends due to normal releases from Thurmond Dam; and about .30 feet on weekends due to pumpback withdrawals. Based on these numbers a typical weekend at Thurmond can see levels drop approximately 6.5 inches.
Because of Thurmond’s bowl shape with gradual slopes and many shallow coves, this temporary weekend drop in water level appears accentuated. For example, one inch of water depth may equate to an extra 2 to 5 feet horizontally of exposed lake bed in some of the shallow coves around the reservoir. The animation below that demonstrates this:
The levels are restored on Monday through Friday, when we move the “borrowed” water back through the Russell Dam and into Thurmond. It is calculated as part of our pool balancing process.
Hydropower is not the top priority during drought
We want to emphasize that generating hydropower is not the driver for water releases during drought periods. In fact, we’ve continued to fall short of meeting SEPA’s power contracts by up to 10,000 megawatt hours a week during the current drought. During drought we release the minimal amount of water based on the drought condition. When we do release water during drought, we do it efficiently by running that water through the turbines to generate power as an incidental benefit to the release. Even recreation as an authorized project purpose trumps hydropower during drought operations.
It also helps to understand that as peaking power plants, we do not generate electricity around the clock. Rather, we carefully schedule power production at peak times of the day to maximize benefits to the public. This keeps your electrical prices low. Many hours of the day we have no water releases, and therefore no power generation. The Southeastern Power Administration (SEPA) markets and schedules our releases to match the greatest energy demand times. So as a result, some hours will have very high release rates, but other hours will have no release at all.
The Russell Dam has four traditional generators and four pumpback generators. Currently, only two of the pumpback generators are operational. The other two are undergoing repairs, but we anticipate the work to be completed in April. If the system remains in Level 2, we will have the ability to use all four pumps if required. While we likely wouldn’t need to use all four pumpback units at the same time, any increase in pumpback may increase the temporary weekend fluctuation at Thurmond Lake. (We’ll provide updates on this in future posts on Balancing the Basin.)
Thanks for reading us. ~ By Tracy Robillard, Corporate Communications Office.