Drought or not, every drop counts
When Bill Proctor, a member of the River Operations hydrothermal team, looked at the temperature readings in early June, he saw a preview of the summer to come. Proctor’s team monitors key sections of the river, and a heat wave was already pushing water temperatures well above normal.
Those readings and river-flow measurements are used by Susan Jacks and her team in Operations Evaluation.
“We use temperature data and computer models to help the fossil and nuclear plants decide how to operate their plants without exceeding their permitted temperature limits,” says Jacks.
There has been more rain this spring than last. But rainfall above Chattanooga is still just 77 percent of normal for this fiscal year.
And, notes Jacks, “With the soil so dry from several years of drought, the rain we do get isn’t running off into the river.” Runoff, and thus hydro generation, is significantly below normal.
Since February of 2007, RO has been operating in a conservation mode, adhering to “minimum flow” guidelines in TVA’s reservoir operating policy. Under theses guidelines, TVA releases only enough water to maintain commercial navigation, water quality, water supply, protection of aquatic life and to cool the power plants. This supports TVA’s overall mission of managing the river system to meet those needs as well as needs for flood control, hydroelectric generation, land use and recreation.
The minimum-flow guidelines vary depending on the amount of water stored in the tributary reservoir system and the time of year. If water in storage is below the minimum system operating guide level, TVA provides a weekly average flow of 13,000 cubic feet per second (cfs) at Chickamauga Dam from June 1 to July 31 and a weekly average flow of 25,000 cfs from August 1 to Labor Day.
If there isn’t enough rain to meet the minimum-flow requirement, TVA has to pull the needed water from tributary reservoirs, which can cause water levels to drop through the summer.
When this happens, balancing guides established for each reservoir help ensure that everyone sees the same impacts. More water is drawn from reservoirs that are higher relative to their balancing guide levels – which means they have more water – and less from reservoirs that are nearer or below their balancing guides.
Even with the river system operating in conservation mode, the challenges for River Operations just keep coming.
“When we bump up against water-temperature limits, it can get intense,” says Jacks.
River Operations is already using set flow conditions to help Sequoyah and Browns Ferry nuclear plants stay within their hydro-thermal-compliance limits – not exceeding their permitted temperature limits for discharging water to the river.
They also suggest when the plants should start operating cooling towers – often making last-minute adjustments in response to unanticipated changes in the weather. Sequoyah, for example, was recently forecast to need a cooling tower, but then a nighttime thunder shower brought temperatures down. Browns Ferry has one unit in “helper mode,” that is, using towers to help the water cool down before it is released back into the river.
Among the fossil plants, recent RO water-temperature forecasts have suggested that John Sevier Fossil Plant may need to “derate,” cutting back on power production to stay within temperature limits, and RO is closely monitoring Kingston to help it avoid exceeding its limit.
“This is an ongoing activity,” says Jacks. “We are actively doing everything we can—working within our guidelines and the flexibility we have—to continue finding the best balance we can to support power production and meet our environmental goals and other river-system demands despite these tough drought conditions.”