TVA‘s Grid Transformation Pairs Solar and Natural Gas to Phase Out Coal

The Tennessee Valley Authority (TVA) is engaged in a massive grid transformation project. It is phasing out coal, which has been the backbone of its system since the 1950s. Coal is being replaced by solar, wind, and natural gas generation. TVA already has 4.2 GW of solar and another 1 GW under development, in addition to contracts to bring in 1.2 GW of wind energy.

To support its renewable energy ambitions, the TVA is either upgrading or adding a great many natural gas plants. This includes:

• The 1.3 GW Kingston Fossil Plant, once the largest coal plant in the world, will be retired in 2027 to be replaced by a 1.5 GW complex. The new facility will consist of a combined-cycle plant powered by GE Vernova LM600 PF+ gas turbines, 100 MW of onsite battery storage, and 4 MW of solar generation.
• A 1,450-MW natural gas-fired combined-cycle plant to replace the Cumberland coal plant.
• TVA is adding 500 MW of peaking GE LM6000VELOX gas turbine packages at its Johnsonville Combustion Turbine Plant.
• Overall, TVA plans to add 3.8 GW of natural gas generation to the grid between 2023 and 2028.

Caption: Aerial rendering of Kingston Energy Complex that will be fueled by natural gas. It is designed to balance the grid while the TVA adds a large quantity of renewable assets. Credit: TVA

The TVA’s Roots

President Roosevelt established the TVA in 1933. Almost a century later, it is the nation’s largest public utility with a portfolio of 30 dams or hydroelectric facilities, four coal plants, 16 natural gas plants, three nuclear plants, 14 solar energy sites, and one wind farm.

At its peak in 2007, coal made up 58% of all TVA generation. Today, it still provides about 7 GW of TVA’s 35 GW of generation from four large plants: the 1.3 GW Kingston plant, the 2.5 GW Cumberland station, the 976 MW Gallatin plant, and the 1.2 GW Shawnee facility. All of that is to be shuttered by 2035 and some much sooner. The shortfall is to be made up by solar, wind, natural gas, battery energy storage systems (BESS), and some nuclear.

With coal being steadily withdrawn from the grid, the agency’s policy is to add renewables while beefing up its natural gas generation capabilities. TVA currently operates over 100 natural-gas- and fuel-oil-fired generators at 17 sites across Tennessee, Mississippi, Alabama, and Kentucky. That adds up to more than 12 GW—a number that will increase in the coming years.

In 2007, natural gas accounted for 10% of TVA capacity. That rose to 20% in 2017. Eventually, the plan is for it to provide 23%. New-build developments could add more than that, but TVA doesn’t intend to operate them continuously as it wants to favor renewables when they are available.

Caption: Sixteen new GE Vernova LM6000 PF+ aeroderivative gas turbines will replace retiring coal units at the Kingston power plant. Credit: GE Vernova

Kingston Making the Coal-to-Gas Switch

To support its renewable energy ambitions, the TVA is either upgrading or adding a great many natural gas plants. This includes the historic Kingston Fossil Plant, once the largest coal plant on earth. It will be retired in 2027 and replaced by a 1.5-GW complex made up of a combined-cycle plant powered by GE Vernova LM600 PF+ gas turbines, 100 MW of onsite battery storage, and 4 MW of solar generation.

Kingston’s coal units are suffering under the demands of the modern grid—frequent starts and stops are the norm. Its aging steam turbines are just not designed for that. As a result, maintenance issues are common. The new facility will comprise 16 dual-fueled aeroderivative gas turbines that will operate in combined-cycle mode to provide the bulk of its power output. These GE Vernova LM6000 PF+ units each have an output of 57.1 MW and a net heat rate of 8,357 Btu/kWh (lower heating value). They offer a simple-cycle efficiency of 40.8% (56% in combined cycle) with a ramp rate of 30 MW per minute and a cold startup time of five minutes. They can turn down to 50% load. They incorporate a dual-fuel dry-low emissions (DLE) combustor that can meet emissions limits in compliance with the regional air district requirements. Further, the system does not need to use water for NOx emissions abatement.

Caption: TVA’s 1.3 GW Kingston Fossil Plant was once the largest coal plant in the world. It is slated for retirement in 2027 and is being replaced by a new natural gas plant on the same site. Credit: TVA

“Natural gas units are cleaner than coal-fired generation, and we can operate them when other sources of generation, like solar, aren’t available,” said Jamie Cook, TVA’s General Manager of Major Projects. “They supplement those sources with reliable power when we need it most.”

Work on converting the site to natural gas is well underway. The first unit has been commissioned, and others are scheduled over the coming months. The Ridgeline Pipeline, too, is being expanded to bring enough natural gas fuel to the plant. The extension should be in service by the fall of 2026.

“The Kingston Energy Complex highlights the way diverse generation works together to ensure TVA can provide more reliable, resilient, and affordable power,” said TVA Chief Operating Officer Don Moul. “These aeroderivative units will help us meet demand during peak energy usage and supplement solar generation on days when sunshine is limited.”

Caption: The TVA is adding SSS Clutches to its gas turbines so they can operate both as power producers and as synchronous condensers. This 240T SSS Clutch destined for TVA is being inspected prior to shipment. Credit: SSS Clutch

Building a Stable Grid

The decision to limit natural gas production and prioritize renewables has caused TVA to specify units that can also provide voltage support to maintain system stability and reliability. Each LM6000 PF+ at Kingston is being fitted with synchro-self-shifting (SSS) clutches from SSS Clutch Co. of New Castle, Delaware. The clutches are built into the load gear as the turbines operate at a speed higher than the generator. The gear helps to create synchronization with the grid.

Grids tend to become less reliable after rotating generation plants are closed and replaced with wind and solar. The grid needs the rotating inertia and system stability benefits of traditional power plants.

“By enabling these LM6000s to run as synchronous condensers, the TVA is ensuring it has enough inertia, system stability, and reactive power support,” said Morgan Hendry, President of SSS Clutch. “Regulators and utilities are starting to realize the folly of retiring natural-gas-generation assets when they are vitally needed for peaking power and for grid stability.”

The original SSS Clutch design was conceived 90 years ago for automotive transmissions. It was adapted for use in power generation and marine propulsion systems. The SSS Clutch is a freewheel with models ranging from 100 kW to 400 MW. As much as 4 MNm of torque can be transmitted at 600 rpm and rotational speeds can go as high as 16,000 rpm with the smaller clutches. That enables any generator system to function as a synchronous condenser when not producing active power. By disengaging the turbine from the generator when it isn’t producing electricity, the generator provides the grid with stabilizing voltage, inertia, and voltage control.

Hendry added that the TVA successfully tested an SSS Clutch on one of its PF+ units as part of the commissioning process. New clutches will be sent to the site whenever the agency is ready to commission additional clutches as part of its timeline for a 2027 grand opening.

Johnsonville Combustion Turbine Plant

TVA has operated the Johnsonville combustion turbine plant since 1975. It sits on 700 acres in the town of New Johnsonville, Tennessee. It has long featured 20 GE Vernova simple-cycle combustion turbines with a combined generation capacity of 1,269 MW. Now, the facility is being expanded courtesy of 500 MW of peaking aeroderivative combustion turbines.

Caption: 10 GE Vernova LM6000VELOX packages are being installed at a TVA site in Johnsonville, TN. Credit: GE Vernova

TVA has ordered 10 GE Vernova LM6000VELOX packages with the DLE option for its dual-fuel capability, high-cyclic life, and fast start time. This enables the utility to add anywhere from 50 to 500 MW to the grid within five minutes whenever needed due to an emergency, peak utilization, or a sudden falloff of wind or solar. Based on GE Vernova’s LM2500XPRESS, the LM6000VELOX can largely be assembled in the factory due to its modular pre-packaged design.

“The LM6000VELOX solution offers greater than 99% start and operational reliability and over 98% availability,” said Olamide Ogunduyile, GE Vernova’s Gas Power, Aero Technical Sales Executive. “Its two-spool design results in lower overall maintenance costs.”

The LM6000VELOX package contains the LM6000 PF+, which offers anywhere from 53.9 to 57.1 MW, depending on whether it uses the SPRINT version to produce an additional power boost. Its heat rate is 8357 Btu/kWh (LHV), and it operates at 40.8% efficiency. It can provide 30 MW of power per minute as it ramps up and can start up from cold condition in five minutes. It can also be turned down to as low as 25% of full load, if desired. The LM6000VELOX is available in both simple- and combined-cycle configurations, as either a DLE or the water-injected Singular Annular Combustor (SAC) combustor, and for both 50- and 60-Hertz markets.

Caption: The specifications for the various versions of the LM6000 in simple-cycle mode. Credit: GE Vernova

The LM6000VELOX includes a variety of features, including a package-mounted air filter, a base-mounted generator and gearbox to reduce foundation labor, a 50% reduction in the number of accessories that need to be shipped to the site, a prepackaged roof skid that is shipped as one piece, an improved cable tray configuration, reduced lube oil flushing including pre-flushing, and an improved drive-train alignment procedure that cuts alignment time in half. Various platforms and ladders are provided to simplify accessibility and maintenance. It can also burn up to 100% hydrogen, if desired. So far, TVA reports that three of these LM6000VELOX units are currently undergoing testing onsite before entering commercial operation.

Additional TVA Gas Turbine Deployment

As TVA closes more of its coal plants, it is steadily adding natural gas. In recent times, TVA has added about 1.5 GW of new gas-fired generation. These include:

• Three combustion turbine units, a combined 750 MW, at the Colbert combustion turbine site in North Alabama.
• 750 MW with three new units at the Paradise combined-cycle plant near Drakesboro, Kentucky that have replaced the Paradise coal plant.

On the immediate horizon, the 2.47-GW Cumberland coal plant in Tennessee is scheduled for closure by 2028. It is being replaced by a 1.4-GW natural gas-fired combined-cycle plant, which will be online by the end of 2026—the point where the first one of Cumberland’s two coal units is retired.

“Natural gas is an important part of our transition to a carbon-neutral future while maintaining reliability,” said TVA President and CEO Jeff Lyash. “These state-of-the-art units will allow us to respond quickly to load demand and improve flexibility as we add more renewable energy, which is not always available on demand.”

Further plans include the 500-MW new Caledonia simple-cycle plant in Lowndes County, Mississippi. Overall, TVA’s ambitious plan is to add 3.8 GW of natural gas generation to the grid between 2023 and 2028.

Drew Robb is a freelance writer specializing in engineering and technology. Email Drew at drew@robbeditorial.com