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How a perfect storm of freezing cold and aging power plants led to Tennessee blackouts

Anila Yoganathan, Knoxville News Sentinel  


    The Tennessee Valley Authority thought it was prepared for the single-digit temperatures and wind from the fierce winter storm that descended days before Christmas.

    But not a few hours into Dec. 23, errors had already set off a series of miscalculations and failures that led to the first-ever rolling blackouts across the Southeast, leaving 10 million residents frustrated, cold and questioning the reliability of the federal utility.

    How did the agency responsible for keeping the lights on in its seven-state region miscalculate the demand for energy and its ability to keep power humming during a major storm?

    In the weeks since TVA administrators had to rely on rolling blackouts to reduce energy demand, two major setbacks have come to light:

    • Starting at midnight Dec. 23 and throughout most of the next 48 hours, TVA underestimated the amount of electricity it needed to provide to residents across the region, according to the Energy Information Administration.
    • A significant number of the power plants TVA planned to fire up proved to be unreliable during the cold weather.

    In the wake of the storm:TVA creates independent panel to review rolling blackouts before Christmas

    Under normal circumstances, the utility can generate more than 32,000 megawatts of energy. In addition to the 32,000, TVA can purchase 6,000 more megawatts from neighboring electric grids. A megawatt is enough to power about 585 homes, TVA spokesperson Scott Brooks said.

    So it shouldn’t be that heavy a lift to provide up to 33,425 megawatts, the amount that was needed on a freezing cold Dec. 23, even though that marked the highest ever single-day demand TVA had experienced. Going into that stretch of extreme cold weather, TVA knew it would have to draw on all of its resources, and planned to run all of its power plants.

    Then, in the early hours of Dec. 23, TVA experienced a major setback. Its largest power plant, the coal-fired Cumberland Fossil Plant, on which it relies consistently, shut down after being damaged by the cold.

    Events went downhill from there.

    Unable to keep some of its backup natural gas plants or its only big gas plant in East Tennessee running consistently, TVA struggled to keep the lights on. The only category of power plants that did its job uninterrupted was nuclear plants.

    Customers and elected officials were furious about the rolling blackouts. They're demanding answers:

    • Why wasn’t the utility prepared for every possibility with the weather?
    • Why is the grid so reliant on fossil fuel power plants?
    • Why did those plants fail when they were needed most?
    • Can TVA customers expect to see rolling blackouts again?

    All this happened in the wake of TVA’s decision - confirmed on Jan. 10 - to replace the Cumberland Fossil Plant with a natural gas combined cycle plant, raising questions about whether that's the right option for the future of the power grid.

    TVA administrators said they are investigating the infrastructure failure that occurred amid the bitter cold leading up to Christmas. Here’s how events unfolded that left Tennesseans in the dark.

    Midnight-6 a.m. Dec. 23: Cumberland plant shuts down just as extreme cold moves in

    • Starting at midnight EST, TVA underestimates the amount of electricity it needs to generate by 655 megawatt hours, according to the Energy Information Administration. This trend will continue throughout most of the frigid weather over the next two days. A megawatt is an output of energy while a megawatt hour is the amount of energy used in an hour.
    • The temperature at the Knoxville airport measures 46 degrees just after midnight, the warmest it will be for the next 48 hours. In Nashville, the temperature at the airport drops to 10 degrees by 12:30 a.m.
    • One of the two units at TVA’s coal-fired Cumberland Fossil Plant goes offline at 2:57 a.m. because “critical instrumentation” at the top of the boiler has frozen due to the cold temperatures and high winds, according to TVA.
      • Cumberland generates a lot of electricity for the Tennessee Valley at a maximum capacity of 2,470 megawatts, and TVA likes to run that plant as often as possible. To lose even one of its boilers during a major winter storm means electricity generation is going to be more challenging for the grid.
    • The temperature drops to 21 degrees at the Knoxville airport by 3 a.m. At the same time, coal generates its largest amount of electricity for the day at 4,482 megawatt hours.
    • By 4 a.m., the temperature at the Nashville airport is down to just 5 degrees. TVA’s trend of underestimating how much electricity is needed continues, now off by 1,099 megawatts hours, the same amount of energy that could be produced by one of its bigger natural gas plants.
    • The second unit at the Cumberland plant goes offline at 4:55 a.m., effectively shutting down the plant at a crucial time. During the winter, TVA typically sees a peak in power demand in the morning as people wake up, turn up their heat and start using appliances.
    • On top of that, TVA is unable to start its coal-fired Bull Run Fossil Plant in Anderson County in East Tennessee. Unlike most of its other coal plants, TVA does not run Bull Run 24/7. Instead, the plant is intended to run during peak seasons - summer and winter - but the plant is old and often unreliable, according to a 2022 report by the TVA Office of Inspector General.
      • The TVA inspector general's report revealed TVA has been more reliant on Bull Run than it initially planned to be. Bull Run is one of TVA’s oldest coal plants and its lack of upgrades and maintenance has caused the plant to deteriorate, be unreliable and pose safety concerns to workers, according to the report. Bull Run is set to retire in December 2023.

    TVA struggled to start its coal-fired Bull Run Fossil Plant in East Tennessee during the winter storm. Bull Run is one of TVA's oldest remaining coal plants. The plant has become unreliable as it nears its retirement in December 2023.

    • TVA’s coal plant generation drops by 1,044 megawatt hours between 5 and 6 a.m., according to the Energy Information Administration.

    6 a.m. Dec. 23: TVA struggles to meet full capacity with natural gas plants

    • As many people are waking up, the temperature is 10 degrees at the Knoxville airport by 6:05 a.m., and it’s only getting colder.
    • From 6 a.m. until 5 p.m., natural gas is generating electricity for TVA, but its output fluctuates between about 8,550 and a little over 9,600 megawatt hours.
    • Adding to its growing list of problems, TVA can’t get one of its big power-generating natural gas plants, the John Sevier Combined Cycle Gas plant in East Tennessee, to run properly. In the next week, TVA will say it is investigating the causes but is unsure which days John Sevier was struggling.
      • As with some of TVA’s coal plants, TVA likes to run its combined cycle gas plants continuously because they can generate a lot of electricity at once and can serve TVA’s 24/7 need for energy. John Sevier is the only combined cycle gas plant located in East Tennessee and has a maximum capacity of 871 megawatts.
    • The big plants aren’t the only facilities giving TVA trouble. At least some of the smaller backup natural gas plants won’t stay on consistently during the winter storm. In the next week TVA will say it is investigating the causes of this as well.
      • These smaller plants can be switched on and off relatively quickly so TVA does not run these plants all day, every day of the year. TVA primarily uses them during high demand for electricity like during cold or hot seasons - such as, say, the coldest days of the year. During winter months, TVA sees a peak in demand in the morning. Having 10 million people crank up their heat in the morning causes a strain on the grid, so turning on some of these smaller gas plants can help reduce that strain … if they work.

    9-10 a.m. Dec. 23: The sun comes up and so does solar power

    • By 9 a.m., it’s about 6 degrees at the Knoxville airport and below 0 degrees in Nashville. With the sun up, solar begins adding 75 megawatt hours of energy to the grid by then.
      • Earlier in the morning, solar power was providing a negative amount of energy to the grid because the sun was not shining. TVA’s solar portfolio is limited to 1,600 megawatts. Solar’s ability to generate electricity changes based on time of day and weather.
      • For most of December, TVA’s solar does not generate more that 1,000 megawatts at any given time, according to the Energy Information Administration. Additional solar panels could generate more megawatts for the grid but solar requires a lot of land and planning ahead. TVA plans to have 10,000 megawatts of solar on its grid by 2035.
      • Battery storage takes energy generated from solar panels on the grid and stores it for later use, like when the sun isn’t shining. The current technology for batteries, however, can store the energy for only up to four hours. More solar panels paired with storage could contribute more megawatts to the grid but TVA will need to build more of those sites first. TVA’s first battery storage site is set to be in Loudon County, in East Tennessee, and will hold up to 40 megawatt hours of energy. The facility was supposed to be operational in 2022 but has not yet come online.
    • TVA underestimates the demand for electricity by 1,911 megawatt hours at 10 a.m.

    10:30 a.m.-6 p.m. Dec. 23: TVA requests rolling blackouts for the first time ever

    • TVA asks 153 local power companies to reduce their energy demand by 5% immediately at 10:31 a.m. EST, according to the Knoxville Utilities Board and TVA. This request starts the process of rolling blackouts across the region.
    • The temperature is 8 degrees at Knoxville airport at 12:04 p.m., as people’s heat and lights go off and back on while blackouts roll through. Less than 30 minutes later, TVA sends a message on social media asking residents to reduce their electric use.
    • TVA tells local power companies at 12:42 p.m. that they can turn everyone’s lights back on at 12:43 p.m. EST, according to KUB, bringing an end to the blackouts for this day.
    • After TVA’s calls for rolling blackouts, the utility’s estimated demand for electricity is much closer to the actual demand at 4 p.m., underestimated by only 35 megawatts.
      • Solar generates its largest amount of energy for the day at 458 megawatt hours, while hydro generates its smallest amount at 2,083 megawatt hours.

    Snow begins to fall on Lower Broadway in Nashville on Dec. 22.

    • TVA sends another round of messages on social media asking residents to reduce their electricity use at 5:10 p.m.
    • The temperature at the Nashville airport drops to 5 degrees at 5:53 p.m. CST , and it’s going to stay that way for most of the night. Solar generates its last amount of energy for the day with 31 megawatt hours at 6 p.m. EST / 5 p.m. CST.

    7 p.m.-midnight Dec. 23: TVA back on track through the night

    • The temperature is down to 5 degrees at the Knoxville airport around 7 p.m., and TVA is back to underestimating the demand by more than 2,000 megawatts.
      • Meanwhile, hydro generates its largest amount of energy for the day at 5,673 megawatt hours. TVA uses its single pumped hydro facility on Raccoon Mountain during the extreme cold weather. TVA later says it will investigate whether Raccoon Mountain ran on both the 23rd or the 24th, or just one day. Raccoon Mountain works like a battery; water is pumped to the top of a mountain, and when TVA needs additional energy added to the grid, it releases the water, which turns a turbine, generating additional energy.
    • Natural gas generates its largest amount of energy for the day at 11,216 megawatt hours at 8 p.m. EST.
    • All of TVA’s nuclear plants have been running at full power around the clock on Dec. 23.
    • Initial numbers show TVA received about 1,500 megawatts of relief from participants of its “demand response” programs, according to TVA. Demand response asks participants such as corporations or industry to use electricity only during nonpeak hours to reduce additional demand and stress on the electric grid. For example, industries could conduct operations at night when most residents are asleep and not using additional electricity.
    • TVA purchases on average 5,433 megawatts of energy per hour from other utilities and market operators on both the 23rd and 24th. Because neighboring grids - including Duke Energy - also are experiencing high demand during the winter storm those days and have to institute their own blackouts, TVA is unable to purchase as much as the 6,000 megawatts it normally would be able to, especially on Dec. 24.

    1-6 a.m. Dec. 24: Early hours play like a rerun of the day before

    • TVA continues to have issues with Cumberland and is unable to start the Bull Run plant.
    • TVA continues to have issues with different natural gas plants.
    • All of TVA’s nuclear power plants are still running throughout the day.

    Watts Bar is one of three nuclear powerplants TVA has on the grid. Nuclear generated electricity throughout the December winter storm and was TVA's only power plant fleet that did not experience interruptions.

    • The temperature is 4 degrees at the Knoxville airport at 1 a.m. EST / midnight CST and TVA underestimates the demand for electricity by almost 3,000 megawatt hours, mimicking the issues from the day before.
    • An hour later, TVA sees its highest peak in power demand on a weekend at 31,756 megawatts at 2 a.m.
    • Natural gas is generating 11,462 megawatt hours at 6 a.m.

    5-7 a.m. Dec. 24: TVA asks local utilities for more rolling blackouts

    • TVA underestimates the demand for electricity by about 2,400 megawatt hours just as people are waking up at 5 a.m.
    • Less than an hour later, TVA requests KUB reduce electricity demand by 5% again by 5:51 a.m., according to KUB. Within the same hour TVA asks the local utility to reduce electricity by 10% by 6:12 a.m.

    A man walks his dog near Market Square in downtown Knoxville on Dec. 23. The temperature hovered around 7 degrees early that day after a winter storm moved into the region.

    • As the blackouts continue and electricity demand consequently falls, TVA’s underestimation of electricity is now only off by 309 megawatt hours at 7 a.m.
      • Hydro and natural gas generate their largest amount of electricity for the day at 5,345 and 11,541 megawatt hours respectively.

    8 a.m. Dec. 24-2 a.m. Dec. 25: Blackouts bring TVA’s electricity demand projections more in line with reality

    Steam comes off of the Tennessee River in downtown Knoxville on Dec. 23. TVA requested first-ever rolling blackouts from local utility companies to get electricity demand projections more in line.

    • TVA estimates demand will be at 31,261 megawatt hours, and the actual demand is 29,819 at 8 a.m. For the first time since Dec. 22, TVA is overestimating the demand for electricity. This continues until 1 p.m.
    • Solar starts adding around 96 megawatts hours of energy to the grid by 9 a.m.
    • The continued rolling blackouts in Tennessee draw national attention when Nashville Mayor John Cooper cites them in a 9:46 a.m. CST announcement on Twitter that he has asked the Tennessee Titans to postpone kickoff against the Houston Texans from noon until 1:02 p.m. CST.

    Snow falls on Nissan Stadium in Nashville on Dec. 22.

    • TVA notifies KUB it can restore electricity to residents at 11:30 a.m.
    • The temperature has risen to 16 degrees at the Knoxville airport by 1 p.m. TVA begins underestimating demand again by 70 megawatt hours.
    • By 2 p.m. it’s 25 degrees in Nashville, and that’s the warmest it’s going to be there for the entirety of the winter storm.
    • At 4 p.m. solar generates its largest amount of electricity for the day at 498 megawatt hours, while natural gas generates its lowest amount of electricity for the day at 8,691.
    • Coal generates its lowest amount of electricity for the day at 1,436 megawatt hours at 5 p.m. An hour later, solar and hydro generate their lowest amount of electricity for the day at 38 and 1,659 megawatt hours respectively.
    • TVA overestimates the demand for electricity by 448 megawatt hours at 2 a.m. Dec. 25. This is the first time TVA has overestimated demand since 1 p.m. Dec. 24.

    Changes in the Tennessee Valley region are affecting the power grid

    Before it began ordering the rolling blackouts on Dec. 23, TVA went through an established 50-step process intended to protect the power grid. The days before Christmas marked the first time the utility had ever reached step 50. The federal utility’s main objective is providing reliable electricity, a fact its leaders have touted to explain their actions and decision-making, but on Dec. 23 and 24 they struggled to do so.

    “We have invested hundreds of millions of dollars in hardening this system and preparing it for events like this,” TVA CEO Jeff Lyash said in a call with elected officials on Dec. 24. “And so when we experience equipment issues, like we did in this case with some of our generators, stations that although we thought we had prepared them for low, long-duration temperatures, in some cases, the preparation wasn't effective. And so we clearly need to do more.”

    Cars slide across an ice-covered White Station Road in Memphis on Dec. 23. Memphis roads were coated in snow and ice after a cold front dropped temperatures into single digits.

    TVA is still reviewing the problems that led to the events before Christmas. It might seem like an isolated incident, but December’s chain of events warns of the potential for worsening electricity reliability in the future.

    • Growing population equals growing demand: The demand for electricity in the Tennessee Valley is only increasing, whether from growing population or more industries moving to Tennessee, not to mention the push to electrify the economy to reduce carbon emissions. This demand was continuously underestimated throughout the winter storm.
    • Replacing aging coal-fired plants: TVA also was reliant on one of its coal plants as a large generator of electricity during the storm. But its coal plants are old, and as they get older they become more unreliable. While TVA can update and maintain the plants, at some point keeping them running becomes more expensive than it’s worth, which is why TVA is retiring all of its coal plants. What replaces those plants will be the key to deciding how reliable and clean the power grid will be, a debate TVA has been having with environmental groups since it began retiring coal plants.
    • Increasingly extreme weather: Climate change impacts are becoming more prevalent, as evidenced by the December storm, with more extreme increases and dips in temperatures. This means TVA will need to learn from its mistakes from this winter quickly, before it faces its next seasonal peak in summer when its customers will need their air conditioning during the increasingly extreme Southern heat.

    Anila Yoganathan is a Knox News investigative reporter. You can contact her at, and follow her on Twitter @AnilaYoganathan. Enjoy exclusive content and premium perks while supporting strong local journalism by subscribing at

    Commercial Appeal reporter Samuel Hardiman contributed to this report.

    This article originally appeared on Knoxville News Sentinel: How a perfect storm of freezing cold and aging power plants led to Tennessee blackouts


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