Tornadoes vs Wind Turbines: A Wake-Up Call?
Mother Nature handed out several invaluable lessons over the last two weeks of May 2024: on May 17, strong weather events downed multiple transmission line towers in the Houston area, and on May 20-21, tornadoes destroyed multiple wind turbines in southwest Iowa. What lessons should we draw from these back-to-back events? Is this a wake-up call? If it is, what is the message of the call? What are the consequences associated with these types of failures and what are the weak links in our nation’s power system? Do we need to rethink how we design wind turbines, or do we need to work harder to slow down the accelerating pattern of more frequent and more extreme weather events? Let’s explore!
Tornadoes picking the turbines they want to destroy
Multiple Downed Transmission Line Towers in Harris County, Texas
Eight people died, nearly 1 million homes and businesses endured life without electricity and classes were cancelled for over 400,000 students for nearly a week after violent tornado-bearing storms downed multiple transmission line towers in Houston on May 17 (AP source). The devastation in Texas did not end on May 17. Similar tornado-bearing storms continued for the month of May killing at least 25 people over Memorial Day weekend in Texas, mostly in the Houston and Dallas areas (VOA source). For the May 17 events in Houston, the National Weather Service (NWS) confirmed that a tornado with peak wind speed of 110 mph had touched down in the area. NWS also reported straight-line winds of up to 100 mph for these events. As reported by Fox Weather, the May 17 events downed 1O transmission line structures in the Houston area (“According to Harris County Executive Judge Lina Hidalgo, 10 energy transmission lines are down.”) While it is possible one of those towers suffered a direct hit by a tornado, it is unlikely all 1O structures were downed by tornadoes, but rather by straight-line winds from the storms. It is also an unavoidable fact that a transmission line is out of commission when only one of its towers is down. The lack of redundancy and the high consequences of failure all point to the critical role transmission lines play in the overall integrity of the power system.
Stronger Events Feel Out Iowa Wind Farms
On May 20-21, a tornado-laden weather system went through the nation's wind energy heartland of southwest Iowa. Strong tornadoes were popping up and taking slow strolls across wind farms. At least 14 tornadoes have been documented across Nebraska and Iowa ranging in Enhanced Fujita (EF) intensity scale from EF-0 to EF-4 (source). For reference, tornadic wind speeds for an EF-4 tornado range from 166 to 200 mph. Overall, 10 Wind Turbine Generators (WTGs) belonging to 3 wind farms were damaged or completely destroyed: 5 WTGs out of 243-WTG Orient Wind Farm (2 percent) owned by Mid-American, 1 WTG out of 140-WTG Arbor Hill Wind Farm (0.7 percent) belonging to Mid-American, and 4 out of 13-WTG Prescott Wind Farm (30 percent) belonging to RPM Access (sources: Wind Watch and USGS Wind Turbine Database). Scenes of WTGs collapsed like straws or burning on the ground are spectacular but no one was hurt at the facilities as technicians rode out the events in designated storm shelters in the operations and maintenance buildings of the farms. Power continued to flow to homes and business during the events!
Resilience of Wind Energy Projects
Wind energy farms acting as power plants consisting of integrated but distributed power generation points are remarkably resilient. As demonstrated by the full-scale test of tornadoes strolling across a dense wind farm area (the May 20-21 Iowa tornado outbreak), damage to the typical wind farm (“power plant”) is limited to knocking out a small percentage of its capacity. The remaining WTGs in the wind farm continued to generate power and the power grid remained stable. The worst case from the above real-life test, courtesy of Mother Nature, is the Prescott Wind Farm with 30 percent of its capacity having been knocked out. At least 3 of the 4 affected WTGs at this wind farm were close to each other and were most likely destroyed by the same tornado illustrating the exceedingly rare occurrence of the tornado track aligning with a WTG string. This occurrence would be an outlier based on this coincidence and given the typical size of wind farms (usually much larger than 13 WTGs).
Individually, WTGs have remarkable strength and ductility in the face of high wind speeds, especially given the slender and lightweight elements making up their structures (tower and blades). WTGs are equipped with mechanisms that enable them to shut down or adjust their blades to minimize the impact during extreme weather. These controls ensure WTG survival without significant damage up to code-mandated hurricane wind speeds in straight-line winds typical of regular wind, hurricanes, and typhoons. However, when a WTG is hit directly by a tornado, these controls cannot keep up with the rapidly changing direction of tornadic winds which can be as high as 300 mph. Regardless of the structure risk category, very few structures can survive such tornadic wind speeds. In fact, during a tornado, the focus is on saving lives. Small and very sturdy spaces such as safe rooms, monolithic domes, or Insulated Concrete Forms (ICF) constructions are created as shelters to save lives during a tornado. A shelter in the operations and maintenance building of a wind farm is created for that purpose.
Rare Failures Despite Unprecedented Event Intensity and Frequency
MidAmerican Energy Company described the Iowa events as unprecedented, noting that they have operated wind farms since 2004 without experiencing such damage. Several of the turbines at the wind farm recorded wind speeds of more than 100 mph as the tornadoes approached just before the turbines were destroyed, the company said in a statement. The tornadoes were rated at EF-4 which would mean that the tornadic wind speeds were between 166 to 200 mph. A WTG is simply not designed to withstand such tornadic winds. Nevertheless, it is important to note that these events are very rare. For example, there have been nearly 900 tornadoes in the US so far in 2024. Only 10 out of the 6,375 WTGs operating in Iowa were damaged in the May 17 Iowa tornado outbreak and none were damaged during recent rounds of severe weather in the state or region. Over 72,000 wind turbines are operational across the United States, with an overall failure rate that is extremely low despite the trend of increasing severity and frequency of extreme weather events, especially in the Midwest, aka “tornado alley.” This is part of what scientists say is a trend characterized by rising average temperatures associated with severe storms becoming more frequent and powerful.
Balancing Failure Probability and Consequence of Failure in Wind Energy Projects
Engineering design is done with the objective of balancing the probability of failure and the consequences of failure based on the concept that risk is equal to the probability of failure multiplied by the consequence of failure. This is referred to as reliability-based design or designing to a target risk level (or to a target reliability index). The design of wind farms and WTGs is no exception.
WTGs are not designed to resist a direct hit by an EF-4 and EF-5 tornado for good reason. First, building a WTG that could withstand a direct hit by an EF-4 or EF-5 tornado would be nearly impossible and/or extremely expensive. Designing WTGs to withstand such rare occurrences would significantly increase the initial investment costs, undermining the economic viability of wind energy. Second, a tornado or a tornado outbreak will rarely directly hit every WTG in a wind farm, typically affecting a small percentage of the farm’s output capacity. The likelihood of a tornado hitting multiple turbines is reduced by spreading them out, typically affecting only a few turbines rather than an entire wind farm. The impact on energy supply from the collapse of a few turbines in a large wind farm is minimal. The consequence of failure is unlikely to involve life safety and is limited to the loss of the turbine. In contrast, if a tornado strikes a gas or nuclear facility, the destruction will likely halt energy production completely or cause severe environmental damage.
A Wake-Up Call?
The extreme weather events of May 2024 affecting a few wind turbines in Iowa do not reveal any weaknesses in the resilience of wind power projects. So what’s the wake-up call about? It is about the urgency to act to slow down the increasing frequency and intensity of extreme weather events by combating climate change by all means, including the sustainable delivery of more wind and solar energy projects. Mother nature opened our eyes to the resilience and unparalleled functional recovery of wind energy projects, pointed to the weaker links in the power system, and underscored the urgent need to accelerate the transition to clean energy to mitigate the impacts of climate change on weather and infrastructure.
Photo Credit: Donald Chamot, Civil Renewables, Inc., Colorado, May 2024.
As shown in this photograph by our own Don Chamot in Colorado, dark clouds are hanging over a bright background. It is for the reader to interpret but for us, we see the light.