Study: Offshore wind farms can mitigate hurricanes

Sun, 12/13/2015 - 8:30am

With the growth and evolution of offshore wind energy, including the installation of the Deepwater Wind Block Island Wind Farm, marine science and environmental researchers all over the world have been conducting studies on the impact offshore wind farm arrays have on the environment and the weather.

One of the more intriguing studies was conducted by a group of professors who used a simulated model to determine that offshore wind farm arrays could mitigate the impact of hurricanes and super-storms. The findings were published in a study called “Taming Hurricanes,” in Nature Climate Change, co-authored jointly by professors at the University of Delaware and Stanford University. The study concluded that "an army of offshore wind turbines could reduce hurricane wind speeds, wave heights and flood-causing storm surge."

University of Delaware professors Cristina Archer and Willett Kempton, and Stanford University professor Mark Jacobson, utilized their sophisticated climate-weather model in order to simulate hurricanes Katrina, Isaac, and Sandy to see what would happen if large wind farms, with tens of thousands of turbines, had been in the storms’ paths. The study’s results demonstrated that offshore wind turbine arrays could possibly buffer damage to coastal cities and diminish landfall impact during hurricanes.

The researchers calculated the global potential for wind power, taking into account that as turbines are generating electricity, they are also siphoning off some energy from the atmosphere. Using a model simulation, the researchers found that as a storm approached, the wind farm array would remove energy from the storm’s edge and slow down the fast-moving winds. The lower wind speeds at the hurricane’s perimeter would gradually trickle inwards toward the eye of the storm, slowing it down.

Archer, who is an associate professor in the University of Delaware’s College of Earth, Ocean, and Environment, told The Block Island Times that, “The little turbines can fight back the beast.”

According to the computer model, the simulated offshore wind farm decreased storm surge — a key cause of hurricane flooding — by up to 34 percent for Hurricane Sandy and 79 percent for Hurricane Katrina. The winds reduced by the wind farm would in turn lower the height of ocean waves, reducing the winds that push water toward the coast as storm surge. 

“The conceptual idea is brilliant and results of the simulations performed are consistent with what we could expect from energy conservation,” said Annette Grilli, Associate Research Professor from the Department of Ocean Engineering at The University of Rhode Island. “The wind farm acts as a sink of energy, diminishing wind speed and consequently, wave height. Additional indirect positive feedback effects modify the dynamics of the hurricane by increasing the atmospheric pressure and therefore taming the storm. However, it is important to insist on the scale at which the taming process is efficient and the resulting impact on the farm design.”

Grilli said: “The Jacobson, Archer and Kempton simulations are performed for wind farms on the order of 200,000 to 550,000 turbines, forming a belt of about 70 kilometers (approx. 43 miles) in width and up to 3,500 kilometers in length. Such a large scale raises many social and ecological issues that are not considered in the authors’ simplified cost-benefit analysis.”

While offshore wind farm arrays wouldn’t completely dissipate a hurricane, the milder winds would prevent the wind turbines from being damaged. Wind turbines are designed to keep spinning up to a certain wind speed, above which the blades lock and feather into a protective position. The study showed that offshore wind farm arrays would slow wind speeds so that they would not reach that threshold.

“This concept is extremely appealing and wind turbines designed to sustain a Category 2 hurricane could theoretically tame a Category 4 to 5 hurricane to a Category 2 storm before the high winds reached the turbines,” said Grilli.  “However, wave breaking is a potential additional source of damage to the wind turbines and it is important to carefully simulate and predict waves, wave breaking zones, and induced impulse force under all potential wave conditions to optimize the turbine siting based on design characteristics of the turbines.”

According to Archer, Kempton and Jacobson’s study, wind turbines will have some ability to protect both themselves and coastal communities from storm damage. The primary question then may be: will a small wind farm be overwhelmed by a Category 5 hurricane? And, therefore, would the cost of construction for storm defense purposes be prohibitive if offshore wind farms incur damage from super-storms to wind turbines and equipment?

"The study shows that it indeed works for very large farms, at 300-plus gigawatts," said Grilli. "For a smaller farm the wind inside the farm will be reduced locally, but the array size will not be large enough to change the dynamic of the storm, which means that a Category 5 hurricane will stay a Category 5 hurricane when it will hit the first wind turbine. In that case the turbines need to be designed to sustain whatever conditions the wind climate is in the area, as is currently the case in any wind farm project."

Grilli explained that if a developer were to build a "mega-farm, I do not think that anybody would, and should, take the risk to install turbines with lower standards that the one prescribed for the current wind climate based on a theoretical study. As any theoretical study it needs to be validated against measured data, so the first step towards validation would be to build an experimental mega-farm and check the model against measured data." 

Grilli said, "In terms of cost-benefit analysis, it is important to note that indirect costs, such as damage to the fishery industry, and non-monetary costs, like ecological, social usage, etc., are not taken into consideration. Those are minimized in a small farm."

Jacobson believes that an offshore wind farm array of any size will have an impact on reducing storms and serve as a defense force. Traditionally obstacles and variances in topography slow and dissipate hurricanes. That’s why when a hurricane reaches landfall it loses steam and weakens.

“A wind farm of any size will have a benefit, with little risk of turbine destruction for east-coast hurricanes,” said Jacobson, professor of Civil and Environmental Engineering at Stanford University. “Research should be focused more on where to site wind farms offshore to have the best benefit in terms of hurricane dissipation and power generation.”

Storm damage to coastlines ends up costing the federal government and cities, states and municipalities billions of dollars in relief funding to rectify damage to infrastructure, not to mention the unspeakable loss of lives and property to those people living in these coastal communities from the damage that those storms inflicted. Hurricane Katrina cost approximately $108 billion in damage, making it the costliest hurricane in U.S. history. Hurricane Sandy cost about $80 billion in damage, and was the second costliest hurricane in U.S. history. 

Despite questions associated with the size of offshore wind farm arrays as a defense force and their effectiveness on a smaller scale, Grilli says that siting wind farms as a storm defense mechanism should be considered by federal, state and local agencies.

“Since the size of the wind farm is critical, if proven effective, such a belt should definitely be considered as part of a general federal siting policy, providing society accepts such a different usage of the ocean and the necessary compromises it will require,” noted Grilli. “However, before this can be implemented, we would need additional modeling for alternative scenarios, including simulating a fragmented belt at higher spatial resolution.”

The question locally is whether or not a wind farm the size of the five turbine, 30-megawatt Deepwater Wind Block Island Wind Farm would have an environmental impact during a hurricane or super-storm.

“In our paper (study) we looked at the sensitivity to the number and size of the turbines and the good news is that the benefits were not linear,” noted Archer. “In other words, we reduced the number of turbines down to a half of the original case and the benefits did not get reduced to a half, they were better than half. We definitely need more funds and more research, but my gut feeling is that even a small farm will have a benefit.”

“The scale of the taming farm in the study is much larger than the scale of the farm planned by the current developers (Deepwater Wind),” said Grilli. “The study demonstrates the efficiency of the system for extremely large farms, on the order of 100,000 to 400,000 turbines for their east coast scenario. Additional modeling at a regional scale including the necessary gaps in the turbine’s belt should be performed with additional storm scenarios, to assess the benefit of a more realistic fragmented belt on taming the storms.  If such results are as encouraging as the initial results are, then definitely new federal siting guidelines or policy should be developed.”

The Block Island Wind Farm, which is a pilot project, is situated three miles off the southeast coast of Block Island. Deepwater Wind plans on constructing a larger 1,000-megawatt wind farm in the future that will contain about 200 wind turbines, and be located in federal waters between Block Island and Martha’s Vineyard.