A new study reports on offshore wind turbulence in the northeastern United States. (CREDIT: University of Delaware)
According to a new study by researchers from University of Delaware (UD), offshore wind in the northeastern region of the United States could be more powerful, yet more unstable than expected.
Their findings were published in a paper in the Journal of Geophysical Research: Atmospheres.
The study could have significant implications for the development of offshore wind farms in the U.S., including the evaluation of the quantity of wind power that can be generated, the type of turbines used, the number of turbines to be installed, and the spacing between each one.
Cristina Archer at UD and Brian Colle at Stony Brook University led the study. The team explored historical data from the years 2003 to 2011 at the Cape Wind tower located near the center of Nantucket Sound off the coast of Martha’s Vineyard, Massachusetts, and gathered complementary data at the same location for the years 2013 and 2014.
The research paper titled “On the Predominance of Unstable Atmospheric Conditions in the Marine Boundary Layer Offshore of the U.S. Northeastern Coast,” was co-authored by UD professors Dana Veron and Fabrice Veron, and Matthew Sienkiewicz from Stony Brook.
The key finding is that atmospheric conditions surrounding Cape Wind are mostly unstable, which is in contrast of prevailing data relating to European offshore wind farms in the North Sea and the Baltic Sea.
The atmospheric conditions mentioned in European studies of offshore wind state that they were mostly neutral - meaning neither too still nor too windy - but balanced, with turbulent wind conditions occurring only 20% of the time.
By contrast, our study found that wind conditions at Cape Wind are unstable between 40 and 80 percent of the time, depending on season and time of day.
Christina Archer Associate Professor, UD
Stable, Unstable and Neutral
Archer states that explaining the reasons behind why wind is neutral, stable, or unstable is a difficult task.
Winds are consistent and smooth when the atmosphere is stable. Winds become choppy causing high winds from above and slow winds from below to collide into each other and mix together, leading to a bumpy and volatile ride for the air current when the atmosphere is unstable. In the neutral state, winds hover in the middle, with an average quantity of turbulence and wind speed difference.
The advantage of these turbulent conditions is that, at the level of the turbines, these bumps bring high wind down from the upper atmosphere where it is typically windier. This means extra wind power, but that extra power comes at a cost: the cost of more stress on the turbine’s blades. If you have increased turbulence, you’re going to design a different farm, especially with regard to turbine selection and spacing. And guess what? Even the wind turbine manufacturing standards are based on the assumption of neutral stability,
Christina Archer A ssociate P rofessor, UD
Archer, an expert offshore wind farm designer, states that the findings are likely to have implications on how offshore wind farms will be built in the region in the years to come.
Is Something Special Going on in the Northeast?
As there have not been any offshore wind turbines in the ocean until now, there are no measurements available regarding wind and turbulence around the rotor blades of the turbines.
The first U.S. offshore wind farm is being built at Block Island, off the coast of Rhode Island. Data has therefore been estimated according to numerical simulations or by using existing buoy data to deduce the conditions that may be found at the height of the wind turbine’s rotor blades.
However, wind data measurements spanning between 2003 and 2009 at the Cape Wind Tower, a multi-level tower in built with sensors and positioned about 65 feet, 131 feet and 196 feet in height, have revealed that the Cape Cod region’s atmospheric conditions are generally turbulent. Neutral conditions only happen about 20% of the time.
In 2013 and 2014, financially supported by the U.S. Department of Energy, the UD-led team conducted a campaign they christened as ‘IMPOWR’ - Improving the Modeling and Prediction of Offshore Wind Power Resources. The aim was to confirm the Cape Wind tower findings with supplementary measurements.
The IMPOWR team flew a plane 19 times over two summers across Cape Wind, above and around the tower, and calculated wind, humidity and temperature during different weather conditions. They also set up new wind and wave sensors on the Cape Wind platform.
Along with buoy data from just above the water’s surface and with the historical data from Cape Wind, the IMPOWR field data highlighted a surprising and yet reliable information of the area’s wind resources.
We get the same message whether we look at the turbulence from the flights or from the sonic anemometers on the tower, or whether we consider the wind speed data alone. The marine atmosphere is more likely to be unstable than neutral. Having three separate pieces of evidence that support the same finding allows us to be confident that there is something special going on.
Christina Archer A ssociate P rofessor, UD
Smooth or turbulent wind conditions are also controlled by direction of the wind. Data relating to wind from the southwest reveal that wind is quite stable and tends to increase in speed from the bottom to the top of the rotor blades. However, wind from the north, northwest or offshore is mostly unstable and tends to have a comparatively even speed across the turbine rotor, Archer explains.
Additional research is required to understand if this occurrence is localized in the Cape Cod area, along the East Coast in general, or present across the U.S., as other offshore wind farms on the East Coast are planned for New Jersey, Maryland, and Massachusetts.
“It’s important to ensure that science doesn’t underestimate the possible wind resources,” Archer says. “Now that we know what is happening in Cape Cod, we have more work to do to determine whether it exists anywhere else or whether it is unique to this region.”