The Winding Road to Net Zero Leads Offshore

Physics

By: Hannah Pell

On 29 March 2021, the Biden administration announced another ambitious clean energy goal: deploy 30 gigawatts of offshore wind by 2030. According to the Office of Energy Efficiency and Renewable Energy, the U.S. offshore wind capacity was 28,521 megawatts (or 28.5 gigawatts) in 2019. Deploying an additional 30 gigawatts over a decade would more than double the current U.S. offshore wind energy generation capability.

How much power is 30 GW? It could be enough to power 10 million homes for a full year, offsetting 78 million metric tons of CO2 emissions (equivalent to the greenhouse gas emissions from 16,851,398 cars driven over a year and CO2 emissions from more than 8.7 billion gallons of gasoline consumed or 20 coal-fired power plants over the course of a year).

So how does energy generation from offshore wind work, and how is it different from land-based wind? What role does offshore wind play in our national energy portfolio? And what are the particular considerations that need to be taken into account when planning offshore wind projects?


Current status of wind power 

Collectively, land- and offshore-based wind contributed 7.2% to our nation’s energy supply in 2019. According to a report from the National Renewable Energy Laboratory, there is an estimated 4,200 GW of potential new electricity generation from offshore wind power in the U.S. The cost per megawatt-hour of wind power has also decreased to nearly one-third of its price a decade ago, down to $30/MWh from roughly $85-90/MWh, due to significant advances in wind technology.

Because wind varies depending on the season or even the time of day, how do we decide where to build turbines? The U.S. Energy Information Administration lists several favored criteria: (1) average wind speed is at least 4 m/s for small turbines or 5.9 m/s for utility-scale turbines, and (2) locations at higher elevations, open plains or water, and gaps between mountains that intensify the wind blowing through.

The first utility-scale offshore wind farm in the U.S. was constructed in 2016. Located off the coast of Rhode Island, the Block Island Wind Farm has five turbines that produce 30 MW of power to 17,000 homes. Additional offshore wind projects are under review by the Bureau of Ocean Energy Management (BOEM). BOEM oversees jurisdiction of the Outer Continental Shelf region where these projects may be constructed, which is limited to 200 nautical miles “seaward of the baseline from which the breadth of the territorial sea is measured.” 

 Offshore versus land-based wind 
Land turbines are much smaller than their offshore counterparts; the average land-based wind turbine is roughly 175 meters tall, while offshore wind turbines can reach 260 m (about three times the height of the Statue of Liberty!). Despite the relative size differences, the turbines operate the same way: wind spins the rotor blades, turning internal gears that rotate the generator, converting kinetic and rotational energy into electrical energy. Additionally, both land- and offshore-based wind turbines have a mechanism that automatically shut off when wind speeds exceed 55 miles per hour measured by an anemometer, protecting them from severe weather events like hurricanes.

Offshore wind turbines are connected to the grid via a series of cables buried under the sea floor. More than half of our offshore wind turbines are located in water so deep that a variety of foundations are constructed to affix the turbine to a stable platform, including monopile, “twisted jacket,” and “mono bucket” design structures (pictures below).

Image credit: U.S. Department of Energy.
 
Pros and Cons
As with any source of energy, there are particular advantages and disadvantages, and weighing these is an important challenge as we navigate a just and large-scale transition to greener energy sources.

Wind is both a clean and renewable source of energy. Once a wind farm has been constructed, the turbines require no fossil fuels to operate. However, there are other environmental effects, including area use, noise and visual pollution, and disruption of natural habitats. The American Bird Conservatory estimates nearly 140,000 to 500,000 birds a year are killed by wind turbines (however, collisions with communications towers and power lines far exceed this number). As we construct more wind turbines, migratory bird patterns are an important factor in determining proper locations.

Another advantage is that we don’t have to worry about running out of wind, given its natural occurrence in the atmosphere. Although future supply isn’t a concern, the availability of wind is; its intermittence means that wind is not as reliable or steady compared to nuclear power plants (which can run 24/7 and are used for baseload supply). Nonetheless, the exciting increase in investments, attention, and resources focused on expanding wind power implies that the pros outnumber the cons.

Will all these efforts get us closer to net zero emissions? It seems the answer might just be blowin’ in the wind.

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