Coastal Phenomenon Could Disrupt or Enhance Offshore Wind
A vastly complex computer simulation helps researchers develop strategies for dealing with wind events that could damage offshore wind turbines.
The U.S. aims to deploy 30 gigawatts (GW) of offshore wind power by 2030–or enough electricity to power 10 million homes.
Offshore wind turbines. Image used courtesy of Ionna22, CC BY-SA 4.0, via Wikimedia Commons
Much of this will come from offshore wind farms located along the Atlantic coasts of New Jersey, New York, Massachusetts, and Rhode Island, although other offshore sites in the Gulf of Mexico and on the West Coast, and in Hawaii will also contribute to the total.
Low-Level Jets
The Atlantic coasts can be counted on for steady winds that help to maximize energy production, but they are also subject to an atmospheric phenomenon called coastal low-level jets (CLLJs). These are fast-moving currents of air that occur near the coast. Unlike the jet stream, which typically travels at 30,000 feet and can reach speeds over 200 mph, CLLJs are much closer to the Earth’s surface and rarely reach over 50 mph. CLLJs are typically several kilometers wide and can extend up to 10 kilometers inland. A combination of factors causes them, but they primarily occur from a difference in temperature between land and water. During the day, the land heats up faster than the water. This creates a pressure gradient, with the air pressure being lower over land than water. This pressure gradient causes the air to flow from the water to the land, creating wind.
CLLJs are most common in the summer when the temperature difference between land and water is greatest and at night when the land cools more rapidly than the water. They can also occur in winter but are not as strong. CLLJs can significantly impact weather and climate and can be a hazard to aircraft. They can also have substantial effects on wind turbines and wind energy production.
Effects on Wind Energy
On the positive side, CLLJs can increase wind speed at the turbine site, leading to increased power production. In some cases, CLLJs can even double or triple wind speed because CLLJs are typically stronger than the surrounding air and can help to align the wind with the turbine blades.
However, CLLJs can have negative impacts on wind turbines. Because the typical CLLJ is about 2000 feet high at sea level, the wind effects are felt by offshore wind turbines, typically between 300 feet and 500 feet high. One of the biggest concerns is that CLLJs can cause increased turbulence. Turbulence can lead to fatigue damage in turbine blades and towers, making it difficult for the turbines to operate efficiently.
In some cases, CLLJs can even cause turbines to shut down because CLLJs can cause excessive loads on the turbines, which can exceed the design limits. In these cases, the turbines must be shut down until the CLLJ passes.
Computer Simulations
Researchers at the General Electric Global Research Center (GE-GRC) and the National Renewable Energy Laboratory (NREL), supported by the National Offshore Wind Research and Development Consortium and GE Offshore Wind, are studying LLJ impacts behavior on Atlantic coastal wind farms using highly complex exascale computing algorithms and models for multiscale atmospheric flows created through computational fluid dynamics (CFD) techniques.
Research into coastal low-level jet streams shows turbulence can affect downstream wind turbine performance. Image used courtesy of Nicholas Brunhart-Lupo, NREL
The simulations were so complicated that they required using Oak Ridge National Laboratory’s Summit supercomputer and NREL’s Eagle supercomputer. The researchers examined the impact of CLLJs within a small five-turbine array and a large 20-turbine wind farm spanning a region of 10 kilometers. The simulation required 2 billion grid points and was one of the largest ever done examining wind studies.
The study revealed that the turbulent wake resulting from the interaction of the CLLJ with a wind turbine also had effects on downwind turbines, often reducing their performance. In the worst cases, the CLLJ could cause the wind turbines to shut down to prevent damage to the blades and structure, reducing its life expectancy. Using this data, the team can create real-world strategies to improve electrical energy output and increase the lifespan of offshore wind installations.
