The U.S. wind industry has put its toe in the water, and conditions feel right for continued growth. Off the coasts of the United States, gigawatts of electricity-generating capacity are blowing in the wind. This extensive offshore resource could provide clean energy to supply nearby population centers and the utility grid for generations to come.
Larger turbine sizes and other refinements to components are helping reduce wind energy costs and generate more power, but more research needs to be done to improve capacity, functionality, efficiency, and affordability at the plant level. The complex environments of offshore installations pose additional challenges to performance, reliability, and commercial viability.
Researchers at the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) are taking a plant-level, systems-engineering approach to maximize offshore wind systems’ energy output, lifespan, and economic feasability, while minimizing forces on individual wind turbines. The laboratory’s wind plant optimization work, in collaboration with academic and industry partners, develops the methods and tools to make plant-level control with wake steering a reality, further augmenting gains from optimized turbine layouts.
“Optimal controls and configurations are critical for offshore wind systems,” said NREL Senior Researcher Paul Fleming. “Turbine wakes persist much longer offshore than on land due to a number of factors, such as relative thermal stability and the lack of terrain, vegetation, and buildings that typically present obstacles to airflow. This means there is more energy lost to wakes offshore, unless you can address them with plant control strategies.”
As described in another recent article, these complications are magnified in floating offshore wind installations. Floating turbines move and pitch in the water, making it extremely difficult to predict wake behavior and devise plant control strategies. NREL is tackling this issue as well as refining wake control strategies and layout designs to create offshore systems able to meet real-world, site-specific operational demands.
Controls To Reduce Energy Losses: Wake Steering
Turbine design optimization is particularly vital to the success of offshore wind plants, whose operators contend with the operational and cost challenges posed by tossing waves and shifting sands. Control systems can optimize the efficiency, power output, lifespan, and safety of wind systems while minimizing maintenance expenses.
Traditionally, control systems have been designed to maximize the individual power production of each turbine. Wind turbine controls govern the generator speed, blade angle (pitch), and rotation (yaw) of individual turbines. NREL is working with partners from around the world to make the leap from control systems that focus on individual turbines to control systems that can help optimize the whole plant.
“The benefits of high, steady, less-turbulent wind typically found in offshore locations can be undermined by wakes—decreases in wind speed downstream from turbines—that extend greater distances than found over land,” said NREL Senior Research Engineer Garrett Barter.
The standard practice of yaw control keeps turbines aligned to the incoming wind direction to maximize the output of each individual turbine. Without coordinated control, however, this approach yields suboptimal power production for wind plants as a whole.
Wake steering allows plant operators to somewhat counterintuitively turn the front row of turbines out of the wind to improve wind flow to other turbines, as well as overall plant production and profits. Field trials conducted by NREL and its partners suggest that wind-plant-level wake steering controls can deliver annual average production gains of 1%–2%—and grow annual profits for a single plant by as much as $1 million depending on plant size and design.
Field trial results also validate the predictions of NREL’s FLOw Redirection and Induction in Steady State (FLORIS) model, a tool used to devise plant control and wake steering strategies. FLORIS predictions suggest that wake steering strategies in conjunction with turbine layout optimization could make it possible to shrink the space between turbines by as much as 25%, enabling bigger systems capable of producing more energy to fit in smaller spaces. While there are commercial tools that include turbine siting as part of standard project planning, accurately taking wake steering strategies into account requires the specialized FLORIS tool.
Strategies To Provide an Additional Boost: Collective Consensus Control
Data sharing through collective consensus control allows turbines to more quickly find optimal positioning and synchronize movement in relation to changing wind directions in real time across entire plants, as well as reduce wear on yaw motors. When wake steering is combined with collective consensus controls, the output of individual turbines as well as the entire plant can provide a 1%–5% gain in annual energy productivity in addition to that supplied by wake steering alone.
Turbines in large-scale wind plants currently operate independently of one another, which means slight differences in the calibration of wind direction sensors or the failure of even one turbine can disrupt the flow in a whole plant.
“Wake steering works best if all turbines agree on the incoming direction of the wind,” said NREL Senior Researcher Jennifer King. “Measurements on the back of turbine nacelles can be prone to errors because the data is gathered after the wind passes through the blades—which means turbines may end up pointing in the wrong direction, resulting in energy losses.”
Collective consensus also diminishes turbine downtime, improves performance by identifying sensor failures quickly (a key consideration with more costly offshore maintenance), and can lengthen turbine lifetime by identifying conditions such as unintentional yaw misalignments that increase loads.
On the Horizon: New Innovations and Partnerships
NREL and its partners continue to explore new methods and technologies that hold promise for greater optimization of offshore wind plants, refining wake steering capabilities, raising power density, more closely coordinating turbine controls, and enhancing layouts.
Researchers also continue to look for new ways to optimize individual turbine designs for greater benefits to overall plant performance. Scientists are just beginning to assess the potential of hybrid plants to combine offshore wind with solar photovoltaics and energy storage to enable more cost-efficient and grid-friendly production and distribution of energy from a range of renewable sources.
Grid services, where wind plants provide benefits to the electrical grid beyond just simple kilowatt-hours, represent an exciting benefit and research opportunity for plant control and optimization. Plant control enables wind plants to maximize their utility to the grid by coordinating output across time and space.
As it pushes into these new areas, NREL will continue to rely on industry, academic, and government partners to identify how future optimization efforts can benefit offshore wind development. The laboratory has worked closely with industry partners including Shell, Equinor, Vestas, Siemens Gamesa Renewable Energy, GE, RES, NextEra Energy, EDF Renewables, Engie, RESurety, and WindESCo to develop new optimization tools, strategies, and technologies.
In addition, academic heavyweights from Brigham Young University, Cornell University, the University of Colorado Boulder, Stuttgart University, Delft University of Technology, and Technical University of Denmark have contributed their expertise. All of the NREL offshore wind optimization projects involve ongoing collaboration with DOE and other national laboratories.
“When you go to the trouble and expense of installing not just one but hundreds of wind towers in the middle of the ocean, you need to make sure you’re getting as much juice as possible from that offshore plant,” Barter said. “Optimization buoys our chances that these installations will perform as strongly as possible at a cost the market can support.”
NREL Offshore Wind Optimization Tools
- ExaWind is a suite of physics codes and data libraries powered by high-performance computing that enables the highest-fidelity simulation of offshore wind turbines and wind power plants using a new moving-wave boundary condition that simulates ocean waves.
- FAST.FARM is a computer-aided engineering tool to simulate each wind turbine, capture relevant physics for prediction of wind farm power performance and structural loads, and predict the ultimate and fatigue loads of each wind turbine in a plant.
- FLOw Redirection and Induction in Steady State (FLORIS) is a modular tool to model steady-state wind plant wake characteristics, optimize turbine interactions, and increase energy yield.
- Offshore Renewables Balance-of-system and Installation Tool (ORBIT) is a modeling tool that evaluates how major balance-of-system costs vary as offshore wind project characteristics, technology solutions, and installation methodologies change.
- Simulator for Wind Farm Applications (SOWFA) is a computational fluid dynamics tool for investigating wind power plant performance under a full range of atmospheric conditions and terrain types.
- Wind Energy with Integrated Servo-control (WEIS) is a toolset currently under development that will be designed to optimize floating offshore turbines.
- Wind Plant Integrated Systems Design and Engineering Model (WISDEM®) is a comprehensive optimization tool that includes modules for turbine aerodynamics, component structural analysis, component costs, plant operations and maintenance costs, financial models, wind plant layouts, and wind turbine aeroelastic simulations.
- WindSE is a systems engineering model linking wind-turbine-focused design to plant-level flow characteristics and power production for blade design optimization and wake deflection.