There is an all too common misconception among policy makers, regulators, and power market participants that wind energy isn’t reliable, and that turbines can’t provide essential grid reliability services. In reality, wind turbines can provide many of the services needed for reliable grid operations, including voltage and reactive power control, frequency response, active power control, and voltage and frequency ride-through. That means wind energy can help boost the overall reliability of America’s electricity grid.
In fact, in some cases, wind provides these services better and more economically than traditional power plants, while in other cases conventional generators currently provide those services more economically. Now, a new report from Sandia National Laboratories, in collaboration with Baylor University, highlights the ways in which wind helps grid operators keep the lights on.
Harnessing wind’s rotational energy
The new report showcases the ability of wind turbines to provide some of these essential grid reliability services using the kinetic energy stored in the rotating wind turbine blades and drivetrain. Leveraging existing literature as well as operational data from a Vestas V27 wind turbine at the Scaled Wind Farm Technology (SWiFT) facility, researchers demonstrated that wind turbines have significantly higher amounts of accessible storage energy per megawatt than a synchronous generator.
Accessing this stored, rotational kinetic energy enables wind turbines to provide regulation, frequency stabilization, and other frequency management services for the grid. Frequency regulation is used to balance out fluctuations in electricity supply and demand that occur over seconds to minutes. Frequency stabilization services, like primary frequency response, are used to stabilize the system’s supply and demand balance in the seconds after a grid disturbance, which is typically the failure of a large conventional generator or transmission outage.
The researchers explain that a wind turbine is essentially two resources – a wind turbine and a flywheel storage device. A flywheel is a mechanical device that stores rotational energy where the amount of energy stored is proportional to the square of its rotational speed. By speeding up or slowing down a wind turbine, the stored rotational energy can be accessed to provide the grid services described above.
However, this comes at a cost. Accessing the rotational energy necessarily means that the efficiency of the wind turbine to generate energy decreases. And while the study doesn’t consider the economics of using the wind turbine’s flywheel capabilities to deliver grid services, it’s straight forward enough to understand that when the value of the grid service exceeds the value of the energy, it makes economic sense for the turbine to deliver the service.
Wind project operators can access stored rotational energy through turbine modulation, i.e. changing the turbine’s rotor speed. Unlike a traditional synchronous generator, wind turbines are decoupled from the grid system’s frequency, which means they need power controllers to determine when to access the stored energy. A synchronous generator spins at the same frequency as the grid, so it can automatically detect a change in frequency due to a sudden increase or decrease in power demand.
This decoupling presents an opportunity, as controllers can be designed to both respond quickly and release the energy efficiently. In fact, wind plants are at least 10 times faster than conventional power plants in changing their output in response to operator or market signals, and their response is far more accurate.
Renewable resources’ fast response is particularly valuable for arresting the frequency drop in the milliseconds and seconds following the loss of a large conventional power plant, and for preventing the power system from falling into a cascading outage.
For example, because of their fast and accurate response, wind plants in Texas already provide a large share of the downward frequency response when system frequency is high. And when wind turbines are already curtailed due to transmission congestion and other reasons, they often provide upward frequency response. As a result, the North American Electric Reliability Corporation has documented that the Texas power system’s frequency response is much better when wind output is high.
The controller can also be programmed to optimize the release of stored kinetic energy. In part because of this efficiency, and because the rotational speed of a wind turbine is not limited by grid frequency like that of a synchronous power plant, the ‘accessible’ amount of stored energy in a wind turbine is shown to be six times greater than a traditional generator, with essentially no efficiency loss, and up to 75 times greater at a 10 percent loss in efficiency.
Market reforms needed to properly value wind’s reliability attributes
Importantly, the research shows that wind turbines can access a portion of the stored kinetic energy and deliver grid services without sacrificing energy production. Consequently, the study concludes that “the wind industry is leaving money on the table, and the U.S. is overlooking a significant source of flexibility and resilience.”
A key part of the problem is market design – there is simply no market for many grid reliability services. Or, as the report notes, the markets are “inherently biased towards certain resources.” In many regions, market rules prevent wind plants from even participating in markets to sell these services. Developing the right price signals and participation models can help ensure these services are procured cost-effectively. This could be another way that well-designed markets deliver the most affordable and reliable electricity to consumers.
In some cases, it may also make sense for wind plants to sacrifice some energy production to provide grid reliability services, particularly as renewable resources grow to provide a larger share of the generation mix. In Colorado, for example, the utility Xcel Energy has been using wind plants to provide frequency regulation for many years, particularly when the wind output would have been curtailed anyway.
The Sandia-Baylor study is another confirmation of wind’s ability to deliver grid reliability services – often better and at lower cost than traditional resources. The study also highlights the need to rethink market designs to properly incentivize the delivery of these services and open the market to full participation.