Study of Eastern U.S. Shows Wind Energy Could Stabilize the Grid
By Robert Fares |
The views expressed are those of the author and are not necessarily those of Scientific American.
Last month, General Electric (GE) consulting presented the results of a U.S. National Renewable Energy Laboratory (NREL) sponsored study testing if wind turbines can be controlled to manage the stability of the electric grid. The authors found that wind turbines might actually be a valuable tool for controlling and stabilizing the grid in the future, disputing the conventional notion that wind energy doesn’t play well with the grid. To understand the source of this counterintuitive result—and its implications—let’s review the key aspect of power grid control at play here: frequency regulation.
Frequency regulation is the process through which the grid operator maintains the frequency of the grid’s alternating current at a precise, predetermined level. In the United States, for example, grids are strictly controlled to put out electric current with a frequency of 60 Hertz. To maintain this level of frequency, the grid operator must carefully ramp power plants up and down so that the total amount of electricity flowing into the grid is perfectly balanced with the total electricity being withdrawn by electricity customers.
The balance and frequency of the electric grid can be illustrated with the analogy of a spinning merry-go-round. The grid operator’s goal is to keep the grid’s electrical frequency constant, or to keep the merry-go-round in our analogy spinning at a constant speed. To increase the speed of the merry-go-round, the grid operator can order generators to increase their power output—or literally increase the torque on their spinning turbine shafts to “push” the grid up to speed. Electricity withdrawn from the grid by customers slows down the merry-go-round in our analogy, decreasing the grid’s electrical frequency. The inertia of the merry-go-round—or its tendency to stay in motion—is determined by the mass and momentum of all of the spinning turbines and generators feeding power into the grid. The job of the grid operator is to keep the whole system in balance by regulating the flow of power into the grid so that it always matches electric load.
The grid’s alternating current varies like a sine wave. The frequency of this wave is controlled by all of the spinning generators feeding power into the grid. The grid operator controls the output of each generator to maintain a specified electrical frequency, e.g. 60 Hertz in the United States. (Source: Wikimedia Commons)
Concern about the effect of wind energy on the grid stems from the fact that wind turbines cannot produce power on demand, so intuitively it seems like adding too much wind energy might reduce the grid operator’s capability to keep the grid’s frequency balanced—but GE and NREL’s study suggests otherwise.
The study sought to model what would happen if the wind penetration of the eastern U.S. grid increased to 25 percent, a sharp increase versus today. The authors modeled the frequency and stability of the grid using equations similar to those for a spinning mass, like the merry-go-round referred to in our analogy above. Moreover, they modeled wind turbines equipped with commercially-available mechanical controls, which can tweak the pitch of the blades or the torque of the generator to slightly adjust the turbine’s power output as described in this recent paper.
Despite what intuition suggests, the authors found that wind turbines could actually help stabilize the grid if they hold back just 5 percent of their power output using governor and inertial mechanical controls. By doing so, turbines unlock the ability to rapidly increase or decrease their power output by a small amount if called upon to do so by the grid operator. The net effect of employing these controls is that the grid operator sees a relatively steady and predictable aggregate output signal from all of the wind turbines connected to the grid.
U.S. National Renewable Energy Laboratory researchers performed real-world tests to show to what extent the output of a wind turbine can be regulated with governor and inertial controls. (Source: NREL)
While the study’s results are counterintuitive at first glance, they make sense when you consider the inertia associated with a spinning windmill over 100 feet (30 meters) in diameter in the context of our merry-go-round analogy. While wind turbines might not have electrical energy storage (e.g. batteries), all wind turbines have a significant amount of passive mechanical energy storage in the inertia of their spinning blades. GE and NREL’s study shows that with the right mechanical controls, this inertia can be harnessed to keep the grid’s frequency under control—even in a high wind penetration scenario—disrupting the notion that wind energy will destabilize the grid.
Despite the study’s findings, there are still challenges involved with transitioning to a 100 percent renewable grid. While mechanical controls can be used to manage the second-to-second variations in a wind turbine’s output, longer-term energy storage might be required to build a renewable energy portfolio capable of reliably providing electricity 24 hours a day and 7 days a week.
Nevertheless, GE and NREL’s study shows that the entire eastern U.S. grid could achieve a dramatic increase in wind penetration without suffering any major destabilizing effects, without threatening electric reliability, and without installing any costly energy storage. For now, at least, wind energy’s intermittent nature should not be held up as a barrier to its development.