People want renewable energy, but wind and solar energy are too intermittent to rely on completely, Jerry Hudgins, director of the Nebraska Wind Applications Center, told NRT students in a March 29, 2023 talk.
“There's a limit to how much wind and solar energy can be attached to the electrical grid, if you want the grid to be reliable,” the University of Nebraska professor in electrical and computer engineering said. “Nobody knows exactly how much that is. Everybody is pretty sure it's somewhere less than 50 percent of the total generating capacity of the grid because, right now, if you look across the United States, about 10 percent of the generating capacity in the United States is from wind energy. So, engineers that are trying to understand this think that, probably, you can get to 20 to 30 percent renewable overall in the system and still be reliable, but after that, nobody really knows.”
Electrical demand in Nebraska is highest in the summer when people use air conditioning and irrigate, but turbines produce the least energy then because wind speed is low, Hudgins said.
Solar is the best renewable energy to use in the summer, but it works less well on cloudy days and drops off at night.
Basing decisions on such weather conditions and current demands, regional transmission authorities manage multistate electric grids and the energy sources feeding into the grids.
“Wind and solar generation is first in line when it’s available,” Hudgins said. “Second in line is always nuclear and natural gas, and third in line is always coal.”
Hybrid systems of wind and solar make the best use of their strengths, but the energy produced must be used the same day because of lack of means to store it.
“Batteries are not efficient enough,” Hudgins said. “They’re too expensive, their lifetimes are not that great and, so, batteries are not an energy storage solution for utility-scale systems.”
Besides intermittency and storage troubles, wind energy can be difficult to site. Turbines need good winds to operate, but wind doesn’t blow much in the southeast part of the United States. Mountainous regions can be windy, but they’re hard to get to and too far from transmission line systems. Offshore turbines raise environmental issues, pose problems for the fishing industry and are expensive. Buildings cause too much turbulence, so urban areas make poor sites. Large, open areas are ideal, but parks and historic and Indigenous sites are off-limits, and turbines can kill bats and birds if sited incorrectly.
“Siting a wind farm is a pretty laborious effort in that there are a lot of environmental studies that go on,” Hudgins said. “You have to stay away from bird habitats, migration paths, roosting areas, wetlands, and so forth.”
Iowa, Kansas, Oklahoma and Texas now produce the most wind energy, and Hudgins estimated Nebraska was 15 years behind them.
“It was state law that external agents could not come in and build a generating facility in the state,” he said. “So, it took a while to get the laws changed in the state to allow wind farm development.”
Hudgins teaches “Introduction to Wind Energy Systems.” He asks students to practice siting a turbine, challenges them to work on energy storage technology and teaches them the physics of wind energy.
One of the first things he explains is the limits of turbines in drawing energy from air. When wind enters a turbine, the turbine draws energy from the air such that the air on the backside of the rotor has less movement.
“The power that you extract from the wind reaches a peak, and then it drops off and, so, that theoretical maximum is about 59 percent,” he said. “That's called the Betz Limit.”
A turbine cannot draw more than 59 percent of the energy from wind regardless of how efficient it is, he said.
“So, if you have somebody who's trying to sell you a wind turbine that says, ‘I’ve got a new breakthrough. I've got a turbine that can get 75 percent of the power coupled into the turbine.’ That's a universe we don’t live in,” he said.
— Ronica Stromberg, NRT Program Coordinator