Wind power has become a major electric generating source in the U.S. and elsewhere, based on the reality that this renewable energy technology, beloved of the environmental community that has long embraced the notion that small is beautiful, has consistently grown bigger and better.
A then-obscure German-born British economist working for the UK’s National Coal Board in 1973 published a book that would become one of the seminal texts of the worldwide environmental movement and an inspiration to advocates of renewable energy. The book by E.F. Schumacher (1911–1977) was Small Is Beautiful: A Study of Economics as if People Mattered. Coming at the time of the first Arab oil embargo and the emergence of economic globalization, the book was an instant bestseller. The Times Literary Supplement dubbed it “among the 100 most influential books published since World War II.”
A committed socialist working for Britain’s gigantic state-owned coal and electric power ministry, Schumacher focused his argument on “appropriate technology” or “Buddhist economics,” and “sustainable” resources. Resources such as coal, oil, and gas were not sustainable, as they were subject to eventual depletion. Schumacher proposed a softer path, later taken up by Friends of the Earth’s environmental guru Amory Lovins in his 1976 book Soft Energy Paths: Toward a Durable Peace.
The works of Schumacher and Lovins became organizing principles for many environmental organizations in the 1970s and 1980s, as they began attacking large hydro projects and power production that relied on extracted minerals (fossil fuels and nuclear), lauding solar and wind as smaller, more-sustainable, and more-beautiful options. For some years, those ideas were ascendant among the supporters of renewable energy and anathema among the incumbent electricity producers, who relied on fossil, nuclear, and hydro to produce the power they sold profitably.
But when it comes to the renewable technologies Schumacher’s and Lovins’ disciples touted, it has become clear that smaller is not necessarily better. That’s particularly the case with wind power, where getting bigger has meant getting better throughout the industry’s development.
Wind Power Growth Continues
Today, wind has become a major contributor to U.S. electricity production, with enormous centralized (although spread over vast stretches of land) wind power stations often far from electric loads, and new transmission lines to move the power to market. According to the American Wind Energy Association (AWEA), the U.S. in 2018 had more than 57,000 wind turbines in place, with a generating capacity of 97 GW. In an August 2019 press release, AWEA CEO Tom Kiernan said, “American wind power’s record growth continues to accelerate with over 200 wind farm projects underway in 33 states.” Those projects add up to 41,801 MW of new capacity, up 10% year-over-year, according to AWEA.
1. Brandon Fitchett, senior project manager for Renewable Energy Research and Development with the Electric Power Research Institute (EPRI), said, “Growing rotor size has increased the land suitable for wind power.” Courtesy: EPRI
As the industry has grown in the numbers of wind turbines, it has also grown in the size of the turbines and, most spectacularly, in the size of the blades, the rotors of the turbines. The two have gone hand-in-hand. Brandon Fitchett (Figure 1), who leads wind power research at the Electric Power Research Institute (EPRI) in Palo Alto, Calif., a former leader of the wind turbine blade development group, told POWER, “In most places, growing rotor size has increased the land suitable for wind power.”
According to Fitchett, the growing size of the blades has meant an increased size in the “swept area” of the rotor, and the volume of air and kinetic energy that passes through the blades to the turbine. The swept area is a function of the diameter (and, of course, the circumference) of the circle created by the rotation of the blades on the rotor. At the same time, the poles that support the rotors have also increased, but blade size “has outpaced the height of the blades.” Among other factors, if the overall height of a U.S. wind machine—the top of the blades—exceeds 500 feet, the windmill can fall afoul of Federal Aviation Administration rules on intrusion into aviation airspace.
“In most places, the rotor size has increased the high profile of wind, and meant that greater areas of land are suitable for wind development in a cost-effective manner,” Fitchett said.
Limits to Wind Exist, but Not Yet Reached
A limit to hypothetical wind turbine power production is described in the 1919 work of German physicist Albert Betz (1885–1968). He showed that given the fundamental laws of conservation of mass and energy, no more than 59.3% of the kinetic energy of wind can be captured. Modern designs are approaching 80% of this limit, according to many analyses.
2. Adwen’s 8-MW platform is based on its tried and tested 5-MW platform, shown here. The AD 8-180 has a 180-meter diameter rotor and the highest annual energy production in the industry, according to the company. Courtesy: Adwen
A 2015 report by the Department of Energy—Enabling Wind Power Nationwide—concluded that taller turbines with longer blades will boost wind power dramatically. Since then, the size of the largest wind power rotors has grown considerably. According to Arcadia Power, a website that follows energy development, the Adwen AD 8-180 wind machine, with a generating capacity of 8 MW, designed for offshore installations (Figure 2), has a rotor diameter of some 600 feet.
A key issue in wind turbine blades is materials technology. A turbine blade, much like an airplane wing or a helicopter blade, must be strong enough to absorb the forces it faces, yet flexible, and not cost so much as to render the machine uneconomic. The bigger the blade, the greater the forces it must sustain.
A 1991 National Academies of Sciences book, Assessment of Research Needs for Wind Turbine Rotor Materials Technology, laid out the basics of the materials challenges. “This means that the basic materials must provide a lot of long-term mechanical performance per unit cost and that they be efficiently manufactured into their final form, including the cost of sufficient quality control. Unless a material choice and fabrication system can satisfy both of these requirements, it will not be appropriate for advancing the state of the art for economical production of power from the wind,” it says.
So far, EPRI’s Fichette said, fiberglass has proven the most economical and flexible material for building wind turbine blades. Carbon-fiber technology is promising and has been used in some cases, but it is expensive and the industry had been successful with advances in fiberglass technology. Carbon-fiber wind blades, he said, can be three times stiffer, which is good, but “10 times more expensive,” adding that “incremental changes in fiberglass materials have kept costs low.”
Are There Limits to Increasing Size?
So, when it comes to wind, at least so far, bigger is better. There are economies of scale both in the construction and deployment of the machines, and the economics of power production. That’s been a common and important factor in the development of electric generating technologies from coal to nuclear to gas to solar to wind. But are there limits?
So far that hasn’t been the case, as the wind industry has been able to increase the size of its rotors and the performance of its turbine blades (see sidebar) without crossing the threshold into uneconomic realms where costs overcome the efficiencies.
How Many Blades Should a Wind Turbine Have?
The first electric-generating wind turbine, in 1888, was a product of American inventor Charles Brush (1849–1929). It featured 144 wooden blades and a generating capacity of 12 kW. The Brush machine was good at capturing the torque in wind’s kinetic energy, but today’s wind machines need to spin far faster than Brush’s behemoth. That means more electric generation from the turbines.
About 90% of commercial wind turbines today have three blades. Why is this? According to engineers and physicists who work with wind energy, the answer is that three blades are an economic and technical tradeoff.
Stan Megraw, an environmental researcher scientist, explained on the website CurioCity: “The more blades there are on a wind turbine, the higher will be the torque (the force that creates rotation) and the slower the rotational speed (because of the increased drag caused by wind flow resistance). But turbines used for generating electricity need to operate at high speeds, and actually don’t need much torque. So, the fewer the number of blades, the better suited the system is for producing power.
“Theoretically, a one-bladed turbine is the most aerodynamically efficient configuration. However, it is not very practical because of stability problems. Turbines with two blades offer the next best design, but are affected by a wobbling phenomenon similar to gyroscopic precession.
“Since a wind turbine must always face into the wind, the blades will have to change their direction vertically when there is a shift in wind direction. This is referred to as yawing. In the case of a two-bladed system, when the blades are vertical (i.e., in line with the tower and the axis of rotation) there is very little resistance to the yawing motion.
“But when the two blades are in the horizontal position, the blades span a greater distance from the axis of rotation and so experience maximum resistance to yawing (notice how a spinning figure skater slows down when they bring their arms away from their body). As a result, the yawing motion starts and stops twice per revolution, and this leads to stress on the turbine due to blade chattering.
“On the other hand, a turbine with three blades has very little vibration or chatter. This is because when one blade is in the horizontal position, its resistance to the yaw force is counter-balanced by the two other blades. So, a three-bladed turbine represents the best combination of high rotational speed and minimum stress.”
EPRI, Fitchett said, is looking at whether economics of scale for wind might collide with the “square-cube law.” The law, according to its legendary creator Galileo, posits: “When an object undergoes a proportional increase in size, its new volume is proportional to the cube of the multiplier and its new surface area is proportional to the square of the multiplier.” It’s important in biology, as a key to the size of animals. But its economic context has import to the question of the limits of economics of scale. When does the cost of the increase in size of a wind rotor blade exceed the value of the increase in power?
“Here at EPRI,” said Fitchett, “we are studying where does that benefit cross the line” of the increased costs. “At what point does the growing size of the machine” become an economic liability. So far, he added, the industry has innovated in terms of blade materials and design. The wind industry “has a fairly predictable” growth path of 60 GW/year for the foreseeable future. ■
—Kennedy Maize is a long-time energy journalist and a frequent contributor to POWER.