Wind energy is the fastest-growing source of electricity, yet no other renewable energy arouses so much opposition. In recent years, the size of the standard horizontal axis wind turbines (HAWTs) has become overwhelming, and many people don’t want to look at them—especially when they’re in scenic areas.
They are also too noisy to be built in populated areas. Is there another way to harness the power of the wind?
Many companies have started developing a lot of different designs—and some have gone bankrupt trying. Some startups have been spun off from university research efforts. Here is a sample of what’s happening.
Vertical axis wind turbines
The familiar HAWTs look like airplane propellers and are mounted on a tall tower. The tower is vertical, but the blades turn on an axis that is horizontal to the ground. This design is a lineal descendant of the type of windmills developed in Medieval Europe.
Vertical axis wind turbines (VAWTs) are even older. The ancient Persians set multiple sails atop a rotating tower. The first modern VAWTs, built in the 1920s by Frenchman G.J.M. Darrieus, more resemble an eggbeater.
To the left is a different design that more nearly resembles what the Persians made. It illustrates the VAWT concept more clearly than the Darrieus model.
VAWTs are inherently less efficient than HAWTs, but they have some theoretical advantages. For one, the drive train is nearer to the ground and therefore easier and less expensive to maintain.
But Darrieus-type blades are difficult to manufacture and present engineering problems no one has yet solved. There is not yet a single commercially viable VAWT wind farm.
Recently, however, researchers led by John O. Dabiri of Cal Tech have conducted experiments with a still different blade design. More important, he set up pairs of VAWTs to counter-rotate, with one moving clockwise and its partner moving counterclockwise.
The counter-rotation is based on a study of shed vortices produced in the wake of a school of fish. The same mathematical model, applied to VAWTs, suggests new advantages compared to HAWTs.
When HAWTs are grouped together, the performance of each individual turbine suffers. Because the width and height of the area they sweep must be identical, the taller they become, the greater the distance must be between them.
Counter-rotating VAWTs, on the other hand, do not interfere with each other. Their geometry does not require that the width and height be identical, so they can be built taller without having to be spaced farther apart.
Will it work on a commercial scale? We’ll have to wait and see.
Aerial wind turbines
Doubling wind speed produces an eight-fold increase in power yield. Conventional HAWTs generate increased power in proportion to the area their rotors sweep. An airborne airfoil can be the same length as a single blade on a rotor, yet sweep a much larger area. It can also be raised or lowered as necessary to take advantage of changing wind conditions.
Airborne wind systems take three basic forms: soft kites, hard-wing machines that resembles a glider, and lighter-than-air designs. Each of these forms comes with its own set of engineering options and challenges.
Airborne turbines would not necessarily compete with ground-based turbines. They can work in tandem. But airborne turbines can conceivably operate in locations where ground-based machinery would not work. The locations include remote parts of the world with no usable infrastructure (including the open ocean) and close to population centers where conventional HAWTs are too noisy.
The engineering choices fall into four sets of options: the shape and operation of the flyer, whether the generator is with the flyer or on the ground, whether to use single or multiple tethers, and the operating altitude. Any combination of these choices presents engineering challenges that have not yet been solved.
Successful engineering cannot guarantee successful operation. No technology can survive in the market place if it cannot deliver its product at a reasonable price. Work has barely begun with identifying regulatory and liability issues. Nevertheless, it is certainly too early to predict that airborne technology cannot find successful solutions to the problems.
So far, only the Buoyant Air Turbine (BAT) built by Alteros has reached the pilot project stage. It is a helium-filled doughnut with a turbine in the middle.
Modeled on blimp technology used for communications, surveillance, and weather monitoring, its relatively tried and true technology enables Altaeros to adjust the height and alignment of the turbine to respond to changes in the wind.
Because it works at a higher altitude, it can produce up to three times as much electricity as the standard tower-mounted turbine.
The Alaska Energy Authority awarded a $1.3 million grant to Altaeros to test the equipment for 18 months. It will generate enough electricity to provide power for more than a dozen homes.
So far the BAT is probably not cost effective anywhere else besides Alaska’s remote regions. But there, if successful, it will be much less expensive than installing anything on land. It will probably provide energy at about 18¢ per KWH—far too expensive for most of the country but much cheaper than the 35¢ to $1 per KWH many residents in the remote areas of Alaska pay now.
If the testing succeeds and Altaeros establishes a business in Alaska, the experience will probably lead to finding ways to cut costs so the BAT can become competitive for larger-scale projects.
Bladeless wind turbines
Why should a wind turbine use spinning rotors at all? Is there not some other way to capture wind to generate electricity?
Research on bladeless turbines began in 1913, when Nicholas Tesla took out a patent for a rotor with a set of smooth discs and nozzles within a housing. In principal, the nozzles move gas to the edge of the discs. There, according to principles of viscosity and adhesion of gas, the velocity of gas slows and causes the discs to spin. That, in turn, forces the gas to the exhaust at the center.
The spinning discs were supposed to provide the mechanical energy to run a generator. In order to work, however, they had to be very thin and very closely spaced. Tesla could not find metals of the quality necessary for him actually to build a working prototype.
Interest in bladeless wind turbines has renewed recently, but no design is as far along in development as the airborne projects. Company websites gush with optimism, and it’s easy to find articles based uncritically on their claims. Otherwise it is difficult to find optimistic descriptions of any of them.
The words “scam” and “failure” occur frequently in articles that look more carefully into the physics of the designs and the engineering problems to overcome. It is important to remember, however, that the United States Congress pulled the plug on further investment in rotor technology because of consistent failure just when HAWTs began to demonstrate their feasibility.
Ironically, the most vocal critics of airborne and bladeless technologies have ties to the HAWT industry. Just as fossil fuel companies guard their turf from newfangled alternative sources of power, so the relatively well-established HAWT industry seems to be protecting its turf from upstart rivals.
3 PDFs from Prof. John O. Dabiri’s web page
High-Altitude Wind Energy: Huge Potential—and Hurdles / Dave Levitan (Yale Environment 360)
Tesla Technology: 5 Bladeless Turbines About to Revolutionize Energy Production / alecope88 (We Are Change)
Darrieus rotor. Public domain from Wikimedia Commons.
Non-Darrieus VAWT rotor. Source unknown
BAT. Alteros Energies