Harnessing wind power is an endeavor humans have been undertaking for thousands of years, so not surprisingly just about every type of machine design has been attempted. Some of the more common types are the Savonius, Darrieus, Dutch windmill, American multiblade (water-pumper), and modern propeller styles. The efficiencies of these machines has been determined by theory and confirmed by experience (see “Turbine Types & Typical Power Coefficients” table).
The propeller-style wind turbine achieves the best efficiency, so it is not surprising that this is the most commercially successful wind turbine type. Engineers use the superior efficiency of the propeller wind turbine, and combine it with a large rotor area—the other governing parameter that they have direct control over—to optimize performance of their designs.
There is a persistent myth that wind power is a new technology ripe for innovation. This thinking leads to wild claims of doubling (or more) the efficiency of “old” propeller designs. This, frankly, is impossible. At 45% aerodynamic efficiency, modern propeller designs are already achieving more than 75% of the theoretical Betz limit, leaving little room for “breakthroughs” in the aerodynamic efficiencies of wind turbines. Using our understanding of wind turbine fluid dynamics, we can expose some common wind turbine hype.
It can be counterintuitive that a propeller-style wind turbine rotor achieves the best efficiency. After all, those three (typically) thin blades let so much of the wind pass between them. Wouldn’t more blades help? That seems logical on the surface, and leads some designers to include an array of blades. This idea, however, ignores the laws of fluid dynamics. Remember, the wind turbine must strike a balance between slowing the wind and allowing it to pass. And it turns out that fewer, thinner blades spinning fast does this best. The American multiblade is effective at what it does—mechanically pumping water—but efficiency is not its goal.
“Augmenters” attempt to leverage the cubic function of wind speed in the energy equation by speeding up the wind into the rotor through ducts or shrouds. This method is akin to the nozzle on a hose that effectively turns a fat, trickle of water into a forceful, faster stream. The difference with wind is that it is not confined to a hose, so it does not have to pass through the nozzle. And in fact, because the wind turbine at the end of the “nozzle” is trying to slow the wind (remember, that is its job), the wind simply sees the augmented turbine as an obstruction and passes around it rather than going through it—taking the path of least resistance. Although a duct does serve to increase the capture area of a wind turbine, a more effective way to accomplish that goal is with longer blades.
One of the benefits touted for vertical-axis turbine designs, such as the Savonius and Darrieus rotors, is that they can take wind from any direction. While it is true that these designs can be simpler because they do not need moving parts to respond to changes in wind direction, the benefit stops there. Wind only blows in one direction at any given moment. Although wind does indeed change direction, sometimes abruptly, the flow through a vertical-axis turbine takes time to react to this changing flow—even if it is not as evident as with a propeller rotor reorienting itself. The claim that vertical-axis designs are better suited for the directional turbulence experienced on short towers is strictly a myth. With any wind turbine design, efficiency increases with steadier wind flow, which is another reason to place any wind turbine, regardless of type, on a tall tower.
Keep in mind the fundamentals of wind energy physics next time you are evaluating wind turbine claims. There are two points most worth remembering:
All wind turbines extract kinetic energy from the moving air by slowing it down, and those that find the best balance between slowing the wind and not disrupting its flow are the most efficient—up to the Betz limit of 59.3%.
The governing equations are dependent on two factors we have some control over—wind turbine swept area (bigger captures more energy) and wind speed (which is a huge factor because it is cubed in determining power). Use these factors to your advantage when choosing and siting a wind energy system.
David Laino puts his aeronautical and mechanical engineering knowledge to use at work designing wind turbines for Endurance Wind Power, and for fun, sailing on the Chesapeake Bay in Maryland.