Nobody has all the answers to the world’s energy questions, so New Scientist has teamed up with Statoil to search for solutions from New Scientist’s audience.
The question posed is: Why do wind turbines have such thin blades? Wouldn’t it be more efficient for them to have wider blades in order to capture more of the wind’s energy?
Not everyone was happy with the question: one criticized the use of “thin” and “wide” as extremes of the same dimension because in specialist design circles they relate to different things. Happily most people got the gist of the question. Many respondents pointed out that wind turbines rotate not because of air striking the blades but rather by the air flowing around them, so space is needed between the blades. A favorite analogy was the aircraft wing, and just as the vortices from one aircraft can affect those that follow, so one turbine blade can affect its trailing neighbor. That’s another reason to spread out the blades.Oliver Jackson from Cambridge, UK, took issue with the whole notion that blades are thin. “At their widest point a typical wind turbine blade is around 2.8 meters wide – that’s approximately the same as two, 13-year-old boys stacked on top of each other.” Happily he then explained why blades are not “even wider”.
Not all answers were related to aerodynamics. Len Croney from Cornwall, UK, suggested that blade width is all down to fashion. In the 18th century, the fashion was for broad arms encased in linen. “Today’s turbine is tall, elegant and bare armed,” he argued.
We also received several ideas for improving turbine design. VB Likhachev, from Krasnoyarsk, Russia, sent in a proposal for wind generators using high magnetic saturation. Ronald Pearson suggests that blade efficiency can be greatly improved by adding winglets to the tips of blades. His inspiration is the tip feathers of birds and he is so convinced of the idea that he’s applied to patent it. Excellent though these ideas may be, they did not answer the question.
To that end, here are our three favorite answers:
Contrary to popular belief, a wind turbine is not pushed round by the wind but pulled round by the aerodynamic lift generated on the blades by the flow of air across them. This transforms the kinetic energy of the wind into rotation of the turbine’s shaft. It also slows the airflow, causing the wind to “pile up” in front of the turbine and deflect some of the flow around it. The limit for the energy that can be harvested by a perfect turbine in ideal conditions, published in 1919 by German physicist Albert Betz, is 59.3 per cent of the available wind energy.
The actual design of the blades is, like most engineering solutions, a complex compromise. Simply put, the amount of energy extracted within Betz’s limit depends on the ratio of the area swept by the blades in the time it takes the air to pass through the turbine, to the total circular area traced out by the blade tips.
For wind turbines that have low-speed, high-torque uses, such as for pumping water, the best efficiency is achieved by a high ratio – a few wide blades or a large number of narrow blades. You see these most often on farms. For electrical generation, the current produced is dependent on the rate at which a conducting coil spins in a magnetic field, so high speed is good.
Of course, the area swept by a narrow blade traveling at high speed (in the time it takes the air to pass through) is the same as the area swept by a wide blade traveling slowly. But there’s an additional factor that must be taken into account: at high speeds, drag becomes an important consideration, and the smaller the blade area, the less energy is lost to drag. So narrow is better. The blade only needs to be wide enough to produce enough torque to keep the turbine’s hub turning. A modern blade is often designed to taper from root to tip. This gives a good starting torque from the wide part while reducing drag at the faster moving tip. Increasing the width or the number of blade s only reduces efficiency.
Burton-on-Trent, Staffordshire, UK
There is only so much energy that a wind turbine can extract from the air stream entering the area swept by the blades. If the blades are broader, their aspect ratio (the length to the breadth) is lower, which makes them less aerodynamically efficient. Such blades would be heavier, harder to ship and install, and would need a stronger tower, all of which piles on the expense.
The larger blades would have to rotate more slowly in order to operate at the most efficient angle of attack to the incoming wind. This slower rotation would need to be stepped up to run the generator, necessitating a higher-ratio gearbox, which would be larger, heavier and also more expensive. What’s more, the blades’ larger area would be more prone to reflect radio waves, increasing radio interference.
Potomac, Maryland, US
Wind turbine blades are thin for the same reason that there are fewer foxes than rabbits – the hunter mustn’t consume all the hunted or there is nothing left to feed on.
The blades extract power from the wind, thereby slowing it, and this slow wind behind the turbine causes the wind in front of the turbine to spill around it. Make the wind too slow and most oncoming wind is lost, making the turbine inefficient. Finding the optimum “solidity” – the solid surface area of the blades presented to the wind as a percentage of the total area swept by the blades – needs to be done empirically. The typical optimum is just a few per cent. This small amount of surface area must be spread across three blades in a standard three-blade design, though you could have wider blades by only having two.
Indeed, you could put the whole surface area on one single blade – but aesthetics dictate that it would look distinctly odd! Alternatively, turbine blades might be thinner still if a design with four or more blades is used. However, structural problems, such as the strength of the blades, then begin to spiral and you might be accused of throwing caution to the wind.