Top Image, transporting a Vestas V112 blade in Vermont to the Kingdom Community Wind Farm, from http://www.nawindpower.com/issues/NAW1312/images/168191.jpg. This blade is 180 feet long, and is not even a "super sized" one these days one (see below). A bit of a challenge to transport it to its new home, but one that was successfully overcome, and that electricity now being made s helping to replace the pollution sourced electricity that used to come from the Vermont Yankee reactor (a Fukushima clone). Yes, it is actually possible to make the world a better place every once in a while…
One of the best ways to squeeze more electricity from the wind resource in most places like NY State is to make the blades longer on a given sized nacelle (the device where rotation is made into electricity). After all, the blades only cost about 25% of the total wind turbine machine - increasing those by 25% only raises the turbine cost by 6.25%, and the installed cost by even less. And if the cost of those blades is increased by 25% (basically, this would be the mass of them, as it would be roughly the same amount of labor involved to make them as for a smaller blade), and the mass is proportional to the 2.5 power of the length, this means that the length will go up by roughly 9%, and the area covered by the rotor circle goes up by 19.5%. So, increasing the manufacturing cost by 6.25% can lead to an increase in energy output by almost 20%. see http://www.perihq.com/documents/WindTurbine-MaterialsandManufacturing_FactSheet.pdf. Sweet...
Now, that is a good business deal - a 6.25% increased investment leads to roughly 20% more money from sale of electricity. After all, it also costs roughly the same to install a 45 meter blade as it does a 50 or a 55 meter (as in the V112) one… Of course, it is NOT QUITE that simple, but the overall business case is a simple one, and since business types don't do numbers and details much, all that is needed is a nice graph on a Power Point presentation and one should be "good to go…"
But, the trick is to actually make such blades that work. And there will be some extra costs - bigger bearings to deal with the added loads, and probably taller towers that will keep those blades in a "happy zone" for the wind for more of their rotation. For those turbines with gear speed increasers, a slight redesign will be needed, because the larger rotor diameter will operate at slower rpm's to maintain the correct "tip speed ratio" (TSR) of around 6:1. The TSR is the ratio of the anglar velocity of the tip (rpm times radius times 2 times Pi) divided by the velocity of the oncoming wind. For example, a 10 m/s wind would need a 60 m/s tip speed, and for a 100 meter rotor that works out to be is around 11.4 rpm. A 90 meter rotor would need a revolution rate of around 12.7 rpm for that same wind speed. The slower rpm also translates into greater torque on the shaft and gearbox, which probably means some beefier gearbox parts and slightly different gearing…. A taller tower and more weight focused on the top (bigger blades, gearbox, etc) also means that the tower has to be correctly "tuned". And while these are not trivial things, that is what wind turbine manufacturing companies are in the business of doing…
The forces on and inside the blade also go up with the square on the length, just for starts, and the object is to get the right mix of stiffness, weight, area and shape that also results in a long lasting turbine blade. One of the preferred materials to use is "exotic fibers" in the resin (in effect, similar to the resins used to make surfboards and water craft/skis) like carbon fiber or boron nitride/boron carbide, but those are expensive. This blade also has to be kept cheap, or at least cost competitive with other blades, so the exotic stuff tends to be used for critical items like the "spar" - a polymer composite beam that enhances stiffness but is only a small percentage of the overall blade weight. Most of the fiber used in the polymer mix is woven glass, as it has the right mix of price, strength and weight.
The blades act like wings on an airplane they provide lift as air passes over them, and the higher pressure on the one side pushes against the lower pressure on the other side. The difference per unit a area is not much of a "push", but there is a lot of blade area when it is 50 meters long and averages 4 meters wide (about 2150 sq ft) and that's why 3 MW of electricity can be harvested from one of these turbines. These blades also have to flex constantly for 20 years, at least, and they have to be fatigue resistant. When operating, they will flex around 10 times per minute, and have to do this for at least two decades. And then there is lightning to deal with, which means they need to have grounding rods in them connected to the rotor hub/tower/ground. There are also a lot of metal rods embedded into the polymer/fiberglass composite, which are used to attach the blades to the hub. The blades also need to be able to be turned as a function of wind speed, and increased weight and torque also means a beefier set of rotor position actuators. Sometimes the blades can be heated to deal with any ice accumulation….
But, if the engineering and science is done correctly and the blades are manufactured correctly, a bigger and more efficient wind turbine can be arranged. In many cases, the size of the generator does not have to change much, or at all - instead, the average efficiency of the wind turbine goes up, as does the annual amount of electricity produced by a turbine with exactly the same generator size as one with a smaller set of blades. And with that comes the difficult task of pricing this new model so that customers overcome the hesitancy of buying something new and so that the owners of the company can squeeze more profit, or at least maintain market share/keep customers, and pay for some more R&D to see if even BIGGER blades can be made…. Yes, continuous improvement and actual business competition is a perpetual treadmill. But, if done correctly, we get lower cost or at least not rising cost electricity and more dependability as well as the ability to place them on more sites, especially ones with a lower annual average wind speed.
Two onshore wind turbines can be used as examples of this phenomena. These are the latest offerings from Vestas (V110) - http://vestas.com/en/products_and_services/turbines/v110-2_0_mw#!technical-specifications - and Nordex (N-131) - see http://www.nordex-online.com/en/produkte-service/wind-turbines/n131-30-mw.html - which involve 53 meter (174 feet) and 64 meter (210 feet) long blades, respectively.
The V110 is an extension of the V100 x 1.8 MW unit that recently has proven to be a big seller in the US and Canada. The new unit is rated at 2 MW, and it has a 4.75 m^2/kw of capacity power ratio versus a 4.36 m^2/kw ratio for the smaller V100. This larger turbine pushes the ability to tap lower wind speed even further, and it comes with an 80 or 95 meter tower in the US or a 95 and 125 meter tower in Europe. At a fairly fast hub height wind speed of 7,5 m/s it should be able to produce an average of 1 MW (almost 9000 MW-hr/yr, a bit over 50% of its rating), and be able to extract 7000 MW-hr/yr (800 kw, or 40% net output) at average winds of 6 m/s. Essentially, this opens up almost ALL of NY State to reasonable priced wind power (except for wind shadowed areas, such as on the east side of mountains/steep hills), especially at 95 meter hub heights. Coming soon to the USA (Texas, initially) in 2014.
Nordex recently announced its N131 x 3 MW turbine for low wind speed areas, and this is a scale-up of their recently commercialized N117 x 2.4 MW unit (lots of them in Michigan). It has a 131 rotor diameter, and a power ratio of close to 4.5 m^2/kw. This one comes with either a 99 meter or 114 meter tall steel tower though higher heights with hybrids may soon be available (120 and 141 meter ones are made for some Nordex (N117 x 3 MW units). The tip of the rotor blade will reach to almost 180 meters (close to 590 feet), where it can be far above the effects of surface turbulence.
So far these are all complete blades as shipped from the factory; some companies (Enercon, Gamesa) actually can split their blades into a pair of sections to make transport easier (especially for the E126 big boy). But, regardless of how they are transported, these blades have to work. In this highly competitive business, failure is not much of an option….
Anyway, these two pictures show where the state of the art is these days - these blades are 80 meters long (going for a demo unit in Denmark for the Vestas V164 x 8 MW turbine). There is a lot of money riding on these these blades (another one is still in a test bench for fatigue studies…). Oh well, who says life is not without risks? By now, the engineers, scientists and business people with their jobs on the line have put their best case forward, and in the near future we will see if it passes the audition. The first V164 goers up on land in the very near future at a test sight next to the ocean in western Denmark really soon…. Pictures from the Vestas Facebook page.
Above, the 80 meter long blades being unloaded for the Vestas V164 demo unit in Denmark. Each blade weighs about 39.5 tons, about what 3 typical blades weigh in at... and unless it is really foggy, there is no hiding these puppies from view.