If you or one those who care about environment or energy resources, of course you have spent some times on wind turbines. Well, they are wonderful! They have no pollution (Let’s not consider the construction part), they can produce more power than other renewable energy systems and finally they are beautiful! But there are some disadvantages that makes us (as an small country which is deciding on their power solutions) say deep in our hearts “mmm…is this really the best option I can choose as my energy source?!”. As you can guess these disadvantages are Low Efficiency and Unreliability.
As you may know according to Betz Law, in best case we can only extract 59.3% of wind’s power, using an ideal wind turbine. So as you see it’s not that much even at its best, so we should do whatever we can do to increase our efficiency. Another serious problem is low reliability of wind turbines which is mostly because of wind speed natural uncertainties and also cut-in, cut-out speed range of turbine. So who wants inefficient and unreliable source of energy as main power source?! We still want it because there are numeral studies and prototypes on building much better wind turbines which can work in a wide range of wind speeds with higher efficiency!
Here we have 5 interesting innovations on designing smart wind turbines which react to the environmental situation to extract more power in wide range of wind speeds.
1- Variable Diameter Rotor
We all know that extracted power from wind has direct relation with surface of swept area by the blades, and area is (pi*r^2). So if you need more power just let your blades extend. Of course it will increase loads on blades and whole turbine structure but measurements on prototype in low wind speeds showed 20 to 50% increase in extracted power. I’m sure we’ll see more of this smart turbine in near future.
2- Variable angle of attack on blade segments
Wind turbine’s Blades should have optimum angle of attack against apparent wind vector (Apparent Wind Vector=natural wind vector+ rotational wind vector). In best case, Blades have a pitch controller to change their angle and to make it better; the blades have their own twist. But it causes some sacrifices in different parts of blade to make some other parts great which means we’ll have some part of blade with optimum angle and some other parts with not-very-optimum angles. Well there has been an study in AmirKabir University of Tehran on blades with autonomous parts which can choose their own optimum angles. The study showed up to 27.8% increase in output power of a single (1m wide) segment in distance of 30m from hub on a test blade at low speeds.
3- Blower on top of Blade
If you are familiar with FLUENT software you can test it yourself! Select an airfoil and put an small velocity input segment on top of it and adjust it as it blows for instance, with 20 m/s speed. Then try different winds and AoAs and check the lift force on it. As you see a simple blower (with optimized placing) can increase the torque on the blade and we can get more power of it (even more than what the blower burns!). in fact the blower makes the boundary layer more dynamic and it will postpone the separation on the blade surface, so we’ll have less chance of stall and more torque which leads to more power as output! FYI, Suction at the trailing edge can do the same.
4- Trailing Edge Flaps
This technology is the most common used technique in smart turbine’s world! There are dozens of articles about blades with flaps and actually you have seen them on the wings of airplanes (it’s really scary when you see how much tension they are bearing during a flight!). The main procedure is somehow like previous part. The flaps change the boundary layer to provide more (or even less) lift force. There are mainly two technologies used in flaps, mechanical and piezoelectric system. The first one is a solid surface which changes the angle with totally mechanical system inside the blade but the second one uses voltage to control the shape of piezoelectric part.
Well, this one is less practical than the others and it’s in its theoretical levels. The plan is like this, we have a cathode and anode placed in optimum position (they are close to each other), and we also have AC or DC voltage source (several KV). When we turn it on, it makes and ionic wind in that small area and postpone separation exactly like what blower does! Because of high voltage source, used in this method, there is still researches to lower this amount to make this technique practical.
There are more smart blade solutions in articles and maybe books, but as a quick start on this topic you can read Active load control techniques for wind turbines by Scott J. Johnson and Dale E. Burg Prepared on Sandia national Laboratories.
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