Why can’t we have a second unit further down the shaft? Is it bird reasons?
In: Engineering
The tower part is just really cheap. There is no need to place a second turbine at an non optimal place when you can just make a second tower.
It’s just not worth it. As you get farther from the ground, wind speed becomes more constant. Closer to the ground it can get a big gusty. Wind turbines don’t perform well with gusts.
Also, for physics reasons, the tips of the propellers generate more power than the parts closer to the center. So, doubling propeller length will more than double power output.
So, the ideal wind turbine will be huge, and will have propellers that can reach up high where there’s constant air movement. A smaller unit down the shaft would generate almost nothing compared to the one on top.
It’s for wind reasons. The wind is stronger up high so the blades are at a height where they get more wind.
If there was enough wind down low for a lower turbine to work well, they would simply make the turbine shorter.
Making a turbine *taller* than it needs to be so you can put 2 sets of blades on it isn’t cost effective because the blades themselves (and gearbox, generator, etc) cost a lot and also the shaft gets more expensive the taller it is and the more weight it holds.
As you go up in height, the windspeed generally increases. A lower turbine wouldn’t be as effective.
Because it’s all about disc area
A wind turbine with 20 meter blades catches 4x more wind than one with 10 meter blades. You really want the largest single wind turbine you can get so you get the most power out of the tower.
Being higher gives an advantage which is why wind turbines are often built on hills, but the tower height determines the maximum blade length which determines the power you get from your pricy construction project
If I build a 40 meter tower and my blades need 10 meters of ground clearance then I can have a 60 meter diameter fan! If you replace that with a smaller 30 meter diameter fan, that only leaves you space to add a 15 meter diameter fan. Your dual turbine configuration catches 5 units of wind while the single big one catches 8 with similar construction costs
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