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Thinking about the complications of building and maintaining turbine rooms installed at high altitudes in horizontal axis wind turbines (plus the difficulties related to it when, for example, a fire happens), I wonder if there is any working design where an axle, transmission belts... or any other means is employed to keep the turbine even at ground level. Too much power loss? Too expensive to build? Where does the complexity come from?

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    $\begingroup$ One problem is that the reactionary torque on a vertical drive shaft will tend to turn the turbine out of the wind. $\endgroup$
    – Transistor
    Aug 26 at 8:28
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    $\begingroup$ Complex, no. But the structure needs to deal with huge torque transients, thus need to be very heavy. Heavy = much more expensive, not just in the rotor and shaft but also in the tower foundations, transporting, etc, etc.. A light mechanical torque transmission system is what is complex. $\endgroup$
    – PcMan
    Aug 26 at 9:49
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    $\begingroup$ modern windmills are huge. These mechanical components would be very large and thus very expensive. $\endgroup$
    – Tiger Guy
    Aug 26 at 12:58
  • $\begingroup$ What problem would this solve? More moving parts means more maintenance. Larger structure means greater cost. If all you are going to do is turn it into electricity, you increase losses. MW of power means large mechanical transmission. It's one thing to build a mechanical transmission as you build tower, but you'd have to plan for maintenance. $\endgroup$ Aug 26 at 16:07
  • $\begingroup$ No, but it is cheaper to transmit electricity. This is why you don't have a shaft going into your house. $\endgroup$
    – Phil Sweet
    Aug 27 at 9:46
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TL;DR:IMHO, the driver for keeping the electrical generation closer to the hub is for primarily for eliminating -as much as possible- moving parts for safety concerns. The secondary benefit is the reduced losses.


IMHO the problem is the distance and the complexity. For the following examples I will use as an example a Wind turbine with the following characteristics:

  • Power : 2MW
  • hub height : 90 m
  • rotational velocity @max output: 20 rpm (usually between 9 and 19)
  • gearbox : planetary 1:100.

The torque on the shaft of the wind blades is about 950 [kN.m].

After the gearbox, the rpm increase to 2000, and the torque drop to 9.5 [kN.m]

For arguments sake let's use those values.


To transfer the energy from the hub to the ground you'd need either:

  • Right angle gearbox and a shaft going the entirety of the hub height (Which can be several tenths of meters high - a 2MW wind turbine is about 80 meter height).
  • A chain
  • or belt to transmit the power.

Right angle gearbox and a shaft

Lets assume we'd make the shaft running down the tower from ordinary structural steel (yield stress 235 MPa). Then the diameter of the shaft (based on simple torsional stress calculation) would have to be :

$$d = 2\cdot \sqrt[3]{\frac{M_t}{\pi\cdot \tau_{allow}}} = 60mm$$

where:

  • $M_t$ is the torsional moment (9.5 kN.m)
  • $\tau_{allowed}$ is the allowable shear stress (assuming a VonMises yied Criterion 0.57*235 MPa= 133 MPa)

That shaft (with a length of 90m) would weigh at least 2000kg (safety factor is still 1), and it would rotate at about 2000rpm. So you would need a lot of support along the length of the beam (in the form of bearing to allow for lateral support and avoid oscillations of the shaft).

chain

Assuming the biggest roller chain (in this table from kcm), (Chain no. 200), which has rollers 40 mm diameter, and weighs approximately 16 kg/m you'd need approximately 16*(90*2)= 2880 kg of chain. Because the max allowable load of that chain is about 72kN, in order to transfer the 2 MW, you'd need to have that chain move at least at 28 m/s (~ 100 kph).

In this particular case you'd have to have a bearing at the top that is able to carry the weight of the chain 2880kg plus the additional dynamic loads.


Bottom line

Any mechanical transmission would have large masses moving at high speeds along great lengths. Having rotating masses like that is an accident waiting to happen, and the results can be catastroptic.

So, IMHO, the driver for keeping the electrical generation closer to the hub is for primarily for eliminating -as much as possible- moving parts for safety concerns. The secondary benefit is the reduced losses.

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Some just have a rod that is driven vertically by a crank.

Used for water pumping windmills - an old design still available new.

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