The presented drum is only a shell, without an centre axle, inspired by hub-less wheels. It rotates on 4 off-center axles, each connected to two hydraulic motors. The 4 axles are supported by a heavy steel support structure. It effectively relocates the structural mass from the rotating component to a static component to reduce inertial losses. The shell is considered of CFRP material, with a mass of 1500 kg. It is connected to the 4 shafts through 1 or 2 gear ring(s) on the inside of the drum.
The reel-out (productive phase) tether speed is a fraction of the wind speed, conventionally being 1/3. This leads to a huge generator at low rpm or a generator with a large gearbox when the system scales up. Hydraulic technology could allow both continuous power in spite of alternating reel-in and reel-out phases, and scaling concern solving. Problems of leakage can occur, as mentioned in the conclusion of the paper below: https://www.researchgate.net/publication/340480351_Review_of_the_application_of_hydraulic_technology_in_wind_turbine
Hydraulic technology does not appear to be currently applied for large wind turbines where the weight reduction of the nacelle is however desired. Thus conclusion mentions also: “Secondly, the development status of hydraulic components is limited.” Perhaps the costs can be a concern as a complete installation is a bit complex. I maybe see some other possibilities.
I think one may see that the scaling of motors allow for faster acceleration and retardation of the ground station drum, thereby removing the need for hydraulic motors.
It seems to me hydraulic systems have some advantages, but where they are used, they are replaced by electric systems as soon at that is feasible and cost efficient. So my hunch would be to just use electric motors until you are sure they are not useful for this purpose.
To make that decision, you will need a better model of how the drum must run during all phases and for all windspeeds. It may not be as simple as “1/3 windspeed out and then really fast back in”. Or to put it in other words: if that is what you need from the ground station winch, then your rig is probably not going to be very good. I can’t go into too much detail here without disclosing sensitive information, but the little things are very important in this case.
The report therefore presents the impacts of a potential future 5 MW system.
Page 35:
The diameter of the drum is taken at 55Ă— the diameter of the tether, making the drum diameter 4 meters in diameter.
We can see that the angular speed of the drum is very low. The question is therefore whether we can really do without hydraulic technology on such a large scale, or a gearbox of high ratio. Perhaps using the drum as a ring gear being a part of a planetary gearbox (see at 0:48), then multi-stage transmission inside the drum…
Indeed, the devil is in the detail. And I am far from having your knowledge in the matter. For the moment I am trying to find out if we can avoid hydraulic technology for a large installation. I hope so, even if the numbers given in the thesis are rather encouraging.
A positive starting point, the large diameter of the drum already makes it a first “gear”, and the “shell” (drum) is even used as such (page 35).
4.3 Design Parameters page 33, Table 4:1. There are a lot of components. The mass of the flywheel (326.3 kg) is about 3 times that of the generator (115 kg). Perhaps the flywheel could be removed if the reel-in phase is shorter, or/and by staggering the phases in a kite-farm.
Description of the ground station below:
3.2 Driveline component design
The driveline components consist of winch, motor, flywheel, chain drive, freewheel clutch,
centrifugal clutch, generator etc. An illustration of the driveline is given below: