Within the tethered, energy via tensile-force approach we're taking, we needed to decide what kind of work the linear force of the kite lines is going to perform. Again, numerous approaches have been and in some cases are still being pursued, from a laddermill, to a carousel, to other notions. Again, trying to keep things as simple as possible, we choose to emulate the well-proven interaction of kite surfers with large kites. One way to do this is to have the line(s) wind and unwind spool(s).
Our mechanical-force based approach is basically a long-stroke, reciprocating engine, driven by the tensile force on the kite lines. We steer the kite in a cyclic trajectory such that the lines reel out as there are large forces on them, and reel back in when there is a smaller force. As we do this, there's an energy surplus, as the force is greater when the wind is pulling the lines out.
We still need a means of temporarily storing the energy needed to reel the lines back in, and a means to harness the surplus.
We provide an overview of the physical components of the system by working our way from the kite lines' interaction with the system to the consumption of energy, grouping components by the challenge they're meant to address.
Challenge 1A: Steering
First, we have to control the kite. A typical kiteboard kite has 4 lines: left/right leading/trailing lines. The lines are attached to a kiteboarding bar so that, relative to the pivot point:
- The left and right leading kite lines can change their lengths relative to one another, decoupled from the net force on the lines...the net force is exerted on the pivot point.
- The left and right trailing kite lines can change their common length relative to the mean length of the leading lines, also decoupled from the net force.
If we have a single spool for the lines going directly to the kite, we can't steer in either way.
We describe one proposal for how to do this in Steering, but I haven't yet built any version of this and think there's probably a simpler and more practical way.
Challenge 1B: Pivot
Next we consider the motion of the kite in the sky, and how to ensure that the whatever spools or pulleys we use to effect steering. Systems to guide the lines so that they feed to/from stationary spools seem elaborate and likely to place considerable strain on the lines, which we want to be able to scale in size and tension capacity.
Thankfully, if the lines are unwinding from spools whose axes are perpendicular to the direction of the lines, we get one degree of kite freedom for free: the angle between the kite lines and the ground. To get the other degree of freedom, we need to change the 'compass' angle of the spools, i.e. allow them to swing about the vertical axis so they're always aligned with the kite lines
Challenge 2: Drive/Recoil (D/R)
With our solution to challenge 2, our interface is a single line that pulls hard, and then pulls not so hard. When it's pulling hard, we want to have it impart energy to the system as it pulls out, then when it's pulling not-as-hard we want expend energy to pull it back in.
A hypothetical drive/recoil oscillation. The outer ring is coupled to a flywheel, the planet ring is coupled to a brake, which when engaged pulls kite line back in.
The sun gear's rotational position is proportional to length of kite line unspooled.
If the lines are unwinding from spools whose axes are perpendicular to the direction of the lines, we get one degree of kite freedom for free: the azimuth, i.e. the angle between the kite lines and the ground. We get another degree of freedom by changing the 'compass' angle of the spools, i.e. allow them to swing about the vertical axis so they're always aligned with the kite lines