Introduction

We assume that there's a lot of power to be harnessed with a kite, even at a modest scale.  We want to design a kite flying machine for as cheap as possible, as simply as possible, using mechanisms that can scale and be automated, though we deliberately ignore challenges of automation as these seem orthogonal to a robust foundational design.

There are two basic ways to use a kite and hope to harness wind energy.

1. Use a kite to carry turbines and generators up high and transmit the energy back electrically.
2. Use a kite as a wing and harness the power transmitted mechanically to the machine that it's attached to.

Both are being pursued actively.  The simpler solution seems to be the latter, if for no other reason than that the part that's flying around is commonly available (traction kites), and the fewer things you need to keep in flight, the simpler.

Once you decide on this approach, you have to decide what kind of work the linear force of the kite lines is going to perform.  There are several variations being pursued, from a laddermill, to a carousel, to other notions.  Trying to keep it as basic as possible, we emulate the proven interaction of kiteboarders 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 force on the kite lines.  We steer the kite such that the lines reel out as there is a large force on them, and we reel them back in when there is a smaller force.  The kite trajectory is correspondingly cyclic.  As we do this, there's an energy surplus, as the force is greater when the wind is pulling the lines out, distance_out  = distance_in, and work = force * distance.  Of course, if the system is closed, we need a way to temporarily store the energy needed to reel the lines back in, and to harness the surplus.

Design Challenges

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 1: 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 relative lengths, decoupled from the net force on the lines (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  decoupled from the net force.

If we have a single spool for the lines, we can't steer it in this way.

We describe how to do this in Steering.

Challenge 2: Pivot

Next we consider the motion of the kite in the sky, and how to ensure that the line spools are aligned with the kite lines.  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.

In Pivot, we describe how we avoid having to pivot the entire system while maintaining kite line and spool alignment.

Challenge 3:  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.

In Drive/Recoil, we describe how this is done.