Thank you for that! I think utilising more of the wind window, and accepting reduced power as being better than parking the kite in certain wind directions would be a valuable step in increasing the system’s usefulness.
The difficulty I see with the net-headwind situation is that the forces on the kite will be determined by the apparent wind, and a headwind will increase those, but if the resulting tether tension can be resolved into the -X direction (in opposition to the direction of the ship), they will act to slow the ship down, requiring energy input from elsewhere to maintain that motion. The kite will be generating more power, but now additional power will be needed from the ship to maintain that ability.
I’m in the process of revising all my maths for this, but so far it seems that we only get a few degrees of apparent headwind where the additional load on the ship’s main engine (at 50-55% maximum efficiency) gives us a net energy benefit over just running an independent diesel generator for the same electrical output. This is what I’ve been thinking of as a kind of “wind-assisted alternator”, where trading-off some increased fuel combustion against additional wind power harvested is still a net ‘win’. I haven’t got this right in my head yet, though.
Something I’m really struggling with on this is how to account for the energy from the kite if the resultant tether tension (in an apparent tailwind) is pulling the ship forward. It feels like double-accounting to say that we can use the reeling out of the tether at constant tension to spin a generator and yield electrical power, but also that this tension multiplied by the ship’s velocity (relative to the ocean) is work the kite’s doing to contribute to the ship’s motion versus the water.
Intuitively, saying that we’re extracting two distinct sets of energy from the wind for the same physical action of the air wind ‘pulling’ on the kite seems wrong. I’ve gone back to the chapters on reference frames in my old University Physics textbooks, and have been digging through discussions of the physics of sailing, and especially on the Downwind Faster Than The Wind project, but all of those are, I think, subtly different situations, and I’m getting more & more confused. Do you have any insight that could clear that up?
There are multiple complex problems to solve. Because current shipping was never intended for kite traction, its a suboptimal platform in several critical parameters. Future shipping needs to be designed for optimal kite use. Ship kites need to scale up another 3x or so (~1000m2). Retrofitting ships is a worthy but marginal “valley of pain” business niche.
The huge opportunity is long term, for radical new kite-ship and ship-kite designs in a decade or two. This is the fastest timeframe that the industrial marine engineering required could be validated and in major production. The “cultural” side is pretty solid- the global energy market is up for grabs, and those who prove AWE at industrial-scale will be heroes.
A small start-up must play a really long game at sea, with several potential competitors already active. It would be helpful to define some key advantage(s) over other ship-kite players, whether of a technical or business nature.
The R&D market for AWE is what is hot now, with ALL our application niches open for investment. Be agile, “don’t go down with the ship” if hotter options come up.
When the ship is moving, for lift mode, the winch speed should be 1/3 of the apparent wind. So if the ship is heading downwind, you must reel out slower. No double bookkeeping. In headwinds reeling out when you are not directly downwind will make the kite drift some degrees towards the power zone (thus transitioning to tether pull becoming vessel drag). It will all add up when you get it right. There is no free lunch unfortunately. The thought that you might accept some tether pull against the direction of movement is interesting. Its all a matter of derivatives - the optimum angle will always be somewhere. It gets even more difficult when elevation wrt the horizon and geometric constraints of your flight pattern are taken into account.
Its not going to be easy. This is why I think tight now that a high efficiency kite is going to be necessary to have a fighting chance of succeeding at this.
A theoretic way to sail a ship directly to windward by kites is by a kite-paravane pair. The power kite flies a crosswind pattern driving the paravane’s mirrored traction motion underwater, and the ship thus towed directly upwind by connection to the tether between the kite-paravane pair.
A kite-paravane or kite-kite pair can of course fly any direction relative to the wind, as long known in theoretic AWE circles (following Wilson and German).