Can kites be economically viable for propelling commercial shipping in mainstream applications?
Lets discuss this interesting blog post from '09. Is it true? The claim is that upwind tracking for container ships is not viable with kite power
Iâll start by stating my view on this: The apparent wind considering a vessel travelling at a speed of 12 m/s (https://www.quora.com/Merchant-Navy-What-is-the-average-speed-of-a-ship) will most of the time be facing more or less in the direction front to back. Any kite should be working mostly perpendicular to apparent wind and itâs going to be difficult to produce much energy in those directions. If energy is produced more downwind, the drag must be overcome by the propeller force, and there will be significant losses in that chain.
For other ships travelling at half that speed, I believe some energy may be harvested by producing kite power perpendicular to the shipâs direction of travel. But this will probably not be enough to cater for all power needs. I guess this is Skysailâs niche, which is fine.
To conclude: I mostly agree with Peter Lynn on this matter
1 Like
To have a better discussion I will now number Peter Lynnâs claims:
[Skipping the onboard energy gen for now]
[Also skipping over âhas not shown to be working yetâ without evidence of someone trying]
- developing autonomous flying (kite auto-pilots) that will operate reliably in marginal and turbulent conditions is probably unachievable
- When thereâs too much wind, above 40knots, kites cannot usually be flown safely.
- When thereâs too little wind (less than 6 or 7 knots), kites fall into the water.
- Until there is 15knots or so they donât develop useful pull.
- the âsteady mid range windsâ that I have not often experienced in my kite flying life, not yet anyway, though I live in hope.
- Itâs not believable that âjust-in-timeâ, customers will accept 5 weeks plus or minus 3 weeks instead of the 10 days to-the-hour trans Atlantic delivery schedule that internal combustion power allows. And, such delayed and irregular schedules would be the reality if kites (or sails) were used now instead of diesel motors.
- As well as unacceptable reliability because of wind variability, there is a technical reason why kite power cannot be useful for commercial shipping- unless container ships change to submarine form so as to reduce their above-water-line drag profile. Aerodynamic drag is the killer. A largish container ship, with containers stacked up 15m and more above deck level can have side area above the water line of about 5000sq.m. Even if itâs kite is also 5000sq.mâs (5 x larger than any kite thatâs yet been built, but probably possible), the shipâs superstructure drag will reduce the efficiency of the kite/line/ship system (measured by lift to drag ratio, L/D) to less than one, and options for upwind courses will fade away. When other other inefficiencies, such as hydrodynamic drag, are also allowed for, even if the apparent wind strikes the shipâs superstructure at 45degrees rather than the worst case of beam-on, upwind sailing will still not be achievable to any bankable extent.
- Fuel use per tonne km is perhaps 70 times better for a container ship than for a truck or train- theyâre VERY fuel efficient.
- wind speed is never easy, and especially not over any sustained period. So, the wind speed will have to be 25knots or more if kites are to propel ships at the speeds the market demands.
[OK, so say kites are only used when the wind is appropriate by direction and strength?]
- Hmm, for downwind sailing, is the wind in a useful direction 35% of the time, and in a useable strength range for 10% of the time? Letâs be generous and say itâs going to work for one day in every 25.
- Are ships owners going to install the equipment, add extra crew, and supply specialist training for some kite assistance barely one day a month? And then thereâs the associated reduced load capacity- and the downtime from running over the lines etc, and the risks to the boat itself and its crew and to other craft from out of control and escaping kites (which will happen sometimes, no matter what fail safe systems are installed) -more costs.
- Oil is so effective and efficient for shipâs propulsion that commercial shipping will be the last transport mode to switch to alternatives should oil prices ever climb to unaffordable levels again.
1 Like
I do believe pulling container ships with kites is a great idea.
Some ralated information: Skysails has had a working system for some years and has gone âbankruptâ but theyâve got some company structure that allowed for skysails yacht to stay alive and they are basically using all of Skysails IP. My assumption was that the reason was mainly because the people who own the ships are not the people who pay for fuel. Udo Zillman hinted at the possibility that the technology was not ripe. Airseas was recently founded. A spinoff from Airbus. They have done their research and concluded that thatâs a worthwhile endeavor and we can safely assume that they know more than we do atm.
Some general claims by me about awes for ship propulsion:
- (1) The sea is a good place for awes. Higher windspeed. No obstacles. More constant wind.
- (2) Price of energy on a ship is higher than price of grid energy. Even if ship engines were as efficient as power plants, they need to transport their own fuel to where it is used. This costs energy and reduces load capacity.
- (3) When pulling a ship downwind there are hardly any transmission losses.
(Compare to generation of electrical energy and usage of the same) - (4) Kites should be used as auxillary power for up to 0.2 of motor load or pull the whole thing. This is to prevent the engines from running inefficiently.
Now on to Peter Lynn:
- Much research is done in that field. I donât have definite evidence, but I believe it to be solveable.
- Thatâs probably based on his own experience flying his kite. I donât see a physics reason why a specially designed kite could not withstand the speed. Weâve got flying things that go much faster.
- Nah. Theyâre landed.
- Donât know cut-in speeds. Any pull is useful.
- Much steadier winds can be expected in higher altitudes at sea. https://en.wikipedia.org/wiki/Atmospheric_circulation
[to be continuedâŚ]
I agree that Peter Lynnâs comments are coloured by his thought that the kite must be a cloth structure mounted on a bridle. If you replace the kitesurfing kite with Kitemillâs high efficiency wing and VTOL, a lot of the arguments are moot.
The argument that still holds very well is: Apparent wind points more or less directly downwind if you sail at 12 m/s. And Kite power production is mostly effective downwind, where the strong forces to withstand tether drag will affect ship speed in a very bad way. If you generate power mostly crosswind or upwards, there is little power to harvest due to the cosine law.
These âfactsâ carry the most weight in this issue.
Flying a common kite behind ship would be useless. It has to be directly in front of ship or as close to that angle as possible.
More exotic system might work, not for traction, but direct mechanical transfer or energy generation. Lifting kite which flies almost directly overhead. It should even be able to fly against the wind. (Imagine a plane descending). On the tether is a vertical wind turbine. The drag from it is offset by the forward force of the lifter kite.
Now that I think about it⌠If the whole flying against the wind thing is ture, then one could just use that for traction.^^ brainfart.
That thing would have to have mad CL/CD. Lift acts mostly upward with a very small component pushing against wind direction while drag is still going directly in wind direction. Maybe a kite can fly that way but not worth considering for traction.
However if one puts the kite a bit downwind and lets it yoyo mostly up/down instead of mostly back and forth, one might gain some power.
Anyway - energy generation with wind going against ship course does not need to be possible for kite propulsion to make sense in other scenarios.
From the post:
Oil is going to become more expensive, and unpopular, and maybe even regulated, so alternatives are going to be looked at, like for example trains from China to Europe. Independently, a shipping company that can make shipping cheaper will have an advantage. So there should be a drive to develop alternatives in working markets.
An idea would be to build fuel efficient electric ships, perhaps partly driven by sails/kites, and have recharging/docking stations/ships along the shipping route.
1 Like
Would prefer liquefied methane generated by kite power as fuel. Energy density much greater, investment cost for storage much lower. Considering the price portion storage is in electric cars, I bet it would be quite an extreme number for ships.
Existing ships could also use vegetable oil.
No matter what process one compares it to, losses are always much higher than just âtying the ship to the windâ and letting it be pulled.
Storing electricity doesnt have to be expensive if your batteries could weigh almost anything and space is a minor issue. Im thinking liquid batteries, cheap and lower capacity.
Anyways, my electric car is far cheaper than a fossile car, so with that argument every container ship would use batteries too
Dont take these pictures to seriously. But the plot shows the availabale power in a 1 m^2 kite in 10 m/s wind on a ship travelling 0âŚ15 m/s in the direction 0âŚ180 to the wind. 0 degrees is straight downwind. The glide number G of the wing is 5 and 10. The power is only produced in perpendicular to the vessel path so as to not cause any drag. And itâs a rotating windmill.
It is quite possibly entirely wrong.
If it is correct though, there is ample power available on downwind tacks as long as youâre moving slightly fast. When moving at 15 m/s, and the wind is 10 m/s, youâre generating power 3/4 of the time, depending on wind direction
1 Like
If anyone has a good map of global offshore wind resource and direction that would be nice.
This one is a great one for land: https://globalwindatlas.info/
This one is a nice visualization but not useful for data extraction: https://earth.nullschool.net/#current/wind/surface/level/orthographic=-285.60,29.91,429
(use mouseover at the scale)
1 Like
Wow.Thx. Hereâs the paper. This will take some work to run and understand. Maybe one day this can run as a web app on awesystems.info
So many conversion losses. I wouldnât be surprised if youâd end up with an efficiency in the low single digit percentages. Now sailing starts to look more interesting.
If you were able to produce LNG onboard a kite ship, its probably better to pump it over into another vessel so it could use it for fuel in that way. To make things more interesting, pump the fuel in transit
Why would you generate it on a ship?^^
Itâs either slowing the ship down by drag or one could just as well use a pure traction kite. Either way gigantic transmission cost.
I was talking about Generating energy and converting it to LNG on platforms.
One good reason would be the fact that ships may travel the seas to find areas with optimal wind and current conditions.
The other reason could the be the possibility to refuel ships in traffic. Remember a cargo boat travels nonstop at 12 m/s.
The third reason could be that maintenance for a ship is a lot simpler than maintenance for a platform.
The fourth reason could be that by using some of the generated power for ship propulsion, the apparent wind speed increases, making it possible to produce with a heavy kite in lighter winds.
In fact, I donât see many good reasons to put the power plant on a platform, other than to have an anchor connected to the bottom of the sea.
Ships are more flexible.
The only way to use kites that I can think of at open sea is if the ocean currents flow in opposite direction of the wind and large sails are deployed in the water (sea anchors) or the ship is anchored to the seabed.
Special tanker/battery boats might be the better solution, depending on a lot of things.
Why is it that maintaining a ship is simpler than a platform? A ship would need to be anchored as well and a platform would not have to be as steady as an oil rig. I agree- as long as it is anchored and not trying to go anywhere itself it might as well be a boat.
The fourth reason just produces losses. I know you were not suggesting a perpetuum mobile, but I donât think getting a kite flying in wind lower than its cut in speed is useful. Little energy generated.
The ship does not need to be anchored. The keel of a sailboat is a very powerful device, once the ship is moving a bit forward.
The fourth reason is valid. Some of the kiteâs power is used for propulsion of the ship (say 30%). This increases the effective wind experiences on the vessel, but also the directio of the wind will turn in a way so that the windmill drag will act as a breaking force to the vessel. So, depending on the angle you are able to fly your AWE rig efficiently to the side of the wind window (obviously less than 90 degrees due to losses and the cosine law), you still may harvest up to 70% of the power. The reason 70% may be better than 100% is that now the minimum wind speed is a bit higher, and you do not have to land your AWE wings.
Anyone experienced in kitesurfing will know this effect. Going on a tack well powered, once you stop moving, the kite falls out of the sky
1 Like