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Kite sails: Can we return to wind powered shipping?

Posted by Alex Barrett on 6 December 2016 at 11:40 am

At one time all shipping was powered by renewable wind energy, but sailing ships didn’t survive long into the era of coal and diesel. Now sailing, in a modified form, could be seeing a return in commercial shipping.

The modern shipping industry is a large contributor to greenhouse gas emissions. The European Commission estimates that “Maritime transport emits around 1000 million tonnes of CO2 annually and is responsible for about 2.5% of global greenhouse gas emissions” [1]. It is estimated that 9.2 billion Tonnes of cargo was transported by ship in 2012 [2] and this figure is set to increase. Most ships use heavy duty diesel engines, which can be quite inefficient and result in high emissions.

Kite sails may provide a way to scale back these emissions. They have begun to be used in the last decade and consist of a lightweight kite with an area of several hundred square metres. The mast has been abandoned and sails instead fly ahead of the ship connected with strong, but light weight cables. [3, 4, 5, 6]

Wind speed increases with height, so it is best to get the sail as high up, and as far from the friction of the water’s surface as possible. By flying higher a kite can capture far more energy than a traditional sail of the same area. Towing the ship with a kite would not have been possible in the 19th century when sails fell out of fashion for commercial shipping. Advances in materials science mean that much stronger ropes and much lighter sails are now easy to produce.

So could a return to sailing ships provide a considerable reduction in greenhouse gas emissions? Certainly, but sailing ships fell out of use for transporting cargo for several very good reasons. Let’s see how many of these can be solved with modern technology.

The first is crew size. In the late 19th century sailing required a much larger crew, cutting down on cargo space. Steamships could be run by fewer people, and so had a distinct advantage. This is less of an issue nowadays as sails can be controlled by electric motors and on board navigation computers, adapting to changing wind conditions far more easily than when they were controlled by hand.

Secondly a sailing ship tips and rolls as it changes the direction of its sails to catch the wind. [7] A modern ship stays mostly upright in the water so can stack shipping containers without worrying about what will happen when the ship rolls. This would not be as practical on a sailing vessel. This is another reason why using a kite, rather than a mast provides an advantage. The sail doesn’t cause the ship to tip, as the attachment can move on a track around the ship, depending on the angle of the sail.

The most significant problem is that wind power is inherently unreliable. It is impossible to predict how fast a sailing ship will make a journey, and the length of intercontinental voyages will be measured in weeks rather than days. The best way to get around this problem is to use wind power to augment diesel engines, rather than to replace them outright. Sails can be used to increase the ship’s speed at times when wind is available, but it can fall back on a conventional engine when it is not.

The German “Skysail” company, who have pioneered the development of kite sails report that ships using their system can save 10-35% on fuel costs, and by extension emissions. During very windy stretches of the journey the sail could result in as much as a 50% reduction in fuel use [4]. Even if strong winds are only available for part of the journey there will still be a reduction compared to using fossil fuels to reach full speed for the entire trip.

The downside is that a ship using this technology could continue to run their engines at full power, and use the sail to increase their maximum speed. This is good for the shipping company, but from an environmental point of view isn’t as good as reducing engine use and using the sail to compensate. It has been reported that ships can travel twice as fast when using this system to augment their engines during windy periods [6]. A number of companies, including the US military have now invested in kite sails [7, 8]. Hopefully this technology will soon be commonplace in the shipping industry.

References

  1. The European Commission
  2. UN Sustainable Development Knowledge Platform
  3. Treehugger.com: Skysails
  4. Nature
  5. Low Tech Magazine
  6. Wired
  7. Treehugger.com: wind powered shipping
  8. Cnet

Image Credit: Treehugger.com

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