According to legend, Ben Franklin flew a kite during a thunderstorm in order to harness the electrical energy in lightning. It seems fitting, then, that two companies are designing kites that "pull" energy from the sky, although not quite the way that wise-old-Ben did it. SwissKitePower and SkySails are two companies independently developing kite-based generators. Their approaches are similar, so I’ll describe the technology generically.
Unlike airborne wind turbines, in which the turbine itself is attached to a tethered wing, the proposed kites will leave the generators at ground level. The kite is tethered by a cable attached to a rotating winch that drives a generator. During the “power phase” the kite is lifted by the wind, causing the winch to spin and generate electricity. When the kite reaches its maximum altitude, an automated control system turns the generator into a motor that reels the kite in (which uses energy, but much less than it generates during the power phase) and the process repeats.
Images (c) SkySails
Kite power is designed to be used for offshore wind farms. They offer a number of benefits, which include:
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The kites reach higher altitudes than ocean-based wind turbines, giving access to much greater wind speeds. At 70 meters, the height of a typical offshore wind turbine, the wind energy is about 1000 watts per square meter. At 400 meters in altitude, there’s 80% more energy available.
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Because the structures are smaller, they can be deployed farther out in the ocean where depth would make traditional wind turbines impractical. The kites won’t disturb the ocean views for coastline residents. Of course, the trade-off to this is that longer wires are needed to bring that electricity back to shore.
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During hurricanes, the kites are simply reeled in for safe keeping.
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Obviously each kite generates less power than a massive turbine would, but many more kites can be deployed. Since they’re so much smaller, they are easier to mass produce and transport.
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Their compact size makes maintenance easier. Damaged units can be swapped out and brought back to shore for maintenance.
SkySails is planning three stages of development: a 55kW model, 1MW prototype (to be constructed later in 2013), and 3.5MW test farm. They expect to have a functional unit on the market in 2015 or 2016.
Since 2009, SkySails has produced kites for the shipping industry. By attaching a “towing kite” to a ship, Skysails can reduce its fuel consumption by 10% to 30%, depending on wind conditions. According to company documentation, “With a good wind the SkySails SKS C 320 can produce a pulling force in the towing rope of more than 320 kilonewton (kN) – a force greater than the thrust of both engines on an Airbus A321. The 32-meter width of the towing kite is just about as broad as the total wingspan of the A321.” That’s a lot of pull!
Here's a short video about SkySails:
Video: AWE Documentary
Ben Franklin said, “The rapid progress true science now makes, occasions my regretting sometimes that I was born so soon. It is impossible to imagine the height to which may be carried, in a thousand years, the power of man over matter.” I think he’d be pretty impressed with how far we’ve come in the 260 years since he flew his kite in a thunderstorm!