Development of a dynamic kite-driven high-altitude wind-energy (HAWE) concept that has shown its mettle as a ship propulsion system is now being stepped up toward a giant 3.5MW design, which its inventors reckon could overtake floating turbines as the technology of choice for future deep-water wind farms.

The SkySails power system, a version of which was used in 2008 on a cargo vessel to offset some 2MW of engine power on a return journey across the Atlantic Ocean, ties together a vast “free-flying” polyamide kite-sail, high-tensile tether, automated control system and generator to harness the high-speed winds 200-800 metres up in the air.

Energy is produced as the kite-sail spins the tether out of a winding drum, charging an electric generator. Once the line is stretched to its full length, the kite-sail is automatically flown into a position where the pulling force is lower, at which point a launch-and-recovery unit reels it back in to start the next power-generating flight.

“If you are designing kites, it is almost inevitable that you begin thinking about what uses they might be put to, and for us the potential for power generation was quite obvious,” says Stephan Wrage, managing director of Hamburg-based SkySails. “But we felt we had to get the first product line [the marine propulsion system] right before we moved on the next one, which is high-altitude wind power.”

The company started with a small-scale 55kW marine prototype, which was powered by a 20-square-metre kite made of off-the-shelf materials. This was then brought ashore, retailored, and put through its paces at SkySails’ testing facility in Wismar earlier this year in winds as strong as five metres per second (m/s).

These trials were the “easy” part, according to Wrage. “The 55kW kite used traditional textiles and, because it was relatively small, it was also light.

“The difficulty in using kites [to generate power from high-altitude wind] comes with scaling them up to be fully automatic, industrial systems.”

The engineering challenge gets exponentially greater with the leap up to a utility-scale SkySail.Using a 400-square-metre kite-sail made of a “patchwork of high-tech materials” and a 1km-long Dyneema tether, the 1MW demonstrator system currently in the works would be able to handle wind speeds of better than 7m/s.

“This kite is being specially designed with our suppliers with UV-radiation coating and anti-degradation coating and so on,” explains Wrage. “Depending on the placement on the part of the kite the type of textile is being altered so that the kite as a whole has both maximum strength and durability as well as keeping the weight to a minimum.

This size of kite has to be able to operate in heavily gusty wind fields, to operate in sudden shifts of winds, to operate reliably, safely.

“There are many more considerations about weight, automation, control systems — how you land a kite in an emergency so it does not crash.”

The launch and recovery apparatus will be much like that built for the marine SkySail, with lift-off, flight control and recovery operations fully automated. Only the “packing and unpacking” will need to be done manually — a design element that will be mechanised for automatic storage in the full-scale offshore version of the technology to come.

The tether will have a copper core to feed power to the SkySail control system and data collection sensors.

No final decision has yet been taken as to the drivetrain for the next-generation system, with geared and direct-drive transmission technologies on the table, probably turning a permanent-magnet generator.

Piloting the SkySail to optimise its wind capture will be central to the efficiency and economics of the system. The kite-sail will be steered via a control pod — outfitted with autopilot and back-up battery — fixed below the airborne structure, allowing for “total control” so that, even if the tether were to break, the kite “could still be brought in for a safe landing”.

Following figure-eights and ovals, the flight path of the kite-sail will be tailored to changing wind conditions, with the programmable control system fine-tuning the angle of attack ten times per second to take account of wind speed, direction, variability, and even shear to stay in the wind-stream’s “sweet spot”.

“One of the nice things about a kite is that you can fly it dynamically, you can get the system up to nominal power very quickly,” says Wrage. “It is much more adjustable to the wind speed than a normal [tower-based] wind turbine as far as cut-in and cut-out go. You can also customise it for the location you are operating in: a smaller-area kite will be better for a spot with low wind speeds, for example.”

Deployment of the flagship 1MW SkySail, which is on track to be in the air by 2014 “at the latest”, is expected to take place off Northern Europe.

By virtue of their lower installation and maintenance costs compared to offshore turbines, HAWE systems are foreseen to come into their own as the technology moves into the water.

SkySails’ long-term view is for a 3.5MW model fixed to a monopile or floating truncated spar foundation anchored by mooring ropes to the sea bed.

Such a unit would weigh no more than 80 tonnes, topped with a kite that could be stowed in a “telephone-booth size” cabinet. A 10MW-rated system — featuring a kite of 2,000 square metres — is on the far horizon.

“Just being able to avoid the need for a high-capacity crane unit for transport and installation offshore — by using a standard anchor-handling vessel instead — means our system has cost advantages that are so high that we feel we could immediately be competitive with deep-water wind turbines,” says Wrage.

By SkySails’ calculations, a full-scale HAWE array located 100km offshore at altitudes of 400 metres could produce power 30% cheaper than the next-generation offshore turbines due to be built in the next few years.

The overall economics when competing with floating wind turbines would be even better, says Wrage.

Further afield, the SkySail has the added advantage of being well suited to installation in hurricane and typhoon regions, as it can be switched into survival mode, with its kite and tether wound in during extreme weather.

“Ultimately, as the SkySail gets bigger, it becomes a question of whether it makes more sense, economically, to build full wind farm developments with fewer, bigger, slower-moving kites, or more, smaller, faster-flying kites,” says Wrage. “Which will be the better overall package? Only time will tell.”

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