Will autopiloted aircraft replace wind turbines?
By: Hidde Tangerman
Technical highlights from the Netherlands. In this section we feature a good example every month. This time, glider planes on a string that generate wind energy.
In the Netherlands, near Emmeloord, a glider flies around continuously. It has no destination, no crew and always flies the same pattern. But the strangest thing is that the glider is attached to a tether. This glider – called the PowerPlane – is a prototype of the company Ampyx Power whose mission is to generate power from the wind. The key is the tether which is also attached to a ground generator (a winch), and the pull of the tether on the winch is converted to electrical energy. The lift generated from the plane in a crosswind flight pulls the tether from the winch. Once the tether is reeled out to a predefined length, the aircraft automatically descends towards the ground and the tether is reeled back in. Then it ascends and repeats the process. Former lecturer in aerospace engineering Richard Ruiterkamp is the founder of Ampyx Power. He borrowed the idea from the now deceased Wubbo Ockels, with whom he collaborated ten years ago at the Technical University of Delft. Ockels devised a construction of kites that generate power in a rope at a certain angle. When Ruiterkamp was involved in the project he was thinking how he could improve the efficiency of the system. He suggested replacing the kite with a glider. “An airplane on a string is also a kind of kite,” Ruiterkamp says at his desk in his office in The Hague. “The physical principles are the same, but aerodynamically a glider is more efficient. That translates directly to the amount of lift (pull – ed.) and thus the power that you generate. Finally we managed to convince Wubbo.”
A glider with an engine on board cannot fly crosswind. However if the glider is attached to a cable, tensile force and lift cancel gravity which means that the plane remains balanced. “In this way, the plane can continue to run its laps forever,” said Ruiterkamp. According to him, this way of generating wind power is cheaper than wind turbines. “In a wind turbine, the vast majority of the power comes from the outer parts of the blades, because they run the fastest. And, if you look at the cost, the parts that contribute the least like the pole and the foundation are the most expensive. If you skip all the static material, all that’s left is the wing flying circles in the air, which comes down to a plane.” Although building a glider still is relatively expensive, Ruiterkamp expects that costs will go down in the long term. “If we continue optimization, we think we can be competitive with energy prices for gas and coal plants.” The PowerPlane flies autonomously with an autopilot in the cockpit. Designing it was the biggest challenge of the whole project according to Ruiterkamp : “How to make a flying robot that is shaken by the wind all the time, while you must take height and speed into account?”
The mission is accomplished. On their website, Ampyx Power shows a tethered autonomous aircraft flying gracefully in a repetitive figure eight-pattern accompanied by waltz music. But, Ruiterkamp has not yet reached his final goal. Next year Ampyx Power will be flying with a larger prototype. And in 2018 the company hopes to market its final model: a glider with a wingspan of 30 meters, which produces as much energy as a wind turbine. In the future Ruiterkamp expects that eight of these aircraft will fly at the same time on one square kilometer. “We are looking for a place with good wind conditions and sufficient infrastructure to sell the electricity. At the same time the fenced area should be in a quiet surrounding, so we can demonstrate the safety first. Australia, Canada and even our own North Sea are optional locations. In Europe we are starting to get serious interest from major energy companies that already have wind turbines at sea.”