Using Wind energy efficiently

An innovative approach, which harvests wind power through the use of comparatively small turbines by means of steerable kites, is setting new standards. Microdrives support the fully automatic control of the kite.

Today, ecologically generated power is a sought-after source of energy. How is "ecological" defined? Does it refer only to the generation of power, or does the manufacture of the turbine flow into the calculation? Cement and steel plants are front and foremost when it comes to industrial energy consumption. Large foundations and giant steel towers have an ecological cost even before the first kWh flows. The EnerKite company is therefore taking a new approach: instead of the dinosaurs of wind technology, it is using slim solutions that concentrate on the key components needed to utilise wind power. A steerable kite transfers the energy of the wind to a generator via a rope. A fully automatic control holds the functional component, i.e, the kite, at high altitude in the best wind window. This ensures high efficiency. To be able to respond quickly to wind gusts, drives from FAULHABER aid in controlling the kite.

New philosophy for new technology

To achieve change in energy policy, innovative technology for power generation is needed. Unfortunately, many alternative concepts are based on inventions that are more reminiscent of the steam engine era than of modern solutions. Without a doubt, this technology also works. However, like the steam locomotive, which is much more resource intensive than modern three-phase locomotives with respect to construction and operation, it rather contradicts an ecological approach. A great amount of material is needed to construct windmills, since the rotor and the heavy generator act on the tower with bending moments and with enormous static loads. Overload events complicate the dilemma: a tower cannot be simply retracted in the event of a hurricane. To safely dissipate the forces, massive concrete or pile foundations, which are a major cost factor and require a large amount of energy, are therefore needed. With respect to their size and expense, the subterranean structures look more like the 2 m thick steel domes of nuclear reactors than ecological structures. Off the coast, the manufacture of the foundations is particularly complex and expensive, and dismantling after use is rather doubtful.
While the kites likewise operate according to an ancient principle for utilizing the wind, the method is refined through the use of modern material and control technology. In order to produce power, a generator is needed in which a magnetic field rotates in a coil. Conventionally, the rotational movement is transferred via heavy, rigid rods and shafts. The developers from Berlin instead use lightweight, high-performance ropes made of heavy-duty fibres for power transmission. Peter Kövesdi, design engineer and specialist for wind systems at EnerKite, offers a visual comparison: "Just like you can use thin spokes placed under tension to make a wheel that uses much less material than one which is solid, ropes can be used to transfer large forces with very little material."

Focus on efficiency

With the EnerKite, a flexible kite – a so-called parafoil – is brought to a height of approximately 150 m. There, unlike on the ground, the wind blows constantly, largely free of turbulence and at higher speed. One load rope and two control ropes transfer the pulling force of the kite to three generator drums. The kite is then "pulled" by the wind automatically from 100 m to 300 m, thereby generating the effective power. Once it has reached the maximum altitude, the kite is controlled in such a way so as to turn it out of the wind and the ropes are quickly drawn in. Very little energy is necessary for this purpose. Afterwards, the kite begins to climb, thereby generating power again.
Peter Kövesdi compares the good aerodynamic properties of the kite to the "down-to-earth" solutions as follows: "The advantage of the kite over windmills is the better utilization of the wind, as there is no turbulence caused by upwind rotor blades or by the tower. The kite is also always at an altitude in excess of 100 m and not, like the rotors, intermittently closer to the ground and intermittently higher than the tower. Thus, the technology can be designed for more uniform loading; in the event of a storm, the kite can be drawn in. This, too, reduces construction costs. The slow movement of the rope while the kite is close to the ground prevents collisions with birds, and the soft parafoil eliminates the risk of falling ice, as ice accumulations quickly flake off."
At sea, simple anchor buoys suffice for securing the generator pontoon; on land the turbine can be both stationary as well as mobile. Large access aisles for giant rotor blades and tower elements are not necessary. A kite can simply be rolled up like a tent; the same applies for the ropes.

Exact control in the wind

In addition to the towing rope, two so-called steering ropes are attached to the kite. In the lingo of kite experts, the EnerKite is a three-liner. The fully automatic control was one of the main problems in making the new technology suitable for practical use. The experts now have a handle on the programming. The best control is only as good as the executing actuator permits. Here, the microdrives from the town of Schönaich come into play. Ropes can only be precisely wound on rope drums while under tension. The wind is a very "dynamic system" with short-term fluctuations. So-called negative gusts can allow the control rope to sag at short notice. This is not a problem for the flight characteristics, but a "no go" for the rope drums. The developers therefore placed a rope tensioner in front of the winding drum that always ensures a defined rope tension at the drum. At winding speeds of 20 to 30 m/s and a pressure roll with diameter of approx. 30 mm, the rope tension motor needs to operate at high speeds that can exceed 10,000 rpm and must be able to very dynamically respond to demands for changes in speed. Here, an electronically commutated standard motor with an output power of approximately 200 W was able to deliver the required performance. The motor is connected to a 32 mm diameter, very robust Planetary Gearhead with all-metal construction. The high required torque for the pressure is thereby ensured. A Motion Controller optimally matched to the motors relieves the EnerKite control of motor management and allows the dynamics of the microdrives to be used optimally. With this application, the motto is "small but efficient", as the microdrives perform a substantial part of the work in controlling the new wind power generator. They ensure that the kite can quickly respond to changes in the wind and that the new material-saving system safely functions in practical operation. In this case as well, drives right off of the shelf could be used to reliably implement the developers' specifications. In difficult situations, simple, small changes to components often facilitate optimum operation. The use of microdrives is limited more by imagination than by technology. The application described here illustrates that even unusual ideas can be practically implemented.

The kite generator operates with a 50% smaller CO2 footprint and twice the energy output of 140 m windmills
Diagram of the flight concept: flying in a figure 8 pattern, the kite rises and produces power, then quickly descends before rising again