Using Wind Energy Efficiently
Microdrives aid in the control of "energy kites” an innovative approach which harvests wind power through the use of comparatively small turbines by means of steerable kites, is setting new standards.
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 utilize 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, the kite, at high altitude in the best wind window which ensures high efficiency. To be able to respond quickly to wind gusts, DC motors 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 as 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. Operating 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 fibers 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 and, 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 such as, in the event of a storm, the kite can be drawn in which also 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 and 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 and 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 and 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 micro drives 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.
Energy Kite Concepts on the Test Bench
Mr. Montnacher, you are an engineering graduate at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart, Germany. What exactly are your duties at the IPA?
The Fraunhofer IPA is a manufacturing technology institute, with 14 departments and a broad range of competence areas. As the Power Industry business unit director at the Fraunhofer IPA, it is my duty to coordinate and focus these diverse competence areas in a customer-specific manner pertaining to energy sector issues and to serve as the principal contact person for all related matters.
Within the scope of a research project, in the field of high-altitude wind utilization you have concerned yourself with energy conversion by way of kites. How did this idea come up at all? And, what are the distinctive technological aspects?
The idea originated with the managing director of NTS, who had already requested technological support from Fraunhofer IPA many years ago. The concept of that time was continuously developed and then implemented in the form of a prototype. A distinctive technological aspect is the combination of the specially-developed cable winch system and the measurement system which enables continuous control of the kite while in flight.
Government policy has, with regard to the development of renewable energy sources, set very ambitious goals. What realistic potential do you see in the concept of the high-altitude wind energy generation, also in comparison with other approaches such as biological fuel cells, for instance?
The concept of high-altitude wind energy generation makes possible an extremely efficient utilization of our "renewable energy sources.” In contrast to many other potential energy sources, wind is available practically without limits and free of charge. In addition, the NTS principle is also suitable for establishing systems of the magnitude of a power plant.
Which critical aspects must be taken into account with respect to the practical feasibility of the concept?
To be practically utilized, many specific problems must first be resolved, starting with the durable strength of the materials employed and further to legal approval issues.
For your concept studies, a kite wingspan with a surface area of 15 m² was constructed. How large must such a kite actually be in order to generate the energy of a conventional wind turbine?
There have been practical trials with kites with a surface area of several 100 m². But it's not necessarily a question of how large an individual kite should be. Based on the NTS principle, multiple kites could – and should – constitute a system. According to current plans, a system consisting of 24 units is being examined.
By reason of that, can all parameters simply be scaled up 1:1?
Technical upscaling is certainly possible, if not exactly 1:1 and undoubtedly not without intensive development work. Up to which magnitude this is possible is currently unknown; this is due to no restrictive criterion having yet been reached.
A particular developmental challenge was posed by the fully automatic control of the kites, which was resolved by means of drive systems from FAULHABER. How will the demands on these technical components change once the kite has been scaled?
An upscaling of the kite surface would, in addition to the requirements that would arise directly due to the greater forces, certainly also have conceptual consequences. Thus, I am convinced that one result will be a clear-cut separation of the control and winch systems. This would then have significant effects on the control concept and also on the control components.
In case of innovative power generation on the one hand, on the other hand a correspondingly efficient usage of energy should also not be neglected, of course. With regard to automated industrial applications, what significance do state-of-the art microdrives, such as those from FAULHABER, have already today – and in view of future development?
Today, efficient energy utilization is largely ignored in detail. There, the potential – especially in production processes – lies in a multitude of small optimizations which, when added up, amount to a worthwhile amount. Here it is imaginable that micro drives will take over many tasks in the energy-optimized process control in the future.
To learn more about energy applications and the micro motors that power them, please contact one of our application engineers, or explore the MICROMO Motion System Selector to view our products and motor configuration options.