KITE WIND GENERATOR
A new concept in wind power generation
ABSTRACT
The Kite Wind Generator Project (acronyms: KiteGen and KWG) proposes a new concept for concentrating on a unique plant huge amounts of aeolic energy. The Project allows to imagine wind machines whose dimensions are not conditioned by structural and dynamical limits.
The KiteGen can compete with conventional electric production systems, including nuclear, both in terms of single plant power production and produced energy cost. The objective of the kite wind generator is to develop, implement and test a completely new concept for electric energy production.
The energy source for the kite wind generator is the high altitude wind of the troposphere. The method of channelling this source makes use of arrays of large tethered kites whose movements are controlled electronically, by highly sophisticated sensors and proprietary software.
The kites are anchored to a revolving structure on a vertical axis, analogous to a giant merry-go-round, which conveys the energy thus generated to the alternators and turbines of a classical power-plant, incorporated within the structure itself.
INTRODUCTION
The power of the sun that illuminates our planet is equal to 10,000 times the raw energy necessary to mankind, the equivalent of 200 million nuclear power plants. (200,000 TW). A good part of this energy is reflected or absorbed by the atmosphere and does not reach the ground: 30% is absorbed on clear days and 99% on overcast days.
The atmosphere transforms part of this energy into mechanical energy, in other words into wind. Wind energy is free and abundant, but we continue to have problems harnessing it efficiently. The present generation of wind power plants is still plagued by the limited availability of windy sites and by the intermittency of the wind. In addition, many people don't like to see giant turbines appearing on hills andridges.
Ideally, we could solve the problems of intermittency and lack of sites if we could place the turbines higher up in the sky, where the winds are stronger and more constant. But with the present technology, the supporting towers have already reached their limits. Structures higher than the present ones (a hundred meters or so) would have to be too bulky and expensive to be practical, to say nothing of the aesthetic problem.
A possible way to overcome the height problem would be to get rid of the tower altogether and use aerodynamic lift to position the wind collecting unit high in the sky. In other words, to use kites. This idea has been already proposed in various forms, but a simple and elegant concept that promises abundant energy at rock bottom costs is the kite wind generator (KiteGen).
The Kite Wind Generator
To understand how the KiteGen works, think of the ancient way of pumping water from a well, where a donkey, walking in a circle, pulled a wooden bar which, in turn, acted on the pump. Think of building something like that, but very large. Think of a whole carousel of steel bars coupled to an electrical generator. To each bar, you attach the cables of an array of kites: that is the KiteGen.
In the KiteGen, the kites reach an altitude of one thousand meters or more. They act very much like the sails of a ship, orienting them in order to optimize the pull on the bars as the carousel turns. Just as the sails of a ship, the kites of the kitegen can "tack" against the wind and keep pulling for nearly three quarters of a complete turn of the bar. Of course, in the kitegen there are no sailors pulling the ropes of the sails, the kites are controlled by a sophisticated system of 3D positioning sensors.
KITES
These power kites are constructed so as to take off in a light breeze. They are essentially giant airfoils whose individual surfaces are several dozen square metres.
THE ROPES
Kites are maneuvered from the ground via a pair of high resistance lines, that control their direction and their angle with regard to the wind. Cables are capable of withstand 30 tons per 2 cm, and these cables weigh only 100kg per km.
SERVO DRIVES
numerical contr
s
1. Brushless servomotor
2. Redection gears
3. Driven reels.
4. Lines
5. Rotating table
6. Base support servo drives numerical control
SINGLE SYSTEM
In the presence of high winds, a single kite is capable of exerting a traction force equivalent to several hundred kilo Newton, travelling at speeds that can exceed 80 metres per second (300km/h.).
The product of the force multiplied by the speed bears witness to the order of magnitude of the potential power generated by the kite. (The calculations needed to obtain the precise values of the power released by the kite, are actually somewhat more complex, as they employ additional variables.)
In the troposphere, around 1,000 metres in altitude, the presence of stronger and more constant winds has been attested to, by decades of measurements. In these winds, the force that the kite transmits to the cables can be channeled to generate electrical energy.
The Project intends to implement an electronic control system that autonomously pilots the kite, so that its flight pattern will maximize the production of energy.
CAROUSEL
The KITE WIND GENERATOR can be envisioned as a giant merry-go-round, solidly anchored to the ground. Its nucleus consists of a central structure, tall enough to support the 'arms' by means of a tensostructure. This 'carousel' is put into motion by the wind itself that drags the kites out from their funnels within the arms, and into the sky.
The rotating central structure contains the automatic winches that release the pairs of cables, which guide the kites. (The cables' length can exceed a thousand metres).
As the kites circle in the air, the vertical rotating axis of the structure activates large-scale alternators, that have been geared down to receive the force exerted on them. At its full capacity the flight of the entire kite array is guided, so as to turn the carousel at the desired speed. An Aeolic power plant conceived in this fashion is capable of producing the energy equivalent to a nuclear power plant, while exploiting an area of few square kilometres, without occupying exclusively it. (The majority of this area can still be used for agriculture, or navigation in the case of an offshore installation.)
KITE GENERATOR CONTROL STAGES
Firstly, the wind causes an upward lift force to the kite, which exerts traction on the cable. This force is transmitted to the alternator-motor pulley and is converted into electrical energy.
Secondly, the controller man oeuvres the kite through the lines in order to reach the quasi stall or an attitude with a low lifting force situation and reel-in the lines with a small energy Consumption.
During both of these phases, an active control of the kite attitude, in combination with the control of the ground pulley, enhances the energy production during the release of the tethers, by increasing the apparent wind. It reduces the energy needed to retract the tethers through an optimisation of the aerodynamic forces on the kites.
The process of knowledge formation required to achieve this objective can be divided into three phases:
The first phase – human control
This stage takes place after having built and completed the system hardware. It consists of sessions where a trained human (for instance a kite-surf trainer or a model aircraft pilot) will practice driving the kite by means of a (joy)stick interfaced to the control machine.
The maneuvering result can be directly seen by the pilot. This phase will be useful to build the base functionality that must be performed by the automatic control and to set-up the responsiveness of the system. It will be possible to harness the first precious wind energy and make the first real evaluations and assessment of the proposed solution.
The second phase – instrumental flight
The trainer will drive the kite in remote by employing only the instrumental information. The base station will provide all the useful information, such as forces, energy, tilt, bending angles, wind speed and direction, winding speed etc. The data will be presented to the trainer in a form that makes piloting as easy as possible.
The data stream coming from the machine and the commands issued by the trainer are routed to build a history database, which could be available, even in real time, in network. The history database will become the off-line training instrument available to all the involved partners for setting-up the control software.
The third phase – automatic flight
The software for automatically control the system will be set up. This is a recursive phase, during which the progress made in software development will be tested and validated. The still active database collection will allow the programmers to analyses the software behavior even in remote, in order to rapidly test and fix the revisions.
Therefore the KiWi Gen will deal with two interdisciplinary aspects, i.e. the wind and airfoils aerodynamics plus sophisticated control mechatronics. For the wind and airfoils aerodynamics part, the project assumes that the market already offers special sport kites, for surfing, karting and snow surf activities. They are categorized as power kites with an important background of included research and characteristics, which are fully suitable for the first project implementation.
Consequently, the project is mainly focused on the executive design and implementation of the control apparatus, which can be conceptually divided in sensor systems, actuation hardware on the ground and at the kite, plus controlling software.
A special set of wind self-powered sensors, such as an inertial platform, anemometer and electronic compass and attitude steering servos are the kite's onboard equipment in wireless connection with the control base. From the hardware point of view, part of the technology is borrowed from the machine tool and robotics field, by deeply exploiting servo motors-drives and numerical controls. Most of the partners involved in this project are specialized in smart sensors, machine tools, robotics, automation, aerodynamics, simulation, flight control and stability of objects in flight. They offer the highly specific skills and experience to face this task.
The kite generator base station that we are outlining has a functional diagram very similar to the one of an industrial machine. It can use more or less the same standard components of a 3 or 5 axis-milling machine: servomotors, servo-drives, encoders, numerical control, rotating table etc. A servo drive with full energy regeneration capability has already been analysed and the brush-less motors act as well as generators, with optimum efficiency.
Further a kite based servo control will allow the kite attitude to be changed according to the needs for optimal power generation, The great accuracy and precision of these components is not required by the proposed application, but it is not harmful. Instead, the robustness and high power management capabilities fit exactly with the requirements, in terms of kites and line manoeuvring, set by the new application.
The Software – analytical vs. cybernetic approach
The control software is the core of the project development and must be built with perception-based decision making, motion and force modeling of the kite in the airspace. An analytical approach is foreseen to set-up the special algorithms required for reducing the developing time. However, it is also an ideal cybernetic application, to be based on neural/fuzzy logic techniques.
Remote interfacing of the numerical control is the main feature on which the method for setting-up the knowledge and the ability for automatic kiting is based. This knowledge is comprehensive of a correct reaction to unforeseen events and to natural wind variations.
In parallel, a comprehensive simulator will be developed, in order to describe the actual kinetic behaviour. This simulator will be used to study optimal strategies for energy production, as well as for the analysis of the motion obtained during the tests. The combination of the tests and simulator will provide an insight into the stability and control parameters related to this concept of kite control, which can lead to further refinements.
The kite generator controller should acquire the skill and copy the behaviour of a trained person driving acrobatic kites and model airplanes. This result will be achieved by pursuing both analytical and fuzzy solutions for the development of software modules:
The analytical approach means that the system will be described and controlled by a set of specially designed algorithms, providing feedback on the lines through the winch/reels, on the basis of the data collected by the sensors, (in combination with the kite's on board attitude control.) This software can be scaled at different levels of coding difficulty and efficiency in exploiting the wind.
On one side, an easier coding can behave in conservative mode, by improving the operational determinism. On the other side, the software complexity can be forced towards acrobatic performance, thereby improving the energy collection attitude. In any case, the computing power available in current microprocessors can support every level of real time software complexity and resource demands; certainly enough to outperform humans in providing fast and calibrated retro-actions.
Many new apparatus with high "MIQ" (Machine Intelligence Quotient) appearing in the near future. In this project, the soft automation solutions based on neural/fuzzy can find a good field of application for developing high MIQ. An optimisation advantage in the amount of energy conversion of the fuzzy vs. the analytical approach can be expected.
SCALABILITY OF THE SYSTEM
The KITE WIND GENERATOR is an installation that produces energy in proportion to its size. As its diameter is increased, the amount of energy captured grows exponentially. The higher altitude of the kites further augments this amount, thus the stronger winds that they are in contact with. Some examples of these values can be:
a diameter of | Is equivalent to a generator of |
100 m | 0.5 MW |
200 m | 3 MW |
300 m | 7MW |
1000 m | 250 MW |
The maximum possible diameter of a KITE WIND GENERATOR is one of the objects of study of this project, but from the initial evaluations, it appears possible to exceed 5,000MW (5 Gigawatts) without great structural risks, with a diameter of slightly more than 2000 metres. It is opportune to consider that 5,000 MW is the equivalent of five nuclear plants, It is possible to demonstrate that a KITE WIND GENERATOR of that size has a construction cost that is a mere fraction.
KITE WIND GENERATOR VS WIND MILL
ADVANTAGES
The advantage of using high flying kites is that it is possible to tap the energy of the strong - and relatively constant - winds of these heights.
A large scale kitegen could produce one to a few Gigawatts of power, the same as a nuclear plant, but at a much lower cost.
Electrical power from a kitegen could cost, perhaps, fifty times less than power produced by presently existingtechnologies.
The KiteGen is also safe: the kites are light sails which would do no damage if they were to fall on the ground. It has a low visual impact, as the kites are invisible from the ground and the carousel is low on the horizon. The footprint of the carousel itself occupies a very small area: the area over which kites fly can still be used for agriculture or as wilderness.
The KiteGen can be built offshore, consuming no land area at all.
CONCLUSION
The kitegen exists only as a set of simulations in the computers. We can do a lot with simulations but, still, technologies have to be tested as actual prototypes and, in a later phase, as industrial products, which must compete in a real market. We know that, in moving from an idea to a product, plenty of things can go wrong and many a beautiful theoretical concept has been slaughtered by an uglyreality.
The KiteGen must pass this reality test. It is an enormous challenge, but it offers also a huge promise. If the test is passed, we face a true breakthrough in energy generation, something that could lead us to a new era, away from pollution, climate change, and resource wars.
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