Technology

What does a kite system look like?

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It consists of three main components: A kite, a tether, and a ground station. The kite flies up in the air, harvesting the wind's energy. The tether attaches the kite to the ground station and transmits the electricity to the ground. The ground station can store the energy in batteries or feed it into the grid. It also houses a landing perch on which the kite rests during no wind or servicing.

Kitekraft systems need 10x less construction materials compared to a conventional wind turbine and our systems cost half as much. The carbon footprint of the produced energy is around 10x smaller compared to wind turbines of similar size.

How does the kite generate energy?

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The kite acts like the blade tips of a conventional wind turbine which harvest most of the energy as they sweep the largest area. The support structures, namely blade roots, nacelle, tower, and foundation are replaced with a thin tether and a small ground station. The kite is braked generatively by 8 small onboard wind turbines. Electricity is conducted through the tether to the ground station and fed in the grid. The kite flies figure eights, which are like two connected circles, instead of flying circles to not twist the tether.

How does it launch & land?

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The kite uses its onboard rotors and the already existing power electronics in reverse power flow direction for launching and landing. This is seamlessly possible. The kite then acts like a multicopter.

How long and how much energy does launching & landing take?

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Depending on the location, the number of launches and landings is in the order of 1 each per day. One launch or one landing maneuver takes about 5 minutes. The required energy is equivalent to a reduction of overall efficiency or annual energy production of about 1 %.

Why use a boxplane structure made from aluminum and not a soft kite or carbon-fiber wing?

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The configuration chosen provides the lowest complexity, best scalability from kW to MW range, overall lowest cost, and offers a fast development time for a fully autonomous low-electricity-cost product.

Power curve and operating conditions?

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Cut-in wind speed is around 4-5 m/s, rated wind speed is 12 m/s, cut-out wind speed is 20-25 m/s. The operating range is -20 °C to 50 °C.

Why 8 Rotors?

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8 Rotors is a sweet spot in terms of economics/simplicity, redundancy, and efficiency: The fewer rotors, the fewer parts, but the higher the required excess power if a rotor fails. Redundancy also requires a certain minimum number of rotors. The more rotors the lower the overall required torque and thus overall electrical machine mass and cost for a certain maximum rotor tip speed.

How fast do the rotors spin?

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The rotor blade tip speed is limited to maximum Mach number of around 0.5 to maximize efficiency and limit noise. So, the 100 kW kite with a rotor radius of 0.8 m has about 2,000 RPM.

What are the main benefits?

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A Kitekraft system requires 10x less building material, is almost invisible, and very simple to transport. This enables competitive wind energy generation already at relatively low installed capacity starting at 100 kW without the need of a multi-million USD investment. Additionally, the reduction of used materials also further reduces the carbon footprint compared to conventional wind turbines.

Alternatives

Why should I not use a conventional-style wind turbine?

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It may also take years of planning and requires many millions of USD of capital when considering MW-scale wind turbines, which may only be realized for larger project sizes. The 100 kW Kitekraft system offers competitive electricity from wind at similar USD/W costs as a MW-scale conventional turbine.

When looking at the ecological footprint, flying wind turbines rank among the best, as you can read up in our blog post Do We Need Flying Wind Turbines?

Why not solar-PV and storage?

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Solar-PV and storage solutions have become low-cost and are expected to further decrease in costs (learning curves, scale-economics, Wright’s Law). It is the best solution now to drive down at least a first chunk of electricity costs and CO₂ emissions at many places. However, the sun only shines during the day and, depending on the location, days are shorter during the winter, and clouds and precipitation reduce PV production. To reduce costs and CO₂ emissions further towards zero, the inclusion of other renewable energies gives an optimal solution. Wind energy generation with Kitekraft systems gives the best and most scalable solution at many places.

What about hydropower, wavepower, or biopower?

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At some locations, those are options that can be tapped. At most locations, however, those are simply not available.

What about hydrogen, electrolysis, fuel cells, or power-to-gas?

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Hydrogen is not an energy source but can only act as an energy storage. It can be made with renewable power, which is then called green hydrogen. This can be sold on the public market or stored and converted back to electricity by fuel cells, i.e., it can act like a battery, though with a rather high cost and low round trip efficiency of only about 50%. Using electrolysis to produce hydrogen can be profitable during times when electricity prices are zero or negative, or when otherwise PV- or wind power would need to be curtailed.

Our analyses and microgrid optimizations indicate, even with very optimistic electrolyzer costs and hydrogen sales prices, total costs cannot be further minimized by including hydrogen compared to a solution only with solar-PV, batteries, and Kitekraft systems.

Power-to-gas is likely to become a large market to decarbonize heavy duty and long haul vehicles or things like steel production.

What about a diesel/gasoline generator?

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Such generators would be used in most microgrids regardless at least as backup or for days with no sun and wind. This is a low-cost backup as the installation costs of such generators are usually very low, while the cost of electricity generation is rather high due to high fuel costs.

The downside of using fossil energy sources is—of course—their carbon footprint. You can find a comparison of the carbon footprints of various energy technologies in our blog post Which Energy Technology has the Lowest CO2-Footprint?

What about other airborne-wind-energy products?

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We believe that our technology approach yields a product with lowest-cost electricity generation. To name a few aspects, this is due to the use of low-cost manufacturing based on aluminum extrusion and the simplicity of the multicopter launch and landing which uses the same rotors and power electronics already installed for power generation.

Application

Is a human pilot required?

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The entire operation of the kite at any time is fully autonomous due to its onboard sensors and computers, and it requires no human intervention.

Is rain or snow an issue?

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No, the entire system is weather-proof.

What about strong storms, tornados, hurricanes? What about thunderstorm & lightning?

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The kite lands and is fixated at the ground station autonomously. So, the kite ducks away from bad weather, unlike a conventional wind turbine which must sustain worst weather conditions and lightning strikes.

What about wind gusts?

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The kite pitches automatically into a gust due to its aerodynamic design with tailplane. Even wild gusts pose no harm.

What do you do about icing?

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All critical components of the entire system are protected from icing with standard technologies such as resistive heating, hot air, or vibration/ultrasonics. As the kite is rather small – much smaller than a conventional turbine of the same power rating – much less power is required for this process. There is neither a danger to anyone on the ground nor to the kite.

Is the power constant for a constant wind speed?

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If the wind speed is below the rated wind speed, the power oscillates during a figure eight trajectory. An almost constant power is obtained, if at least two Kitekraft systems are used and synchronized with a phase shift, which is enabled by the flight control software. We also plan an optional small flywheel energy storage in the ground station to minimize power oscillations.

Can the kite fly in my garden or over my house or over a city? How much space is required?

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No. The 100kW system should be placed on a free field without houses, trees, or other obstacles in a radius around the ground station of at least the tether length (100-150 m) plus several meters safety. There should also be some distance to the next houses to avoid any potential noise or shadowing effects.

How is the kite maintained? Does it have to be cleaned?

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The entire Kitekraft system is designed with a high robustness to minimize any issues that could arise. For instance, the generators are direct drive brushless DC and will likely never need maintenance over the entire 20 years minimum service life. There are only a few parts which will need maintenance and exchange from time to time, which would be included in our service plan subscription. Such parts are easily accessible and swappable at the ground station when the kite is landed.

For larger overhauls, the kite or the entire system can be transported to dedicated facilities. The kite’s wings’ leading edges and the rotors might have to be cleaned from dirt and insects from time to time depending on the location, which however can also be done without climbers or cranes and should take only about an hour.

Is a certification required?

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The certification process depends on the system size and may not be necessary at all for small sizes of a few hundred kW. Once production of the 100 kW product is started, we plan a certification regardless in order to get external quality proof.

What about a permit?

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The permitting process depends on the location. We will help with or take care entirely of all the required paperwork.

Is the kite noisy?

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The noisiest components are the rotors, but it can be kept below a level of annoyance. Features of a quiet design are a high number of rotor blades (we use at least 5), winglets or a blade ring, and/or sawtooth-trailing edges. Another point is that the emitted noise is only of high frequency which attenuates rapidly over distance unlike the low-frequency and infrasonic noise emitted by conventional wind turbines.

Safety

Are the kite and tether dangerous?

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The kite and tether will always be well above the height of a human and, e.g., farming vehicles, so that it cannot be touched or come in contact with anything. In principle, the kite and tether are dangerous as both are moving relatively fast. The tether also carries strong forces and a high voltage, although isolated, and the kite has fast spinning rotors. However, with the design for safety and redundancy, the kite and tether are not dangerous, just like airplanes or cars or blades of a conventional wind turbine are not considered dangerous.

What if the wind dies? Does the kite fall out off the sky?

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The kite can be automatically propelled by the rotors during figure-8 flight. But this only makes sense for short periods of time. If there is no wind for longer periods of time the kite lands autonomously.

How can you be sure that you can get an extremely low probability of catastrophic failure?

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With today’s computers, digital twins/software models of the kite can be built and fly millions of flight hours virtually and detect problems in the system design and software early on. We will also test the 1:4 and 1:2 scale demonstrators as well as the full-scale demonstrator extensively. The flight control algorithms and hardware are today available at very low costs. – A similarly ambitious technology are SpaceX’s rockets which now land safely and precisely practically every single time, something that was unimaginable just a few years ago.

What if the kite crashes? What if the tether breaks?

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A Kitekraft system is designed with no single point of failure or high safety factors, e.g., all power electronics, signal electronics/sensors/computers, flight controller software, and actuators are fully redundant. The mechanical tether core and wing spars have a high safety factor. Overall, the probability of a catastrophic failure will be designed to be extremely low, as low as the probability of a conventional wind turbine falling over or losing a blade or as low as the probability of an airliner crash, not only for safety but also for economic viability.

In the finalized product with all redundancies, quality-controlled manufacturing and software releasing, and proven track record of many flight hours, anyone can safely walk below a kite. – As a further level of safety, the system is designed with a higher safety factor for the tether than for the kite, so that even a catastrophic failure is limited within the vicinity of about one tether radius. Yet another level of safety, in the then unlikely case of a tether break, the kite's rotors would have no power which limits the extend it can go, but small batteries can still power the control system and would guide the kite to an emergency-glide-landing next to the ground station. Moreover, the all systems are currently equipped with a ballistic parachute.

What about birds and bats?

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Depending on the location it can make sense to fit the kite system with an optional camera system on the ground station featuring an AI bird detection system. It is implemented to protect wildlife, but also the kite, as a bird can also be a danger to the kite if it would hit a critical part. If birds are predicted to come to close to the kite or tether, the kite will take corrective actions. In the simplest case, the kite goes to hovering mode, waits until the birds have passed, and continues its operational flight.

What about insects?

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The impact is expected to be similar to that of conventional wind turbines. Collisions with insects cannot easily be avoided. What is possible, though, is to land the kite during days of high insect migration. This would also avoid possible damages or performance degradation of the kite caused by insect accumulation at the wings and rotor blades.

Product

What will be the first product?

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Our first product will be a 100 kW kite system. This size is a sweet spot in being still relatively small allowing a fast and cost-efficient development, while allowing a price competitive to large-scale turbines. For now, wind parks of several 100 kW to several MW could be realized by using several 100 kW systems.

We plan to further increase the system size into the MW-scale, decreasing specific costs significantly and undercutting conventional wind turbines.

When will it be available on the market?

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For the 1:4 scale demonstrator, testing at your site is possible already now. Shipping for the 100 kW product is planned to start in 2024.

I want to be one of the first to get it. Can you put me on a list?

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Yes, please contact info@kitekraft.de.

Got more questions?

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