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Project description

KIDERWIND is an open-source project which aims to harness the wind at high altitude in order to generate power. It is part of the Airborne Wind Energy (or High Altitude Wind Power) technologies which use the kinetic energy of high winds.

1 Airborne Wind Energy (AWE)

1.1 Why using it?

  • Recent studies have shown that there is the potential in the world's wind power to sustain global energy needs, as the winds at an altitude above 200 meters from the ground are stronger and more consistent than those lower to the ground. An atlas of high altitude wind power resource has been prepared for all points on earth.
  • The existing technology, based on wind turbine towers cannot harvest such energy as it is close to reaching its economic and technological limits. The weight and the cost increase exponentially with the height so it makes it difficult to position them at more than 100m above the ground.

This is the motivation for why more than 20 active groups (companies and academic institutions) are currently investigating the new technologies of airborne wind energy. They have to date put a lot of effort into this domain because they know that in the next years the AWE technologies will be able to produce energy in a competitive way in comparison to existing fossil fuel. That's very different from the current renewable energies that are marginal in the global energy market.

1.2 The AWE technology

1.2.1 Presentation

The AWE technology does not use a tower but tethered flying systems (like kites or gliders). There are various architectures which can be classified into two groups: fly-gen (the generator is flying) or ground-gen (the generator is on the ground). In our opinion fly-gen systems will be much more expensive than the ground-gen ones because they require a supplementary flying structure in order to sustain the generator and the electric cable weight. Whereas the ground-gen systems are tethered to a motor/generator unit situated on the ground and can be deployed for two kinds of use case, based on:

In both cases the flying system is piloted automatically so that the tether is pulled with a constant strength but at highest velocity to provide maximum return on mechanical power to the ground station.

For more details on how this works see here.

1.2.2 Advantages of the AWE technology

The main advantages of AWE systems are as follows:

  • Wind speed and consistency increase: they increase with altitude as the effects of the earth's surface decrease. Despite the average on the ground, almost everywhere being quite low (~2m/s), at just 500m the average increases to approximately 6m/s and at 1000m is almost 8m/s, a magnitude which would be regarded as excellent for a conventional wind turbine. Since the power contained in the wind increases with the cube of the wind speed, this results in a very large increase in available power. Contrary to conventional wind tower (max. 100m), AWE systems can reach high altitudes (superior to 500m) and so can guarantee the production of more power.
    Here are the different wind speed and power according to the altitude (source):
  • Greater operational flexibility: the AWE plants have the ability to adjust the working height, allowing them to work in the area where the wind is most favourable; it is also possible to regulate at will the amplitude of the surface perpendicular to the wind that is swept by the wing. The traditional plants operate at a fixed height (that of the tower) and with a constant sweep surface (that of the circle described by the blades of the turbine in rotation). Both of these factors make the AWE plants much more productive at equal intensity and frequency of the wind.
  • Higher capacity factor: as a result of these two advantages the capacity factor is more than twice that of the traditional wind power plants (see here for more details).

Key Benefits:

  • Reduced impact on the environment:
    • No pollution compared to fossil fuels.
    • Dramatic reduction in use of raw material, compared with fixed tower-mounted plants.
    • Minimal visual impact on the landscape.
  • Satisfaction of global energy needs: C. Archer and K. Caldeira discovered that the energy quantity available via AWE could be several times greater than the total global energy requirements thanks to the high wind speed and consistency almost everywhere.

The conclusion of this research indicates that through the development of AWE technologies we can meet the worlds energy needs at a competitive cost with a very low environmental impact.

1.2.3 Drawbacks of the AWE technology

There are some critical issues that should be considered in respect to AWE technologies:

  • The maintenance is likely to be more expensive than traditional wind power plants due to the wearing tether’s heavy use (See here for more details).
  • Safety: Since it deploys a flying object some safety issues should be addressed.
  • The system requires a no-fly zone.

It is therefore important to note that these drawbacks are minimal regarding to the advantages it can bring (i.e all the energy the world needs with a minimal environmental impact).

1.2.4 Costs of AWE energy production

To estimate the energy production costs using AWE systems we can start from those of fixed tower-mounted plants which are estimated approximately at 70 $/MW h.

  • Due to the increased capacity factor described above we can divide the costs by two which ensures a production cost more or less at 35 $/MW h.
  • Obviously since no tower is required costs can be reduced dramatically even further.

As a consequence we can state that with AWE systems the price will be below about 35 $/MW h (for context, consider that 35 $/MW h is the cost of the energy produced by coal based power plants).

2 Our proposal

2.1 Philosophy : an open-source system

It's important that the potential of AWE doesn't remain in the hands of few companies like it is in the case of fossil fuel energy sources. Our proposal is to develop KIDERWIND so that these resources and knowledge can be shared among the global population in a more equitable way, ensuring access to energy to all people. If the costs to build a power plant are lower and at the same time the cost of producing energy is cheaper, access to energy will become more affordable for everyone. A case such as the Arduino (an open-source electronics prototyping platform) shows us that open source hardware allows for the reduction of production costs and consequently enables greater opportunity for many people in being able to buy such products. Furthermore an open-source approach ensures greater availability of the detailed designs and of the know how needed to give people the possibility of building by themselves these power plants. Using Open-source guarantees a system that can be used, studied, redistributed and modified by everyone.

To make this we would like to develop AWE systems using an open-source business model for the hardware part, as well as the software part.

2.2 Description of the system

2.2.1 Working overview

The working mechanics of an AWE installation can be subdivided into three core phases : Take-off
  • The glider will be raised to the suited altitude in order to get enough wind (it should be more than 3 m/s). This could be done using a remote helicopter which would send a signal to the operator when the glider can start its flight. When validated by the operator the helicopter would release the glider (so that it is able to engage an autonomous flight mode). The productive phase

Ultimately the tensile force of the sail is converted into energy through a cycle of intermittent traction and recovery.

  • In the generation phase, the glider is automated to perform a figure-of-eight or circular maneuvers contained in a plane perpendicular to the wind, generating a high cable tension. At the same time, the tether is slowly reeled off a drum which is connected to the generator. This phase is achieved thanks to an algorithm stored in the autopilot. When the glider reaches a maximal altitude, the retraction phase is initiated. This phase should last several minutes.
  • In the retraction phase, this process is reversed, using the generator as a motor to reel in the tether. The glider dives to its initial altitude (the one after the take-off, e.g. 200m). Diving is a low-traction flight mode so that the glider can be reeled in at high speed, spending only a fraction of the energy produced in the generation phase.

This cycle (generation / retraction) will be repeated many times until the wind is stronger than the minimum value with which the plant can work (e.g. 3 m/s) and weaker than the maximum value (e.g 20-25 m/s). This phase should last about one minute. In this video you can watch this cycle. The landing and shut off

If the wind is too strong or too weak or in the event of potential breakage of the main wire, the glider will try to land as soon as possible on the base station or on an authorized landing area.

2.2.2 Detail of the equipment The ground station

Is essentially composed of a winch mechanically linked to an electrical generator and motor. The tether

800-1000m long made with advanced polymers; such as Dineema and Vectra. The Flying Control Unit (FCU) and the Autopilot

Is a combined system that allows the glider to follow a given trajectory (determined by the direction and speed of the wind), so as to maximise energy production but also prevent the tether from breaking. The FCU is mounted at the upper end of the main rope (several hundred meters long) tethered to the ground station. The FCU is also connected to the glider by 4 ropes 10-20m long. The FCU is controlled by an Autopilot (like ArduPilot) which pulls the four ropes in order to optimize the trajectory. Close to the FCU there is a “load cell”which measures the strength applied on the ropes. The System Control Unit (SCU)

An electronic system which forms part of the ground-station designed to monitor the overall performance. The SCU controls the duration of the rising, the descent phases and tracks the energy produced.

  • During the rising phase the SCU monitors the energy produced and checks when this phase should be stopped (maximal altitude reached, too high / weak wind).
  • At the end of the generation phase, the SCU performs simultaneously two operations :
    1. Notify the FCU that it has to begin the descent
    2. Activates the electrical engine of the ground station to rewind the tether.
  • During the descent phase the SCU regulates the electrical power supplied to the motor to adjust the rewinding speed for the main (descent of the glider). The SCU will then calculate how long the descent stage must last.
  • When the wing reaches the minimum operating altitude, the SCU performs simultaneously two operations:
    1. Informs the FCU that it should restart the rising flight technique for energy production.
    2. Activates the motor so as to generate energy.

The maximum and minimum operational altitudes can be changed at each cycle of operation in order to correlate with the speed of the wind, ensuring the plant works for the longest possible to durations at its maximum power capacity. The flying part

Composed of a hybrid system somewhere between a kite and a glider. It will have the advantages of a kite (structural resistance, lightness) combined with the aerodynamic efficiency of a glider. The tensairity technology (see also here for more details) could be considered. The copter or a quad

Used for the take-off (and may also be used for the landing).

2.3 Getting started

2.3.1 Prototyping phase Objective
  • Validate the system by developing and deploying a 1kW plant. Resources needed
  • Material to build the prototype : it won't be expensive (between 500 / 700 € at most) and can be composed primarily of scavenged material (except for the autopilot): electrical generators coming from car scrap yards, glider from model aircraft making…
  • Team to develop the software part and to build the installation (ground station, tether, glider) : to do that, we could imagine organizing a hackathon over few weeks.
  • Spaces dedicated to aircraft modelling in order to fly and test the system.

2.3.2 Intermediate phase Objective
  • Install a 100kW plant.

This phase should validate that the prototype is scalable. Resources needed
  • Material to build the installation.
  • A technical operator to monitor the system.
  • No-fly zones like unused airports, old nuclear power plants, fields used for radio-controlled models.

2.3.3 Production phase Objective
  • Install a 500kW plant. Resources needed
  • Material to build the installation.
  • A technical operator to monitor the system.
  • No-fly zones like unused airports, old nuclear power plants, fields used for radio-controlled models.

2.4 Collaborating

We would welcome collaborations with the following communities to develop the system:


Wind power and wind speed

  • A wind turbine can produce its maximal power when the wind is between 14 and 25 m/s (see the explanation here). As a consequence the production depends on the wind speed. So it is important to consider the frequency at which a wind speed is reached (the power production can't be calculated for an average wind speed value as the power is not proportional to the speed as shown here). For example at 11 m/s the power produced is half of the maximal power (see the diagram here).
  • The capacity factor of a wind turbine installation can be expressed in :
    • Percentage : the capacity factor is the actual output over a period of time as a proportion of a wind turbine or facility’s maximum capacity. For example, if a 1.5-MW turbine generates power over one year at an average rate of 0.5 MW, its capacity factor is 33% for that year (See here).
    • Hours/year : if we say that the production is X hours / years it means that X kWh can be produced for a 1 kW nominal power installation.
  • Different studies show that between 200m and 1000m the capacity factor is at least equal to 4000 hours/year (or 4000 kWh/kW installed) in almost all parts of the world. Given that for traditional wind towers it is about 2000 hours/year we can say that the capacity factor for AWE is double (for more details see p.14-16 of this document).

Lifespan of a tether of an AWE system

The tether is the most stressed part of an AWE system and it will have to be replaced regularly. The fatigue strength of a rope subjected only to tensile (or traction) mainly depends on the force with which it is pulled. The parameter that allows us to predict the duration of the ropes is the tensile strength expressed as a percentage with respect to the breaking force. In the case of ropes subjected simultaneously to bending and tensile, fatigue strength is significantly reduced compared to the case of tensile stress only. The data provided by the companies that produce and sell ropes allow us to predict that they may have a duration of several months. A compromise should be found between production and maintenance costs. If you want to increase the lifespan of the tether you will have higher production costs but will lower the maintenance costs.

Interesting links / References




/var/www/kiderwind/data/pages/project_description/start.txt · Last modified: 25/08/2014 19:32 by Mirelsol