Kitegen a Revolution in Wind Energy Generation-suman

7/27/2019 Kitegen a Revolution in Wind Energy Generation-suman

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SRIRAM ENGINEERING COLLEGE

TAMIL NADU, INDIA.

Department of Electrical and Electronics Engineering

KITEGEN: A REVOLUTION IN WIND ENERGYGENERATION.

PRESENTED BYR.Ram Kumar [email protected] 9790112657M.Balasubramani [email protected] 8098525799

ABSTRACT

Wind is rapidly becoming an important renewable energy

source as the worry about declining fossil fuel reserves and

the global warming associated with their use continues to

grow. This paper presents a new class of wind energy

generators , denoted as KITEGEN , which employ power

kites to capture high altitude wind power. A relatively new

idea for wind power generation that can overcome many of

these shortcomings uses large kites to extract power from

high-altitude winds. In this scheme, very large and relatively

inexpensive kites are tethered to ground-based generators.

Due to high altitude force of wind, the power kites starts

to fly upto the maximum height. Kites will move correctly if

it has the AERODYNAMICFORCES.(i.e. This can beachieved simply by altering the angle between the kite and the

wind in order to make the lift force).By the action of power
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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kites, the lines connected from the kite to rotor generator

makes to rotate the rotor and so ELECTRICITY is generated.

While comparing to all power generating techniques,

kitegen produces the large amount of energy with minimum

cost. Small space is enough for the construction of kitegen.

The performance of the KITEGEN is controlled by using

KSU[KITEGEN STEERING UNIT].KSU is very important to

make aerodynamic forces to act on kite, which makes it to fly.

AIM:The Ultimate Aim of this paper is to generate electric

power with low cost, minimum area, less energy loss, high

efficiency by using power kites. Kite Gen is pioneering a

revolution on how to produce clean energy from wind, withthe aim not only to compete within the current wind industry

but, as still too rarely happens with renewable sources, to

move the battlefield into the territory of fossil fuels.

This paper deals with the following topics:

INTRODUCTION.

NECESSITY FOR CHANGE OVER

FROM WIND FARMS TO KITEGEN

TECHNOLOGY.

STRUCTURE OF KITEGEN.

Yo-yo configuration

Carousel configuration

MECHANISM OF KITEGEN.

WINDS OF HIGH ALTITUDE.

CONTROL AND STEERING UNIT OF

KITEGEN.

FUTURE SCENARIOS.

CONCLUSION.

INTRODUCTION: T he problems posed by electric energy generation fromfossil sources include high costs due to large demand andlimited resources, pollution and CO2 production, and thegeopolitics of producer countries. These problems can beovercome by alternative sources that are renewable, cheap,easily available, and sustainable. However, current renewabletechnologies have limitations. Indeed, even the mostoptimistic forecast on the diffusion of wind, photovoltaic, and

biomass sources estimates no more than a 20% contribution tototal energy production within the next1520 years. Excludinghydropower plants, wind turbines are currently the largestsource of renewable energy [1]. Unfortunately, wind turbinesrequire heavy towers, foundations, and huge blades, whichimpact the environment in terms of land usage and noise

generated by blade rotation, and require massive investmentswith long-term amortization. Consequently, electric energy

production costs are not yet competitive with thermalgenerators, despite recent. Wind is rapidly becoming animportant renewable energy source as the worry aboutdeclining fossil fuel reserves and the global warmingassociated with their use continues to grow. There has been agreat deal of success in generating power from largewindmills.


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KiteGen is a company located near Torino, Italy,that is building kite-powered generators. Kite Gen is

pioneering a revolution on how to produce clean energy fromwind, with the aim not only to compete within the currentwind industry but, as still too rarely happens with renewablesources, to move the battlefield into the territory of fossilfuels. The idea of kitegen is created from seashore gamecalled kite surfing. (as shown in fig.1)

FIGURE 1: Kite surfing. Expert kite-surfers drive kites to obtain energyfor propulsion. Control technology can be applied to exploit this technique for electric energy generation.

To reach altitude wind and exploiting its higher kineticenergy, the Kite Gen project starts from a radical change of

perspective: no longer heavy and static plants like currentwind turbines, but instead light, dynamic and intelligent ones.In the air, to subtract energy from the wind at an altitude of 800 / 1,000 m, power kites, semi-rigid automatically pilotedhigh efficiency air foils. On the ground, all the heavymachinery for power generation. To connect the two systems,high resistance lines transmitting the traction of the kites andat the same time controlling their direction and angle to thewind.

A clear advantage of this technology is visuallysuggested in the illustration below. The essence of the KiteGen concept is comparable with a wind turbine, whose mostefficient part are the wing tips in red where the highestspeeds are reached; but only the truly needed componentsremain, high speed wings and the generator, the latter conveniently moved to the ground. The resulting structure,

base foundation included, is much lighter and cheaper.Moreover the operative height can be adjusted according towind conditions.

NECESSITY FOR CHANGE OVER FROM WIND FARM TO KITEGEN:

Kitegen have a several number of advantages over wind Farm.wind farms are unable to built over high altitudes. Wind at800 m is out of the reach of current and futureaerogeneratingtowers, already struggling at 100 m.Thestructure holding up the rotors becomes exponentially heavier,more unstable and expensive. Unfortunately,wind turbinesrequire heavy towers, foundations,and huge blades, whichimpact the environment in terms of land usage and noisegenerated by blade rotation, and require massive investmentswith long-term amortization.Consequently, electric energy

production costs are not yet competitive with thermalgenerators, despite recent increases in oil and gas prices .

FIGURE 2: DISADVANTAGE OF WIND TOWER

The solution to overcome this problem we implementthe kitegen idea. Solution: A radical shift of perspective: no


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longer heavy and static structures, but a light, dynamic andintelligent machine.In wind towers, the outermost 20%of the bladescontributes for 80%of the power.

FIGURE 3: Comparison between wind turbines and airfoils in energy production. In wind towers, limited blade portions (red) contribute predominantly to power production. In KiteGen, the kite acts as the mostactive portions of the blades, without the need for mechanical support of theless active portions and the tower.

The kite acts as the outermost part of the blades withoutrequiring the heavy tower.

Energy generation with controlled power kites canrepresent a quantum leap in wind power technology,

promising to obtain renewable energy from a source largelyavailable almost everywhere, with production costs lower thanthose of fossil sources.

According to our simulation results, it is estimatedthat the required land usage for a kite generator may be lower than a current wind farm of the same power by a factor of upto 3050, with electric energy production costs lower by afactor up to 1020. Such potential improvement over currentwind technology is due to several aerodynamic andmechanical reasons. For example, 90% of the power generated

by a 2-MW threeblade turbine with a 90-m rotor diameter iscontributed by only the outer 40% of the blade area,corresponding to about 120 m2.

This dependence is due to the fact that theaerodynamic forces on each infinitesimal section of the

blades are proportional to the square of its speed with respectto the air, and this speed increases toward the tip of the blades.In KiteGen, the tethered airfoils act as the outer portions of the

blades, without the need for mechanical support of the tower and of the less-productive inner blade portions. Indeed, amean generated power of 620 kW is obtained in the

simulation reported in Figure 16 for a single kite of 100-m2area and 300-m line length. Figure 5,shows that the torqueexerted by wind forces at the base of a wind turbines supportstructure increases with the height of the tower, the force isindependent of the line length in KiteGen.

Due to structural and economical limits, it is notconvenient to go beyond the 100120 m height of the largestturbines commercially available. In contrast, airfoils canfly at altitudes up to several hundred meters, takingadvantage of the fact that, as altitude over the groundincreases,the wind is faster and less variable ;For example,at800 m the mean wind speed doubles with respect to 100m(the altitude at which the largest wind turbines operate).Sincethe power that can be extracted from wind grows with thecube of the wind speed, the possibility of reaching suchheights represents a further significant advantage of KiteGen.

The carousel configuration is scalable up to severalhundred megawatts, leading to increasing advantages over

current wind farms. Using data from the Danish WindIndustry Association Web site, it follows that,for a site such asBrindisi, in the south of Italy, a 2-MW wind turbine has amean production of 4000 MWh/year. To attain a meangeneration of 9 TWh/year, which corresponds to almost 1000-MW mean power, 2250 such towers are required, with a landusage of 300 km2 and an energy production cost of about100120 /MWh. In comparison, the production cost fromfossil sources (gas, oil) is about 6070 /MWh. Kite-poweredgenerators have many advantages over windmill generators,assuming the same power can be produced.

First, the actual generator is located on the groundinstead of on top of an 80 meter tower, greatly simplifyingengineering and reducing costs. Second, the expensive andlarge rotor blades are replaced by a relatively inexpensive kiteand set of lines. Third, the amount of land required by the kitegenerators is can be as much as 9 times less than the landrequired by a windmill farm producing the same amount of energy.

Kites have the potential to harness wind at muchhigher altitudes than windmills, giving access to stronger andmore consistent winds. Kite generators also have greater ease

of scalability as bigger generators can be built, and more kitescan be added in carousel configuration, but stability andmanufacturing difficulties limit the size of windmills.Furthermore, damage to a kite or lines is less costly thanrepairing windmill blades, and kites could be activelycontrolled to avoid planes and even wildlife. Besides being a

proven and more mature technology, windmills do not haveany major advantages over kite-powered generators. If the

predictions by KiteGen and other companies about the


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potential of kite power are close to accurate, they will likelyhave a large impact upon wind power.

STRUCTURE OF KITEGEN:The vertical axis orientation of the rotation is intended to

eliminate the static and dynamic problems that prevent the

increase in size of conventional wind turbines .The problem of "capturing" the wind is solved by the use of ( Power kites ) whose movements are controlled automatically by a computer .Through cables the kites are anchored to a structure thatrotates, generating electricity. This structure is the turbine of the high altitude wind farm while the kites are the "blades" of the turbine. The kites are flown on a predetermined trajectory,that can transform the exerted force on the cable, to an overallmechanical torque which rotates the vertical axis turbine.

CAROUSEL CONFIGURATION:

About twenty automatically controlled kites can keeprotating a turbine of 1,600 meters diameter at a speed of 15revolutions per hour. This can generate 1 Gigawatt of power,equivalent to a medium size nuclear power station but with anestimated capital cost 10 times lower. In other words 1 cubicKm of sky is able to provide 1 GigaWatt of power for 80% of the time in a year. The kites extra added benefit lies in the factthat the length of cables allows them to reach heights over 500meters, where the high altitude wind flows, withoutintroducing structural weaknesses. Simulations published byKiteGen, estimate that they can achieve 793 kW average

power generation with a kite that is 100 m 2 in size using analtitude-varying windspeed between 8 m/s at ground level and24 m/s at 800 m. [5] By scaling up the size of the kite to 500m2, 2 MW can be generated at 9 m/s constant windspeed, andmuch more power at higher windspeeds.

FIGURE 4 KiteGen carousel configuration concept. Several airfoils

are controlled by the kite steering units placed on the arms of a verticalaxis rotor. The airfoils flight is controlled so as to turn the rotor,which transmits its motion to an electric generator.

In another design, called carousel configuration, they predict that a plant could generate up to 1000 MW mean power with 12 m/s winds and 100 kites that are each 500 m 2 insize. In actual prototype tests with small kites, KiteGen wasable to generate positive net energy using small kitescontrolled by humans. More importantly, the power generatedmatched well to the power predicted by their model,suggesting that their model also provides a good estimate for

higher-power generators.YO-YO CONFIGURATION:

Individual yo-yo configuration generators would beless expensive and lighter than existing windmills since a kiteand tether lines weighing about 3 tons altogether wouldreplace the tower and rotor which weigh about 200-300 tonstogether (the same generator is assumed in both cases).Furthermore, windmills need to be spatially separated in order to achieve maximum performance, while kite-poweredgenerators would extract power from the same amount of air volume with a significantly reduced footprint on the ground.

KiteGen predicts they can achieve energy costs of $0.02-$0.05 per kWh, as compared to $0.05-$0.09 per kWh for fossilenergy and $0.15 per kWh for current windmills.In order totrust these numbers from KiteGen, it is necessary to look deeper into the assumptions they make.

The most important of these is that sufficient wind isavailable, followed by the assumption that enough power can

be generated by the kite and that this power can be transmittedto the ground. Next, that the kites can be sufficiently
http://en.wikipedia.org/wiki/Wind_turbinehttp://en.wikipedia.org/wiki/Power_kitehttp://en.wikipedia.org/wiki/Power_kitehttp://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Wind_farmhttp://en.wikipedia.org/wiki/Gigawatthttp://en.wikipedia.org/wiki/Nuclear_power_stationhttp://en.wikipedia.org/wiki/Nuclear_power_stationhttp://en.wikipedia.org/wiki/Nuclear_power_stationhttp://en.wikipedia.org/wiki/File:Kitegen.jpghttp://en.wikipedia.org/wiki/Wind_turbinehttp://en.wikipedia.org/wiki/Power_kitehttp://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Wind_farmhttp://en.wikipedia.org/wiki/Gigawatthttp://en.wikipedia.org/wiki/Nuclear_power_station


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controlled, and finally, that these generators would havereduced costs compared to other types of generators.

FIGURE 5: KiteGen small-scale prototype of a yo-yo configuration.The kitelines are linked to two electric drives. The flight of the kite is controlled by

regulating the pulling force on each line, and energy is generated as the kiteunrolls the lines.

MECHANISM OF KITEGEN:The two kite lines are rolled around two drums

and linked to two electric drives,which are fixed to theground. The flight of the kite is controlled by regulating the

pulling force on each line. Energy is collected when the windforce on the kite unrolls the lines, and the electric drives actas generators due to the rotation of the drums. When themaximal line length of about 300 m is reached, the drives actas motors to recover the kite, spending a small percentage

(about 12%, see the Simulation Results section for details)of the previously generated energy .This yo-yo configuration is under the control of the

kite steering unit (KSU, see ), which includes the electricdrives (for a total power of 40 kW), the drums, and all of thehardware needed to control a single kite. The aims of the

prototype are to demonstrate the ability to control the flight of a single kite, to produce a significant amount of energy, and toverify the energy production levels predicted in simulationstudies.The potential of a similar yo-yo configuration isinvestigated,by means of simulation results, in and for one or more kites linked to a single cable. In and , it is assumed thatthe angle of incidence of the kites can be controlled. Thus, thecontrol inputs are not only the roll angle and the cablewinding speed, as considered in and in this article, but also thelift coefficient CL.

For medium-to-large-scale energy generators, analternative KiteGen configuration is being studied, namely,the carousel configuration. In this configuration, introduced inand shown in Figure 4, several airfoils are controlled by their KSUs placed on the arms of a vertical-axis rotor. The

controller of each kite is designed to maximize the torqueexerted on the rotor, which transmits its motion to an electricgenerator. For a given wind direction, each airfoil can produceenergy for about 300 of carousel rotation; only a smallfraction about 1%,of the generated energy is used to drag thekite against the wind for the remaining 60.

WINDS OF HIGH ALTITUDE:There are two wind flow bands that envelope the

Earth globe. One passes over the southern hemisphere at thelatitude of Pataganio, while the other passes over the northernhemisphere, over Europe. The flow height ranges from 800meters up to 10,000 meters of altitude, while the width is4,000 or 5,000 km. The average power of the wind is about2 kW per square meter.

High altitude wind is much more powerful and constantwhen compared to that at earth level, which is intense in veryfew places, and at full speed for only about 1,700-1,800 hours

per year, which limits the annual production of energy. Thewind which is planned to be used is around 800 meters heightwith average speeds of 7 m/s and specific power of 200 W/m. For example, a section of wind width of 1,000 meters at analtitude between 600 and 1,000 meters has a power equal to400*1000*200 = 80 MW.

The prototype in the Province of Asti which workswith 9 generators and up to 10,000m generates a peak power of 27 MW. A park of Kitegens with 100 MW peak power should produce500 GWh/year; enough for 86,000

household .The Kitegen can generate about 6000 hours / year.

FIGURE 6 :Wind-speed variation as a function of altitude. These data are based on the average European wind speed of 3 m/s at ground level .


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CONTROL AND STEERING UNIT OFKITEGEN: The KSU (Kite Steering Unit) is the unit that allows toautomatically pilot a power kite or an array of power kitesover a predefined flight path (see prototype video). The power kite is manouvered by differentially unrolling and recovering

the two lines on two winches controlled by engines.Each Kite Gen power plant is composed by several

KSUs pulled by the power kites along a ring-shape circular path at ground level. At the very core of the project stays thesoftware that, receiving data also from on-board avionicsensors (see sensors video), autonomously pilots the power kites, so that their flight patterns can be controlled,synchronized and normally directed to maximise the

production of energy.With such configuration, a single Kite Gen power plant is

able to intercept very large amounts of altitude wind. In the

illustration below, concerning the Kite Generator in theCarousel configuration, is shown the area swept by a plantwith a circular path of 800 m diameter: the same quantity can

be reached by approx. 150 latest generation wind turbines. Ithas to be noted that wind turbines need to be spaced to avoidshading one another and decreasing the total yield, so thesewould require a territory of more than 40 Km. The Kite Gen

power plant, a safe area around included, uses approx. 5 Km.Energy production takes place in a distributed manner at eachunit, thus avoiding unmanageable sizes of the electricalequipment.

The modular approach makes possible to build very powerfulKite Gen plants, where as the diameter at ground level of thecircular path grows, the area swept increases to the square andtherefore the total wind power. 100 MW Kite Gen power

plants, not much larger than the illustrated example, diameter at ground level of the circular path of approx. 1,000 m, areestimated to deliver a cost of energy produced lower than 0.03

Euro per kWh. This value is bound to be further enhanced,since 1 000 MW (1 GW) plants are under study (see Futurescenarios).In the Stem configuration too is possible to reach bigamount of generated power, through wind farm made of several generators, each of them much closer to the other thanit is actually possible to traditional wind mills.

Good automated control of the kite is extremelycritical in order to keep the kites in the sky and generatingoptimal power. KiteGen presents one model which it uses inthe kite simulations presented in Canale et al., but they used ahuman controller during the actual test flights. However, other groups have successfully demonstrated kite control or have

published a number of promising papers on kite control.While difficult, this controls problem does not have anyfundamental physical constraints as long as the kites haveenough forward velocity to keep moving, and that can beoptimized with sufficient research and time.

To ensure the kite will keep moving, we need anaerodynamic force in the forward direction of the kite. Thiscan be achieved simply by altering the angle between the kiteand the wind in order to make the lift force (normally

perpendicular to the effective wind direction) act partially inthe forward direction of the kite. This results in more forwardacceleration with less force on the lines, and so there is atradeoff between keeping the kite flying, and producing

power.If we use the same aerodynamic equations as Canale et

al., we see that this force in the forward direction depends

greatly upon the angle of attack, positive for some angles andnegative for others.However, an average force of zero is allthat is necessary to keep the kite flying, so the angle can becontrolled to ensure there is enough forward momentum. Thisindicates the importance of the control algorithms for flyingthe kites, which is a major area of research. Due to thesuccessful tests and simulations by KiteGen, we can assumethat enough forward velocity can be obtained.

The KiteGen project has designed and simulated a small-scale prototype. The two kite lines are rolled around twodrums and linked to two electric drives, which are fixed to the

ground. The flight of the kite is controlled by regulating the pulling force on each line. Energy is collected when the windforce on the kite unrolls the lines, and the electric drives act asgenerators due to the rotation of the drums. When themaximal line length of about 300 m is reached, the drives actas motors to recover the kite, spending a small percentage(about 12%, see the Simulation Results section for details)of the previously generated energy .

This yo-yo configuration is under the control of the kitesteering unit (KSU, see Figure 3), which includes the electric


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drives (for a total power of 40 kW), the drums, and all of thehardware needed to control a single kite. The aims of the

prototype are to demonstrate the ability to control the flight of a single kite, to produce a significant amount of energy, and toverify the energy production levels predicted in simulationstudies.The potential of a similar yo-yo configuration isinvestigated,by means of simulation results, in and for one or more kites linked to a single cable. In and, it is assumed thatthe angle of incidence of the kites can be controlled. Thus, thecontrol inputs are not only the roll angle and the cablewinding speed, as considered in and in this article, but also thelift coefficient CL.

For medium-to-large-scale energy generators, analternative KiteGen configuration is being studied, namely,the carousel configuration. In this configuration, introduced infigure, several airfoils are controlled by their KSUs placed onthe arms of a vertical-axis rotor. The controller of each kite isdesigned to maximize the torque exerted on the rotor, which

transmits its motion to an electric generator. For a given winddirection, each airfoil can produce energy for about 300 of carousel rotation; only a small fraction about 1%,of thegenerated energy is used to drag the kite against the wind for the remaining 60.According to our simulation results, it isestimated that the required land usage for a kite generator may

be lower than a current wind farm of the same power by afactor of up to 3050, with electric energy

FIGURE 7: Scheme of the kite steering unit. The kite steering unit, which provides automatic control for KiteGen, includes the electric drives, drums,and all of the hardware needed to control a single kite.

production costs lower by a factor up to 1020. Such potentialimprovement over current wind technology is due to severalaerodynamic and mechanical reasons. For example, 90% of the power generated by a 2-MW threeblade turbine with a 90-m rotor diameter is contributed by only the outer 40% of the

blade area, corresponding to about 120 m2.This dependence is due to the fact that the

aerodynamic forces on each infinitesimal section of the bladesare proportional to the square of its speed with respect to theair, and this speed increases toward the tip of the blades. InKiteGen, the tethered airfoils act as the outer portions of the

blades, without the need for mechanical support of the tower and of the less-productive inner blade portions; see Figure 5.Indeed, a mean generated power of 620 kW is obtained in thesimulation reported in Figure 16 for a single kite of 100-m2area and 300-m line length.Figure shows that the torqueexerted by wind forces at the base of a wind turbines supportstructure increases with which transmits its motion to anelectric generator.

CONTROL DESIGN: The main objective of KiteGen control is to maximizeenergy generation while preventing the airfoils from falling tothe ground or the lines from tangling. The control problem can

be expressed in terms of maximizing a cost function that predicts the net energy generation while satisfying constraintson the input and state variables. Nonlinear model predictivecontrol (MPC) is employed to accomplish theseobjectives,since it aims to optimize a given cost function andfulfil constraints at the same time. However, fastimplementation is needed to allow real-time control at therequired sampling time, which is on the order of 0.1 s. In

particular, the implementation of fast model predictive control(FMPC) based on set membership approximationmethodologies.

SENSORS AND SENSOR FUSIONS: The KiteGen controller is based on feedback of the kite

position and speed vector, which must be measured or accurately estimated. Each airfoil is thus equipped with a pair of triaxial accelerometers and a pair fulfilled, the DVS givesthe same accuracy as the theoretical minimal variance filter.Moreover, in the presence of modeling errors andnonlinearities, the DVS guarantees stability and performstradeoffs between optimality and robustness, which are notachievable with EKF.

AERODYNAMIC FORCES:The aerodynamic force _Faer depends on the effective windspeed _We, which in the local system is computed as

where _Wa is the kite speed with respect to the ground. For

both the yo-yo and carousel configurations, _Wa can beexpressed as a function of the local coordinate system (, , r)and the position of the KSU with respect to the fixedcoordinate system (X,Y, Z).Let us consider now the kite wind coordinate system,with itsorigin located at the kite center of gravity, the basis vector

_xw aligned with the effective wind speed vector, the basisvector _zw contained by the kite longitudinal plane of symmetry and pointing from the top surface of the kite to the

bottom, and the basis vector _yw completing a righthanded


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system. In the wind coordinate system the aerodynamic force _Faer,w is given by

where FD is the drag force and FL is the lift force, computedas

where is the air density, A is the kite characteristic area,andCL and CD are the kite lift and drag coefficients. All of thesevariables are assumed to be constant. The aerodynamic force

_Faer can then be expressed in the local coordinate system asa nonlinear function of several arguments of the form

The kite roll angle in (12) is the control variable, defined by

where d is the kite width and _l is the length difference betweenthe two lines (see Figure 8). The roll angle influences the

kite motion by changing the direction of _Faer.LINE FORCES: Concerning the effect of the lines, the force Fc isalways directed along the local unit vector er and cannot benegative, since the kite can only pull the lines.Moreover, Fc ismeasured by a force transducer on the KSU, and, throughcontrol of the electric drives, it is regulated so that the line speedsatisfies r(t) r ref (t), where r ref (t) is chosen. In the case of the yo-yo configuration,Fc(t) = F c(, , r, , , r, r ref , _W e), while, for the carouselconfiguration, F c(t) = F c(, , r,_, , , r, _, r ref , _W e).

MOTOR DYNAMICS:In the case of the carousel configuration, the motion

law for the generator rotor is taken into account by theequation.

where Jz is the rotor moment of inertia and Tc is the torque of the electric generator/motor linked to the rotor. Viscous termsare neglected in (14) since the rotor speed _ is kept low asshown in the Simulation Results section. Tc is positivewhen the kite is pulling the rotor with increasing values of _,

thus generating energy, and it is negative when the electricgenerator is acting as a motor to drag the rotor when the kite isnot able to generate a pulling force.The torque Tc is set by alocal controller to keep the rotor at constant speed _ = _ref.

KITEGEN DYNAMIC DESCRIPTION:The generic system dynamics are of the form

where x(t) = [(t) (t) r(t) _(t) (t) (t) r(t) _(t)]T are themodel states and u(t) = (t) is the control variable. In the caseof the yo-yo configuration, _ = _ = _ref = 0. All of the modelstates are assumed to be measured or estimated for use infeedback control. Mechanical power P generated

FIGURE 8 Forces acting on the kite. The aerodynamic lift and drag forcesare FL and FD, respectively, the gravitational force is mg ,and the pullingforce F c is exerted by the lines. The length difference between the lines givesthe roll input angle .

FIGURE 9: Yo-yo configuration phases. The kite steering unit acts on thekite lines in such a way that energy is generated in the traction phase (green)and spent in the passive phase (red). Each cycle begins when the proper starting conditions (circled in blue) are satisfied. In this simulation the effectsof turbulence are neglected.


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with KiteGen is the sum of the power generated by unrollingthe lines and the power generated by the rotor movement, thatis,Both terms in (16) can be negative when the kite lines are

being recovered in the yo-yo configuration or the rotor is being dragged against the wind in the carouselconfiguration.For the yo-yo configuration the term _ Tc iszero, and thus the generated mechanical energy is due only toline unrolling.Note that (16) is related to a carousel with asingle KSU.When more kites are linked to the same carousel,the effect of line rolling/unrolling for each kite must beincluded.

Yo-Yo CONFIGURATION CONTROLLER:The traction phase begins when the kite is flying in a

prescribed zone downwind of the KSU, at a suitable altitude ZI with a given line length r.

FIGURE 10:(a) Carousel configuration phases. The same rotor arm isdepicted with three subsequent angular values. The pasive phase starts when

the rotor arm reaches the angular position _ 0, and lasts until the rotation angle _ 3 is reached. To maneuver the kite to a suitable position to begin the traction phase (highlighted in blue), the passive phase is divided into 3 subphases(gray orange, and green) delimited by rotation angles _ 1 and _ 2. (b) Kitetrajectory with carousel configuration.The kite follows figure-eight orbits,which maximize its speed during the traction phase (green), while during the

passive phase (red) the airfoil speed is very low to reduce drag forces. Thekite steering unit follows a circular trajectory at ground height, with radius R..

When the traction phase starts, the kite flies as line length r increases due to a positive value r ref of the line.

FIGURE 11: Simulation results for the yo-yo configuration. Kite trajectoriesare reported during the traction (green) and passive (red) phases of a completeyo-yo configuration cycle in the presence of wind turbulence. Note that the

behavior is similar to Figure despite the turbulence.


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FIGURE 12: Simulated power obtained with the yo-yo configuration. Acomplete cycle is considered in the presence of wind turbulence.Theinstantaneous course of the generated power during the traction phase (green)is reported together with the power spent for the kite recovery in the passive

phase (red). The mean value of the power generated during the cycle, which isrepresented by a dashed line, is 11.8 kW. The corresponding generated energyis 2613 kJ per cycle.

speed reference provided by the local motor controller. Sincea traction force Fc is created on the kite lines, the systemgenerates mechanical power. The predictive control law

computes the line angle (see Figure)in order to vary Fc andthus optimize the aerodynamic behavior of the kite for energygeneration. The line angle is obtained by varying _l according to (13) by imposing a setpoint on the desired linelength achieved by the local motor controller.

The value of the reference line speed r ref is chosen asa compromise between obtaining high traction force actionand high line winding speed. Basically, the stronger the wind,the higher the value of r ref that can be set while obtaininghigh force values. The control system objective in the traction

phase is to maximize the energy generated during the prediction interval [ tk, tk + TP ]. Since the instantaneousgenerated mechanical power is

P(t) = r(t)Fc(t) , MPC minimizes the cost function.

The traction phase ends when the length of the linesreaches a given value r and the passive phase begins. The

passive phase is divided into three subphases. In the firstsubphase, the line speed r(t) is controlled to smoothlydecrease toward zero. The control objective is to move thekite into a zone with low values of and high values of | |(see Figure 7), where the effective wind speed _ We and force

Fc are low and the kite can be recovered with low energyexpense. Then, in the second subphase, r(t) is controlled tosmoothly decrease from zero to a negative value, which

provides a compromise between high rewinding speed andlow force Fc . During this passive subphase, the controlobjective is to minimize the energy spent to rewind the lines.This second subphase ends when the line length r reaches thedesired minimum value.In the third passive subphase, r(t) iscontrolled to smoothly increase toward zero from the previousnegative setpoint.The control objective is to move the kite inthe traction phase starting zone. The passive phase ends whenthe starting conditions for the traction phase are reached.

CAROUSEL CONFIGURATIONCONTROLLER: In the carousel configuration ,the torque Tc given bythe carousel motor/generator is such that the rotor moves atthe constant reference angular speed _ ref, which is chosen tooptimize the net energy generated in the cycle. Since theangular speed is constant, each kite can be controlledindependently, provided that the lines never collide. Thus, asingle kite is considered in the following. The traction phase

begins at the rotor angular position _ = _ 3, where the nominalwind direction is such that the kite can pull the rotor arm [seeFigure 10(a)]. A suitable trajectory for the line speed r duringthe traction phase is set to further increase generated power.Recalling that mechanical power obtained at each instant isthe sum of the effects given by line unrolling and rotor movement, MPC minimizes the cost function


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FIGURE 13: Line speed reference imposed during a complete carouselcycle. The commanded line speed r (t) is chosen on the basis of simulationdata to increase the mean generated power and to ensure that the lengths of the lines at the beginning of each cycle are the same.

FIGURE 14: Figure-eight kite orbits during the traction phase for thecarousel configuration. Such orbits are imposed by means of suitableconstraints on the angles and to avoid line wrapping

FIGURE 15: Power generated with the carousel configuration. Twocomplete cycles are considered in the presence of wind turbulence. The

instantaneous course of the generated power during the traction phases(green) is reported together with the power required for the kite recovery inthe passive phases (red). Note the nearly null values of energy usage duringthe passive phases. The mean value of the power generated during the twocycles is 621 kW and is represented by a ashed line. The correspondinggenerated energy is 234 MJ per cycle.

When the rotor arm reaches the angle _ 0, the kite canno longer pull the carousel, and the traction phase ends. Then,the passive phase starts, and the electric generator linked tothe rotor acts as a motor to drag the carousel between angles

_ 0 and _ 3. Meanwhile, the kite is moved to a suitable positionfor initiating the next traction phase.

Yo-Yo CONFIGURATION:For simulation, we consider a yo-yo configuration

similar to the physical prototype. The numerical values of thekite model and control parameters are reported in Table 1,while Table 2 contains the state values for the start and endconditions of each phase as well as the values of the state andinput constraints. Figure shows the trajectory of the kite, whilethe power generated during the cycle is reported in Figure 12.The mean power is 11.8 kW, which corresponds to energygeneration of 2613 kJ per cycle.

CAROUSEL CONFIGURATION:A carousel with a single KSU is considered. The

model and control parameters employed are reported in Table3, while Table 4 contains the start and end conditions for each

phase, as well as the values of the state and input constraints.The line speed during the cycle is reported in Figure. Thisreference trajectory is chosen on the basis of the previoussimulation to maximize the mean generated power and toensure that the length of the lines at the beginning of eachcycle is the same. Figure shows the trajectories of the kite andthe control unit during two full cycles in the presence of

random wind disturbances. Figure depicts some orbits traced by the kite during the traction phase, while the power generated during the two cycles is reported in Figure.

The mean power is 621 kW, and the generated energyis 234 MJ per cycle. Figure 17 depicts the course of theeffective wind speed | _ W e| (see the section Kite Generator Models for details). It can be noted that during the traction

phase the mean effective wind speed is about 14 times greater than the tangential speed of the rotor connected to thegenerator, which is 18 km/h.

Since the fixed coordinate system (X,Y, Z) is defined

on the basis of the nominal wind direction, a measurablechange of the latter can be overcome by rotating the wholecoordinate system (X,Y, Z), thus obtaining the same

performance without changing either the control system parameters or the starting conditions of the various phases.


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FIGURE 16: Simulated effective wind speed for the carousel configuration.The course of | _ We| during the traction subphases (green) and the passivesubphases (red) is related to two complete carousel cycles. The average valuesare 250 km/h during the traction phase and 85 km/h during the passive phase.

FUTURE SCENARIO:Given a fixed optimal aspect ratio of the plant, as the

diameter of the circular path at ground level grows, the areaswept by the power kites increases to the square, reaching alsohigher and more powerful winds. Such exponential growth of the total wind power that can be harnessed is the main reason

behind building bigger Kite Gen power plants.And due to theinner modularity of the technology, that foresees themultiplication of single steering units producing energy on alarger circular path, the scalability of the Kite Gen power

plants comes without significant structural and costconstraints.

In a way, the difficulty in growing the size of a plantcan be compared to what it takes, having built an automobile,to build a long line of the same automobiles.This is the reasonwhy 1 000 MW (1 GW) and larger Kite Gen plants arestudied, where the aim is not just increasingly cheaper cost of energy produced, but providing a solution to effectively reach(a not only dream of) a global energy mix where a consistent

part of the baseload comes from clean, renewable source:tropospheric wind. It would be like installing thousands of current wind turbines into the same site, something impossible

because of the surface spacing needed to maintain a sufficentefficiency.

Kite Gen power plants overcome this limit, eachunit exploiting different regions of the huge volume of windswept by the whole machine. It has to be added that, in scalingup the dimensions of the plant, one key technology that KiteGen Research plans to explore is the conversion of mechanical energy into electrical energy through linear magnetic engines in a reversible fashion.The theoretical

boundaries of such configuration appears to be a ring of approx. 25 km diameter, very similar to a railway bridge,which is the base, or technically the stator, on which rotates amag lev Kite Gen; the tethered high power kites flyautomatically at up to 10 km height in a controlled formation,generating a power of more than 60 GW. Right now, however,is deemed that opportunity evaluations will probably suggestto consider power plants of smaller dimensions.

CONCLUSION:According to the calculations above, the predictions

published by KiteGen appear quite optimistic, but perhaps notunrealistic. Significant improvement seems to be achieved bygoing to 400 m altitude, but the real potential for consistentand powerful wind lies at an even higher altitude. The dragand weight of lines will play an even more important role atthis height, and so more detailed studies will need to be

performed. However, the kites are able to extract sufficient power from the wind, and control systems are already

effective enough to keep a kite flying.Also, Kite systems have the potential to becompetitively inexpensive, and to be more scalable thancurrent windmills. Wind intermittency will still be a very large

problem, and so wind power will need to be coupled withmore consistent power plants unless the current technology for energy storage is greatly improved. In conclusion, While kite-

powered generators will likely not replace traditional power plants immediately, they have a lot of potential to startreplacing windmill farms in the near future.

Kitegen a Revolution in Wind Energy Generation-suman

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