Technical Paper Int’l J. of Aeronautical & Space Sci. 12(4), 354–359 (2011) DOI:10.5139/IJASS.2011.12.4.354 An Airborne Cycloidal Wind Turbine Mounted Using a Tethered Bal- loon

In Seong Hwang*, Wanggu Kang** and Seung Jo Kim*** Korea Aerospace Research Institute, Daejeon 305-333, Korea

Abstract This study proposes a design for an airborne wind turbine generator. The proposed system comprises a cycloidal wind turbine adopting a cycloidal rotor blade system that is used at a high altitude. The turbine is mounted on a tethered balloon. The proposed system is relatively easier to be realized and stable. Moreover, the rotor efficiency is high, which can be adjusted using the blade pitch angle variation. In addition, the rotor is well adapted to the wind-flow direction change. This article proves the feasibility of the proposed system through a sample design for a wind turbine that produces a power of 30 kW. The generated wind power at 500 m height is nearly 3 times of that on the ground.

Key words: Cycloidal wind turbine, Tethered balloon, High altitude

1. Introduction altitudes have been studied. Kim and Park (2010) proposed a parawing on ships, in which the underlying concept is Wind energy is one of the most popular renewable a tethered parafoil that pulls and tows a ship. The ship is energies in the world. However, the generation of a strong equipped with hydraulic turbines below the water line. and steady wind flow is not easy to achieve. For this purpose, Another concept is the use of controlled tethered airfoils to wind turbines are fixed on top of a tall tower; typically, in extract energy from high-altitude wind flows (Fagiano, 2009). daytime, wind speed rises in proportion to the seventh In this method, known as KiteGen®, the tether line makes a root of altitude (Wikipedia, 2011b). Therefore, doubling the linear translational motion and drives an electric generator altitude of a turbine increases the expected wind speeds that is placed on the ground. Besides these two models, by 10% and the expected power by 34%, which means that many other concepts for high-altitude wind generators can the wind power generated is proportional to the cube of the be found in journals and patents (Bronstein, 2011). wind speed. Moreover, at nighttime, or when the atmosphere Although the idea of using high-altitude wind energy becomes stable, doubling the altitude may increase wind is very useful, there exist several technological and policy speed by 20%-60%. However, the height of the tower is barriers (Bronstein, 2011), such as electricity transmission, limited because of building cost involved. In general, to system safety, and cost effectiveness. The electricity from avoid buckling, doubling the tower height requires doubling the airborne power generator can be transmitted by a tether the diameter of the tower as well, increasing the amount of cable, which is reliable, cost-effective, and low-weight. A building material by a factor of at least 4. This means that high-performance synthetic fiber, such as vectran or aramid the cost of such a tower becomes 4.3 times for the tower and with an aluminum conductor, is an affordable option for the foundation construction only (Fingersh et al., 2006). For 100~200 kg/km tether cable (Bronstein, 2011; Kang, 2008). this reason, some novel concepts using wind energy at high System safety is another big engineering design problem for

Copyright © 2011. The Korean Society for Aeronautical and Space Sciences *** Senior Researcher, Corresponding author *** E-mail: [email protected] This is an Open Access article distributed under the terms of the Creative Com- *** Principal Researcher mons Attribution Non-Commercial License (http://creativecommons.org/licenses/by- *** President nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduc- tion in any medium, provided the original work is properly cited.

Received: August 16, 2011 Accepted: December 06, 2011 Copyright ⓒ The Korean Society for Aeronautical & Space Sciences 354 http://ijass.or.kr pISSN: 2093-6742 eISSN: 2093-2480 In Seong Hwang An Airborne Cycloidal Wind Turbine Mounted Using a Tethered Balloon the airborne wind turbine. Th e main source of system failure designed. Table 1 shows the top-level requirements. Th e cut- can be the rotating parts of the wind turbine. After studying in and cut-out wind speeds were determined considering many diff erent turbines, the authors concluded that the the characteristics of the cycloidal rotor. Th e maximum current horizontal-axis wind turbine is not appropriate for operating altitude was limited to 500 m by the tethering cable airborne power generators. Th e tips of the horizontal-axis weight and statistical wind speed profi le data. wind turbine blades make a big circle around the rotating Th e wind profi le power law defi nes the relationship center. Th is rotating arch can unexpectedly collide with any between wind speeds at two diff erent heights. Th is law part of the airborne wind turbine and result in system failure. is often used in wind power assessments. Th e following For this reason, vertical-axis type wind turbines also known equation expresses the profi le of wind speeds with reference as the cycloidal wind turbines are proposed as an alternative to the altitude (Wikipedia, 2011a). in this study. In this article, an airborne cycloidal wind turbine is (1) proposed as a new high-altitude wind power system. Th is method is relatively easier to be realized and stable because where, u is the wind speed and z is the altitude. the rotor and electrical generator are located below the Th e exponent value is an empirically derived coeffi cient balloon. Th e unique characteristic of the cycloidal rotor is that varies depending on the stability of the atmosphere. that the rotating axis and blades are parallel to the tethering For neutral stability conditions, p is approximately 1/7 or cable, which in turn increases the system stability. Th e 0.143 (Wikipedia, 2011a). Although wind conditions vary effi ciency of the rotor can be enhanced by varying the with regional diff erences, the profi le pattern remains almost cycloidal blade pitch. In addition, the rotor is well adapted similar. Th is means that wind power becomes approximately to the wind-fl ow direction change. Figure 1 shows the overall 2.7 times at 500 m height compared with that at 50 m height. conceptual view of the proposed system. Th e balloon is fi lled Moreover, research by Archer and Caldeira points out that with buoyant helium gas to produce lift. Th e cycloidal wind rapid increase in wind power density with altitude occurs turbine with electric power generator is located below the between 80 and 500 m (for median winds, +0.25 W/m2 for balloon. A set of winch devices holds the airborne system each meter increase in altitude) (Archer and Caldeira, 2009). using the tether line on the ground. A disk is attached to the Th is research also suggests that for several sites in Europe, generator for the purpose of balancing the torque caused by there may not be much benefi t in going higher than 500 m, the rotation of the cycloidal blades. unless the height reached is more than 2,000 m. Th is is one factor that determines the operating altitude as shown in 2. System Requirement and Wind Environ- Table 1. ment Another parameter to be considered in wind environment is the wind speed distribution. For this purpose, the Weibull distribution is widely used. As a feasibility analysis model, 30 kW class wind turbine is

(2) the stability of the atmosphere. For neutral stability conditions, p is approximately 1/7 or 0.143where (Wikipedia, k is a shape 2011a).parameter Although and c is a scale wind parameter. conditionsFigure 2vary shows with the regionalwind speed differences, frequency thedistribution for profilea given pattern set of wind remains data recorded almost similar. in a period This of 30 years in meansHong Kong that (Lun and wind Lam, 2000). power Th e becomesthree curves represent approximatelythree diff erent 2.7types times of locations–an at 500 m open height sea area (e.g. an comparedisland), a with completely that at exposed50 m height. area (e.g.Moreover, high up in the city research by Archer and Caldeira points out thatTable rapid 1. System increase requirement in wind power density with altitude occursParameter between 80 and 500 m Value (for median winds, +0.25 W/m2 for each meter Rated power 30 kW Fig. 1. Conceptual design view of high- increaseWorking in altitude) wind speed (Archer and Caldeira,3~20 m/s Fig. 1. Conceptual design view of high-altitude cycloidal wind tur- altitude cycloidal wind turbine. 2009). ThisOperating research altitude also suggests that300~500 for m bine. several sites in Europe, there may not be much benefit in going higher than 500 m, unless the height reached is more than 2,000 m. This is 2. System Requirement and Wind http://ijass.org Environment 355one factor that determines the operating altitude as shown in Table 1. As a feasibility analysis model, 30 kW Another parameter to be considered in class wind turbine is designed. Table 1 shows wind environment is the wind speed the top-level requirements. The cut-in and cut- distribution. For this purpose, the Weibull distribution is widely used. out wind speeds were determined considering kk−1 the characteristics of the cycloidal rotor. The ⎛⎞⎛⎞kv⎡⎤ ⎛⎞ v fv() =−⎜⎟⎜⎟exp ⎢⎥ ⎜⎟ maximum operating altitude was limited to ⎝⎠⎝⎠cc⎣⎦⎢⎥ ⎝⎠ c 500 m by the tethering cable weight and (2) statistical wind speed profile data. where k is a shape parameter and c is a scale parameter. Table 1. System requirement Figure 2 shows the wind speed frequency Parameter Value distribution for a given set of wind data Rated power 30 kW recorded in a period of 30 years in Hong Kong Working wind speed 3~20 m/s (Lun and Lam, 2000). The three curves Operating altitude 300~500 m represent three different types of locations–an open sea area (e.g. an island), a completely The wind profile power law defines the exposed area (e.g. high up in the city center), relationship between wind speeds at two and a city area within the territory. The mean different heights. This law is often used in wind speeds become 7.0, 4.5, and 2.7 m/s, respectively, for the aforementioned locations. wind power assessments. The following Applying a value of 0.143 as the exponent equation expresses the profile of wind speeds value in Eq. (1) and assuming a reference with reference to the altitude (Wikipedia, height of 50 m, the wind speeds at 500 m 2011a). height become 9.7, 6.3, and 3.8 m/s, p uz⎛⎞ respectively, for the three types of locations. = ⎜⎟ (1) uz00⎝⎠ where, u is the wind speed and z is the altitude. The exponent value is an empirically derived coefficient that varies depending on

conservative compared with the result of the previous research; however, 30% is acceptable because of the consideration of uncertain factors. In a cycloidal rotor, in general, a symmetric airfoil is used, and a relatively thick airfoil is preferred for structural reasons. The blade pitch angle should be large, more than 30° when the rotor starts rotating; Int’l J. of Aeronautical & Space Sci. 12(4), 354–359 (2011) however, it becomes smaller, almost as small as 10° during steady rotation.

Fig. 2. A comparison of Weibull probability conservative compared with the result of the distributions of the three locations in the previous research; however, 30% is acceptable territory. because of the consideration of uncertain factors. In a cycloidal rotor, in general, a symmetric airfoil is used, and a relatively 3. System Design thick airfoil is preferred for structural reasons. The blade pitch angle should be large, more 3.1 Characteristics of cycloidal rotor than 30° when the rotor starts rotating; however, it becomes smaller, almost as small The cycloidal wind turbine shown in Fig. 1 as 10° during steady rotation. adopts the cycloidal blade system for variable Fig. 2. bladeA comparison pitch control of Weibull (Yun, probability 2004). Figure 3 Fig. 2. A comparison of Weibull probability distributions of the three distributionsshows of the the schematic three locationsview, which in consists the of locations in the territory. territory.several blades rotating with periodic pitch angle variation. The characteristics of the rotor

could be changed by variations of the pitch angle (θ) and the phase angle (φ), which is center),3. Systemand a city Designarea within the territory. The mean wind Fig. 3. Schematic view of cycloidal rotor defined as the location of maximum pitch Fig. 3. Schematic view of cycloidal rotor blade system. speeds become 7.0, 4.5, and 2.7 m/s, respectively, for the blade system. 3.1 Characteristicsangle. This mechanism of cycloidal has rotor also been applied aforementioned locations. Applying a value of 0.143 as the to aircraft and marine propulsion systems Table 2. SelectedTable 2.rotor Selected design parameters rotor design parameters exponent value(Bartels in Eq. (1) and and Jurgens, assuming 2004; a reference Boschma, height 2001; of The cycloidal wind turbine shown in Fig. 1 ParameterParameter ValueValue 50 m,adopts the wind Hwang,the speeds cycloidal 2009).at 500 blade m Althoughheight system become for finding variable 9.7, 6.3, optimal and Tip speed ratio 2.2 Tip speed ratio 2.2 3.8 m/s,blade respectively, pitchvalues controlof for the the rotor (Yun,three is types 2004).important, of locations. Figure in this 3 study, Solidity 0.5 shows theseveral schematic aerodynamic view, which and structural consists of variables Rotor efficiencySolidity 0.30.5 several were blades adopted rotating without with periodic further pitch parametric BladeRotor airfoil efficiency NACA00180.3 3. Systemangle variation.studies, Design The with characteristics reference to of the rotor results of NumberBlade of airfoil blades NACA00184 o could beprevious changed research by variations (Hwang ofet al., the 2009). pitch Table PitchNumber angle of blades 10 4 2 shows the determined values. The tip speed o angle (θ) and the phase angle (φ), which is Fig. 3.Phase SchematicPitch angle angle view of cycloidal 5 rotor 10° 3.1 Characteristicsratio, which of cycloidal is defined rotor as the ratio between defined as the location of maximum pitch blade system. rotor blade tip speed and wind-flow speed is Phase angle 5° Theangle. cycloidal This mechanism wind turbine has shown also been in Fig.applied 1 adopts the 3.2 Aerodynamic and structural analysis assumed to be 2.2. This value determines the cycloidalto aircraftblade system and for marine variable propulsion blade pitch systems control (Yun, Tableof 2. cycloidal Selected rotor design parameters rotor solidity. Rotor solidity is defined as the 2004).(Bartels Figure 3 and shows Jurgens, the schematic 2004; Boschma,view, which 2001; consists of Parameter Value ratio between the area of the blades and rotor severalHwang, blades rotating 2009). with Although periodic pitch finding angle optimal variation. The of the previous research; however, 30% is acceptable because cylinder area. This variable is a function of the Tip speed ratioOn the basis of the2.2 selected design characteristicsvalues of ofthe the rotor rotor is could important, be changed in this by variationsstudy, of Solidityof theparameters, consideration theof uncertain rotor 0.5factors. configuration In a cycloidal was rotor, several number aerodynamic of blades, and structural blade chord variables length, and in general, a symmetric airfoil is used, and a relatively thick the pitch anglerotor (θ) and diameter. the phase When angle the(φ), which rotor is solidity defined is Rotor efficiencydetermined as shown in Table0.3 3. Under these were adopted without further parametric Bladeairfoil airfoilconditions, is preferred power for structural NACA0018 generation reasons. using The blade the pitch as thestudies, locationhigher, of with maximum the reference tip pitchspeed angle. to ratio the This becomes results mechanism smaller. of has In also been appliedthe to cycloidal aircraft and rotor, marine slower propulsion rotation systems is Numberanglecycloidal shouldof blades be windlarge, more turbine than was 430° when estimated the rotor as starts previous research (Hwang et al., 2009). Table Pitch angleshown in Fig. 4. The rated10o power 30 kW, (Bartels and preferable Jurgens, 2004; because Boschma, of the 2001; centrifugal Hwang, force 2009). on rotating; however, it becomes smaller, almost as small as 10° 2 shows the determined values. The tip speed Phase anglewhich is a requirement of 5 thiso system, could Althoughratio, finding whichthe blades. optimal is defined The values rotor as of the efficiencythe ratiorotor is between isimportant, assumed in to during steady rotation. be 0.3 in this research. This value is be achieved at 17 m/s of wind speed. The this study,rotor several blade aerodynamic tip speed and and wind-flow structural speedvariables is were 3.2 Aerodynamic and structural analysis assumed to be 2.2. This value determines the 3.2 Aerodynamic and structural analysis of cycloidal adopted without further parametric studies, with reference of cycloidal rotor to therotor results solidity. of previous Rotor research solidity (Hwang is defined et al., as2009). the Table rotor ratio between the area of the blades and rotor 2 shows the determined values. The tip speed ratio, which is On the basis of the selected design parameters, the rotor cylinder area. This variable is a function of the On the basis of the selected design configuration was determined as shown in Table 3. Under definednumber as the ofratio blades, between blade rotor chordblade tip length, speed andand wind- parameters, the rotor configuration was flow rotorspeed diameter.is assumed When to be 2.2. the rotorThis value solidity determines is the determinedthese conditions, as shown power in Tablegeneration 3. Under using these the cycloidal wind rotor higher,solidity. the Rotor tip solidityspeed ratio is defined becomes as smaller.the ratio Inbetween conditions,turbine was powerestimated generation as shown in Fig. using 4. The the rated power 30 the areathe of cycloidalthe blades and rotor, rotor slower cylinder rotationarea. This isvariable cycloidalkW, which wind is a requirement turbine was of this estimated system, could as be achieved shown in Fig. 4. The rated power 30 kW, is a functionpreferable of thebecause number of the of blades,centrifugal blade force chord on length, at 17 m/s of wind speed. The rotating speed of the rotor is which is a requirement of this system, could the blades. The rotor efficiency is assumed to approximately 120 rpm for the power generated. and rotor diameter. When the rotor solidity is higher, the tip be achieved at 17 m/s of wind speed. The speedbe ratio 0.3 becomes in this smaller. research. In the Thiscycloidal value rotor, is slower When the cycloidal wind turbine is in operation, the dominant load on the blades is the centrifugal force. Assuming rotation is preferable because of the centrifugal force on the blades. The rotor efficiency is assumed to be 0.3 in this that blades are simply supported at both ends, displacement research. This value is conservative compared with the result can be calculated using the following equation:

DOI:10.5139/IJASS.2011.12.4.354 356 In Seong Hwang An Airborne Cycloidal Wind Turbine Mounted Using a Tethered Balloon

(5), S is disk area and l is disk location.

(3) Resulting values of Eqs. (4) and (5) should be the same; therefore, where w is the distributed load and L is the blade span length. (6) Figurerotating 5 shows speed the ofblade the section rotor isview. approximately As mentioned, the necessary on the electric power generator. For 120 rpm for the power generated. this purpose, a simple circular plate is attached airfoil120 NACA0018 rpm for wasthe powerused in generated. this study, and it has one spar this purpose, a simple circular plate is attached When the cycloidal wind turbine is in as shownTh e result in Fig. of 1.Eq. The(6) is positionindependent and areaof the ofwind speed and at 1/4th chord position. Th e weight of one blade of aluminum operation, the dominant load on the blades is thetherotor disk disk rotating could could be bespeed. calculated calculated Assuming easily easily a drag using using coeffi the the cient of 2.0 for alloy thetheis approximately centrifugalcentrifugal force.force. 24.5 Askg.Assuming Th e maximum that blades displacement are followingthe disk, equations: the area of the antitorque disk is calculated as 1.2 is calculatedsimply supported as 0.187 m at from both Eq. ends, (3) displacement under a uniformly m2 when it is at a distance of 6.0 m fromP the rotating center. distributedcan be load calculated and maximum using operating the followingwind condition. rotor torque = ω Althoughequation: this calculation is very simple, it is necessary and 3.3 System weight and balloon sizing 4 (4) suffi cient to check 5whether⎛⎞wL the4 rotor blade structure design d = (3) On theantitorque basis of the b rotory the design disk and previous research, the is appropriate ord not.= ⎜⎟ (3) 384 EI whole system was roughly2 modeled using the 3D-modeling 384 ⎝⎠EI = F⋅= l(0.5ρ v2 SC ) ⋅ l (5) (5) When the cycloidal wind turbine is rotating, an antitorque = F⋅= l(0.5ρ v SCd ) ⋅ l where w is is the the distributed distributed load load and and L is is the the program CATIA. Table 4 shows the component weight device would be necessary on the electric power generator. where ω isis rotorrotor rotationrotation speedspeed andand P isis rotorrotor blade span length. estimation of the cycloidal wind turbine. Total weight of the For this purpose, a simple circular plate is attached as shown power. In Eq. (5), S isis diskdisk areaarea andand ll isis diskdisk Figure 5 shows the blade section view. As rotor is almost 300 kg. in Fig. 1. Th e position and area of the disk could be calculated location.location. mentioned, the airfoil NACA0018 was used in A spherical balloon was designed to make the 300 kg Resulting values of Eqs. (4) and (5) should easilythisthis using study,study, the following andand itit hashas equations: oneone sparspar atat 1/4th1/4th chordchord be weightthe same; rotor therefore, airborne. Th e balloon is fi lled with helium gas position. The weight of one blade of to generate a static lift andP tethered1 to the ground by a cable. aluminum alloy is approximately 24.5 kg. The (4) S ⋅l = (6) Th e design altitudeS ⋅l for= the balloon2 is 500 m. (6)Th e air density at maximum displacement is calculated as 0.187 ω (0..5ρv C d ) m from Eq. (3) under a uniformly distributed 500 m altitude is 1.167 kg/m3 when thed ground temperature antitorquem from byEq. the (3) disk under a uniformly distributed The result of Eq. (6) is independent of the load and maximum operating wind condition. is 15°C. Th e air density at 15°C is 1.225 kg/m3 at the ground, load and maximum operating wind condition. (5) wind speed and rotor rotating speed. Although this calculation is very simple, it is Although this calculation is very simple, it is Assuming a drag coefficient of 2.0 for the disk, necessary and sufficient to check whether the wherenecessary ω is rotor and rotation sufficient speed to andcheck P is whetherrotor power. the In Eq. thethe areaarea ofof thethe antitorqueantitorque diskdisk isis calculatedcalculated asas rotor blade structure design is appropriate or Table2 4. Cycloidal wind turbine weight estimation rotor blade structure design is appropriate or 1.2 m2 whenwhen itit isis atat aa distancedistance ofof 6.06.0 mm fromfrom not. not. thethe rotatingrotating center.center.Parameter Weight (kg)

Table 3. Rotor design confi guration Blade 98 Table 3. Rotor design configuration Parameter Value 3.3 System weightHub arm and balloon sizing 64 Parameter Value Rotor hub 16 RotorRotor diameter diameter 6 m6 m On the basis of the rotor design and BladeBlade span span lengthlength 6 m6 m Rotor axis 36 previous research, the whole system was BladeBlade chord chord lengthlength 0.750.75 m m Rotor blade control unit 50 roughly modeled using the 3D-modeling Additional unit 30 program CATIA. Table 4 shows the component weightTotal estimation of the cycloidal294 wind turbine. Total weight of the rotor is almostTable 3005. The kg. balloon system weight estimation

Parameter Weight (kg) Remarks Table 4. Cycloidal wind turbine weight Balloon fabricestimation130 12 m diameter, 250 g/ m2 fabric Parameter Weight (kg) BladeBalloon pressure 30 98 controller Hub arm 64 RotorThe hub rigging ropes 20 16 and patches Fig. 4. Power generation curve of cycloidal Rotor axis 36 Fig. 4.Fig. Power 4. generation Power curve generation of cycloidal curve wind of turbine. cycloidal Tether cable 150 Line density 200 g/m × wind turbine. Rotor blade control unit 50 Additional unit 30 500 m Additional unit 30 Load terminating Total 294 structure 50 kg

Cycloidal blades 294 Table 4 A spherical balloon was designed to make Generator units 100 Assumed Fig. 5. Blade airfoil (NACA0018). thethe 300300 kgkg weightweight rotorrotor airborne.airborne. TheThe balloonballoon Total 724 Fig. 5. Blade airfoil (NACA0018). isis filled filled with with helium helium gas gas to to generate generate a a static static When the cycloidal wind turbine is liftlift andand tetheredtethered toto thethe groundground byby aa cable.cable. TheThe rotating, an antitorque device would be design altitude for the balloon is 500 m. The 357 http://ijass.org Int’l J. of Aeronautical & Space Sci. 12(4), 354–359 (2011) while the helium density is 0.169 kg/m3. Th e static lift by 1 as 150 kg including the weight of a load termination bracket. m3 of helium is about 1.0 kg at 500 m altitude (Khoury and Th e load termination bracket is used to transfer loads from Gillett, 1999). Th e total weight of the tethered balloon system the balloon to the tether cable (Kang, 2008). can be divided among three components: balloon, tether Although the electrical power generator has not been cable, and payload and comprises the weight of the balloon designed as yet, the weight including some other related fabric, pressure control system, and the rigging cables and units is assumed to be 100 kg. A summation of all these patches. Th e weights are summarized in Table 5. weights gives a total payload of 724 kg. Th e volume of the Usually, a tethered buoyant balloon is designed to have exactly spherical-shaped balloon is 904 m3 with a 6 m radius. a 10%~15% additional lift to the total weight of the balloon Th e weight of this balloon is estimated on the basis of Kang system. Th e envelope of the balloon should be divided into (2008). two diff erent cells: One is the helium cell, which is used to store the buoyant gas, and the other is the ballonet, which 3.4 Power generation capacity is used for the pressure control of the balloon (Khoury Table 5 shows total wind power generation capacity and Gillett, 1999). A ballonet is used to maintain the inner at three regions for a period of 1 year. Th e wind profi le pressure of the balloon envelope, which is caused by ambient distribution in Fig. 2 was used for this calculation. Th e temperature and pressure change of the surrounding air. reference height from the ground is assumed to be 50 m. Helium leakage can be a major source of pressure loss in Assuming that the exponent value in Eq. (1) is 0.143, the the envelope. Th e volume of the ballonet is 10%~30% of the wind speed distribution at 500 m height was obtained; then entire balloon envelope. For the tethered balloon used in the wind power was calculated within the operating wind speed airborne wind turbine, a ballonet volume of 10% is enough. range according to the following equation: Th e major characteristics of the balloon can be summarized as shown in Table 6. (7) Th e tether cable is about 20 mm in diameter and comprises three diff erent layers as shown in Fig. 6. Th e outer sheath of where Cp is the rotor effi ciency, which is assumed as 0.3 the cable is made of a polyurethane coating, which protects in this study. it from ultraviolet light and abrasion and prevents water As indicated, if the proposed wind turbine system is used, absorption. Th e middle layer is made of synthetic fi bers such it will generate a much larger energy. Th e additional energy as aramid or vectran. Th e midlayer carries loads that are generated is approximately 84, 40, and 8 MWh per 1 year, transferred from the balloon to the ground. Th e core layer is respectively. made of a metal wire, which is used for transferring electric current. Th e number of metal wires are 4~6. Th ree wires are used for transferring electric current and another is used for 4. Conclusions electric grounding and lightning protection, the other wire is redundant and it is used when some cables are broken in Th is article describes an airborne cycloidal wind turbine operation. As shown in Fig. 6, one or two communication mounted using a tethered balloon. Th e preliminary study cables can be added to the core layer for controlling and about high-altitude cycloidal wind power generation system monitoring the balloon and the airborne wind turbine in this article shows that it is possible to obtain much more system. If a wireless communication system is used, these wind energy. Th e sample calculation shows that the amount cables can be omitted. When the density of the tethering of additional energy becomes almost 200% of that at ground cable is 0.2 kg/m, it becomes approximately 100 kg for the level. If we can obtain 80 MWh more of electrical energy 500 m long line. Overall tether cable weight can be assumed in 1 year as indicated in Table 7, and if the electricity per

Table 6. The balloon system characteristics more of electrical energy in 1 year as Parameter Values indicated in Table 7, and if the electricity per household is assumed to be 360 kWh/month, Balloon diameter 12 m which is the average usage in Korea in 2009, Balloon volume 904 m3 this additional amount of energy will serve Ballonet volume @500 m 90 m3 approximately 20 houses. The rotor size in Static lift 814 kg Table 3 is relatively small for a town of Net lift 90 kg Fig. 6. Tether cable construction. Fig. 6. Tether cable construction. approximately 100 houses; however, the proposed system could be extended to any Although the electrical power generator number of houses and can be regarded as a has not been designed as yet, the weight thoroughly feasible plan for an alternative DOI:10.5139/IJASS.2011.12.4.354 358 including some other related units is assumed electric power generation system. to be 100 kg. A summation of all these weights gives a total payload of 724 kg. The Table 7. Power generation capacity volume of the exactly spherical-shaped (MWh/year) 3 balloon is 904 m with a 6 m radius. The @ 500 m @ ground weight of this balloon is estimated on the basis Open sea 134.4 50.0 of Kang (2008). Exposed city 64.0 23.8 area 3.4 Power generation capacity City area 12.2 4.5

Table 5 shows total wind power generation capacity at three regions for a period of 1 year. References The wind profile distribution in Fig. 2 was used for this calculation. The reference height Archer, C. L. and Caldeira, K. (2009). Global from the ground is assumed to be 50 m. assessment of high-altitude wind Assuming that the exponent value in Eq. (1) is power. Energies, 2, 307-319. 0.143, the wind speed distribution at 500 m Bartels, J. and Jurgens, D. (2004). Latest height was obtained; then wind power was developments in Voith Schneider calculated within the operating wind speed propulsion systems. Proceedings of range according to the following equation: the 18th International Tug & Salvage 1 Convention and Exhibition, Miami, P = ρAv3C (7) 2 p FL. Boschma, J. (2001). Modern aviation where Cp is the rotor efficiency, which is assumed as 0.3 in this study. applications for cycloidal propulsion. Proceedings of the1st AIAA Aircraft, As indicated, if the proposed wind turbine Technology Integration, and system is used, it will generate a much larger energy. The additional energy generated is Operations Forum, Los Angeles, CA. pp. AIAA-2001-5267. approximately 84, 40, and 8 MWh per 1 year, Bronstein, M. G. (2011). Harnessing rivers of respectively. wind: a technology and policy assessment of high altitude wind 4. Conclusions power in the U.S. Technological Forecasting and Social Change, 78, This article describes an airborne cycloidal 736-746. wind turbine mounted using a tethered balloon. Fagiano, L. (2009). Control of Tethered The preliminary study about high-altitude Airfoils for High-Altitude Wind cycloidal wind power generation system in Energy Generation. PhD Thesis, this article shows that it is possible to obtain Politecnico Di Torino. much more wind energy. The sample Fingersh, L. J., Hand, M. M., and Laxson, A. calculation shows that the amount of S. (2006). Wind Turbine Design Cost additional energy becomes almost 200% of and Scaling Model [NREL/TP-500- that at ground level. If we can obtain 80 MWh

In Seong Hwang An Airborne Cycloidal Wind Turbine Mounted Using a Tethered Balloon

Table 7. Power generation capacity (MWh/year) Change, 78, 736-746.

@ 500 m @ ground Fagiano, L. (2009). Control of Tethered Airfoils for High- Open sea 134.4 50.0 Altitude Wind Energy Generation. PhD Thesis, Politecnico Di Exposed city area 64.0 23.8 Torino. City area 12.2 4.5 Fingersh, L. J., Hand, M. M., and Laxson, A. S. (2006). Wind Turbine Design Cost and Scaling Model [NREL/TP-500- 40566]. Golden: US National Renewable Energy Laboratory. Hwang, I. S. (2009). Numerical and Experimental Study household is assumed to be 360 kWh/month, which is the on Design and Development of Quadrotor Cyclocopter. PhD average usage in Korea in 2009, this additional amount of Thesis, Seoul National University. energy will serve approximately 20 houses. The rotor size Hwang, I. S., Lee, Y. H., and Kim, S. J. (2009). Optimization in Table 3 is relatively small for a town of approximately 100 of cycloidal water turbine and the performance improvement houses; however, the proposed system could be extended to by individual blade control. Applied Energy, 86, 1532-1540. any number of houses and can be regarded as a thoroughly Kang, W. (2008). Gust Response Analysis of 32m Aerostat. feasible plan for an alternative electric power generation PhD Thesis, Korea Advanced Institute of Science and system. Technology. Khoury, G. A. and Gillett, J. D. (1999). Airship Technology. References Cambridge: Cambridge University Press. Kim, J. and Park, C. (2010). Wind power generation with a Archer, C. L. and Caldeira, K. (2009). Global assessment of parawing on ships, a proposal. Energy, 35, 1425-1432. high-altitude wind power. Energies, 2, 307-319. Lun, I. Y. F. and Lam, J. C. (2000). A study of Weibull Bartels, J. and Jurgens, D. (2004). Latest developments in parameters using long-term wind observations. Renewable Voith Schneider propulsion systems. Proceedings of the 18th Energy, 20, 145-153. International Tug & Salvage Convention and Exhibition, Wikipedia (2011a). Wind profile power law. http:// Miami, FL. en.wikipedia.org/wiki/Wind_profile_power_law (Accessed Boschma, J. (2001). Modern aviation applications for Aug 16 2011). cycloidal propulsion. Proceedings of the1st AIAA Aircraft, Wikipedia (2011b). Wind turbine design. http:// Technology Integration, and Operations Forum, Los Angeles, en.wikipedia.org/wiki/Wind_turbine_design (Accessed Aug CA. pp. AIAA-2001-5267. 16 2011). Bronstein, M. G. (2011). Harnessing rivers of wind: a Yun, C. Y. (2004). A New Vertical Take-Off and Landing technology and policy assessment of high altitude wind Aircraft With Cycloidal Blades System: Cyclocopter. PhD power in the U.S. Technological Forecasting and Social Thesis, Seoul National University.

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