NASA Wind Energy Airborne Harvesting System Study

ANNOTATED BIBLIOGRAPHY

Relevance Score: 10 

Bilaniuk, N. (2009). Generic System Requirements for High Altitude Wind Turbines. Ottawa, Canada: LTA Windpower. http://www.ltawind.com/Downloads/HAWT%20requirements%20whitepaper%20v1.pdf

  • The introduction defines some common terms in the wind turbine field
    • Efficiency
    • Coefficient of Performance-often used for efficiency
    • Betz limit (16/27, or slighter over 59%)
    • Capacity Factor
    • Nameplate capacity- rated maximum output
    • Drag design-motion of blades is in same direction as the wind
    • Axial flow design-motion of blades is perpendicular to direction of wind
  • The axial flow design has emerged as the preferred option. A similar shakeout will likely occur for airborne wind turbines.
  • Winds are stronger and steadier at higher altitudes. Wind speed increases with altitude up until about 12 km (Figure 1). Wind energy is proportional to the cube of wind speed and is directly proportional to air density, which decreases with altitude.
  • Figure 2 shows that the size of surface features influences the reduction of wind speed at the surface. Locations with the best surface winds tend not to coincide with the locations of electricity markets.
  • Reaching to higher altitudes will result in higher capacity factor and reduced cost per kWh.
  • High speed/low torque is better. A low speed/high torque source requires stronger parts (expensive) and high step-up ratio transmission. This results in a high “cut-in wind speed”
  • Mechanical Power transmission:
    • Undulations in tether tension-can have low angular frequency or intermittent power
    • Ganged tethers-prop on kite, allows kites to gather power by flying cross-wind and deliver power at higher frequencies (patent 4,251,040)
    • Torsion in tether-patent 6,616,402, difficult to achieve from strength of materials point of view
  • Balance of cost seems to favor using an electrically conductive tether instead of mechanical transmission
  • Tether lessons learned (section 3):
    • Use stranded copper wire
    • Focus on designs that fly a generator less than 1 km up and be careful with heat concentration around a winch
    • For scalability to large sizes or higher altitudes, avoid winches to preserve possibility of using non-unified tether.
  • Bernoulli lift generation by itself is not adequate for wind calms that are likely to occur some of the time.
  • Using buoyant lift alone requires large surplus buoyancy to maintain clearance above the ground. Large surplus buoyancy means a winch is required, which means that a unified tether must be used.
  • Excess wind, lightening strikes, and precipitation are hazards that airborne wind turbines will have to deal with.
  • During routine operation, wind farms must be able to get by with zero staff in order to be economically viable. Routine unattended takeoffs and landings must be possible.
  • Overarching requirement: efficiency with autonomous survivability.

Relevance score: 10

Williams, P., Lansdorp, B., and Ockels, W. (2007). Flexible Tethered Kite with Moveable Attachment Points, Part II: State and Wind Estimation. AIAA Atmospheric Flight Mechanics Conference and Exhibit. Hilton Head: AIAA.http://repository.tudelft.nl/view/ir/uuid:dd1c4167-8c32-4cf1-b087-ba91608ca9a6/ 

 

  • It is part of the same paper series as AIAA-2007-6628. 
  • An overview of the dynamic model derived in AIAA-2007-6628 is given in section II.
  • In III a, a square-root unscented Kalman filter is developed to estimate the kite state vector using the dynamic model, and III b. applies this filter to station keeping, cross-wind motion, and variable-wind kite motion. This type of filter is good because there is no need to derive analytic Jacobians, which would be difficult because the system is highly nonlinear.
  • The controller and filter are very accurate in controlling and estimating, respectively, the motion of a kite.

Relevance Score: 10

Williams, P., Lansdorp, B., and Ockels, W. (2007). Flexible Tethered Kite with Moveable Attachment Points, Part I: Dynamics and Control. AIAA Atmospheric Flight Mechanics Conference and Exhibit. Hilton Head: AIAA. http://repository.tudelft.nl/view/ir/uuid:c37fdb5b-0845-49b0-9ff8-eafc4daf7866/

 

  • This report focuses on the dynamics and control of a flexible kite and does not discuss tether characteristics (especially applicable to the Laddermill project).
  • The roll and pitch of a kite can be controlled by tilting the total lift vector with a slider system that changes the attachment points of the kite tether.
  • Section 2 derives a model for a kite that consists of two rigid plates that are connected by a frictionless hinge. The two plates are able to roll and pitch freely around the hinge, but are constrained to the same yaw angle. The primary forces acting on the plates in the model are aerodynamic, gravity, and tether tension forces. The tethers are modeled as elastic springs.
  • Section 3 derives equilibrium configurations when the tether is attached to the quarter chord of the plates. With the tether in this position, the pitch angle decreases and the roll angle increases with increasing wind speed in order to achieve equilibrium. The system is naturally unstable, so active control is necessary for stability.
  • Sections IIIa. And IIIb. Derive a controller based on full state feedback (station keeping control) and a controller based classical PID control, respectively.
  • Arbitrary time-varying periodic trajectories for the system are generated in IVa. and trajectories are tracked using the state-space controller derived in the previous sections. Tracking proves to be more difficult than station keeping control due to actuator input becoming more saturated due to variations.
  • Different types of cross-wind motion are simulated in IVc. and the results are provided. Some improvements to the control algorithms are necessary to fly the kites in an autonomously robust manner.
  • Section V extends the two plate model to multiple plates, with torsional springs used to couple the pitch and roll motions of the plates.

Relevance Score: 10

Jung, T.P. (2009).Wind Tunnel Study of Drag of Various Rope Designs. 27thAIAA Aerodynamics Conference.San Antonio: AIAA.http://www.aiaa.org/agenda.cfm?lumeetingid=2123&formatview=1&dateget=22-Jun-09

 

  • The paper describes a study done on six different rope designs (section IIa.) to determine the characteristics that most affect drag.
  • The momentum deficit method is used to determine the drag coefficients for each rope design based only on measured dynamic pressures (Equation 2).
  • The ropes are modeled as right circular cylinders and the flow regime is subcritical (star on Figure 5).
  • A rope is assumed to have a Strouhal number between 0.21 and 0.3, which is used to determine the sampling rate (500 Hz).  In order to use the momentum deficit method, the dynamic pressure must be measured at least three rope diameters behind the rope so that the near wake is avoided.
  • Studies have shown that small surface variations (roughness) only affect the boundary layer and result in less drag for a cylinder (Figure 8), but greater roughness can cause the flow to separate (Figure 9), leading to greater drag.
  • Section III describes the test set-up, instrumentation, and uncertainty/repeatability considerations.
  • The results at 90 ft/s show that material properties of the rope are the primary determinant for drag characteristics.  Adding latex reduces the drag significantly in all cases.  The fibers on the 5thrope increase the drag, but are not as much a factor as the material.  The weave designs act to reduce the drag (small surface variation), but does not have as large an effect as material.  Reducing porosity results in increased drag, but does not affect the drag as much as the material.

Relevance Score: 9.5

Lansdorp, B., & Ockels, W. (2005). Comparison of concepts for high-altitude wind energy generation with ground based generatorBeijingChina International. http://www.tudelft.nl/live/binaries/fe263f84-29af-4010-8222-2f1112c8f223/doc/Comparison%20of%20concepts_bas.pdf

 

  • Compares the Laddermill (LM ) and Pumping Mill (PM) concepts
  • PM power generation is not constant which could be troublesome if connecting to a power grid
  • For LM, half of it is unused at anytime. For PM, all of it is unused for about 33% of the time
  • LM: Tether and Kite must be the same; PM: Kite can be made for their position on the tether. The tether can be tapered as well to be as durable as necessary.
  • LM: the wings could be switched if necessary for weather. PM: the tether can be changed (lengthened) but the kites would be difficult to switch out.
  • PM: loss in power generation because of the re-stretching of the tether
  • LM: the whole tether needs to be strengthened because of the interaction with the generator. PM: only the part which come in contact with the generator
  • LM: everything can be inspected/ replaced in one rotation. PM: the whole system would have to be grounded
  • LM: complicated ground station for obvious reasons. PM: very simply ground station
  • Vwind = 3.48 +0.00573h, 0 <h < 988 m
  • Vwind = 7.85 +0.00146h, h >= 988 m
  • LM wing mass will be 2.2 times higher and the LM tether: 2 times more massive
  • Adapting the designs to incorporate crosswind would be beneficial
  • The PM will result in a more lightweight option

Relevance Score: 9.5

Stiesdal, H. (1999, Fall). The Wind Turbine: Components and Operation. Bonus Energy A/S Newsletter , pp. 4-24.http://www.windmission.dk/workshop/BonusTurbine.pdf

 

  • Basic outline of why a turbine is designed like it is
  • “The air flow around a wind turbine blade is completely dominated by the head wind from the rotational movement of the blade through the air.”
  • “The blade aerodynamic profile produces lift because of its streamlined shape. The rear side is more curved than the front side.”
  • “The lift effect on the blade aerodynamic profile causes the forces of the air to point in the correct direction.”
  • “The blade width, thickness, and twist is a compromise between the need for streamlining and the need for strength.”
  •  “At constant shaft speed, in step with the grid, the angle of attack increases with increasing wind speed. The blade stalls when the angle of attack exceeds 15 degrees. In a stall condition the air can no longer flow smoothly or laminar over the rear side of the blade, lift therefore fall and drag increases.”

Relevance Score: 9.5

Williams, Paul (2006).  Optimal Wind Power Extraction with a Tethered Kite. AIAA Guidance, Navigation, and Control Conference and Exhibit. Keystone: AIAA. http://pdf.aiaa.org/preview/CDReadyMGNC06_1305/PV2006_6193.pdf

 

  • The article states that wind speed obeys a power law at low altitudes and power is proportional to the cube of the wind speed. The article introduces several different airborne wind generator concepts, but only a kite-tether system is explored in more detail.
  • First, a multi-link lumped mass tethered kite model is derived for planar motion. External forces on the tether are applied at the lumped masse. The kite is controlled by changing the angle of attack, and the attitude and flexibility of the kite are ignored (Section III).
  • Then, a simpler pendulum model of the system is used to perform preliminary analysis of cross-wind dynamics (three-dimensions). In this model, the tether is modeled by a single straight, inelastic rod (Section IVa).
  • Since a kite system is very sensitive to gusts and turbulence, a simple linear receding horizon based feedback controller is used to track trajectories for the kite system (Section IVb).
  • The effect of certain system parameters on the static stability of the kite-tether system is examined (Section V).
  • Numerical optimal control techniques are used to optimize wind energy extraction using the planar and cross-wind models (Section VI)
  • Numerical results are given for the planar and cross-wind models (Section VII).
  • Results show that cross-wind motion is more efficient at generating power than planar motion (simple angle of attack change). Numerical simulations also show that it is possible to track desired trajectories even in the presence of large variations in wind speed around the mean.

Relevance Score: 9 

Diehl, M. (2010, January 20). Wind Power Generation Via Fast Flying Kites. Belgium: Optimization in Engineering Center (OPTEC) & Electrical Engineering Department (ESAT), K.U. Leuven. http://asta.fs.tum.de/asta/…10/rivo_diehl_flugdrachen_ws200910.pdf

  • High torques on turbine blades and tower limit size and height of conventional wind turbines
  • Flying kites in cross-wind motion can result in forces 100 times greater than on a static kite.
  • On-board generator:
    • Pros: fast spinning generators
    • Cons: heavy, moving parts on board, electric cables, conversion losses
  • Pumping cycle:
    • Power generation phase and retraction phase (slide 10)
    • Pros: all heavy and electric parts are on the ground
  • Lloyd paper demonstrates that power grows quadratically with glide number (lift to drag ratio) in cross-wind flight--slide 12
  • In cross wind motion a kite moves faster than the wind (by a factor equal to the glide number)
  • Dancing kites are more efficient:
    • Absolute line drag is reduced because only short lines move fast in cross-wind direction
    • Centrifugal forces help negate effect of kite mass
    • Kites can compensate each other during retraction, without lift control
    • 40% more power compared to single kites
  • Starting/landing methods (difficult due to low winds at low altitude): statically with telescope mast (skysail), artificial blower (KiteGen), helicopter style, conventional aircraft style.
  • Slides 32-45 talk about different groups working on high altitude wind power.
  • Leuven team is focusing on rigid airfoils and rotational arm-like device (slide 46&47):
    • Centrifugal and lifting forces keep the kites in the air and aid in take-offs/landings
    • Lines from multiple kites are connected to reduce line drag (slide 54)

Relevance Score: 9

Furuya, O., & Maekawa, S. (1982). Technical and Economical Assesment of Tethered Wind Energy Systems (TWES). Golden, CO: Solar Energy Research Institute. http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JSEEDO000106000003000327000001&idtype=cvips&gifs=yes&ref=no

 

  • 16 KW/m^2 at 10 km as compared to 0.5 KW/m^2 at ground level
  • Introduced a hybrid method, in which the Tethered Wind Energy System, TWES, was a mix between balloon and wing concepts
  • Introduced a VTOL concept, which would operate like a normal wing concept but at low speeds it would have power supplied to the turbines. Said to be the best design by the author by fulfilling the requirements listed below
  • The highest power is available at 300 mb
  • Conditions and Requirements for the a TWES
    • Lift at low and high winds
    • Effect of air density reduction on lift generation mechanism
    • Deployment and landing methods
    • Safety
    • Maintenance
    • Available current technology
    • Cost
    • High voltage transmission to minimize loss through the tether cable.
    • Should use AC generators with step-up transformers.
    • A.C. synchronous generator/ step-up transformer/rectifier-inverter system
      • The heat dissipation mechanism can be significantly smaller because of cold T
  • Eliminate conductance and inductance in cable.
    • No synchronization.
    • Superiority of DC transmission
  • Constant rotational speed was selected.
    • Requires blade pitch which has been used widely (reliable).
    • Superior to variable speed generators both in performance and operation.”
    • “The weight and cost comparison of these two generators will also give and advantage to CRS.”
  • Aluminum tether with Kevlar insulation
  • Recommended a 9 month flying time. With the excluded three months having the lowest wind energy. The TWES grounded time can be used for repair and maintenance to increase reliability and lifetime of the system
  • Cost formulas for their design listed and explained. (p.55)
  • Cost of electricity (p.57)
  • 8.5 c/ kWh
  • Largest cost is from the generating subsystem. (Most room for improvement)

Relevance Score: 9

Bolonkin, A. (2004). Utilization of Wind Energy at High AltitudeProvidence: AIAA. http://arxiv.org/ftp/physics/papers/0701/0701114.pdf

 

  • Wind rotor power increases at the cube of wind speed
  • The use of artificial fibers instead of copper
    • For 20 MW and voltage 1000 V the cross sections are Cu: 20,000 AF: 37 mm^2
    • Cable length of 25 km, weight of Cu: 8930 tons, AF: 3.33 tons
    • AF is also much cheaper
  • High altitude wind is stable and constant. “This is true practically everywhere”
  • At a height of 12-13 km, the average wind speed in jet stream is 41 m/s. Can reach 103 m/s.
  • The 40 m/s can produce 64 times more energy than surface wind speeds at 10 m.
  • Power of wind energy equation (p.11)
  • Wind speed equation depending on the height and surface roughness (p.11)
  • Many different projects are listed with their possible power production, economic efficiency, and technical parameters
  • Advantages as listed in paper:
    • Energy at least in 10 times cheaper then energy received from current wind installations
    • Inexpensive (no expensive tower)
    • Can theoretically capture wind energy from an enormous area
    • Power per unit is some hundred times more than current wind installations
    • Does not require large ground space
    • The installation may be located near customers.
    • Ocean going vessels can use this installation for its primary propulsion source.
    • No noise and bad views.
    • The energy production is more stable because the wind is steadier at high altitude.
    • The installation can be easy relocated in other place.

Relevance Score: 9

Lansdorp, B., Ruiterkamp, R., and Ockels, W. (2007). Towards Flight Testing of Remotely Controlled Surfkites for Wind Energy Generation. AIAA Atmospheric Flight Mechanics Conference and Exhibit. Hilton Head: AIAA. http://repository.tudelft.nl/view/ir/uuid:2daec500-3379-46a7-a747-90baf6044f58/

 

  • This paper first introduces the Laddermill concept, as in the paper, “The Laddermill-Innovative Wind Energy from High Altitudes in Holland and Australia.”
  • Section III discusses certain parameters that would be important for testing kite performance, such as wind speed, position, line length, apparent wind speed at the kite, tether tension, L/D, etc.
  • Section IV contains a discussion of three different types of steering mechanisms that were examined, as in the paper, “The Laddermill-Innovative Wind Energy from High Altitudes in Holland and Australia.” However, more details are provided for the third concept, which involves an sliding mechanism for the tether attachment point on the kites.
  • Section V lists different parameters measured during a kite flight and the sensors that are currently being used to perform the measurements.
  • Initial testing was started and all sensors were operational, except for an apparent wind speed sensor. 
  • Analysis of the data should result in useful relationships such as the dependency of lift and drag on the angle of attack.

Relevance Score: 9

Williams, P., Lansdorp, B., & Ockels, W. (2007). Optimal Trajectories for Tethered Kite Mounted on a Vertical Axis Generator. Hilton Head: AIAA. http://www.google.com/url?sa=t&source=web&cd=1&ved=0CBIQFjAA&url=http%3A%2F%2Fwww.tudelft.nl%2Flive%2Fpagina.jsp%3Fid%3Dfe263f84-29af-4010-8222-2f1112c8f223%26lang%3Den%26binary%3D%2Fdoc%2FVertical%2520axis%2520kite%2520wind%2520generator_bas.pdf&ei=JzQRTIi_GoeANreuuZ4D&usg=AFQjCNFlQj5_YAu7Cm3PAtWNEWhiKvseuA

 

  • Attach the cable to a lever arm on a vertical axis generator
  • The model is based on the assumption that the tether remains straight, which would be controlled by changing angle of attack and roll angle
  • It also does not account for the mass or drag of the tether
  • It is a model that attempts to determine the optimal trajectories for this kind of system
  • A cross-wind pattern for high wind speed and large kite areas
  • Lower speeds and lower kite areas: the kite tends to move in an elongated ellipse in a plane normal to the wind with predominant motion occurring in the vertical direction

Relevance Score: 9

Williams, P., Lansdorp, B., Ruiterkamp, R., and Ockels, W. (2008). Modeling, Simulation, and Testing of Surf Kites for Power Generation. AIAA Modeling and Simulation Technologies Conference and Exhibit. Honolulu: AIAA. http://www.google.com/search?q=Modeling%2C+Simulation%2C+and+Testing+of+Surf+Kites+for+Power+Generation&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&safe=active

 

  • The article begins with a discussion of the current energy situation, renewable energy sources, and airborne wind generation. The Laddermill is also explained.
  • The Laddermill cannot be reeled into too fast because the lift force is increased, which increases cable tension.
  • Section IIIa. describes a point mass model for modeling a kite’s dynamics. This model is useful for preliminary analysis of flight trajectories and performance, using only the lift and drag of the kite.
  • Section IIIb. derives a rigid body model of the kite and tether system. This model has the advantage of needing to use forces/moments to control the attitude, which results in a less complicated control design. However, the flexible characteristics of a kite are neglected. The sliding cable attachment point is also taken into consideration.
  • Section IVc. discusses the plate model for a kite, which was detailed in AIAA-2007-6628.
  • Alternative methods for modeling a kite are a lumped parameter or finite element implementation (Section IVd.). Section IVe. uses the Tornado vortex-lattice software to determine rough aerodynamic parameters for a kite.
  • The kite dynamic model was tested via an actual flight test, in which GPS was used to determine the kite position, attitude, etc.  A controller based on the dynamic model was designed to keep the kite on a desired trajectory and it provided robust tracking control with large wind variations (Fig. 16 & 17).

Relevance Score: 8.5

North, D. High Altitude Wind Power. HamptonNASA Langley Research Center. Download Paper.

 

  • Extremely low mass, low cost, highly maneuverable kites
  • High altitude winds systems would most likely be in the 70-80% range vs. 20-30% for ground based wind turbines in regards to overall electrical power supply
  • Current best estimates for this type of system project a life cycle cost at 0.5 to 1.5 cents per kilowatt hour compared to current costs of 5 to 12 cents per kilowatt hour.
  • Four types: ladder mill, rotor kite, rotating kit, turntable kite
  • The “Pumping” Laddermill or Kite Reel;  Kite Gen; FlyGen; Magenn; Flying Electric Generator are described
  • FlyGen has generators attached to the kite and the kite dives from high to low altitudes
  • Magenn is meant for small-scale power production
  • Listed Challenges
    • Aerodynamic control of flexible airfoil for maximum energy extraction
    • High altitude airspace restrictions
    • High strength-to-weight ratio tether materials
    • Reversible reeling / generator systems
    • Airfoil / tether recovery and deployment methods (permanent deployment with lighter-than-air features)
    • Optimum power station siting for jet stream

Relevance Score: 8.5

MacKay, D. J. (2009). Sustainable Energy - without the hot air. Cambridge: UIT Cambridge Ltd.  http://www.withouthotair.com/

 

 Part 1: Numbers, Not Adjectives.

  • Offshore wind, which could generate 25% of the UK’s electricity by 2020.”
  • Wind farms will devastate the countryside pointlessly. -James Lovelock
  • The maximum contribution of onshore wind, albeit “huge,” is much less than our consumption
  • Why not offshore… At the big Danish wind farm, Horns Reef, all 80 turbines had to be dismantled and repaired after only 18 months’ exposure to the sea air.
  • See 60-67 for more details on why not.
  • This article describes the inability for offshore wind and onshore wind to produce enough power to be worth the price

Part II: Making a Difference.

  • Pages 74-90
  • Wind Power fluctuates
  • The biggest fluctuation in Irish wind power had a change of 84 MW per hour. On a country scale, 3.7 GW per hour would need to be added with this fluctuation. Every morning, British demand is increased by 6.5 GW per hour.
  • If a lull of five days without wind power were to occur. Every person would have to use 4 kWh per day less
  • Two solutions for intermittency: The first solution stores up energy, and then copes with fluctuations by turning on and off a source powered from the energy store. The second solution works by turning on and off a piece of demand.
  • Describe hydroelectricity and such to deal with lulls
  • Smart chargers could cut down on electricity use. i.e. using wind power to charge car only when the wind is blowing and in the time limit set by the user. Aka switching demand on and off
  • Turn your refrigerator down a little less when there is a lull
  • Ways of storing energy: Flywheels, Supercapacitors, Vanadium Flow Batteries

Part III: Technical Chapters.

  • P. 263-268
  • If the departing wind speed is 1/3 of the arriving wind speed, the power extracted is 16/27 (59%) of the total power in the wind.
  • Efficiency factor X Power X Area= _ _% X [ (0.5)(rho)(v^3)] X [(pi/4)(d^2)]
  • Real wind turbines don’t actually deliver a power proportional to wind-speed cubed. Rather, they typically have just a range of wind-speeds within which they deliver the ideal power.
  • The way that wind speed increases with height is complicated and depends on the roughness of the surrounding terrain and on the time of day.
  • Double in height= 10% increase in wind-speed=30% increase in power
  • V(z)=V10 *(z/10m)α; where α is usually around 1/7 or 0.143
  • V(z)=Vref *(log(z/z0)/log(zref/z0)); Vref is the speed at zref. z0 is the roughness length (around 0.1)
  • Van den Berg suggests the different wind profiles often hold at night.
  • Capacity is usually 1 MW for windmills. But a good capacity factor is 30%
  • A microturbine delivers on average 0.2 kWh per day.

Part IV: Useful Data.

  • 140 turbines will create 322 MW- enough to power 200,000 homes
  • A home is defined as 4700 kWh per year, 0.54 kW or 13 kWh per day
  • 1 kWp: “p” indicates peak power
  • 1 kWh(e): “e” indicates just electrical power
  • 1 kWh(th): “th” indicates just thermal energy

Synopsis.

  • Synopsis of the Wind Energy Paper (Some of the below information may have been included in other papers bibliographies
  • Covering 10% of Britain with wind farms would yield 20 kWh per day per person on average.
  • We require either a radical reduction in consumption, or significant additional sources of energy – or, of course, both.
  • Roof-mounted micro-wind turbines are an awful idea 

Relevance Score: 8.5

Lansdorp, B., & Williams, P. (2006). The Laddermill-Innovative Wind Energy from High Altitudes in Holland andAustraliaWindpower 06. AdelaideAustraliahttp://administration.ewi.tudelft.nl/live/binaries/fe263f84-29af-4010-8222-2f1112c8f223/doc/The Laddermill - Innovative wind energy from high altitudes.pdf

 

  • The article states that wind speed obeys a power law at low altitudes and power is proportional to the cube of the wind speed. The article introduces several different airborne wind generator concepts that have been created over the years, which leads to a discussion of the Laddermill, a concept created by Delft Universityand RMIT University.
  • The Laddermill uses a system of kites that are tethered to a ground-based drum connected to a generator. The power generation equipment is located on the ground, as opposed to other concepts. Lift generated by the kites is translated into tension in the cable that pulls it off the drum, thus driving the generator. Only 10% of the cable moves around the drum, while the remaining tether length remains airborne all the time.
  • Spiral and figure eight trajectories are possible kite paths that could generate large amounts of power, but more research is required.
  • Simulations based on wind data in Holland indicate that lulls would require the system to be down about 40 times a year.
  • Section 4.1 and 4.2 provide an overview of system dynamics and control modeling introduced in the reference “Optimal Wind Power Extraction with a Tethered Kite.”
  • Three control mechanisms for adjusting the kite attitude have been tested: drag flaps, winch servos for changing the angle of attack of the kite wingtips, and a sliding mechanism that changes the location of the power line attachment point.
  • The ground station is used to convert tension into electrical energy.  Lessons learned: 1) a layer wound drum will damage the cable because of cutting , 2.) micro slip occurs between the top layer and one below, and 3.) normal slip occurs when the kite flies in geometric patterns.
  • A rotating-platform type ground station has been developed to address 3 and produce more net power (4 kW compared to 2 kW).
  • A list of future work and goals is given at the end of the report.

Relevance: 8.5

Canale, M., Fagiano, L., & Ippolito, M. (2007). KiteGen Project: Control as Key Technology for a Quantum Leap in Wind Energy Generators. American Control Conference. New Yorkhttp://www.kitegen.com/pdf/ACCNewYork2007.pdf

 

  • This article introduces the KiteGen project, which, like the Laddermill project, uses flexible, tethered kites to generate power. The focus on this paper is on a carousal-like configuration, where the kites are attached to the arms of a vertical axis rotor and their attitude is controlled by Kite Steering Units (KSUs).
  • A complete power cycle consists of a traction phase, in which the kites generate maximum power, and a drag phase, in which power must be used to position the kites at the beginning of another traction phase.
  • Control design is carried out using a Fast implementation of a Predictive Controller (FMPC).
  • A simple model for the vertical axis generator/kite system is developed in section II that accounts for wind variations. Viscosity components are neglected in the generator rotor motion law because the rotor speed is kept low.
  • Section III derives an MPC controller for the traction and drag phases.
  • Simulations show that in two retraction phases, 478 kW of power could be produced in nominal conditions and 475 kW could be produced with lateral wind disturbances for a 100 m2 kite area. These results show that the control system has good tolerance to lateral wind disturbances.
  • More complicated system dynamics need to be considered in future studies to accurately model the generator-kite set-up, such as the flexibility of the kites, but the results so far are positive.

Relevance Score: 8.5

Colozza, A. (2003). Initial Feasibility Assessment of a High Altitude Long Endurance AirshipClevelandNASA GlenResearch Centerhttp://www.google.com/url?sa=t&source=web&cd=1&ved=0CBIQFjAA&url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.81.7220%26rep%3Drep1%26type%3Dpdf&ei=USsRTPGiG-jtnQeF37TtBw&usg=AFQjCNF7t5CjemC0nzDvxasbQf0Vs8vxrQ

 

  • Airships do not need to stay in motion
  • The ability to carry heavy payloads
  • Presenting the case for a long endurance high altitude airship
  • Managing the collection, storage and consumption of energy (from the sun) will determine the feasibility of the vehicle.
  • The airship must overcome the wind speed to maintain its location
  • Power could be beamed from the surface to the airship. Eliminates the need for energy storage
  • An airship with its heavy lifting capacity provides the potential for carrying certain types of payloads that would not be practical for other types of high altitude long endurance vehicles.
  • The shape of the airship needs to be taken into careful consideration depending on the mission associated with the airship
  • The statistical mean and 99th percentile wind speeds will vary with the time of year, latitude, longitude and altitude.
  • In the lower stratosphere, strong winds occur as part of defined circulation patterns and are mostly horizontal with little mixing.
  • Page 18. 10-15 km
  • Pages 20-22. Mean and 99th percentile wind velocity equations for each season.
  • The development of the power / propulsion system can be simplified and its production cost can be reduced by designing and producing a system module at a specified power level.
  • How to power the airship with solar cells and hydrogen fuel cells is explained in depth
  • Detailed propeller information
  • Solar intensity, wind speed and direction, time of year, altitude, latitude
  • The larger the airship the greater the drag it has to overcome to maintain station over a particular location.
  • At the 42° North latitude point, these high wind speeds in conjunction with the winter time low sun angles and short day lengths produce conditions in which none of the airship sizes examined would operate. (high wind speeds is a detriment to the HALE)
  • Winds provide a significant increase in drag and therefore power requirement
  •  Larger airship to sustain flight in high wind conditions

Relevance Score: 8.5

Williams, P., Lansdorp, B., and Ockels, W. (2007). Modeling and Control of a Kite on a Variable Length Flexible Inelastic Tether. AIAA Atmospheric Flight Mechanics Conference and Exhibit. Hilton Head: AIAA. http://repository.tudelft.nl/view/ir/uuid:1ce8347b-16b2-4d96-a62f-e22d09027f7e/

 

  • The Laddermill concept is introduced once again.
  • In section II, a lumped mass approach is used to model the tether as inelastic rods connected by point masses. This approach is similar to the method in AIAA-2006-6193, but the model in this paper includes variation in the length of the tether when it is retracted and the internal tension. The cable is treated as inelastic to enable larger times steps compared to that of an elastic cable.
  • Basic controllers for manipulating the motion of the kite system are developed in section III. The key variables considered were kite altitude, kite cross-wind position, and tether tension. The kite altitude can be controlled by the amount of tether deployed. The cross wind position is controlled by the roll angle. The lift produced by the kite governs the amount of tension in the cable. All three are also related to one another.
  • Simulations are run in section IV that combine the kite-tether model and the controller.  Some problems occur due to the fact that the tether is modeled as inelastic, when in actuality it is elastic. For example, large fluctuations in tether tension occur in the simulations, but in real life the tension would change more smoothly. The simulations also take gusts into consideration.
  • Overall, the controller works well to control the kite’s altitude and tether tension.

Relevance Score:  8.5

Dadd, G., Hudson, D., & Shenoi, R. (2010, January-February). Comparison of Two Kite Force Models with Experiment. Journal of Aircraft , 47 (1)http://www.aiaa.org/content.cfm?pageid=406&gTable=jaPaper&gid=44738

 

  • Provides some useful kite dynamics analysis.
  • Compliments Lloyd’s cross-wind paper.

Relevance Score: 8 

Ockels, W. J., Lansdorp, B., Breukels, J., & Spierenburg, G. (2004). The Laddermill: Work In Progress. European Wind Energy Conference. London. http://repository.tudelft.nl/view/ir/uuid%3Abacf3e21-1cae-4722-93e2-55f15fdcd3ed/

 

  • Goal of 10% electricity from renewable energy in the Netherlands by 2020 and 1500 MW of installed wind power.
  • Two different Ladder mill concepts: Ladder mill (2.1) and pumping mill (2.2)
  • A 3-D, 6 DOF simulation software package was developed to model the Ladder mill (Section 3).
  • Two types of wings are explored: inherently stable and controlled (Sections 5.1-5.3).

Relevance Score: 8

Bardi, U. (2009, July 6). High Altitude Wind Power: An Era of Abundance?. Retrieved September 1, 2009, from The Oil Drum: Europehttp://www.theoildrum.com/node/5538

 

  • There is an abundant amount of renewable energy available.
  • “New” renewable energy, such as photovoltaic and wind, make up only a small fraction of the world’s total primary energy.
  • The energy return of energy invested (EROEI) is low for renewable energies (20 for wind) compared to fossil fuels in it its golden days (100).
  • Fossil fuels are still required to build non-fossil energy plants, and the decline fossil fuel sources make it even more difficult to sustain alternative sources.
  • The wind speed increases with height according to the “Hellman exponent” = 1.7 and wind power is proportional to the cube of wind speed. At higher altitudes, wind is also more constant.
  • Possible high altitude systems include kites, kites with rotors, and balloons. The balloons are limited by helium, a non-renewable source.
  • Kitegen has two possible configurations: a lower-power yo-yo configuration (1 MW) and a high power carousel configuration that could output as much as 1 GW.
  • Page 3 calculates the EROEI for a conventional wind turbine (20) and for Kitegen. Kitegen has the potential of an EROEI of 375 for a lifespan of 30 years.
  • Total energy in atmosphere stored in form of winds is about 2000 TW (Tera-watts) and current energy projection for the entire world is about 16 TW.
  • Problems with high altitude wind power: effect on atmospheric wind circulation, reduction in precipitation (0.1 % if power is produced equivalent to today’s needs)—it is recommended that no more than 10 times the amount we need to today is produced.
  • The article asserts that lack of electric power is not the only problem facing humanity: food and mineral abundance will still be problems, and both the economy and population need to be stabilized at a stationary level.

Relevance Score:  8

Verheul, R., Breukels, J., & Ockels, W. (2009). Material Selection and Joining Methods for the Purpose of a High-Altitude Inflatable Kite. 50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. Palm Springs, CA. http://pdf.aiaa.org/preview/CDReadyMSDM09_2047/PV2009_2338.pdf (copy and paste in browser)

 

  • Calculations of expected stresses on a long-endurance kite are performed.
  • These calculations are used to determine the requirements for fabrics and joints used on these kites.
  • Results of some tests are given.

Relevance Score: 8

Breukels, J., & Ockels, W. (2008). Analysis of Complex Inflatable Structures Using a Multi-Body Dynamics Approach. 49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. Schaumburg, IL. http://home.tudelft.nl/?id=847&q=Analysis+of+complex+inflatables+structures+using+a+multi+body+dynamics+approach&searchradio=on&x=32&y=14&option=START&min=10&start=0&L=1

 

  • This paper outlines a simulation of inflatable structures through the use of multi-body dynamics.
  • The analysis uses very little mechanics of materials; rather the simulation uses test data from simple cases and extrapolates in order to simulate more complex structures.
  • Provides good insight into flexibility of kite.

Relevance Score:  8

Houska, B., & Diehl, M. (2006). Optimal Control of Towing Kites. 45th IEEE Conference on Decision & Control, (pp. 2693-2697). San Diego. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4177402&userType=inst (copy and paste in browser)

 

  • A good paper on the optimization of a tethered towing kite’s traction force by controlling its roll angle. 

Relevance Score:  8

Fagiano, L. (2009). Control of Tethered Airfoils for High-Altitude Wind Energy Generation. Doctoral Dissertation. Politecnico Di Torino. http://www.awec2010.com/public/img/media/fagiano.pdf (copy and paste in browser)

 

  • Fagiano’s doctoral dissertation, which focuses on all aspects of the KiteGen concept and contains information from previous KiteGen papers.
  • Very comprehensive

Relevance Score:  8

The Magic of a Tether

  • Explores different aspects of tethers for airborne wind generation.

Relevance Score: 8

Archer, C. L., & Caldeira, K. (2009). Global Assessment of High-Altitude Wind Power. Energies , 307-319. http://www.mdpi.com/1996-1073/2/2/307/pdf

 

  • 7-16 km of altitude
  • The total wind energy in the jet streams is roughly 100 times the global energy demand
  • Kite Gen, Laddermill, Sky Windpower
  • Wind power density: .5*rho*V^3
  • Describes where the winds are strongest at 1000 m and 10000 m via maps (p.312)
  • Obvious benefits would arise if a high-altitude technology were able to dynamically reach this “optimal” height.
  • The area of optimal wind power density > 10 kW/m2 to the east of Asia near Japan experiences wind power densities of at least 1 kW/m2 95% of the time (Figure 4e), practically unthinkable near the ground even at the windiest spots.
  • They discuss reducing intermittency by (a) storing energy, (b) increasing the area intercepted, (c) interconnecting devices
  • A smart combination of storage, larger devices, and interconnected systems at appropriate locations might provide firm power.
  • Results suggest that no significant impacts [on the climate] can be expected unless high-altitude wind harnessing is implemented massively on the global scale.

Relevance Score: 8

Fagiano, L., Milanese, M., and Piga, D. (2009). High-Altitude Wind Power Generation for Renewable Energy Cheaper than Oil. EU Sustainable Development Conference. BrusselsBelgiumhttp://ec.europa.eu/research/sd/conference/2009/papers/15/lorenzo_fagiano,_mario_milanese_and_dario_piga_-_high_altitude_wind_power_generation_for_renewable_energy_cheaper_than_oil.pd

 

  • Introduction contains some interesting facts about wind power productions and its current shortcomings.
  • The outer 20% of the blades on conventional wind turbines produce 80% of the net power due to the higher tangential velocity. High altitude kite wind power generation replaces the tower and blades found on conventional wind turbines with cables and kites, thus saving weight and cost (See figure 1).
  • The Kite Steering Unit (KSU) consists of the electric drives, the drums, and all the hardware needed to control a single kite.
  • KG-YoYo configuration consists of two phases: a traction phase, in which wind speed creates lift on the kite and pulls the line off of a drum and generates power, and the passive phase, in which the kite is made to generate less lift, which reduces the power required to reel in the cable by 20%. In the traction phase, the kite flies in a cross-wind direction to maximize the power output.
  • The control mechanism keeps the kite in a limited space region (Figure 2b), while optimizing power.
  • For a kite with the values given in Table 1, 2 MW of power is produce in a 9 m/s wind, while a traditional wind turbine produces only 1 MW (Figure 3).
  • The results from the testing of a 40 kW prototype agree well with simulation results, which gives a high level of confidence in the latter for performing a realistic study on the potential of larger scale systems.
  • Table 2 shows that the capacity factor (CF) for a KG-YoYo is much better compared to conventional wind turbines at all the sites listed.
  • A KG-farm with four 2-MW KG-yoyo and limited aerodynamic interferences could potentially have a power density of 32 MW/km^2, compared to 12 MW/km^2 for current wind turbines.
  • Scale factors may positively affect the production costs of KG farms, resulting in estimates of 50$/MWh for a 100 MW farm and 15$/MWh for a 500 MW farm, which is drastically lower than other power generation methods.

Relevance Score: 8

Canale, M., Fagiano, L., & Milanese, M. (2007, December). Power Kites For Wind Energy Generation. IEEE Control Systems Magazine , pp. 25-38. http://www.kitegen.com/pdf/IEEECSM200712.pdf 

 

  • Unfortunately, wind turbines require heavy towers, foundations, and huge blades, which impact the environment in terms of land usage and noise generated by blade rotation, and require massive investments
  • KiteGen: two kite lines are rolled around two drums and linked to two electric drives. The kite is controlled by regulating the pulling force of each line
  • 12% of total energy is consumed in reeling it back in
  • For 300 degrees it is producing energy from the wind and it is being dragged the other 60
  • 90% percent of the power generated comes from the outer 40% of blade area
  • Each airfoil is thus equipped with a pair of triaxial accelerometers and a pair of triaxial magnetometers placed at the airfoil’s extreme edges, which transmit data to the control unit by means of radio signals.
  • They attempt to model and simulate the KiteGen: yo-yo and carousel ideas
  • The yo-yo model shows the traction and passive phase and the power relative to these stages. Mean generated power: 11.8 kW (Figure 12)
  • The carousel model has a mean generated power of 621 kW (Figure 16)

Relevance Score: 7.5 

MacCready, P. B. (2006). Paul MacCready Sees Great Promise In Using Kites to Tap Power of Wind. Drachen Foundation Journal , pg. 3-6. http://www.drachen.org/journals/journal22/Journal-22/PaulMacCready.pdf

  • A kite can remain in the air and interact with stronger relative winds than is available if it were attached to a slow moving vehicle.
  • Although decreasing the performance of a kite, a propeller can be used to provide an independent source of energy.
  • Kites can be controlled to move fast.
  • Moving kites act like the swept area of a moving turbine blade.
  • The motion of kites can be used to stay aloft even in light winds due to tether tension.
  • In some circumstances an airborne wind turbine could obtain 27 to 64 times the energy obtainable at the surface.
  • Challenges: avoiding aircraft and natural variations of wind.
  • The article speculates that two air vehicles connected by a long line could generate power by operating at different altitudes, speeds and directions.

Relevance Score: 7.5

Van Dam, C.P. (2010). Wind Energy: Status, Challenges, Opportunities. NASA Internal Workshop on Wind Power Capabilities. Download Paper.

  • Slide 4 shows that the price of electricity produced by wind power has decreased over the years, and lists some advantages of wind energy.
  • Slide 6 shows that the U.S. was a leader in wind power in 80’s, but from the 90’s to more recent times has lagged behind. The installed capacity of the U.S. is currently increasing, however (Slide 7).
  • Slide 9 shows the progression of wind turbine size and power output over the years.
  • The standard architecture of current wind turbines consists of three blades, an upwind rotor, active yaw, and a freestanding tower.
  • The technical specs for the Vestas V90 are given in slides 12-13.
  • Slide 14 demonstrates that current technology can only extract a portion of the wind energy available at a certain altitude (capacity factor).
  • Highly efficient rotors have low solidity (ratio of blade area to swept area) and high tip speed ratio (TSR)—Slide 15.
  • Slides 16 and 17 list the challenges associated with wind energy. Slides 19-23 discuss critical performance challenges such as reducing capital costs, increasing capacity factor, reducing O&M costs, and increased reliability.
  • Offshore wind power generation is fairly new and developing (Slides 24-26).
  • Slide 28: Airborne wind energy shows the potential of 20 kW/m^2 compared to <0.8 kW/m^2 for terrestrial systems. However, cost competitiveness, lack of demonstration systems, and lack of cost of energy models are hurdles facing airborne wind energy systems.

Relevance Score: 7 

Breuer, J., Ockels, W., & Luchsinger, R. (2007). An Inflatable Wing Using The Principle of Tensairity. 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. Honolulu. http://repository.tudelft.nl/view/ir/uuid%3A306785d0-f75b-4378-a145-75905c83efa7/

 

  • Introduces  the structural concept of tensairity as it is applied to flexible kites
  • Tensairity has been used in the past on bridges and roofs. 

Relevance Score: 7

Breukels, J., & Ockels, W. (2007). Design of a Large Inflatable Kiteplane. 48th AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and Materials Conference. Honolulu. http://repository.tudelft.nl/view/ir/uuid%3A5a24676e-ef16-43c6-b8cf-a72bba08d269/

 

  • Focuses on the structural characteristics of a kite plane.
  • Scaling up a foam kite plane is difficult because the weight of the foam becomes too great.
  • An alternative method is to construct the kite out of inflatable material.
  • Tube kites are also undesirable because the Dacron structural layer becomes too thick and heavy.
  • A single layer laminate of Aramid and glass-fiber weave in a Mylar matrix is examined as a possible solution.
  • An optimum placement of the bridle line is also found based on wrinkling pressure. 

Relevance Score:  7

Williams, P., Lansdorp, B., & Ockels, W. (2008). Modeling of Optimal Power Generation Using Multiple Kites. AIAA Modeling and Simulation Technologies Conference and Exhibit. Honolulu. http://pdf.aiaa.org/preview/CDReadyMMST08_1855/PV2008_6691.pdf (copy and paste in browser)

 

  • Similar to other analysis for the Ladder mill, except multiple kites are taken into account in this paper.
  • Two configurations: all kites are connected to same line and multiple kites are connected to branches in the cable.
  • Focuses on development of optimal open-loop trajectories.

Relevance Score:  7 

Williams, P., Lansdorp, B., & Ockels, W. (2008). Optimal Crosswind Towing and Power Generation with Tethered Kites. Journal of Guidance, Control, and Dynamics, 81-93. http://repository.tudelft.nl/view/ir/uuid%3Ae1c9e380-5170-45dc-a10c-664e1b388576/

 

  • Another paper that revolves around determining optimal trajectories and power generation for tethered kites using a simplified model at different wind speeds and system parameters.

Relevance Score:  7

Williams, P., Lansdorp, B., & Ockels, W. (2008). Nonlinear Control and Estimation of a Tethered Kite in Changing Wind Conditions. Journal of Guidance, Control, and Dynamics , 793-798. http://www.aiaa.org/content.cfm?pageid=406&gTable=jaPaper&gid=31604

 

  • Develops a preliminary, nonlinear control algorithm for a tethered kite using an approximate dynamic model.

Relevance Score:  7

Lansdorp, B., Remes, B., & Ockels, W. (2005). Design and Testing of a Remotely Controlled Surfkite for the Laddermill. World Wind Energy Conference 2005. Melbourne, Australia. http://repository.tudelft.nl/view/ir/uuid%3A927a1c12-ae63-4f6f-b2a1-14ac6da22e2e/

 

  • This paper deals with the design and testing of a remotely controlled surfkite for Ladder mill.
  • The control actuator design (drag flaps) and results from testing are presented.

Relevance Score:  7

Meijaard, J., Ockels, W., & Schwab, A. Modelling of the Dynamic Behaviour of a Laddermill, A Novel Concept to Exploit Wind Energy. Delft, The Netherlands. http://audiophile.tam.cornell.edu/~als93/Publications/MeiOckSch99.pdf(copy and paste in browser)

 

  • A simple model of the Ladder mill is presented.
  • The kites are modeled as rigid bodies and the cable is assumed to be flexible.

Relevance Score:  7

Breukels, J., & Ockels, W. (2007). Past, Present and Future of Kites and Energy Generation. Delft, The Netherlands: Acta Press. http://repository.tudelft.nl/view/ir/uuid%3Aba566c70-ccf2-43f1-8c31-484441b59984/

 

  • Describes the Ladder mill and lays down a path for the development of kite technology.

Relevance Score: 7

Podgaets, A., & Ockels, W. (2007). Comparison of Two Mathematical Models of the Kite for Laddermill Sail Simulation. World Congress on Engineering and Computer Science, WCECS 2007. San Francisco : IAENG International Association of Engineers. http://repository.tudelft.nl/view/ir/uuid%3Aa6e0f41c-fb92-4638-b9a8-9b2df345cd48/

 

  • Includes more mathematical models for kite dynamics and control.

Relevance Score:  7

Ockels, W. (2001, June-September). Laddermill, A Novel Concept to Exploit the Energy in the Airspace. Aircraft Design 4 (2-3), pp. 81-97. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VK4-445GG6V-1&_user=141910&_coverDate=09%2F30%2F2001&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1399392930&_rerunOrigin=google&_acct=C000011779&_version=1&_urlVersion=0&_userid=141910&md5=55268b4d8c40e12a217c457bd977144b(copy and paste in browser)

 

  • One of the first Ladder mill papers ever.
  • Describes the Ladder mill concept and provides some initial power output and cost estimates.

Relevance Score:  7

Canale, M., Fagiano, L., & Milanese, M. (2006). Control of Tethered Airfoils for a New Class of Wind Energy Generator. 45th IEEE Conference on Decision & Control, (pp. 4020-4026). San Diego. http://www.kitegen.com/pdf/IEEE200611.pdf (copy and paste in browser)

 

  • Description of the dynamics and control of tethered kites to be used in the KiteGen concept.

Relevance Score:  7

Krenciszek, J., Akindeinde, S., Braun, H., Marcel, C., & Okyere, E. (2008). Mathematical Modeling of the Pumping Kite Wind Generator: Optimization of the Power Output. http://www.win.tue.nl/casa/meetings/special/ecmi08/pumping-kite.pdf (copy and paste in browser)

 

  • Provides a mathematical model for optimizing the power output of a pumping mill type airborne wind turbine.

Relevance Score:  7

Manalis, M. (1975). Airborne Windmills: Energy Source for Communication Aerostats. AIAA, Lighter Than Air Technology Conference. Snowmass, Colorado: University of California, Santa Barbara. http://www.aiaa.org/content.cfm?pageid=406 (search title)

 

  • This paper investigates the possibility of using aero generators on board aerostats for supplementing the power generated by the primary propulsion system (diesel generators).

Relevance Score:  7

Roberts, B., Shepard, D., Caldeira, K., Cannon, M., Eccles, D., Grenier, A., et al. Harnessing High Altitude Wind Power. Ramona, CA: Sky WindPower. http://www.jp-petit.org/ENERGIES_DOUCES/eolienne_cerf_volant/eolienne_cerf_volant.pdf (paste in browser)

 

  • Provides analysis for the aerodynamic, electric, control, and tether aspects of a flying generator (prop on kite).

Relevance Score:  7

Fletcher, C., & Roberts, B. (1979, July-August). Electricity Generation from Jet-Stream Winds. Journal of Energy , 3 (4), pp. 241-249. http://www.aiaa.org/content.cfm?pageid=406 (search title)

 

  • Explores the potential of high altitude wind power.
  • Some analysis on diffuser-augmented wind turbines and tethers.

Relevance Score:  7

O'Gairbhith, C. Assessing the Viability of High Altitude Wind Resources in Ireland. Loughborough University. http://www.carbontracking.com/reports/High_Altitude_Wind_Resource_in_Ireland.pdf (copy and paste in browser)

 

  • Different technologies for exploiting wind power are discussed and a kite-based system is studied in more detail.
  • The wind power density over Ireland is modeled using recent data.

Relevance Score:  7

German, W. (2010, May 24). Tethered Airfoils: An Enabling Technology. Retrieved July 13, 2010, from EnergyKiteSytems.net: http://www.energykitesystems.net/Soaren/TetheredAviation/TetheredAirfoilsMay242010.pdf(copy and paste in browser)

 

  • Explores several different uses of tethered airfoils, such as power generation, pumping water, transportation, hydrogen synthesis, and radio signal relaying.
  • Power generation is mentioned in sections 3.1, 3.9, and 3.10.

Relevance Score:  7 

Archer, C., & Caldeira, K. (2008). Atlas of High Altitude Wind Power. Stanford, CA: Department of Global Ecology, Carnegie Institute for Science. http://www.mdpi.com/1996-1073/2/2/307/

  • Wind power density “heat” maps at different altitudes and times of year.

Relevance Score:  7

O'Doherty, R., & Roberts, B. (1982). The Application of U.S. Upper Wind Data in One Design of Tethered Wind Energy Systems. Golden, CO, USA: Technical Report, Solar Energy Research Institute. http://www.wired.com/images_blogs/wiredscience/2009/06/1400.pdf (copy and paste browser)

 

  • A statistical assessment of the U.S. upper wind resource.
  • Power density can be as high as 16 kW/m^2 in northeastern states, with a maximum at a pressure altitude of 300 mb.

Relevance Score: 7 

Lansdorp, B., Ruiterkamp, R., Williams, P., & Ockels, W. (2008). Long-Term Laddermill Modeling for Site Selection. AIAA Modeling and Simulation Technologies Conference and Exhibit. Honolulu. http://www.aiaa.org/content.cfm?pageid=406 (search title)

 

  • Includes a modeling and optimization approach that can help to design Ladder mill for particular sites around the world.

Relevance Score: 7

Succar, S. (2005). Global Prospects for Wind Energy: Addressing the Challenges of an Intermittent Energy Source. IAC International Workshop on Energy. Princeton Environmental Institute. http://www.princeton.edu/~ssuccar/recent/Succar_IACDurban_Oct05.pdf

 

  • Slide 3 shows that there is plenty of wind energy resources to meet all of the world’s electricity demands. However, slide 4 shows that wind energy and other renewable resources currently and in the near future only make up a small portion of the total energy production.
  • The best resources for wind energy do not always lie close to demand centers (Slide 5 @ 80 m), so extra transmission infrastructure would be required, resulting in large capital costs.
  • Due to wind intermittency, most current wind turbines only deliver their rated power 20% of the year (slide 8: capacity factor = 0.30).
  • Slide 12 states that excess energy could be stored and used at a later time, so that the system uptime is brought to 90% and capable of providing the baseload more efficiently.
  • Some energy store options are shown on slide 14 (&31), with Compressed Air (CAES) as the “clear” choice due to its storage time and capacity. Description follows in next slides.
  • Slides 18-22 deal with costs.
  • Possible ways to reduce costs for low wind speed turbines are introduced in slide 25.
  • The wind potential in Africa is not completely known (slide 26). Africa has less large scale wind power production than Europe, but is having greater growth in that area (slide 27).
  • Exploitation of lower wind classes and small scale turbine technology requires further research to reduce costs (slides 28 & 29).
  • High altitude wind power is mentioned on slide 35.

Relevance Score: 7

Fournier, P. (1970). Low-Speed Wind-Tunnel Investigation of All-Flexible Twin-Keel Tension-Structure Parawings.WashingtonD.C.: NASA. http://ntrs.nasa.gov/search.jsp?Ne=20&N=4294967142+50&Ns=HarvestDate|0&as=false

 

  • This paper has a detailed analysis of many kites with the following conclusions:
  • Low-speed wind-tunnel tests were made to determine the static aerodynamic characteristics of several tension-structure all-flexible twin-keel parawings.
  • Those having 5' to 15' canted keels showed higher values of lift-drag ratio throughout most of the resultant force range
  • There was no great change in the value of lift-drag ratio for a given resultant-force coefficient between the model with the narrow center panel and the basic width center panel
  • The model with the widened center panel indicated somewhat lower lift-drag ratio
  • An increase in the keel-to-payload distance from 1.00 keel length to 1.25 keel length indicated an improvement in performance
  • A further increase to 1.50 keel length did not increase maximum lift-drag ratio

Relevance Score: 7

Lang, D. (April 2009). Energy and the Possible Application of Kites. Drachen Foundation. http://www.drachen.org/pdf/april09-discourse.pdf (pp. 65-74)

 

  • This is a white paper presented by the Drachen Foundation to NIST/TIP.
  •  “Hydrogen must be created at the expenditure of actual intrinsic energy sources before it can fulfill its role in the economy.”
  • The article gives several reasons why hydrogen is not an efficient means of powering consumer vehicles such as cars and aircraft (pages 3-4).
  • Two major issues associated with the conventional “hydrogen economy” are listed on page 4. 
  • The first issue could be addressed by discovering a plentiful, cheap, renewable source of hydrogen and means of producing sufficient electricity without using fossil fuels.
  • The second issue can be solved by converting current power plants so that they that they use hydrogen as a heat source.
  • Page 7 outlines the steps for transitioning to a “hydrogen assisted economy:” essentially, hydrogen would be used as a heat source for power plant to generate electricity and then the electricity would be used to power commercial vehicles.
  • “Creation of the required hydrogen with minimal resulting carbon footprint implies water electrolysis via renewable electrical power (page 8).”
  • Issues with wind power are discussed on pages 8-9.
  • Kite based systems suffer from wind magnitude variability and wind direction variability.
  • Ocean based systems could be a solution to these problems, but then the power would have to be brought back to land somehow.
  • Water could be converted to hydrogen and oxygen via high pressure electrolysis and then transported back to land to fuel HAE power plants (page 11).
  • The end-to-end efficiency in this type of system needs to be determined.
  • The article concludes with a very good description of how kites could be used to generate electricity (or at least for presenting the idea to the public, who is less familiar with the technical aspects).

Relevance Score: 7

Venturi Wind. Turbine 110 - 500. Deventer-Colmschate: Venturi Wind. http://www.venturiwind.com/pdf/datasheet%20Venturi%20Wind%20Turbine%20110%20-%20550.pdf

 

  • Gives information on the specs of a turbine
  • Gives us an idea of what a typical small wind turbine is like in terms of power output, what its made of, and the overall size of the turbine

Relevance Score: 7

U.S. Patent Application No. 11/830,769. Publication No. 20080048453 (published Feb. 28, 2008) (Douglas J. Amick, applicant) http://www.amickglobal.com/wind_turbines/twt/twt_ppa.htm

 

  • This is a patent application that introduces a lighter-than-air airborne wind turbine that can remain aloft in all wind conditions.
  • The wind turbine has a relatively-higher RPM, but a smaller diameter.
  • The wind turbine is tethered to the ground with an electrically conductive cable.
  • The system includes an ultra-low weight onboard weather diagnostic computer to keep the system in a position to optimally extract wind energy and determine whether to retract the system in case of bad weather conditions. This system could also be located on the ground.
  • Rear wing stabilizers and forward mounted lifting wings could be used to improve stability and performance.
  • A ground station with hangar doors would be used to store the turbine system when not in use. This station could also be built on top of a building or underground.
  • The lower portion of the invention has attachment brackets that connect the harness and tether to the main body casing.
  • The application indicates that the length of central member of a three point harness could be adjusted to control the invention’s angle of attack through mechanical servos. The angle of attack would be adjusted to reduce drag on the turbine blades.
  • A control module controls the harness/tether, control surfaces, generator, and electrical output so that optimal power generation is achieved.
  • The physical aerodynamic, wing-like body shape would be inflatable and contain lighter than air gases, such as helium.
  • The application details possible methods for providing structure to the system, such as ribs.
  • The required airspace is described as an inverted cone that depends on different factors, such as buoyancy force, wind speed, etc.
  • A lever arm, possibly wish bone shaped, would move the tether up when launching the turbine and move it down for retraction.
  • The tether would be unraveled from a reel contained in the ground station.
  • Reel-to-power box cables, located in the ground structure, deliver electricity from the tether to a power/conditioning box.
  • The tether would consist of tensile members and electrical wires sheathed in insulation jackets. All members would ideally be made of carbon nanotubes for increased conductance, strength, and weight savings. Alternative materials are copper core, Spectra fiber, Kevlar fiber, or polyester fiber.
  • Outlet air could exit through slots positioned in the tail boom for that type of configuration.
  • The inlet and out turbine shape would be optimized to provide the greatest flow rates and power output.
  • The system would not need a gearbox, tower, or nacelle, which would result in weight reduction.

Relevance Score: 7

Dumas, A., Anzillotti, S., & Trancossi, M. P.S.I.C.H.E.; The Concept of a Stratospheric Airship for Energy Production, Telecommunications and Territorial Survellance. SAE Aerospace. http://www.dismi.unimore.it/download/Presentation%20PSICHE.pdf

 

  • Aerostatics balloon to operate at 15 to 18 km
  • Problems with previous attempts (as stated): insufficient energy, cannot hover statically, non-lenticular shapes
  • Lenticular semi-rigid structure covered by PV panels is proposed
  • Engines on four side of balloon

Relevance Score: 7

National Renewable Energy Laboratory. (2007). Small Wind Electric Systems: A Virginia Consumer's Guide.Washington D.C.U.S. Department of Energy. http://www.windpoweringamerica.gov/pdfs/small_wind/small_wind_va.pdf

 

  • Good picture on two if focusing on Virginia
  • Wind energy systems represent one of the most cost-effective, cleanest, home-based renewable energy technologies available today.
  • Homes need to become more efficient
  • Most zoning ordinances in Virginia contained height limits of 35 feet in the fall of 2004
  • Establish an energy budget to help define the size of turbine you will need
  • The blades are usually made of a composite material such as fiberglass.
  • In general, the higher the tower, the more power the wind sys tem can produce
  • Small wind turbines generate DC electricity. An inverter is necessary in order to use in households. (Loss in overall efficiency)
  • 3-10 kW turbine on an 80 foot tower is $15,000 to $50,000
  • P = k Cp ½ p A V3
    • k=0.000133 for kW
    • Cp=Maximum power coefficient
    • p=Air Density, lb/ft3
    • A=Rotor swept area, ft2
    • V=Wind speed, mph
  •  AEO = 0.01328 D2 V3
    • AEO= Annual energy output, kWh/y
    • D = Rotor Diameter, ft
    • V = Annual average wind speed, mph

Relevance Score: 7

Sky Windpower (2010). SkyWindPower Flyer. Sky Windpower. www.skywindpower.com .

 

  • This is a flyer that introduces Sky WindPower’s Flying Electric Generator (FEG) concept.
  • There are several pictures that show the development of the FEG, beginning in the 1980s, and some facts about airborne wind energy.

Relevance Score: 6.5

Van Dam, J. (2004, December 14). Wind Turbine Noise. 2004 California Wind Energy Collaborative Forum . Underwriters Laboratories Inc.  http://cwec.ucdavis.edu/forum2004/proceedings/index.html#presentations

 

  • This pdf presentation provides some key terminology on wind turbine noise. The difference between sound power and sound pressure is explained. Typical source strengths are provided and the dB scale is defined.
  • Several noise level measurement techniques are listed and illustrated.
  • Tonality is mentioned, but not described in much detail.
  • Models for noise propagation are listed, along with shortcomings.
  • Allowable noise regulations for several countries are provided, including DenmarkGermanySweden,Netherlands (guidelines), FranceUK (guidelines), Greece, and the US.
  • The point is made that sound perception is just as important as sound level
  • Background noise is discussed and noise sources on wind turbines are listed and illustrated.
  • The diameter size vs. decibel level is plotted and further information is given for small wind turbines (SWTs)

Relevance Score: 6.5

National Renewable Energy Laboratory. (2009). United States- Wind Resource Map. Washington D.C.U.S.Department of Energy. http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf

 

  • I think this is much more useful than the 80 m map
  • This shows that at the coast, there are high wind speeds while the other did not.
  • They should be almost equivalent (which they are) since this is at 50 m while the other is at 80

Relevance Score: 6.5

Jacobson, Mark Z. (2008). Supplementary Information Appendix from Review of Solutions to Global Warming, Air Pollution, and Energy Security. pp. 45-53. http://www.cleanairalliance.org/files/active/0/EnergyEnvRev1008.pdf

 

  • Pages 2 and 3 list several characteristics of wind turbines and vehicles powered by wind turbine technology.

Relevance Score: 6.5

Johnson, S. (2009). Small Wind Installer Survey. DavisUniversity of CaliforniaDavishttp://cwec.ucdavis.edu/smallwindreports/documents/Completed_Installer_Surveys.pdf

 

  • Shows the difficulties on installing small wind turbines.
  • There needs to be more help from the government to speed up the process in putting in wind turbines. If these small wind companies are having this much trouble, one can imagine the amount of red tape for airborne wind turbines
  • Complaints on expensive fees and time consuming processes
  • 80% of time is jumping through hurdles to put up wind turbines

Relevance Score: 6.5

Johnson, S. (2009). Small Wind Permitting Challenges. DavisCalifornia Wind Energy Collaborative.http://cwec.ucdavis.edu/smallwindreports/documents/CWEC-2009-01-Installer_Survey.pdf

 

  • This is simply a summary of the above information

Relevance Score: 6.5

Miller, Stoia, Harmala, & Atreya. (2005). Operational Capability of High Altitude Solar Powered Airships. Arlington: AIAA. http://pdf.aiaa.org/preview/CDReadyMATIO05_1137/PV2005_7487.pdf

 

  • The CIGS technology is projected, in a few years, to produce cells with an efficiency of 15% and a mass of 32 g/m^2. This makes it very attractive for application in airships.
  • Lithium based battery technology offers the most promising solution for solar powered airships.
  • Airships attempt to avoid windy conditions while we are trying to find windy conditions. May be too much of a contrast to mend them together.
  • Solar cells on the airship will increase its Hull weight because of the heating of the gas
  • Boeing developed an airship performance tool which integrates the solar collection model, wind vignettes,aerodynamics, energy model, and hull weight to assess airship station keeping performance.
  • Planar Solid Oxide Fuel Cells was the only energy storage device that could perform over various latitudes and during the winter

Relevance Score: 6.5

Nishimura, J., Yajima, N., and Yamagami, T. (1996). Long Duration Flight Systems in ISAS. AIAA 34th Aerospace Sciences Meeting and Exhibit. Reno: AIAA. http://www.aiaa.org/content.cfm?pageid=406

 

  • The paper introduces balloon flight tests carried out by the Institute of Space and Astronomical Science (ISAS) in Japan in the 80’s and 90’s. All of the flights were started in Japan and consisted of flying over the Siberian Polar Regions and landing in Mongolia. These tests lasted 2-3 weeks. The tests were occurring at the same time as American tests and were very valuable as far as observing geophysical phenomena, as well as rare events such as cosmic rays, and solar phenomena. The balloon ballast consumption was also monitored.
  • The paper also introduces EVAL (Ethylene-Vinyl-Alcohol) film as a similar, but alternative material to Mylar film on polyethylene balloons.  EVAL can be heat shielded and has a helium gas permeability that is 2 orders of magnitude smaller than that of Mylar.  IR (Infra-red) absorption is directly related to ballast consumption on balloons and EVAL is capable of covering the IR absorption band (7.5-14.5 µ) that occurs at the altitude at which most balloons operate. Absorption data shows that EVAL results in a halving of the gas temperature between night and day compared with polyethylene without the film.  Tests also show that EVAL is capable of withstanding 10 times more overpressure.
  • OZP balloons are normal polyethylene zero pressure balloons without exhaust ducts. The volume does not change in these balloons, but large circumferential stress is a problem when the balloons are over pressurized.
  • Over-pressurized pumpkin shaped balloons (OPS) have a pumpkin shape at high pressures and alleviate the stress problem because the major stresses can be designed to occur at certain locations.  A volume change occurs in OPS balloons with overpressure, but this problem is not too serious and in mid-latitude flights, the OPS balloons have similar ballast saving performance compared to the OZP. 
  • The duration of flights are extended by about two times by putting 2-3% overpressure in OPS and three times with 5% overpressure compared to zero pressure balloons
  • In summer Antarctic flights, the duration is doubled compared to zero pressure balloons by over-pressurizing OZP by 2% and OPS by 3%. The result is greater payload capacity, duration of flight, or higher altitude flight.
  • The conclusion is that OPS is a better balloon shape due to its comparable performance to an OZP balloon and better structural characteristics.

Relevance Score: 6

Gav, B. (2008, January 13). Alternative Wind Power Experiments - SkySails and Airborne Wind Turbines. Retrieved September 1, 2009, from The Oil Drum: Australia/New Zealand: http://www.theoildrum.com/node/3500

 

  • The first half of the article explains the concept of kite sails that can be attached to ships for increased fuel efficiency.
  • The SkySails consist of a towing kite with rope, a launch and recovery system and an automatic control system.
  • The kites usually fly around 1000 ft above sea level and sea trials suggest that 10-15% of the oil normally burned on the ships could be saved. Larger kites could bring these numbers up to 30-35%.
  • The second half relates directly to airborne wind turbine technology.
  • Magenn power is developing a lighter-than-air turbine that spins around a horizontal axis and is connected to the ground by a copper tether. The current turbine design would be capable of providing power to a rural village (about 10 kW).
  • Sky WindPower is working on clusters of Flying Electric Generators (FEGs) that operate in the jet stream between 15,000 and 30,000 ft. The clusters would consist of 4 to 8 rotors. The clusters would initially use energy from the FEGs to get to the required altitude. Then, by changing their pitch, the FEGs would then start generating power. GPS would track the position of the clusters and onboard computers would control the attitude.
  • Capacity factor is the percentage of energy actually captured relative to what would be captured if the wind turbines were operating at full capacity all the time. In the jet stream the capacity factor is about 70% in the southern parts of the U.S. and 90% in the north.
  • Balloons tethered up to 15,000 ft exist at fifteen sites along the U.S’s southern border.
  • The article states that if 0.4% of U.S. airspace were reserved for airborne wind turbines, all our power needs could be met.
  • The strength to weight ratio of tethers has improved greatly and vendors are selling their products to the military and NASA.
  • Lightning strikes are a potential problem that needs to be addressed.
  • Sounds could be used to keep birds away.
  • The cost to produce electricity using airborne wind turbines could be 5 to 10 times less than for conventional wind mills.
  • Wubbo Ockels from Delft University is working on a system that consists of an arrangement of two or more tethered kites that work together to produce a steady supply of power. The kites can be difficult to control though and Ockels is designing kites that resemble conventional airplanes with standard control surfaces.

Relevance Score: 6

Miller, Tim; Mandel, Mathias. (2002). Airship Envelopes: Requirements, Materials and Test Methods. Dover: ILCDoverhttp://www.ilcdover.com/products/aerospace_defense/supportfiles/lta_envelopes.pdf

 

  • Current Non-rigid and semi-rigid airships all employ the pressure envelope design principle, so this principle must be considered as a main structural element of these airships.
  • At the beginning of development, it is necessary to specify all requirements. The FAA –ADC (Airship Design Criteria) or German LFLS (Lufttuechtigkeitsforderungen fuer Luftschiffe) provided minimum requirements for non-rigid and semi-rigid airships. These requirements aid in determining which materials are best suitable for an airship.
  • A partial test matrix was developed for airship qualification.
  • One of the material test methods for airships is cut slit tear testing (similar to fracture testing), in which an initial slit is placed on a sample material piece and stressed until failure. However, this method has no direct correlation to tear propagation in an actual airship.
  • Equations have been developed that relate cut slit tear strength to critical slit length.
  • Tests were performed on LZN07 material and good correlation was obtained between data and theory. The tests showed that LZN07 material would have an equivalent level of safety compared to others.
  • Further investigation is required to determine the affect of time on slit propagation and to test slits that occur dynamically during the flight of an airship. New methods need to be developed to ensure a robust, cost effective airship.

Relevance Score: 6

Pattinson, J. HALE Airship. Shropshire: Lindstrand Technologies Ltd. http://www.sstd.rl.ac.uk/Appleton_Space_Conference/Pattinson.pdf

 

  • This (High altitude, Long endurance) airship shows promise if an airship is chosen to be used to keep a concept aloft.
  • More information on top of this simple slideshow would be needed in order to understand the idea completely
  • Manufacturing and Flight of balloon is given. Not much technical information is included.

Relevance Score:  6

DeLaurier, J. (1972). A Stability Analysis of Cable-Body Systems Totally Immersed in a Fluid Stream.  NASA Contractor Report. Stanford: Stanford University. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720014350_1972014350.pdf (paste in browser)

 

  • A very detailed stability analysis of a tether-body system in a moving fluid.

Relevance Score:  6

Varma, S., & Goela, J. (1982). Effect of Wind Loading on the Design of a Kite Tether. Journal of Energy , 6 (5), pp. 342-343. http://www.aiaa.org/content.cfm?pageid=406 (search title)

 

  • Considers the effect of wing drag on the static profile and force transmission efficiency of a tether connected to a kite-like system.
  • Does not take into account varying wind speeds and density with altitude.

Relevance Score:  6

Skinner, S., Fujino, A., & Ruhe, B. (2004). Using Kites to Generate Electricity: Plodding, Low Tech Approach Wins. Drachen Foundation Journal , 14-16. http://www.drachen.org/journals/journal16/Journal16.pdf (copy and paste browser)

 

  • Different kite power generation concepts are introduced.

Relevance Score: 5.5

Energy Efficiency and Renewable Energy. (2008). 20% Wind Energy By 2030. ArlingtonU.S. Department of Energy. http://www1.eere.energy.gov/windandhydro/pdfs/20percent_wkshp_proceedings_5-19-09.pdf

 

  • The only reason that we found this article applicable was the fact that the U.S. Department of Energy is pushing to have 20% wind energy by 2030
  • Other than that fact, the relevance of the actual substance of the article does not really help in helping our project

Relevance Score: 5.5

Seitzler, M. (2009). The Electrical and Mechanical Performance Evaluation of a Roof-Mounted, One-Kilowatt Wind TurbineCalifornia Wind Energy Collaborative. Report No. CWEC-2009-003. http://cwec.ucdavis.edu/documents/CWEC-2009-003.pdf

 

  • Small wind systems typically have capacities of 1 kW to 50 kW, rotor diameters of 2-7 meters, and tower heights up to 30 meters.
  • They are increasingly being installed at residences or on building rooftops where the power is used on-site. They have smaller capital costs than their larger counterparts.
  • The CWEC partnered with the California Energy Commission’s Pier program to implement a rooftop mounted Small Wind Energy Demonstration system at UC Davis that could be used for outreach and education, as well as a research platform for wind turbine performance analysis.
  • The paper focuses on an experiment on the electrical and load performance of the small, 1 kW wind turbine, a Bergey Windpower XL.1 with a stand-alone configuration. The device is three-bladed, upwind, auto-furling, and horizontal axis.
  • Section 2 describes more details about the wind turbine as well as modifications that were made.
  • Section 3 provides a detailed description of the experiment set-up, instrumentation, data analysis, uncertainty calculations, and role of turbulence in the results.
  •  XFOIL was used to predict the lift and drag on a XL.1 blade. Blade element momentum theory was used to predict the thrust and torque of the XL.1 blades and WT_Perf was used to validate these predictions.  Efficiencies were used to convert ideal power to actual electrical power (Fig. 36).
  • The experimental results are given in section 5. According to Figure 45, the higher the resistive load, the lower the alternator winding current, the lower the resistive torque, and the higher RPM, for a given wind condition.
  • Overall relevance: identifies useful methods for analyzing the performance of a wind turbine.

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