01-12-2010, 08:43 AM
FLYING WINDMILLS OR FLYING ELECTRIC GENERATOR (FEG) TECHNOLOGY
PRESENTED BY: NABEEL.SUBER.KUNJU
Batch EEE-7
Sree Buddha College Of Engineering
Pattoor
INTRODUCTION
FEGs are proposed to harness kinetic energy in the powerful, persistent high altitude winds.
It is tethered to the earth and churning out electricity from the jet streams.
This energy source is well suited for rural or low populated areas.
FEG technology is just cheaper, cleaner and can provide more energy than the environmentally unhealthy methods which leads to inevitable destruction of earth.
High altitude wind power is efficient tether technology that can reach 15,000 feet in the air.
Various systems that capture high altitude power includes tethered balloons, tethered fixed-winged craft, tether climbing & descending kites and tethered rotorcrafts.
TETHERED ROTORCRAFT
Fig.1: Early two-rotor prototype in flight
Fig.2: Four-rotor demonstration craft
INTRODUCTION FOR ROTORCRAFTS
Uses conventional rotors which generate power & simultaneously produce sufficient lift to keep the system aloft.
Tethered rotorcraft is the preferred option which is a variant of the gyroplane.
Tethered rotorcraft with four or more rotors in each unit could harness the powerful, persistent jet streams and should be able to compete effectively with all other energy production methods.
The best tether for the rotorcraft appears to be a single composite electro-mechanical cable made of insulated aluminium conductors & high strength fiber.
The rotors are inclined at an adjustable angle to the on-coming wind generally a 40° angle.
The wind on the inclined rotors generates lift, gyroplane style & forces rotation , which generates electricity, wind mill style.
Electricity is conducted down the tether to a ground station.
DESCRIPTION OF THE TETHERED ROTORCRAFTS
The tether’s insulated aluminium conductors bring power to ground, and are wound with strong Kevlar-family cords.
Conductor weight is a critical compromise between power loss and heat generation.
Aluminium conductors are used with the tether transmission voltages.
FEG units envisioned for commercial power production have a rated capacity in the 3 to 30MW range.
Generators arrays are considered for wind farms in airspaces restricted from commercial and private craft uses.
When operating as an electrical power source, four or more rotors are inclined at an adjustable, controllable angle to the on-coming wind.
Rotors have their open faces at an angle up to 50° to wind.
High altitude wind speeds and other conditions are measured at 12 A.M & P.M at major airports world wide by radiosonde weather balloons and are reported on NOAA and other government websites.
The plan-form of the rotor centerlines is approx. square, adjacent rotors rotate in opposite directions; diagonally opposite rotors rotate in the same directions.
In these four-rotor assembly , craft attitude in pitch , roll, and yaw can be controlled by collective rotor pitch change.
No cyclic pitch control is needed to modify the blades’ pitch as they rotate, this reduces maintenance costs.
Rotor collective pitch variation varies the thrust developed by each rotor in the format using GPS/Gyro supplied error signal data
FLYING GENERATORS AERODYNAMIC PERFORMANCE
Fig.3 is the flying generator’s side view for atypical flight configuration in a wind of velocity V.
A single tether of length Lc is attached to the craft at a point A on the craft’s plane of symmetry.
Aircraft’s centre of mass is at C.
For low altitude, around 1500ft (< 500 m) , the assumption of a straight , mass-less tether is reasonable.
For high altitudes, the analysis has been extended to include tether mass and tether air-loads.
Roberts & Blackler and Robert & Shepard shown that higher altitudes are achievable using an aluminium-Kevlar composite or an aluminium-Spectra composite for the electro-mechanical tethering cable.
NOMENCLATURE
c = Rotor’s control axis angle
= Angle of cable to the horizontal
T, H, P = Thrust, force, and power output of a single rotor
Cp, = Power coefficient and tip speed ratio, component of the wind normal to the rotor’s control axis divided by the speed of the rotor blade’s tip
R, = Tip radius and angular velocity of rotors
V, = Velocity & air density of the free stream
M, g = Craft mass & acceleration due to gravity
X, Y, Z = Wire fixed, orthogonal set of axes also forces in
these directions. Alternatively wind axes are used.
x, y, z = Displacements in X, Y, Z directions
, , = Angular displacements about X, Y, Z axes
o = Rotor’s collective pitch angle
Lc = Tether length from ground to craft
a1 = Rotor’s force and aft flapping angle
Fig.4 shows the power output coefficient, Cp, for each rotor where
Cp = P (1)
R . 1/2V3
Cp is plotted against control axis angle, c, for values of constant tip speed ratio,.
By Ref. to fig.3,
c = 0 (2)
and = V cos c (3)
R
Dotted curve represents the maximum power output under conditions of zero profile drag on the rotor blades.
When c = 90° , value of Cp will equal the Betz limit of 0.593 .
AIRBORNE OR GROUND-BASED WIND GENERATOR: images
COMPARISON
GROUND-BASED WIND TURBINE
Experiences surface feature turbulence not present at high altitude.
Rigidly mounted on support towers.
Direct and gust-induced moment loads are significant.
FLYING ELECTRIC GENERATOR
Turbulence reaction is different.
Not mounted on towers.
FEG have ability to reduce gust loads due to tether cable flexibility , built in elasticity and changeable shape.
ELECTRICAL SYSTEM DETAILS
FEG need to ascend and remain aloft for short periods on grid-sourced energy.
Voltages at the terminals to be kept within designed tolerances and/or be adjusted by timely voltage regulation.
If the generators capacity is above a minimum level , a System Impact Study is required to connect a new generator to the grid.
FEG ,in single units of 20MW or more , can achieve about 80% availability with suitable siting at land or sea locations.
FEG transmits power over lengths of between 4 and 8 km.
Flying generator / tether voltages between 11kV and 25kV ac could be used on units of 30MW at the most extreme altitudes.
Arrays of flying generators could move north or south to follow seasonal shifts in wind patterns or power demand.
Tether arrangement contains three conductors-two could form the single-phase circuit , third could be the ground wire and control cabling function.
Generator and tether performance depends on a good lightning storm detection system.
ADVANTAGES & DISADVANTAGES OF FEG
ADVANTAGES
Environment friendly
FEG technology is just cheaper , cleaner and have more efficiency.
Low cost availability of electricity.
FEG’s are unaffected by surface feature turbulence.
The environment impact at high altitude are minimal with virtually no visual or noise intrusion and no bird strikes.
FEG wind farms would give capacity (generating) factors around three times greater than that from conventional wind farms.
DISADVANTAGES
Restricted airspace for airplanes to fly.
Not suitable for highly populated areas, unless there are adequate safety measures provided.
Seasonal variations in jet streams speed across the globe can create dull periods for electricity production by FEG.
CONCLUSION
FEG can supply electricity for grid connection, for hydrogen production or for hydro-storage.
High altitude wind power depends on currently available technologies and engineering, building on decades of experience with wind turbine & gyroplane technologies.
Full-scale facilities, using individual FEG units of rated power around 30MW, could easily form wind farms equivalent in output to regular coal, gas & nuclear facilities.
This energy source can play an important part in addressing the world’s energy & global warming problems.
REFERENCES
1. Caldeira, K., Seasonal, global wind resource diagrams,
skywindpower.com
2. O’Doherty, R. J., Roberts, B. W. Application of Upper Wind data in
One Design of Tethered Wind Energy System. Solar Energy Res.
Institute, TR-211-1400, Golden Colorado, USA, Feb 1982, pp. 1-127.
3. Atkinson, J. D. et al, The Use of Australian upper Wind Data in the
Design of an Electrical Generating Platform. Chas. Kolling Res. Lab.,
TN D-17, Univ. of Sydney, June 1979, pp. 1-19.
4. Roberts, B. W., Blackler, J. Various Systems for Generation of
Electricity Using Upper Atmospheric Winds, 2nd Wind Energy
Innovation Systems Conf., Solar Energy Res. Institute, Colorado
Springs, Dec. 1980, pp. 67-80.
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