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ABSTRACT
High Altitude Wind Power uses flying electric generator (FEG) technology in
the form of what have been more popularly called flying windmills, is a
proposed renewable energy project over rural or low-populated areas, to
produce around 12,000 MW of electricity with only 600 well clustered
rotorcraft kites that use only simple autogyro physics to generate far more
kinetic energy than a nuclear plant can.
According to Sky WindPower; the overuse of fossil fuels and the
overabundance of radioactive waste from nuclear energy plants is taking our
planet once again down a path of destruction, for something that is more
expensive and far more dangerous in the long run. FEG technology is just
cheaper, cleaner and can provide more energy than those environmentally
unhealthy methods of the past, making it a desirable substitute/alternative.
The secret to functioning High Altitude Wind Power is efficient tether
technology that reaches 15,000 feet in the air, far higher than birds will fly, but
creating restricted airspace for planes and other aircraft.
The same materials used in the tethers that hold these balloons in place can
also hold flying windmills in place; and with energy cable technology getting
ever lighter and stronger .Flying windmills appear to be 90 percent more
energy efficient in wind tunnel tests than their land-based counterparts; that is
three times more efficiency due to simple yet constantly abundant and
effective high altitude wind power, available only 15,000 feet in the air by way
of clustered rotor craft kites tethered with existing anti-terrorist technologies
like those used on the Mexican/American border radar balloons.
High Altitude Wind Power offers itself as a clean and more powerful source of
power generation than anything available on-the-grid at present and if Sky
WindPower Corp. has their way, FEG technology and flying windmills will take
the lead of a more sustainable future within the decade.
Flying electric generators (FEGs) are proposed to harness kinetic energy in the
powerful, persistent high altitude winds. Average power density can be as high
as 20 kW/m2 in a approximately 1000 km wide band around latitude 30in both
Earth hemispheres. At 15,000 feet (4600 m) and above, tethered rotorcraft, with
four or more rotors mounted on each unit, could give individual rated outputs of
up to 40 MW. These aircraft would be highly controllable and could be flown in
arrays, making them a large-scale source of reliable wind power. The aerodynamics, electrics, and control of these craft are described in detail, along
with a description of the tether mechanics. A 240 kW craft has been designed to
demonstrate the concept at altitude. It is anticipated that large-scale units would
make low cost electricity available for grid supply, for hydrogen production, or
for hydro-storage from large-scale generating facilities.
INTRODUCTION
Two major jet streams, the Sub-Tropical Jet and the Polar Front Jet exist in both Earth
hemispheres. These enormous energy streams are formed by the combination of
tropical region sunlight falling and Earth rotation. This wind resource is invariably
available wherever the sun shines and the Earth rotates. These jet stream winds offer
an energy benefit between one and two orders of magnitude greater than equalrotorarea,
ground mounted wind turbines operating in the lowest regions of the Earth’s
boundary layer. In the USA,Caldeira and O’Doherty and Roberts have shown that
average power densities of around 17 kW/m2 are available. In Australia, Atkinson et
al show that 19 kW/m2 is achievable.These winds are available in northern India,
China, Japan,Africa, the Mediterranean, and elsewhere.
Various systems have been examined to capture this energy, and these include
tethered balloons, tethered fixed-winged craft, tether climbing and descending kites,
and rotorcraft.
Our preferred option is a tethered rotorcraft, a variant of the gyroplane, where
conventional rotors generate power and simultaneously produce sufficient lift to keep
the system aloft. This arrangement, using a twin-rotor configuration, has been
described and flown at low altitude by Roberts and Blackler (Fig. 1). More recent
developments have produced a quadruple rotor arrangement (Fig. 2).
Commercialization of the quad-rotor technology could significantly contribute to
greenhouse gas reductions.
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. Generators at altitude also avoid community
concern associated with ground-based wind turbine appearance and
noise. Bird strike problems are also less. However, tethered generators would need to
be placed in dedicated airspace, which would restrict other aircraft. Arrays of tethered
generators would not be flown near population centers unless and until operating
experience assured the safety of such a configuration.
At this time, the best tether for the rotorcraft appears to be a single, composite
electro-mechanical cable made of insulated aluminium conductors and high strength
fiber. When operating as a power source, two, four, or more rotors are inclined at an
adjustable angle to the on-coming wind, generally a 40angle. The wind on the
inclined rotors generates lift, gyroplane-style, and forces rotation, which generates
electricity, windmill-style. Electricity is conducted down the tether to a ground
station.
The craft simultaneously generates lift and electricity. However, it can also
function as an elementary powered helicopter with ground-supplied electrical energy,
and with the generators then functioning as motors. The craft can thus ascend or
descend from altitude as an elementary, tethered helicopter. During any lull periods
aloft, power may be supplied to maintain altitude, or to land on a small groundbase. A
ground winch to reel the tether could be used to retrieve the craft in an emergency.
THE BEST SPOTS TO PLACE FEGs
Based on the ERA-15 reanalysis of the European Centre for Medium-Range
Weather Forecasts, we calculated the seasonal-mean, climate-zone wind power
density from December 1978 to February 1994 .Computed power densities in high
altitude winds exceed a 10 kW/m2 seasonal average at the jet stream’s typical
latitudes and altitudes. This is the highest power density for a large renewable energy
resource anywhere on Earth. It exceeds the power densities of sunlight, near surface
winds, ocean currents, hydropower, tides, geothermal, and other large-scale renewable
resources. For comparison, Earth surface solar energy is typically about 0.24 kW/m2 ,
and photovoltaic cell conversion of energy into electricity has an efficiency several
times less than that of wind power.
High power densities would be uninteresting if only a small amount of total
power were available. However, wind power is roughly 100 times the power used by
all human civilization. Total power dissipated in winds is about 15 times 10 W. Total
Human thermal power consumption is about 13 times 10 W. Removing 1% of high
altitude winds’ available energy is not expected to have adverse environmental
consequences.
High altitude winds are a very attractive potential source of power, because
this vast energy is high density and persistent. Furthermore, high altitude winds are
typically just a few kilometres away from energy users. No other energy source
combines potential resource size, density, and proximity so attractively.
The wind speed data from across the globe is recorded at heights from 263 feet to
almost 40,000 feet over the last 30 years, and calculated which regions would
generate the most power. According to the study, Tokyo, Seoul, Sydney and New
York City all sit on a goldmine of stratospheric wind power.
During the summer months, Delhi and Mumbai could also benefit from sky high
turbines. But unfortunately for India, the gusts die down in the fall and spring,
reducing the energy density in the atmosphere.
IV DESCRIPTION OF THE PREFERRED ENERGY CONVERSION
SYSTEM
The currently proposed new tethered craft consists of four identical rotors
mounted in an airframe which flies in the powerful and persistent winds. The tether’s
insulated aluminum conductors bring power to ground, and are wound with strong Kevlar-family cords. The conductor weight is a critical compromise between power
loss and heat generation. We propose employing aluminum conductors with tether
transmission voltages of 15 kV and higher, because they are light weight for the
energy transmitted. To minimize total per kWh system cost and reduce tether costs,
the design allows higher per meter losses and higher conductor heating than does
traditional utility power transmission. Depending on flight altitude, electrical losses
between the tether and the converted power’s insertion into the commercial grid are
expected to be as much as 20%, and are included in energy cost estimates
described in Section IX.
The flying electric generator units (FEGs) envisioned for commercial power
production have a rated capacity in the 3 to 30 MW range. Generators arrays are
contemplated for wind farms in airspace restricted from commercial and private
aircraft use. To supply all U.S. energy needs, airspace for power generation is
calculated to restrict far less airspace than is already restricted from civil aviation for
other purposes. While similar in concept to current wind farms, in most cases
flying generator arrays may be located much closer to demand load centers.
When operating as an electrical power source, four or more rotors are inclined
at an adjustable, controllable angle to the on-coming wind. In general the rotors have
their open faces at an angle of up to 50to this wind. This disk incidence is reduced in
various wind conditions to hold the power output at the rated value without exceeding
the design tether load.Rotorcraft can also function as an elementary powered
helicopter as described in section II.
The capacity, or generating factor calculations account for wind lulls or
storms during which the generators must be landed. However, the projected capacity
for flying electric generators is far higher than for the best ground-based wind
turbine sites because of the persistent winds at high altitudes.
High altitude wind speeds and other conditions are measured at 12 A.M. and P.M. at
major airports worldwide by radiosonde weather balloons, and are reported on NOAA
and other government websites. It is thus possible to calculate what the past capacity
of flying generators at those locations would have been.
The U.S. average capacity factor would have been about 80% for craft flying
at 10,000 meters. At Detroit’s latitude, the capacity factor was calculated at 90%, at
San Diego’s, 71%. This compares to capacity factors of about 35 percent for
ground-based wind turbines operating at the best sites.
Fig. 2 above and Fig. 3 below show the four-rotor assembly with four identical
rotors arranged, two forward, and two aft. The plan-form of the rotor centerlines is
approximately square. Adjacent rotors rotate in opposite directions; diagonally
opposite rotors rotate in the same direction.
In this particular 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, as is needed in helicopter technology.
This should help reduce maintenance costs. Rotor collective pitch variation then
varies the thrust developed by each rotor in the format described below using
GPS/Gyro supplied error signal data.
(1) Total craft thrust (and total power output) is controlled by simultaneously equal,
collective pitch action on all rotors.
(2) Roll control is by differential, but equal, collective pitch action between the port
and starboard pair of rotors.
(3) Pitch control is by differential, but equal, collective pitch action between the fore
and aft pair of rotors.