Power Electronics

The Sky's the Limit for Wind Power Conversion

Harnessing the energy in the high altitude winds is a technology that can support the world’s energy needs. Capturing this energy can be accomplished by further development of existing technologies and may not require any fundamental scientific breakthroughs.

Considering all costs, including the true costs of nuclear fission and the external costs of fossil fuel energy sources, airborne wind energy could be the world’s cheapest energy source. (Possible exceptions are limited hydro sources and limited situations where surface-based wind turbines may be the most economic for supplying relatively local needs.)

High-energy winds are at altitudes high above us, not just at a few hundred feet where they can be tapped by tower-based turbine rotors. Airborne Wind Energy technologies will employ tethered wind energy capture devices that “fly” to these altitudes where wind power is much greater than it is at ground level.

There are several groups developing Airborne Wind Energy (AWE) technologies intended for use up to 2000 ft above ground level (AGL) and others intended for use at altitudes greater than 2000 ft. AGL. Some technologies might be able to bridge this segmentation, but not always in the exact incarnations for above and below that altitude. The 2000 ft. was chosen because that is the altitude above which the FAA is not currently interested in approving what it considers to be “obstructions.” AWE technologies can be flown higher outside the 12 nautical mile limit off the coast into international airspace, but still in the US “economic zone.”

The effects of winds at altitudes miles above was clearly demonstrated in the form of detailed color charts calculated by Dr. Ken Caldeira, formerly of the Lawrence Livermore National Laboratory, now at the Carnegie Institute’s Department of Global Ecology in Stanford, California. A typical chart (Fig. 1) shows the latitudes and altitudes where this energy can be found. To calculate the energy available from wind, see the sidebar, “Wind Power Is Proportional To Velocity Cubed.”

One of the first to work on capturing high altitude wind energy was Australian Dr. Bryan Roberts. He demonstrated that Flying Electric Generator (FEG) technology is practical and should work at high altitudes (Fig. 2). The Roberts’ “rotorcraft” resembled a tethered elementary helicopter with no cabin. It had two rotors, each twelve feet in diameter. Its two contra-rotating rotors were powered by electricity from the ground, enabling it to fly to its desired altitude.

Newer FEGs

FEG technology, located at rural locations not very far from urban centers, in connection with electrical grids can serve many needs. Compared with tower-based turbines, much smaller rotors are necessary per megawatt captured in the high velocity high altitude winds. Rated capacities of each FEG may be expected to eventually increase to the multi-MW range. In the future, instead of two rotors, these FEGs would use four in a square arrangement, or more in bigger arrangements, as shown in Fig. 3. Fig. 4 shows an actual prototype of an experimental FEG made by Sky Windpower.

For example, Sky WindPower’s Flying Electric Generators (FEGs) can employ much smaller rotors than their tower-based cousins. It appears that rated capacities of their FEG will initially be about 1 MW. Their generators produce a high voltage at relatively low current to allow use of small diameter, lightweight tether cable. Some electrical transmission losses will obviously occur with the tether.

Tether Cable Needed

An important solution required for a viable high altitude FEG is an appropriate tether cable that can survive the environment and also the high voltage flowing through it. For example, a 1 MW (1×106 W) system would require generation of 10,000V (104) at 100 (102) A. Obviously, this would be difficult to achieve in a tethered system that could be a few thousands of feet long. This requires a solution that minimizes the voltage drop on the tethered cable. This would involve innovations in tether design and possibly material science. It is possible that tether might not use copper wire. One possibility might be high temperature superconducting wire.

One possible future configuration is to use eight generators, each rated at 125 kW or greater, for a total of 1MW or greater. Fortunately, tether strength-to-weight ratios actually improve as sizes scale up, and guidance control weight goes up less than proportionately with size. In other words, within reasonable limits, efficiency may be expected to improve with scale.

Using more than two rotors permits avoiding the biggest component maintenance problems of two-rotor helicopters, caused by “cyclic pitch”, in which the blades are forced to change pitch back and forth by “swash plates” during every rotation.

Avoiding these problems is accomplished by using “collective pitch”, in which blade pitch remains constant through complete revolutions, using temporary change in the constant pitch of pairs of rotors when direction change (left, right, up, down) is desired, the pairs selected depend on the direction change desired.

Use of this collective pitch approach is crucial in keeping maintenance costs low and assuring FEGs being able to fly for substantial periods of time between landings for maintenance. There is also the potential for use of a direct drive configuration as is being done in some ground wind turbines, now, to reduce maintenance costs.

Weather Effects

Lightning and atmospheric static discharge affect tethers containing conductors. However, the frequency of conditions in which these atmospheric conditions are a potential problem is close to zero in key parts of the world needing energy the most.

Even where lightning conditions do seasonally exist, with good warning provided by current technology, FEGs will be grounded, and returned to service rapidly after the passing storm. Furthermore, lightning problems have been mitigated in other tether applications and will be utilized by FEGs when conditions are not severe.

When encountered, turbulence can be a problem. However, a tethered rotorcraft such as an FEG has the freedom to move a little, like a kite, and settle back. This comes about primarily because of the long tether simply would change its shape a bit when the FEG encounters a wind gust and then gradually resume its natural shape, which would reduce tension on the FEG tether.

In addition, programmed electronic controls, using GPS and gyroscope attitude sensing information, assure that rotor pitch and airfoil control surfaces react in damping fashion to the FEG movement in this situation.

Although an FEG rotor is turning, it does not stop its surfaces from also having a kite-like lifting action. Depending on the angle of the rotor plane to the horizontal, there is an energy division between the wind energy going into lift and the wind energy going into generating power. This angle of attack can be controlled, so it is continuously set automatically to keep the FEG at the desired altitude and generate rated power whenever possible, but not more. FEGs are free to fly at the best altitude at any time, taking into account both wind velocity and expected turbulence.

GPS technology can be used to assure that the rotorcraft stays within a few feet both horizontally and vertically of where it is commanded to be at high altitude. This is very important to assure that multiple FEGs stay separated enough not only to avoid collision, but also to avoid interfering with each other’s efficient access to the available wind energy, a problem known as “wind shadowing.” Fortunately, with three dimensions in which to operate, this is much less of a problem for FEGs than it is for wind turbines based in the two-dimensional ground surface arrays.

FEG tethers going up to high altitudes obviously could pose a problem for aircraft. Therefore, FEGs will avoid taking off or landing near areas where there is aircraft traffic. With the safety measures being implemented, it is expected that the rotorcraft could be bought down like a helicopter in autorotation without harm, even if a tether were severed.

Unlike nuclear fission power plants, another non-global warming energy source, FEG arrays would not be very tempting terrorism targets, and do not pose radiation worries whether from accident or deliberate action.

Capacity Factor

Usually, the wind industry deals in terms of rated capacity of wind turbines or total installed capacity at wind farms. However, this does not cover how much energy is actually captured. 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.

By far the biggest reason for not operating at capacity is that insufficient winds exist at the given site to generate at rated capacity. This is true at any altitude, but the percentage of the time is much less at high altitude.

Capacity factor for FEGs of Roberts’ design operating at high altitude typically range from 70% in southern U.S. and up to in some cases over 90% at the Detroit’s latitude, then starting to taper off going further north. Capacity factors can be improved by expanding rotor sizes relative to generator capacities but at increased cost, and, if carried too far, at greater cost per kilowatt-hour delivered.

At higher altitudes, ground topology has almost no bearing on capacity factor. Fly FEGs any place within about a hundred miles of a city and the capacity factor can be expected to be about the same.

This means that good site selection for FEG arrays depends primarily on being isolated enough not to be over populated areas, but not so far as to make ground transmission to those areas too expensive.

FEG System Operation

The system employs a land-based control unit for the FEG as well as an internal control in the FEG itself (Fig. 5). When starting up the system, the control unit applies a dc voltage via the tether that causes the FEG’s motor-generators to act as motors. Power goes from the ground to the rotorcraft to drive the motors to carry the FEG to the desired altitude for optimal wind generation. Then, the wind drives the rotor blades that act as airfoils creating lift for the FEG. At the point where the wind provides enough energy to maintain the craft aloft at generation altitude the craft goes into autorotation mode in which no energy from the ground is needed.

Any extra wind energy over what is needed to maintain the craft aloft is converted to electricity as the FEG automatically, seamlessly, switches the motor-generator into generation mode. Much like the wheels connected to regenerative braking systems in automobiles, the rotors do not reverse direction in this type of motor-generation system. The voltage produced by the FEG must be as high as possible to minimize the current flowing through the tether. Also, it should minimize heating of the tether and heat energy losses, both of which are undesirable.

The tether cable material consists primarily of conductors and strength elements and is designed to be flexible. The tether cable design is proprietary.

The control unit accepts the high voltage dc output from the tether and applies it to an inverter that produces ac output applied to the utility grid. This output is then synchronized with the existing utility grid and applied to it.

Note that the anchor is essential for power generation as the wind would otherwise blow the craft away and it would descend to the ground, like any other untethered kite.

Multiple FEGs

While other power sources will, of course, always supply some power, FEG arrays, all operating at relatively remote sites with average capacity factors of 50-75% (two to three times the typical ground wind capacity factor), but not located so far from metropolitan areas as to require very long transmission lines, could contribute significantly toward achieving many states’ renewable portfolio standard goals. FEGs could provide substantial power for rapidly growing, energy hungry economies and replacement power for island nations currently heavily importing fossil fuels. It is important to note that future U.S. and world energy needs will undoubtedly increase, and, in addition, that many current energy needs now provided by fossil fuels directly, not through generating electricity, will switch to high altitude, wind-supplied electrical grid energy as the economics for this technology become more favorable and the need to address global warming problems become more generally recognized.

Thus, the number of FEG arrays required to meet future world needs will increase significantly. However, in the U.S., this whole subject should really be addressed from the viewpoint of combined U.S. and Canadian supply and demand, as the U.S and Canadian electrical grids are connected.


Acknowledgement:

We thank PJ Shepard, “Chief Cheerleader” for Sky WindPower and Secretary of the Airborne Wind Energy Consortium, for her assistance with this article, as well as Dr. Bryan Roberts and prototype development engineer Bruce Weddendorf, for their assistance with this article.

References:

-Sky WindPower, “High Altitude Wind Power,” http://www.skywindpower.com.
-Roberts, Bryan et al, “Harnessing High Altitude Wind Power” IEEE Transactions on Energy Conversion, Vol 22 No.1 March 2007.
-Lawrence Livermore National Laboratory, Wind Energy Research at LLNL, California Wind Energy Collaborative University of California at Davis, 10 May, 2010, Jeff Mirocha, Staff Scientist, Atmospheric, Earth, and Energy Department, Lawrence Livermore National Laboratory, LLNL-PRES-415464.

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