The Sky's the Limit for Wind Power Conversion

Jul 28, 2011 11:46 AM
Davis, Sam

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.

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