Harnessing High Altitude Wind Power

Paper by Bryan W. Roberts, David H. Shepard, Ken Caldeira, M. Elizabeth Cannon, David G. Eccles, Albert J. Grenier and Jonathan F. Freidin.

Abstract:
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 30° in 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.

Scalability Considerations
The tethered rotorcraft is inherently scalable in size and output, from small prototype configurations of below 240 kW, through commercially viable systems with competitive costs of energy, in the range of 3 MW to 30 MW per craft. Larger sizes are more economical and may utilize more than four rotors to maintain economy and manageability of materials.

For cost illustration purposes, we use a 100 MW array, comprised of 3.4 MW FEGs. The cost estimates are based on 250 FEGs/year production rate assuming prior production of 150 FEGs, in accordance with NREL guidelines [20]. A 3.4 MW platform-rated craft is estimated to weigh 21,000 lbs (9500 kg) and cost $1,360,000. Adding ground systems and production profits brings the total to $2,260,000 per 3.4 MW. The balance of station costs for the 100 MW
array, including site preparation, facilities and equipment, spare parts and construction is $4,210,000. Taken together these initial capital costs come to $71,200,000 per 100 MW

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.

Claim:
High altitude winds are a very attractive potential source of power, because this vast energy is high density and persistent. 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.

Furthermore, high altitude winds are typically just a few kilometers away from energy users. No other energy source combines potential resource size, density, and proximity so attractively. Removing 1% of high altitude winds’ available energy is not expected to have adverse environmental consequences. The tethered craft proposed in this paper 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.

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.



The flying generator’s side view in the above illustration is for a typical 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. The aircraft’s center of mass is at C. The tether is assumed, herein for simplicity, to be mass-less and non-extendible. For low altitude flight, around 1500 ft



BIOGRAPHIES (short form)

Bryan W. Roberts graduated BE from the University of New South Wales in 1959. He received a Ph.D. from the University of Cambridge, UK in 1962 in the field of aeronautics. His research has involved theoretical and practical studies of the stability and control of tethered rotorcraft. He has some 80 publications and is a Senior Member of the AIAA, a Member of the American Helicopter Society and a Fellow of the Institution of Engineers, Australia.

Ken Caldeira is a staff scientist at the Carnegie Institution’s Department of Global Ecology at Stanford University. Caldeira was a post-doc in the Geosciences Department at Penn State. He earned his Ph.D. in Atmospheric Sciences from New York University in 1991. Caldeira is a lead author of the “State of the Carbon Cycle Report”,and part of the US delegation in climate
change negotiations leading up to the 2005 G8 summit in Gleneagles, Scotland.

Caldeira was a member of the US Carbon Cycle Steering Group, and Coordinating Lead Author of an IPCC report on carbon storage in the ocean. He was a member of the UNESCO International Oceanography Commission CO2 Panel of Experts.

M. Elizabeth Cannon is Dean of the Schulich School of Engineering at the University of Calgary where she conducts research in the area of satellite navigation for land, air and marine applications. Elizabeth is a Fellow of the Canadian Academy of Engineering, the Royal Society of Canada.


Albert J. Grenier obtained his engineering degree from Clarkson University in 1965. Becoming intrigued with the promise of capturing high altitude wind energy, in 2004 Grenier joined Sky Wind Power Corporation as Executive Vice President, where he has contributed to Flying Electric Generator (FEG) design and taken the lead in qualifying components to be supplied by vendors for the planned 240kW FEG high altitude wind energy capture demonstration.

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David H. Shepard
David G. Eccles
Jonathan F. Freidin