High Altitude Wind Power - A Review
The first international conference on High Altitude Wind Power happened last November in Chico, CA. That's a sign this new technology is being taken seriously by academics. Hence, the rest of us concerned about our energy needs should become acquainted with the topic. For those unfamiliar with the concept, the idea is simply that winds are stronger and more reliable as you get further from the earth's surface - peaking near 10,000 meters. Harnessing high level winds could be an abundant energy source, but for the challenges of the technology. Various methods of tapping the resource have been suggested. A few prototypes have been built, and there are a couple of companies on the verge of releasing commercial products.
High Altitude Wind Resources
The winds up at high altitudes are surprisingly strong and steady - much better for power generation than the winds near the surface. A recent paper byCristina Archer and Ken Caldeira looked carefully at National Centers for Environmental Prediction (NCEP) and the Department of Energy (DOE) data and produced high altitude global wind speed maps that factored in the fraction of the time the wind was blowing at speed. The maps clearly show the jet stream bands in the northern and southern mid latitudes where the strongest and steadiest winds are located.
Wind_power density (kW/m2) that was exceeded 50%, 68%, and 95% of the time during 1979-2006 at 1,000 m (left) and 10,000 m (right) from the NCEP/DOE reanalyses. (Excerpt from .)
The energy available from wind scales as the cube of the wind velocity times the density of the air. Peak energy density in the mid latitudes resides between 5,000 m and 10,000 m elevation. The best winds are in the jet streams. There is less advantage to go so high in the equatorial regions. Surface winds are always attenuated near the ground by friction with the surface. Just getting above the planetary boundary layer, typically 300 to 500 m, can easily triple the energy density available compared to winds near the surface.
High Altitude Wind Machines
Various proposed methods for extracting energy from high altitude wind fit into three families of devices. These are lighter-than-air devices, kite-like systems, and rotating propeller machines. We will look at the organizations developing these devices and consider the pros and cons of each system.
Lighter than air machines
Magenn Power Inc. is taking orders for their first commercial product, a 100kW device with a price tag of $500,000. The Magenn machines are large ridged dirigibles that rotate on their tethers. The machines require lots of helium to stay aloft and they are designed to work between 500 - 1000 ft. elevation.
Magenn is targeting the device as an alternative to diesel generator sets for remote and emergency power applications. Relatively low-level operation of this systems means that it can operate just about anywhere without regard to airspace restrictions.
Tethered Airfoils (Kites)
The Europeans seem to like kite power methods. The most advanced system is being developed by an Italian group. The researchers at KiteGen have investigated systems that employ steerable kites in several configurations. In the "yo-yo" configuration, power is generated by the extension of the tether during a fast-flying power stroke. Then the airfoil is feathered while the tether is reeled back before again executing the power stroke. A "carousel" arrangement allows several kites to power a rotating vertical-shaft machine in a continuous manner.
A key advantage for the kite approach is in the efficient deployment of the active airfoil. Ground-based windmills require enormous towers and foundations to support the large rotor blades, and only the tips see maximal wind speed and power production. Kites, on the other hand, can access high winds with a light tether to transfer power from the air foil to the ground. The entire air foil is subject to maximum wind speed.
A similar design called a Laddermill has been studied by researchers at Delft University in the Netherlands. Work continues on developing adequatecomputer models of the kite system. One of the primary challenges is being able to control the kite in changing wind conditions. These control problems are formidable, and account for much of the theoretical work being done.
The kite systems are best suited for harnessing the winds below 1000 m elevation. Machines targeting the upper atmosphere are going after yet another order of magnitude in power density and even steadier winds. These machines are usually designed with spinning rotors.
Rotating Propeller Machines
Two California companies are investigating "flying electric generators" (FEG) asSky Windpower calls them. Bryan Roberts of Sky Windpower is one of the pioneers in the field with papers dating to the late 1970's, and under his direction a two-rotor prototype was flown in 1980. Present plans are for a four-rotor demonstration design capable of 240 kW and operation up to 4600 m.
For all power rotor designs, the tether must contain electrical conductors to transfer power from the rotors to the ground. High voltage transmission is used to minimize the weight of the conductors. The rotors are powered during launch so the tethered craft can be flown to altitude under power.
The Joby Energy modular turbine concept
The other California start-up is Joby Energy, investigating a modular flying turbine concept. The company has built several small prototype devices that demonstrate flight control of their platform concept.
Finally, there is Makani Power, Inc., another California start-up that has received $10 million funding from Google. They have a very capable team, but little information is available about what they plan to do.
Most casual observers of high altitude wind power easily point out the obvious problems. Lightning, air traffic interference, failures causing machines to fall out of the air, and the weight of miles of cable are all frequent criticisms. Fortunately, these are all engineering challenges that can be tackled with standard engineering practices. There is no new physics that we need to learn to develop this resource. Aerodynamics, structural design and materials strength, power electronics, electrical generation, and control theory, are all well-developed disciplines that will be applied to the problem.
The driving motivation is the promise of high returns on investment. Estimates of the Energy Return of Energy Invested (EROEI) are around 100, better than most fossil fuel resources but without the CO2 problems. Cost estimates for the electricity produced range from 2 to 5 cents per kW H. With our insatiable demand for cheap energy, and with a widely dispersed potential solution a few miles overhead, someone will bring it down to earth and make some money along the way - the race is on.
 Global Assessment of High-Altitude Wind Power, Cristina L. Archer and Ken Caldeira, Energies 2009, 2(2), 307-319.
 Harnessing High Altitude Wind Power, B. W. Roberts, D. H. Shepard, K. Caldeira, M. E. Cannon, D. G. Eccles, A. J. Grenier, and J. F. Freidin, IEEE Trans. on Energy Conversion, 2007, 22(1) pp. 136-144.
 The Laddermill - Innovative Wind Energy from High Altitudes in Holland and Australia, B. Lansdorp and P. Williams, Windpower, 2006.
 Control of tethered airfoils for a new class of wind energy generator, M. Canale, L. Fagiano, M. Ippolito, M. Milanese, 45th. IEEE Conference on Decision and Control, San Diego (CA), USA, 2006.
 Design and Construction of a 4 KW Groundstation for the Laddermill, B. Lansdorp and W. J. Ockels, IASTED EuroPES 2007.
 Power Kites for Wind Energy Generation - Fast Predictive Control of Tethered Airfoils, M. Canale, L. Fagiano, M. Milanese, IEEE Control Systems Magazine (12) 2007.
 Comparison of Two Mathematical Models of the Kite for Laddermill Sail Simulation, A. R. Podgaets and W. J. Ockels, Proc. of the World Cong. on Engineering and Computer Science 2007.
 Long-Term Laddermill Modeling for Site Selection, B. Lansdorp, R. Ruiterkamp, P. Williams, and W. Ockels, AIAA Modeling and Simulation Technologies Conference 2008.
 High-altitude wind power generation for renewable energy cheaper than oil, L. Fagiano, M. Milanese, and D. Piga, EU Sustainable Development Conf. 2009.
 High altitude wind power: an era of abundance?, Ugo Bardi, The Oil Drum, July 6, 2009.
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