Jun
18
Ready to Catch Some High Winds?
June 18, 2009 | 1 Comment
Several technologies have been proposed that hope to harnessing wind power from high altitudes. Two basic approaches have been proposed. The mechanical energy can be transmitted from high altitude to the Earth’s surface, where generators would produce the electricity at the ground or electricity could be generated aloft and transmitted to the surface using the tether.
Most concepts are still at an early stage of development, in which patents have been obtained. But neither business entities nor commercial-scale prototypes exist. No high-altitude wind power technology to date has produced a prototype that has been tested long enough to provide a solid record of electricity generation and associated costs.
But the lure is powerful. The jet streams, even with seasonal movements are relatively persistent features of the mid-latitudes in both global hemispheres. The total wind energy in the jet streams is roughly 100 times the global energy demand. The abundance, strength, and relative consistency make jet stream winds particularly interesting in wind power development. It’s a huge resource and it is accessible, even though a giant engineering problem.
Now twenty-eight years of wind data from the reanalysis by the National Centers for Environmental Prediction and the Department of Energy have been analyzed and interpolated to study geographical distributions and persistency of winds at all altitudes. The intermittency issues and global climate effects of large-scale extraction of energy from high-altitude winds have been investigated. Cristina L. Archer at the Department of Geological and Environmental Sciences, California State University in Chico and Ken Caldeira at the Department of Global Ecology, Carnegie Institution of Washington, Stanford whose paper “Global Assessment of High Altitude Wind Power” covers the reanalysis with some interesting points.
As noted the technology is barely past conception stage. One mechanical concept the authors look at is the KiteGen. KiteGen consists of tethered airfoils (kites) connected to a ground-based generator with two lines, which are pulled and released by a control unit [4-6]. The energy generated during the traction phase is greater than the energy needed in the recovery phase. A single unit of 100 square meters is expected to generate 620 kW of electricity. Arrays of several kites can be arranged in a carousel configuration around a circular rail for electricity generation of up to 100 MW. This approach appears most suitable for the lowest few kilometers of the atmosphere.
KiteGen Carousel Concept. Click image for the largest view.
The design proposed by Sky Windpower has four rotors mounted on an airframe, tethered to the ground via insulated aluminum conductors wound with Kevlar-type cords. The rotors both provide lift and power electric generation. The aircraft can be lofted with supplied electricity to reach the desired altitude, but then can generate up to 40 MW of power, with angles of up to 50° into the wind. Multiple high altitude wind turbines (rotorcrafts) could be arranged in arrays for large scale electricity generation. For this approach, the aim would be to capture energy closer to the jet streams.
Sky Windpower's Fying Electric Generators Concept. Click image for the largest view.
The sharp reader will quickly realize that as the altitude increases the density of the air decreases so lessening the power of the wind. The authors give a good rendition of the mathematics and the formulas needed to assess the potential power available. That point is covered, and the results still offer lots of power to harvest.
The authors even briefly cover the interference with aviation.
The researchers found that the regions best suited for harvesting this energy match with population centers in the eastern U.S. and East Asia, but the fluctuations in wind strength still present a challenge for exploiting this energy source on a large scale. Ken Caldeira says, “There is a huge amount of energy available in high altitude winds. These winds blow much more strongly and steadily than near-surface winds, but you need to go get up miles to get a big advantage. Ideally, you would like to be up near the jet streams, around 30,000 feet.”
Jet streams are meandering belts of fast winds at altitudes between 20 and 50,000 feet that shift seasonally, but otherwise are persistent features in the atmosphere. Jet stream winds are generally steadier and 10 times faster than winds near the ground, making them a potentially a vast and dependable source of energy. This is truly competition for ground based wind turbines.
Cristina Archer says, “We found the highest wind power densities over Japan and eastern China, the eastern coast of the United States, southern Australia, and north-eastern Africa. The median values in these areas are greater than 10 kilowatts per square meter. This is unthinkable near the ground, where even the best locations have usually less than one kilowatt per square meter.” The analysis assessments included the high altitude wind energy for the world’s five largest cities: Tokyo, New York, Sao Paulo, Seoul, and Mexico City. “For cities that are affected by polar jet streams such as Tokyo, Seoul, and New York, the high-altitude resource is phenomenal,” said Archer. “New York, which has the highest average high-altitude wind power density of any U.S. city, has an average wind power density of up to 16 kilowatts per square meter.” Now that’s like 16 100-watt bulbs in just over a square yard. This is serious atmospheric power.
Now for the bad news. Caldeira says, “While there is enough power in these high altitude winds to power all of modern civilization, at any specific location there are still times when the winds do not blow.” But the comparisons with near surface winds doesn’t seem fair, even over the best areas, the wind can be expected to fail about five percent of the time. Yet, this is a big change from being offline 60 or 70 percent of the time.
Caldeira continues, “This means that you either need back-up power, massive amounts of energy storage, or a continental or even global scale electricity grid to assure power availability. So, while high-altitude wind may ultimately prove to be a major energy source, it requires substantial infrastructure.”
Maybe, perhaps even probably. But there is a massive resource overhead and ingenuity isn’t in full play just yet. Five percent or even ten percent down time, with the prospects of energy storage breakthroughs may well blow off Caldeira’s conclusion in the future.
High altitude wind is certainly worthy of more creativity and ingenuity. The current examples look good. Meanwhile the leaders in this field need to be looking into the future issues such as land and airspace rights for starters. One has to know soon if the engineering matures can the technology get airborne at all.
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Keep posting stuff like this i really like it