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| Fig. 1: Kitegen kite turbine concept. (Source: Wikimedia Commons) |
Energy generation from wind is by no means a modern technology, as wind has been harnessed as an energy source for over two centuries. Traditionally wind mills and the modern wind turbine have been land based structures consisting of a generator mounted on the top of a tower founded on either land or ocean shelf. This traditional design limits the turbine in two ways, namely height and location. Innovative designs for airborne wind turbines in the recent decade have introduced prototype generators capable of harnessing winds at higher altitudes and extensively more dynamic positioning.
Wind generally increases with height from the earth's surface, and there are major jet streams found in each hemisphere. These massive energy streams are formed by the earth's rotation and the sun shining on different regions of the earth's surface. [1] The Polar Jet Stream at the mid-latitudes is found between 7-12 km above the earth's surface and the Sub-Tropical Jet Stream is found at ± 60° and 6-10 km above the earth's surface. [2]
Archer and Caldeira use the following formula to assess available wind power at different altitudes:
According the above equation the power available in the wind increases a cube function of the velocity at a certain altitude. With the wind velocities in the jet streams a full order of magnitude larger that the wind speeds harnessed by land turbines, the power density increases by three orders of magnitude. [2] According to data collected by Archer and Caldeira at 1000 ft there is an approximate power density of 304 W/m2 , and an elevation of 10,000 ft jumps to power density of 20,084 W/m2. [2] 1000 ft and 10,000 feet are representative of land turbines and air-borne turbines respectively.
Consistency is of large concern when assessing the potential of high altitude wind. If the high velocity wind is full force for a short time annually it is not a viable option over land-based turbines. Based off wind data taken at high altitudes over a three decade time period from 1976 to 2006 show that the most rapid increase in wind speed occurs between 6,000 and 7,000 feet above sea level. At these altitudes it is possible to find a wind power density of around 19 kW/m2 50% of the time annually and 7 kW/m2 68% of the time. [2] Below 1000 meters the wind power density is < 0.3 kW/m2 only 5% of the year. [2] This data shows that at high altitudes there are magnitudes larger amounts of power as well as much greater consistency in the wind than below 1000 ft.
With the realization of the potential of high altitude wind, there are considerable efforts to harness the steady and fast blowing winds of the jet streams. Two emerging prototype stage technologies are Makani Power and Kitegen Energy Systems. Makani Power is focused on a generator called Wing 7 fabricated mostly of carbon fiber and desgined to generator power over large expanses of ocean. [3] The Wing 7 flies in circles perpendicular to the wind and is capable of generating up to 30 kW. Weighing only 120 pounds the device can pull up to three tons, and flies up to 820 feet. [3] Although below 1000 feet, the development of an airborne turbine is promising for harnessing even higher altitude winds. Makani hopes to have a 600 kW capable device by 2016. [3]
Kitegen, a wind power engineering company based out of Italy, has been prototyping kite powered generators since 2007. [4] The basic design consists of a kite about 100 m2 in size that catches wind and pulls out a cable that in turn spins a turbine and generates power. [4] Estimated power generation is about 800 kW between ground level and 800 meters with varying wind speeds up to 24 m/s. [5] A kite with area up to 500 m2 could generate 2 MW of power with a constant windspeed of 9 m/s and exponentially more power with higher wind speeds. Kitegen, like Makani Power however, is limited in terms of altitude, and is not yet tapping into the large wind resources between 6,000 and 7,000 feet.
The development of high altitude wind generation most certainly has large advantages when compared to traditional ground based options. Lower material and installation cost as well as minimal land use make air borne turbines a favorable route for energy generation. [4] Companies in the same vein of Makani Power and Kitegen will undoubtedly have airborne and kite powered turbines harnessing wind in the range of 3000 feet in the next decade at the current rate of innovation. This altitude range however, does not have the same levels of consistency of wind speeds annually as altitudes above 6000 feet. Lightning and electrostatic discharge surely present obstacles to air borne turbines, as well as increased turbulence at high altitudes. An unwieldy part of existing airborne designs is the cable, which tethers the device to land or a sea anchor. Although essentially for transmitting power back to land, the cable itself increases in weight with altitude. The higher the turbine is designed to run, the more weight it must also constantly hold aloft.
Once a reliable and commercially viable airborne wind generator is designed and manufactured it will no doubt render most land turbines dated. Land turbines may still have their niche however with home based energy generation that does not need MW of power. Airborne turbines are certainly a thing of the near future, but are most suitable for large amounts of energy generation, on the size of grid scale. If fully developed and made economically viable, air borne wind generators could be a major energy source for the electricity consuming societies of the present day world.
© Jason Smith Vidaurre. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] B. W. Roberts et al., "Harnessing High-Altitude Wind Power," IEEE Trans. Energy Conversion 22, 136 (2007).
[2] C. L. Archer and K. Caldeira, "Global Assessment of High-Altitude Wind Power," Energies 2, 307 (2009).
[3] D. Biello, "Kinetic Kite," Scientific American 304, No. 4, 74 (2012).
[4] M. Canale, L. Fagiano and M. Milanese, "High Altitude Wind Energy Generation Using Controlled Power Kites," IEEE Trans. Control System Technol. 18, 279 (2010).
[5] D. Sleiter, "Kite Powered Generators," Physics 240, Stanford University, Fall 2010.
