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| Fig. 1: Comparison of high-altitude wind power with other energy options. [2] |
Of the myriad ways of energy capture available in the world today, few are entirely renewable and even fewer require no special materials or technologies. Solar power is renewable but requires specific types of materials known as photovoltaics, which are both expensive and inefficient. Hydroelectric power is renewable but commonly has a detrimental effect on the surrounding physical environment and natural habitat, sometimes causing dangers for living things in the area. Wind, on the other hand, is completely renewable, relies on relatively simple electrical and mechanical principles, and has very little influence on the surrounding environment.
In this report, I will introduce the concept of high altitude wind and identify current companies pursuing designs, but my focus will be on the cost of this new energy source.
Wind power has historically been ground-based; we've all seen the sometimes vast arrays of "windmills" across hillsides or in valleys. Unfortunately, these arrays harness a small percentage of the power present in the wind. [1] High altitude wind, on the other hand, is much more powerful and consistent than surface winds, and because the generated power grows with the cube of wind speed, this means the available energy is much greater at high altitudes. [2] In fact, the best conventional ground-based systems harvest only around 35%, whereas airborne electric generators can achieve about 90% in places like Detroit. [1]
Of course, these numbers depend on location, wind consistency, and weather to an extent, but considerable research is being done to identify the most appropriate locations for these high altitude turbines. One group reports the highest wind power densities are at altitudes near 10,000 meters above the ground in a number of places around the world, like Japan, China, the U.S., and Australia. [3]
Like with all exciting, new, and potentially lucrative technologies, there is a large number of groups pursuing the best design for an airborne wind turbine. I have chosen two to highlight: the flying electric generator (FEG) design, by Sky WindPower Corporation and the airborne kite array created by KiteGen. The former converts mechanical energy to electrical energy in board, and the latter transmits the mechanical energy to the ground for generation. [3]
The FEG prototype is a four-rotor generator rated at 240kW, with each rotor having a diameter of 35 feet. [1] The "rotorcraft" would be tethered to the ground with a single, composite electromechanical cable made of insulated aluminum conductors and high-strength fiber, and the tether would also serve to transport the electrical power from the craft to the ground. [1]
The KiteGen design utilizes tethered kites connected to a ground-based generator. The kites transfer the wind's mechanical power to the ground, and generators there convert it to electricity. [3] This would be an appropriate solution for lower-altitude (first few kilometers) energy capture. [3]
While the designs are different, they take advantage of the same principles: winds are stronger and more consistent at higher altitudes, meaning that there is a higher density of power. By allowing the tether cables to be long enough, these kites and rotorcrafts can reach hundreds or thousands of meters in the air.
Another advantage of more powerful wind is that it requires smaller turbines than surface-based ones. Since they're smaller, they should require less frequent and less expensive maintenance. [2] In fact, due to the increase in efficiency, the relative simplicity of the technology, and the relatively low cost of the materials, it is expected that high altitude wind energy can be cheaper than current forms of capture (see Fig. 1). [2]
In terms of dollars per MegaWatt-hour, these technologies would be vastly superior (i.e. factor of at least two) to current power production methods. Whereas current wind energy is approximately 150 $/MWh, while fossil energy is between 50 and 90 $/MWh, high altitude wind technologies are estimated to cost between 20 and 50 $/MWh. [2] And while these numbers are neither perfect nor 100% certain to be true if every new technology were to be put into action today, they nonetheless demonstrate a promise for cheaper energy that is also clean and renewable.
High altitude wind power holds vast potential for the earth's power needs in the future, especially considering the finite nature of the energy sources upon which we currently rely. Inexpensive, clean, and low-material technologies will be the future of global energy. The technologies currently being developed by companies like Sky WindPower and KiteGen will be the ones we look to when our oil reserves dry up; therefore, these are important technologies to monitor, encourage, and promote.
© 2010 Taylor Cone. 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. Roberts et al., "Harnessing High-Altitude Wind Power," IEEE Transactions on Energy Conversion 22, 136 (2007).
[2] M. Canale, L. Fagiano and M. Milanese, "High Altitude Wind Energy Generation Using Controlled Power Kites," IEEE Transactions on Control Systems Technology 18, 279 (2009).
[3] C. L. Archer and K. Caldeira, "Global Assessment of High-Altitude Wind Power," Energies 2, 307 (2009) 2009.