|Fig. 1: Land-based Windmill. (Source: Wikimedia Commons)|
Humans are consuming energy at an exponentially increasing rate.  Throughout history our major energy resource has been fossil fuels but our consumption of these fuels will not be sustainable as there exists a finite amount of them on earth.  The prospect of renewable energy has grown more appealing as we seek to alleviate the high demand for fossil fuels and as we also seek to address another disastrous problem: climate change.  As renewable energy technology advances, resources such as wind, solar, and geothermal have become more and more economically profitably. Proliferating these technologies will allow for a more sustainable and cleaner power.
A major renewable resource is wind energy. Only 0.002% of our global 15 terawatt usage comes from wind energy. [1,3,4] There is ample potential to increase this percentage as global winds have a combined power greater than 3600 terawatts.  A specific wind resource that has not been tapped into is high altitude wind. Most wind turbines are land based where wind speeds are relatively slower and have less power. Wind speeds tend to increase with increasing altitude and equivalently contain a larger power density. These high altitude winds are more energy abundant but the main challenge to utilizing this energy is our ability to engineer devices that are able to reach such heights and transmit power back down to earth. In this paper we discuss the physics of wind energy, survey current high altitude wind turbine technologies and discuss the economic feasibility of the technology.
Everyone has experienced wind firsthand. Wind is the bulk movement of air in the atmosphere generated by differences in air pressure. The major wind patterns of earth are caused by the differential heating of the poles and equator and are also influenced by the Coriolis effect from the planet's rotation.  The amount of energy in the global winds is enough to power the planet 240 times over.  The kinetic energy of wind can be expressed as
where v is wind speed and m is the mass of the moving air. We can say that the total mass of air with density ρ flowing through area A over time t is equivalent to
|Fig. 2: A typical wind speed profile. |
Thus, we see that power is equivalent to
Furthermore, wind energy is often normalized to area and expressed in units of W/m2.This equation provides insight into how much energy exists in the atmosphere but not all of this can be extracted. German physicist Alfred Betz proved that the maximum percent of energy that could be extracted from wind via wind turbine blades is 59.3% (or 16/27) which is known as the Betz limit. [7,8] Currently, wind turbines are able to use over 40% of the winds energy reaching approximately 70% of the Betz limit. 
The majority of wind turbines today are land based as shown in Figure 1. These wind turbines are stationary and are only able to use the wind energy relatively close to ground. Figure 2 shows that (up to an altitude of 12,000 m) wind speeds increase with increasing altitude and thus (assuming a small decrease in air density) there exists the potential to extract more energy at these higher altitudes.  Only 35% of wind energy exist below 1 kilometre above ground level. 
Engineering a high altitude wind turbine is not a trivial task. Building a structure tall enough to reach high winds is practically out of the question. The best way to get a turbine up in the sky is attaching it to an airborne vehicle. This doesn't alone complete the task as there also exists the challenge of being able to store the energy. It is not practical to store the energy in a storage device such as a battery because this adds unnecessary weight to the airborne vehicle. Sending electricity through a wire down to the ground is the best alternative.
|Fig. 3: Conceptual Design of Airborne Wind Tubine. (Source: Wikimedia Commons)|
Engineers have converged on this problem by developing tethered airborne platforms. One conceptual design of a high altitude wind turbine is shown in Figure 3. It is comprised of an air-to-ground tether, a set of rudders to catch the wind, and a generator. There are several small companies seeking to engineer variations of this contraption but they all employ a common method to achieve the same end goal. One method proposed by the company Altaeros includes an inflatable ring with a wind turbine inside called a Buoyant Air Turbine (BAT). The BAT is lifted to an altitude of 300 meters above the ground and a single structure has a power capacity of 30 kW.  The method proposed by Makani is a tethered glider that flies in circles and has a series of small turbines to generate electricity.  Windlift uses a kite to lift a turbine. 
Windborne energy has several benefits over traditional land-based windmills. First of all, as described in the previous sections, there is more power at higher elevations. With the right engineering it is possible to extract more energy from the wind than is currently being done. Additionally, windborne turbines are portable. This is advantageous for being able to gather energy in remote areas and can act as a mobile generator. These deployable devices can be deployed to remote locations to power micro grids allowing for much lower energy loss in transmission.
A windmill is successful if when economically scaled it is able to provide electricity at a competitive rate. A major metric in wind power is the capacity factor, or the average amount of power that a specific wind turbine is able to provide. The capacity factor takes into consideration all that time that the turbine is generating power as well as the time that it is not. For example, if a wind turbine is operating at peak capacity for half a year and then doesn't operate the other half of the year it has a capacity factor of 50% for that year.
Land based wind mills are advantageous because there is little operation cost between periods of high and low wind; they can remain stationary on the ground. A high altitude wind turbine currently requires much more human operation. If there is no wind blowing then some high altitude platforms, such as kites, may be unable to get off the ground and a human operator much be present in order to relaunch the device once the wind resumes blowing. The cost of human operators to maintain airborne windmills may be more than the value of electricity derived from their usage. Airborne wind turbines are not likely to become a major player in the global power market and are not even likely to soon compete directly with traditional land-based wind power.
There are, however, several niche markets where windborne energy will have an advantage such as remote power and microgrids. A major advantage that windborne turbines have are their transportability. A single platform can be deployed and brought down to the ground, and moved elsewhere much more quickly than a land-based windmill.
Wind energy may not significantly increase in its percentage of the world watt usage but it still remains a useful option to employ. It is clean and renewable and has the potential for high wattage genearation. There are many difficult economic and engineering hurdles to overcome before high wind energy becomes useful for wide-scale energy generation. Nevertheless, there does exist a market for high altitude wind turbines in such applications where a generator needs to be portable or deployed in a remote location. As engineering gets better, high wind energy will be utilized more fully.
© Dustin Gerrard. 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.
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