As energy demand continues to grow in sync with economic growth, and costs of impending climate change become internalized through regulation, wind energy will have increasing prevalence in the global energy mix. Regulatory influence, including renewable portfolio standards, tax incentive programs, and possible carbon credit legislation, along with declining fossil fuel reserves are driving the shift. The US Energy Information Administration projects electricity demand to increase 39% from 2005 to 2030, and the Department of Energy has an overarching proposal to increase wind energy's fraction of this US electricity supply to 20% in 2030. 
Currently, wind power is largely produced through horizontal-axis, land-based and offshore turbines. This technology is relatively mature, and as of 2011, NREL reports that the US currently has about 44 GW of wind turbines installed. To get to 20% in 2030, approximately 300 GW of installed wind power will be needed.  Current technology, however, has several limitations. Namely, wind resource near the ground is often inconsistent and low-speed. This results in a low capacity factor, which represents the actual power produced in a given time period divided by the theoretical rated maximum for the turbine. The highest wind speeds are at much higher elevations than practical to reach for land-based turbines, and these winds are what airborne wind power devices hope to access.
Incremental innovations are continually occurring to decrease capital costs and increase performance, but the prospect of airborne wind power represents a true disruptive change. Airborne wind devices use much less material for a specific rated power output than land turbines. Also, they have a higher capacity factor, making them more suitable for utility scale grid integration. In this survey, I will touch on the basics of wind resource availability and highlight some of the major designs proposed to extract this high-altitude wind energy.
The wind power available in a parcel of air is simply the time derivative of the air's kinetic energy: 
where P is the power in watts, ρ is the air density in kg/m3, and V is the air velocity in m/s. As elevation increases, air density decreases but the median air velocity typically increases significantly. Since power scales by the cube of velocity, there is much greater wind resource available at higher altitudes, despite the lower air density. The wind power density, or power available in a cross-section of air in W/m2, is simply P/A in the equation above. We multiply this by turbine rotor area to get an upper limit on power, PW, for a turbine. The power extractable by a turbine, or PT, is simply a coefficient of performance multiplied by the wind power PW (PT = CpPW). The coefficient of performance, or Cp, is a measure of the turbine's efficiency, which varies greatly with turbine size, design, and wind speed.
As mentioned, both wind power density and consistency generally increase with altitude, motivating the airborne wind power concept. From the height of the order of a ground turbine to a height of 500 m, the median power density increases about 0.25 W/m2 per meter of elevation.  Since FAA regulations currently mandate a maximum flying height for airborne wind contraptions of 2000 ft, most companies are designing products that fly within this range. The strongest and steadiest winds, however, are present in the jet streams which are in the range of 7 to 16 km above the Earth's surface.  Power densities here are one to two orders of magnitude higher than those at the level of ground-mounted turbines, and capacity factors in the jet stream can reach 80% compared to about 35% for conventional wind turbines. 
I will outline a small subset of the numerous design permutations possible here. Some propositions have been a helium-buoyed floating turbine, a kite system with energy conversion on the ground, a rotorcraft, and an autonomous winged turbine. Several companies have promulgated each of these concepts and have proposed various scales of production. For comparison, note that typical wind turbines in the US have a rated output of about 2 MW.
In designing an airborne wind energy extraction system, there are many different characteristics to optimize, including efficiency, safety, and capital costs. Several tradeoffs may exist between these in any one design.
The KiteGen project, an effort out of Italy, is developing kite-surfing style kites that trace figure eights across the sky, transferring energy to a generator on the ground. They are guided by a sophisticated control system that relies on data from onboard and ground-based sensors. In contrast to other concepts, conversion of wind energy to electricity happens on the ground, not in the air. The target height of operation is less than 1000 m. Preliminary tests show that the system can generate up to 40 kW with commercially available kites with an area of 10 m2. Clearly, significant scale up is needed, perhaps in the form of a carousel configuration, to achieve the output of standard turbines.
These next three concepts involve conversion of mechanical wind energy to electricity at the turbine itself, and conduction of that electricity down an insulated tether connecting the turbine to the ground.
This rather futuristic looking design, proposed by Sky WindPower Co., is one of the few targeted toward jet stream altitude winds. Of course, this is also restricted airspace, so the appropriate regulatory approvals must be obtained. In the design, four adjustable, inclined rotors generate power from the high-altitude wind and transmit the electricity at 15 kV down a tether. In periods of low or no wind, the craft consumes power through the tether and uses its rotors to generate lift in a state known as "hover." Sky WindPower Co. has completed a 240 kW prototype design, and has demonstrated generation and hover capability in near-ground tests. To achieve competitive economics, full scale designs of 3 to 30 MW are proposed.
The following two designs are known as "airborne wind turbines." They use mechanics similar to ground-mounted turbines to generate power, but they contend to do so at a higher capacity factor with less material usage and less expense.
Launched out of an MIT Energy Ventures class in 2009, this concept makes intuitive sense. Take a horizontal-axis turbine rotor and frame it with a helium donut to lift it into the troposphere. The company behind it is Altaeros Energies, which was a finalist in the 2010 MIT Clean Energy Prize and won the 2011 ConocoPhillips Energy Prize. The team is still in the prototyping stages, experimenting with a 100 kW concept. Likely challenges include controlling the turbine system at high-altitudes and making it durable enough to withstand rapidly changing wind patterns. Regarding applications, because of its light weight and portability, the design may be particularly well suited to remote, off-grid locations that have a high relative cost of power.
In this emerging space, where there are as many hacks as legitimate concepts, a firm known as Makani Power has attracted by far the most investment for its development of the fixed wing airborne wind turbine. It decided against the kite-based approach, and instead went with a tethered rigid wing that travels in circles at altitudes of several hundred meters, mimicking the motion of a horizontal-axis turbine. Several small propellers are mounted on this wing, which generate power when the wind meets a minimum threshold, and consume power to maintain elevation in temporary lulls. The design is based on the principle that the tips of a turbine rotor are most effective; the wing itself acts as a rotor tip. Makani plans to develop a 600 kW utility scale turbine for onshore applications, and a 5 MW system for full scale offshore applications. The company has already demonstrated fully autonomous liftoff, power generation, hover, and descent with a 30 kW prototype.
Ultimately, these concepts will only be adopted at scale if they can provide power to utilities at rates comparable to fossil fuels. Subsidies such as investment and production tax credits aid greatly, and have recently brought down traditional onshore wind power costs to rates competitive with coal and natural gas in some locales. However, to displace conventional turbines in most locations, levelized costs for airborne wind must be even lower.
Whereas claims of low costs on the order of a few cents per kilowatt-hour are ubiquitous, serious cost analysis remains lacking in the public literature. Compared to conventional turbines, the upfront capital cost for an airborne turbine will likely be much lower due to lower material use, but the ongoing costs are likely to be higher in an airborne system with so many more degrees of freedom than a grounded structure. Still, airborne wind commercialization promises to reduce the overall cost, both from better wind availability and lower material use.
There are also several issues independent of the economics. Avian impacts, while largely overstated, are an oft-voiced concern, even though these turbines would operate above most migratory paths. Furthermore, the image of a massive airborne turbine catastrophically failing in a storm and raining down its lethal guts causes community discomfort. In reality, these devices would be built with several redundancies, high safety factors, and predictive sensors, but the concerns remain. Noise-pollution is also an issue, especially since rotors on high-altitude turbines will be spinning much faster than their counterparts closer to the surface.
Because offshore wind installations are currently significantly more expensive than those onshore, they present an especially compelling opportunity for the introduction of airborne turbines that require less transportation costs and support infrastructure. More niche, localized implementations such as in off-grid or remote applications may be stepping stones to providing utility scale power. Competing with diesel generators is certainly easier than competing with grid prices.
© Bhaskar Garg. 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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