THE FUTURE NOW
LOOKING TO THE SKIES What is airborne wind power and can it revolutionize the way we look at renewable energy?
AIRBORNE WIND TECHNOLOGY FEATURE POWER
looking to the sky for solutions
It’s a bird! It’s a plane! It’s — wait … what is that up there? Is that a shark balloon? Without much context, Altaeros Energy’s newly developed Buoyant Airborne Turbine may draw more than a few questioning glances. However, the company behind the colossal, shark-shaped floating turbine, a Massachusetts Institute of Technology startup, is picking up speed (and not just from the wind). Over the past couple decades, the research and design of alternative energies has seen a tremendous upswing, much of the research being funneled into wind energy. In an effort to better harness the extensive power our spacious skies, companies such as Altaeros Energy, Makani, and KiteGen are pushing the envelope and looking to the skies to exploit more constant winds of higher speed at a fraction of the cost. No doubt their innovations carry the potential to have a serious impact on the world’s energy, but can the new technology overhaul the wind energy field as its inventors hope it will? Or will airborne wind turbines, like many renewable energies, play only a peripheral role?
Article by John Gaumnitz
AIRBORNE WIND POWER
If you have ever driven through the Great Plains or Central Lowlands of the American Midwest, then you have encountered the long stretches of wind farms that have sprung up in the past 20 years (Figure 1). Pushed by international environmental organizations and governments, there has been a large increase in the amount of wind power produced in the United States in an effort to lower the country’s carbon emissions . Of all the renewable resources, wind, from the Earth’s surface up through the atmosphere kilometers above the surface, holds the most potential energy reserves, with an estimated energy reserve of 400 to 1,800 Terawatts (TW) of power . The current rate of global power consumption is estimated to be, roughly, 18 TW. Even using a more conservative estimate of its potential power, wind could be the most powerful, next-to-zero-emission option the world possesses. The question comes down to whether or not the immense amount of power can be used effectively. Currently wind power lags far behind fossil fuel power in cost, efficiency, accessibility in remote areas, and, of course, production in almost all areas of
the world . Even with the consistent growth in wind usage since the early 2000s, the entire production of wind power is still only 370 Gigawatts (GW) globally, according to the Global Wind Energy Council of Brussels, Belgium . To illustrate how powerful wind is versus how little of it is being used, let’s equate it to a more tangible unit. If 370 GW, the total wind power production, were 1 foot long, then 400 TW (the lower-end estimate of available wind power) would be more than the length of four football fields (that is, 1,200 feet). The difference between use and availability is enormous, which is why researchers continue to work toward harnessing it. Improvements in the current wind power turbine system are also needed to make wind a cost-effective replacement for fossil fuels. One of the most promising improvements is the development of airborne wind turbine technology. Research and development in this new field is expanding greatly, but will it truly be a viable option to meet the world’s energy needs? In this article, we will review the current state of wind power, including the downfalls of current wind power systems and how airborne
Figure 2: Map showing the location of wind farms across the continental United States.
wind turbines may address the problems of the conventional wind turbine. We will also briefly discuss potential designs of airborne wind systems and analyze whether this new field could truly propel wind power to the forefront of worldwide energy production. WHAT IS WIND POWER?
When you think of the wind, it’s easy to picture a breeze rustling through the leaves on the trees on a brisk day. In reality, however, wind has an awesome amount of energy that can be harnessed so that instead of moving just a tree branch, it can move the electronics we use every day. The kinetic energy of wind’s movement originates from a variety of atmospheric phenomena, largely from pressure and temperature differences in different geographic regions, which are caused by the Sun’s uneven radiation of the Earth’s atmosphere. Put very simply, with wind power, the kinetic energy of moving air is converted into rotational mechanical energy (the wind rotates the blades of a turbine), which in turn produces electricity (the turbine turns a shaft, rotating an electric generator) (Figure 2). The power that wind produces through a turbine is dependent on the area the turbine’s blades cover as they sweep through the air and, more importantly, on the wind’s velocity as it flows over the blades. To increase the power produced by a wind turbine, either the length of the turbine blades or the wind velocity must increase. Interestingly, wind power generation is cubically proportional to wind velocity, while only linearly proportional to area (see Figure 3 for graphs depicting this). Therefore, doubling the size of a turbine’s sweeping area would only double the power generated, but doubling the wind velocity flowing into the turbine will increase the power by eight times!
important to power production. . Conventional ground-level wind power is limited to the surface wind velocity. Airborne wind power, on the other hand, can operate at a variety of heights above Earth’s surface, and, thus, has the potential to harvest the higher-altitude winds. Although varying greatly globally, the mean wind power density at 1 km above the Earth’s surface is four times greater than a conventional wind turbine height (approximately 100 meters). And even more impressive, at an altitude of 10 km, the power density is more than 40 times greater . Figure 2: Depiction of the conventional wind turbine mechanism.
Therefore, a relatively small rise in wind velocity would have a much larger impact on the power generated than would changing the size of the turbine, which also would clearly encounter structural and material limitations . This means that if the wind is raised by even onefourth of its current speed, the turbine’s area size can be halved and still produce the same amount of power. Wind velocity increases logarithmically
with the height above the Earth’s surface (altitude), reaching a maximum velocity at 7 to 12 km above the surface . The term power density is commonly used to compare geographical and altitudinal wind velocity and air density. Power density is an expression of the potential power per turbine area that the wind possesses in a certain location. This measure is useful because it does not depend on the turbine or power generator, but rather simply on the atmospheric conditions
WHAT IS AIRBORNE WIND POWER?
Airborne wind power, or high-altitude wind power, is an expansion of conventional wind power. Aimed at tapping into the higher-altitude wind velocities, airborne wind power, in theory, uses the same power-generating principles as conventional wind turbine but employs them at higher altitudes in order to harness the faster, more consistent winds. There are numerous designs to achieve this goal, yet all share a few features.
Figure 3: Left: Wind velocity as altitude increases. Right: Power density as altitude increases. Model uses reference data from Central Iowa and assumes open farm land with few windbreaks. The governing equations for these plots:
AIRBORNE WIND POWER
Every airborne wind turbine has two parts: a grounded portion and an airborne, flying portion. The two portions are connected via a tethering cable, which holds the airborne portion in place and prevents it from flying off. The tether also allows the airborne structureâ€™s altitude to be adjusted and periodically brought back to the ground. In addition, depending on the design of the system, the tethering cable can also serve as a conductive wire that connects the turbineâ€™s generator to the power grid. WHY ARE SCIENTISTS WORKING ON AIRBORNE WIND POWER?
Airborne wind power generating systems
hold two main advantages over the conventional wind systems. The first advantage, as mentioned earlier, is that airborne wind power density steadily increases with altitude. This is important because not only does it mean that more power can be generated than would be generated with a conventional ground-level turbine, but that an increase in power will not require an increase in size of the airborne structure. As Figure 4 shows, some airborne turbine designs can use a much smaller structure while still providing energy to the same area because the tether is more mobile than the traditional turbine post and can be adjusted to take advantage of higher-altitude winds. Although not all airborne designs employ
Figure 4: Comparison of the traditional fixed turbine rotational area (left) versus a mobile, tethered airborne wing design.
Figure 6: Schematic view of the two airborne wind power systems. (a) The Ground-Gen system shows the airborne portion transferring mechanical power to a fixed generator on the ground. (b) The Fly-Gen system transfers mechanical power to electric power with an airborne generator and the power is transferred to the ground through the tether.
this concept, many do in order to utilize higher wind speeds without the necessity for a larger flying structure . Often geographic locations that are not suitable for conventional wind farming on the surface level have ample wind speeds at higher altitudes that merit harvesting . Airborne wind power could open the door to making wind power reach areas where it was thought to be previously impractical. The second advantage is that airborne wind power is predicted to slash electric energy costs per kilowatt-hour (a unit of measurement commonly used by energy providers). In large part, both the initial capital and operating costs (the price drivers for energy-supplying companies) are what currently holds back conventional wind power. Massive structural posts and foundations are necessary to support the mammoth blades, and they come at a steep cost. The most common conventional turbine produced, a 1.5 MW model from GE, has three blades, each weighing 36 tons, and a 71-ton tower to support the structure (for a total of 164 tons). In addition, the installation of such a massive structure is a time-intensive, dangerous undertaking. Each blade and tower section must be shipped separately, simply because of their size and mass. Then a crane has to be employed to assemble the turbine on site. The laborand material-intensive projects average $2.7 million for the installation of a single GE turbine . In addition to the front-loaded capital costs, conventional wind turbines take up a relatively large amount of land, resulting in a high land-leasing cost. Airborne wind power designs are constructed from fewer materials, and much cheaper materials to boot. Without the need for enormous structures, airborne systems are projected to cut installation and shipping costs. Plus, an airborne wind power
Figure 5: Left: The specially designed truck beds necessary for turbine transport highlight the difficulty of moving conventional wind turbines to their sites. Right: An oversized anchored crane assembles the pieces of a conventional wind turbine.
system needs only a small ground level connection, so the land-leasing costs will decrease as well. With a variety of different designs being researched, it is difficult to accurately estimate how much cheaper the airborne systems could be; however, with some models, the costs are estimated to be decreased by more than 40% . (These estimates are based on idealistic cost modeling without much of a market for comparison.) Currently, the majority of conventional wind power production has been heavily subsidized through government funding to try to compete with fossil fuels. Even with the costs we have discussed, wind power is still one of the cheapest renewable energy options. And yet it still lags behind. With the drop in costs and added energy output predicted with airborne wind power generation, it has the potential to be the first green technology to truly compete with fossil fuels . WHAT DOES AIRBORNE WIND POWER CURRENTLY LOOK LIKE?
With the possible benefits of airborne wind power in mind, the real question
becomes can the theory and the potential actually come to fruition? And, how are these concepts actually being used in today’s energy research? The field of airborne wind power can be divided into two categories of system, dubbed Ground-Gen and Fly-Gen
“IT HAS THE POTENTIAL TO BE THE FIRST GREEN TECHNOLOGY TO TRULY COMPETE WITH FOSSIL FUELS” (Figure 6). In Ground-Gen system, the electrical power of the system is produced by a generator on the ground; the generator is run by the mechanical power generated by the wind moving the airborne aircraft. The aircraft’s mechanical motion is transferred by a tether connecting it to the ground. An easy way to think of this system is simply that it is a conventional
turbine placed high up in the atmosphere, spinning horizontally instead of vertically and with a single aircraft replacing the multiple blades. Although these GroundGen systems can stand alone, it is likely that multiple aircraft and tethers could be attached to a single generator to maximize output . The aircraft portion of the system can be a variety of structures. Some research is being conducted on gliders with a profile typical of an airplane, as shown in Figure 6. However, one of the most promising and heavily financed aircraft styles is a kite. The company KiteGen has been on the forefront of this form of kite-structure for airborne wind power systems. The kites are almost the same as the kites used in recreational kite surfing and parasailing. Their design uses some of the cheapest materials, without sacrificing productivity. The kite’s lightweight structure can dynamically change with the wind direction and altitude. To counteract the kite moving sporadically, two tethers are placed on opposite ends of the kite and they work to keep the kite open continuously. Fly-Gen, in contrast to Ground-Gen, moves the generator into the air as well.
AIRBORNE WIND POWER
Figure 7: Left: KiteGen’s patented design comparing the kite path with that of the blades of a conventional turbine. Right: A prototype for the production station at the ground-level of a kite generation system.
Figure 8: A cross-sectional diagram of the Fly-Gen’s tethering scheme.
With this system design, it is more like abandoning a conventional turbine’s post and moving the blades and generator into the sky (and that’s not too far from the research that multiple companies are currently investing in). The tether holding the airborne aircraft to the ground plays an additional role in the design of this system. The tether contains conductive insulated cables that run from the generator in the sky to the ground, connecting the system to the power grid (Figure 7). The tether also contains a Kevlar backbone for additional strength.
These rotors act like propellers on a helicopter to raise the aircraft into the sky, using the generators aboard in reverse as engines. Once at the proper altitude, the rotors stop propelling and allow the aircraft to flow with the wind like a kite, relying on air lift to keep it afloat. Once the aircraft is airborne, air flow forces the rotors, causing them to spin in the opposite direction, and generates electricity like a conventional turbine’s spinning blades. The electricity can then run down the tether and into the power grid for consumption. The rotors’ controls can be operated from the ground either manually or through an automated system that can adjust the craft’s flightpath according to wind velocities. Additionally, the rotors can take over as propellers once again, while the craft is gliding, and guide the aircraft safely back down to the ground after use .
Fly-Gen, unlike Ground-Gen, doesn’t use the wind to move an aircraft, but rather it depends on the wind moving propellers mounted on the aircraft, as the aircraft is suspended in the air. So, rather than acting like a blade of the conventional turbine rotating in the air (like in Ground-Gen systems), Fly-Gen incorporates conventional turbine blades
into its design. Another advantage of the Fly-Gen system is that the ground structures necessary are minimal; they only need to weigh enough to anchor the airborne portion so that it does not fly away. Thus, these Fly-Gen systems have the potential to be easily portable, based out of trucks. This is a key driving force behind Fly-Gen’s development. (This will be discussed more in a bit.) One similarity to Ground-Gen is the potential for aircraft variety, giving both systems multiple options for their design. In the past year, two companies’ designs have received media coverage and caught the public’s attention. One company catching the eyes of investors is Makani (not surprising, since its parent company is tech giant Google). Their aircraft design is similar to a traditional airplane glider, but it has an additional layer of rotors on the top and bottom of its wing (Figure 9).
The other company, and the first to be on
Figure 9: Blueprint front-view of the proposed Makani aircraft system, showing the rotors above and below the aircraft’s wing.
the cusp of a marketable final product, is Altaeros Energy. Altaeros’ aircraft design — known as an aerostat — is one of the most unusual in the airborne wind power field. Unlike the other designs that have been proposed and discussed, the Buoyant Airborne Turbine (BAT) design does not use lift from from wings to stay in the air; rather, it uses buoyant force, like a helium balloon. The aerostat design, which is essentially a large donut-shaped blimp, is filled with lighter-than-air gas so that it floats. The inner hollow region of the aerostat houses a conventional three-bladed turbine and generator, which transfers the wind power into electrical power that is then transferred to the ground via a tether connected to the donut portion . Altaeros’ system is promising not only because of its power-generating capabilities. Because the aerostat is stationary, it could double as a telecommunication hub. With electricity on hand, as a generator in and of itself, the aerostat could provide a cellular network connection and even WiFi internet connection to the areas below it . On the climate research front, the hollow region of the aerostat could be used for environmental monitoring. Clearly, the static nature of the lighterthan-air aerostat has its benefits, but the gases required to keep it afloat remain a bit of a problem. Helium, the gas of choice for blimps, is becoming a scarce commodity and is not likely to be ideal for mass production. Additionally, helium will slowly leak through the balloon’s fabric (an area being heavily researched) and needs fairly consistent replacement. One potential solution is to use hydrogen gas, which can be replenished using an electrolytic process aboard the balloon. Hydrogen is, of course, infamous for its volatility — just consider the Hindenburg catastrophe — and needs to be carefully managed . Yet, despite its shortcom-
Figure 10: One of Makani’s current prototypes for their rotor-plane aircraft system. Makani has multiple models, all still very much in the research and design phase.
ings, the potential of the aerostat design is clear to see. In the grand scheme of things, the issue of acquiring gas to fill the balloon is a minor challenge when compared with the structural and mechanical challenges other aircraft development still face. WILL AIRBORNE WIND POWER REVOLUTIONIZE RENEWABLE ENERGY?
Well, investors certainly seem to think airborne wind power will have some impact. The proof-of-concept developments by KiteGen early on and Makani and Altaeros in the past few years have led to many investments in companies exploring the field . However, that doesn’t mean the technology will soon take over the energy market. Ken Caldeira, a leading climate researcher from the Carnegie Institute and Stanford University is quoted as saying, “I would be reluctant to remocvrtgage my house and invest the money in these companies, because I think the probability of them being able to compete in the marketplace at scale in, say, the next decade is pretty small” . Note that he mentions at scale because the scale of this technology is likely to be the limiting factor because of some simple, yet inherent limitations of airborne
wind power. First, aviation around airborne wind turbines would have to be shut down during their operation. All forms of airborne wind power systems use ground tethers and substantial structures in the air. There could be a catastrophic accident if an airplane or helicopter were to have the misfortune of running into either portion. It’s interesting to note that one of the structures most similar to airborne systems, and more specifically to the aerostat design, are barrage balloons. These airborne anti-aircraft balloons have been used during wars to prevent aircraft from flying above certain areas. They were made famous when deployed over London during World War II to prevent low-flying Nazi aircraft from attacking important areas during the Battle of Britain (Figure 13). A collision with any aloft portion of an airborne wind power system, whether it be an aerostat like Altaeros’ BAT or a moving structure like KiteGen or Makani’s designs, or a collision with a tethering cable would more than likely destroy an aircraft. Clearly, any area where these wind power structures fly would need to become a nofly zone to avoid accidents. Unfortunately, that excludes large areas of industrialized
AIRBORNE WIND POWER
Figure 11: The most recent prototype of the BAT system that Altaeros Energy has deployed for extensive testing in remote Alaska.
countries such as the United States. Even areas of the United States referred to as â€œfly-over countryâ€? could not heavily host airborne wind turbines, like they do for conventional wind turbines, because air traffic would have to be rerouted around them. This is a major limitation to airborne wind power because it is unlikely to be adopted on a large scale in areas with high energy needs, simply because of the air traffic already present .
Weather limitations are another key issue with airborne wind energy. Just as you would avoid flying an airplane, helicopter, or blimp during inclement weather, airborne wind power systems will not be able to function in severe weather conditions either. During lightning, hail storms, or tornado-strength winds, airborne wind systems must be landed. They are more fragile than the hulking structures of conventional wind turbines, and there is
also the potential that they might break free from their tethering cable, crash landing or wreaking havoc on local air traffic. Additionally, winter conditions are a huge concern. Icing on the wings could affect the performance of some airborne wind power aircraft types. Just as an airplane may be grounded until its wings are de-iced, the airborne turbines would need to be, adding another major cost to some designs of wind power and possibly
limiting their use to certain seasons . These disadvantages raise the question: On what scale could these systems work? Clearly because of the no-fly issue alone, most U.S. markets for large-scale airborne wind energy producing farms are off the table. But what about areas that are more remote or those that need energy production for a short period of time? This is the market that appears to be airborne wind power’s niche. Currently, remote villages throughout the world rely almost solely on diesel generators to power homes and equipment. This usually makes them dependent on the importation of foreign oil, driving up their energy costs, to live even at a subsistence level. Also, areas that have never had electricity available could now receive a single generator that wouldn’t incur any additional costs after installation. Airborne wind energy would provide these remote places with a renewable energy option and would not oblige them to rely on an outside market.
Figure 12: Altaeros BAT as seen from below.
This energy option could help the climate as well, allowing less developed nations to use less fossil fuel as whole. Because airborne wind energy systems are so much more portable than other renewable options, they also give researchers traveling to remote locations the ability to use renewable energy instead of diesel generators. In addition, since it doesn’t need
“AIRBORNE ENERGY WOULD PROVIDE THESE REMOTE PLACES WITH A RENEWABLE ENERGY OPTION” fuel, an airborne wind energy system can extend the length of expeditions. One more potential niche for airborne wind power systems is emergency relief. The BAT system from Altaeros can be brought
in and started up in a “under a day, without any heavy equipment,” according to company co-founder Adam Rein. Following a disaster, nothing is more important than a quick response, and airborne wind power could quickly be brought in to areas without power, for short periods of time. Again, the airborne wind power systems can replace diesel generators that are usually used for months following disasters until the infrastructure is back up and running . So, the answer to the question is that airborne wind power is not likely to take over power production on a market scale. It is, however, very likely to revolutionize the way we deliver electricity to remote and underserved regions of the world. You probably won’t be seeing a shark-like balloon flying above a metropolitan area any time soon, but people in the most remote regions of the world may have access to cheaper, cleaner-produced electricity for the first time.
Figure 13: Barrage balloon flying over the London Bridge during World War II.
REFERENCES AIRBORNE WIND POWER
1. Fagiano, L., and M. Milanese. “Airborne Wind Energy: An Overview.” 2012 American Control Conference (2012): 3132-143. IEEE Xplore. Web. 13 Apr. 2016. 2. Miller, L. M., F. Gans, and A. Kleidon. “Jet Stream Wind Power as a Renewable Energy Resource: newline Little Power, Big Impacts.” Earth System Dynamics 2.2 (2011): 201-12. ScienceDirect. Web. 13 Apr. 2016. 3. Adhikari, Jeevan, and S. K. Panda. “Overview of High Altitude Wind Energy Harvesting System.” 2013 5th International Conference on Power Electronics Systems and Applications (PESA) (2013): 1-8. IEEE Xplore. Web. 12 Apr. 2016. 4. Cherubini, Antonello, Rocco Vertechy, and Marco Fontana. “Airborne Wind Energy Systems: A Review of the Technologies.” Renewable and Sustainable Energy Reviews 51 (2015): 1461-476. ScienceDirect. Web. 12 Apr. 2016. 5. Koch, Patrick, Heini Wernli, and Huw C. Davies. “An Event-based Jet-stream Climatology and Typology.” International Journal of Climatology 26.3 (2006): 283-301. Wiley Online Library. Web. 12 Apr. 2016. 6. Archer, Cristina L., and Ken Caldeira. “Global Assessment of High-Altitude Wind Power.” Energies 2.2 (2009): 307-19. ScienceDirect. Web. 12 Apr. 2016. 7. Archer, Cristina L. “An Introduction to Meteorology for Airborne Wind Energy.” Airborne Wind Energy. Ed. Uwe Ahrens, Moritz Diehl, and Roland Schmehl. Berlin: Springer, 2013. 81-94. Print. 8. Diehl, Moritz. “Airborne Wind Energy: Basic Concepts and Physical Foundations.” Airborne Wind Energy. Ed. Uwe Ahrens, Moritz Diehl, and Roland Schmehl. Berlin: Springer, 2013. 3-22. Print. 9.
“Why Airborne Wind Energy.” Antonello Cherubini. N.p., n.d. Web. 12 Apr. 2016.
10. “How Much Does a Wind Turbine Cost? - Renewables First.” Renewables First. Windpower Learning Centre, n.d. Web. 16 Apr. 2016. 11. Heilmann, Jannis, and Corey Houle. “Economics of Pumping Kite Generators.” Airborne Wind Energy. Ed. Uwe Ahrens, Moritz Diehl, and Roland Schmehl. Berlin: Springer, 2013. 271-84. Print. 12.
“Makani.” Makani. Google, n.d. Web. 16 Apr. 2016.
Altaeros Energies. Altaeros Energy Inc., n.d. Web. 16 Apr. 2016.
14. Vermillion, Chris, Ben Glass, and Adam Rein. “Lighter-Than-Air Wind Energy Systems.” Airborne Wind Energy. Ed. Uwe Ahrens, Moritz Diehl, and Roland Schmehl. Berlin: Springer, 2013. 501-14. Print. 15. Levitan, David. “High-Altitude Wind Energy: Huge Potential - And Hurdles.”Yale Environment 360. Yale University, 24 Dec. 2012. Web. 16 Apr. 2016. 16. Barnard, Mike. “Airborne Wind Energy: It’s All Platypuses Instead Of Cheetahs.” CleanTechnica. Sustainable Enterprises Media, Inc., 03 Mar. 2014. Web. 16 Apr. 2016.
Images: Cover Spread: http://www.ecnmag.com/sites/ecnmag.com/files/Altaeros_Prototype_Ground_2.jpg Figure 1: http://eerscmap.usgs.gov/windfarm/ Figure 2: https://www.teachengineering.org/collection/nyu_/activities/nyu_windturbine/nyu_windturbine_activity1_figure1.jpg Figure 3: Models and Data from Reference 7 Figure 4: From Reference 9 Figure 5: https://c2.staticflickr.com/3/2516/5797609840_c31f2d200c_z.jpg https://c2.staticflickr.com/3/2516/5797609840_c31f2d200c_z.jpg Figure 6: From Reference 4 Figure 7: http://www.kitegen.com/en/ Figure 8: From Reference 3 Figure 9: http://www.google.com/makani/images/figures/blueprint.png Figure 10: http://www.alternative-energy-news.info/wp-content/uploads/2015/03/makani-energy-kite.jpg Figure 11: http://cache.newsana.com/orig/53352905b5f2d.jpg Figure 12: http://www.gadgetreview.com/wp-content/uploads/2014/04/altaeros-bat.jpg Figure 13: http://www.skylighters.org/barrageballoons/bovertower.jpg Magazine Design by Emily E. Duncan