Cryptogon points to a new video on Google's Makani wind power kites (covered in one of the more popular posts here many years ago Alternative wind power experiments) - Makani: Google’s Energy Harvesting Kites.

Makani is working to accelerate the shift to clean, renewable energy by developing energy kites, a new type of wind turbine that uses lightweight electronics, advanced materials, and smart software to generate more energy with less materials—all at lower cost.

Kevin seems to have a love-hate relationship with Google, also pointing to this piece of investigative journalism on Medium - How the CIA made Google.

INSURGE INTELLIGENCE, a new crowd-funded investigative journalism project, breaks the exclusive story of how the United States intelligence community funded, nurtured and incubated Google as part of a drive to dominate the world through control of information. Seed-funded by the NSA and CIA, Google was merely the first among a plethora of private sector start-ups co-opted by US intelligence to retain ‘information superiority.’

The origins of this ingenious strategy trace back to a secret Pentagon-sponsored group, that for the last two decades has functioned as a bridge between the US government and elites across the business, industry, finance, corporate, and media sectors. The group has allowed some of the most powerful special interests in corporate America to systematically circumvent democratic accountability and the rule of law to influence government policies, as well as public opinion in the US and around the world. The results have been catastrophic: NSA mass surveillance, a permanent state of global war, and a new initiative to transform the US military into Skynet.

The Guardian has an article on airborne wind turbines - Flying a kite for aerial wind power.

According to Schmehl, airborne energy production can be cleaner, cheaper and more effective than conventional wind power generation. "It's now clear that the world needs more energy from renewables. And we need the progress to be faster. Wind is an important resource that so far has been limited, because conventional wind turbines just scratch off the bottom layer of what is actually available in the atmosphere."

While a normal wind turbine is up to 200 metres tall, a kite can catch much higher currents. "We operate between 100 and 300 metres but kites can in fact fly much higher. So far, the altitude record is 9,740 metres," says Schmehl.

Kite wind generation overcomes the problem of intermittent power, typical of conventional wind technologies, for one simple reason: the higher you go, the more constantly the wind blows. Airborne wind turbines provide a more stable energy flow, and they are much cheaper as they need less material than a wind turbine. Instead of a steel tower, you have a system that looks and works like a yo-yo.

"You have a cable going into the sky with a flying harvesting device. Our group has focused on kite power, and specifically the pumping kite power system. We use the traction power of a kite sail to pull a cable from a drum that drives a generator on the ground.

Once the cable has completely unwound it needs to be reeled in again, which requires a certain amount of energy. "You have to design the pumping cycle so as to have a traction and a retraction phase," says Schmehl. He and his team came up with a solution to minimise energy losses. "We rotate the kite into the wind as we pull it back, so essentially the airstream does part of the work for us. This way, we need less energy to reel in the cable."

PCMag reports that Google has bought Saul Griffith's alternative wind power company Makani - Google Acquires Wind Power Firm Makani Power

Google is known for putting its resources into some novel technologies like self-driving cars and wearable computing devices. Now, the Web giant is moving into yet another field—wind power. After previously investing in the company, Google has agreed to acquire Alameda, Calif.-based green energy startup Makani Power. ...

Makani, which takes its name from the Hawaiian word for breeze, develops airborne wind turbines that are mounted on self-piloting flying wings tethered to the ground like a kite. Google will bring the Makani team into its secret "moonshot" research lab Google X, which produced Google Glass. "This formalizes a long and productive relationship between our two companies, and will provide Makani with the resources to accelerate our work to make wind energy cost competitive with fossil fuels.

In a statement to PCMag on Thursday, Google confirmed the acquisition and said it's eager to bring Makani on board. "Creating clean energy is one of the most pressing issues facing the world, and Google for years has been interested in helping to solve this problem," Google X Director Astro Teller, said in the statement. "Makani Power's technology has opened the door to a radical new approach to wind energy. They've turned a technology that today involves hundreds of tons of steel and precious open space into a problem that can be solved with really intelligent software. We're looking forward to bringing them into Google[x]."

Makani said the timing of the acquisition "couldn't be better" since its so-called Wing 7 kite-power prototype just completed its first ever fully autonomous flight. The startup has said its wing-shaped high-altitude kite design can produce 10 times more energy than conventional turbines.

Ugo Bardi has an excellent post up at TOD on high altitude wind power - High altitude wind power: an era of abundance?.

Why should there be an energy problem? After all, there is plenty of energy around us. The sun beams on the earth's surface a daily amount of energy that corresponds to almost ten thousands times the primary energy we generate - mainly - from fossil fuels. And that doesn't include geothermal energy nor the perspectives of nuclear energy, especially in terms of fusion power. Just tap a small fraction of this energy bonanza that surrounds us and we can have more than we need.
But, of course, things are not so simple. We still rely heavily on fossil fuels for our needs and switching to alternative sources is proving to be a very slow and difficult process. Production from traditional nuclear plants is going down (WNA 2009) and fusion power remains far away in the future. Traditional renewable sources, such as wood burning and hydroelectric have very limited possibilities of expansion, while the "new" renewables (mainly photovoltaic and wind power) still produce only a minuscule fraction of the worlds' total primary energy. It was only last year (2008) that for the first time the total power of new renewable plants installed outstripped that of new traditional plants in the US and in Europe (REN21 2009). Renewables are growing fast, but can they grow fast enough to compensate for the depletion of fossil fuels?

We have a problem of cost. That can be intended as monetary costs, but also in terms of energy return of energy invested (EROEI). As shown in Charles Hall's "balloon graph" (2009) the EROEI of renewables can be considered as reasonably good in most cases (with the exception of biofuels). It is around 10 for photovoltaics and around 20 for wind. Similar returns are reported for current nuclear technology. These are good returns on the investment, but not as good as it was for fossil fuels in the golden days. Decades ago, the EROEI of petroleum was of the order of 100 and perhaps even better (Hall 2009). It was this high EROEI that led fossil fuels to acquire the dominance that they have today. Without that kind of EROEI; other energy sources haven't had a possibility to compete. Today, we still need fossil energy to build non-fossil energy plants. But, with fossil fuels starting their decline, it will be more and more difficult to sustain the growth of alternative energies at a rate fast enough to provide a smooth substitution of conventional sources. We can think of an industrialized world that doesn't need fossil fuels, but we don't seem to be able to get there fast enough.

So, we are facing Tantalus' curse: we are surrounded by abundant energy but we can't get it. That is, unless we can develop a technology with a much better EROEI than what we have now. With a very fast energy return on investment, we could free the world's energy system from its dependence on fossil fuels. That is, unfortunately, easier said than done. The internet is full of claims of supposed breakthroughs in energy technologies that promise a lot but turn out to be just dreams; or even outright scams. But there may exist an energy technology that can not only promise, but deliver a high EROEI and that is also based on sound physical principles: high altitude wind power.

The basic idea of high altitude wind power is that wind is more intense as you move up in the atmosphere. The average wind speed increases with height according to an exponent (called "Hellman exponent") which is about 1/7. But the energy contained in a mass of air in movement increases with the cube of speed. From a simple calculation, we see that if we could raise a wind turbine to a height of 800 m, we could increase the power obtained of a factor of 8 in comparison to the same turbine near the ground. Even larger increases are possible at higher altitudes, where winds are also much more constant; easing the intermittency problem of conventional wind turbines. But of course, it is impossible to reach such heights with the current wind technology, limited to about 100 m because of the cost and weight of the tower.

This concept has been clear for a long time and has led to several proposals to tap the wind at higher heights. There are two possible ways for doing that: balloons and wings. You can find a recent summary of the progress in this area in the work by Big Gav (2009) published on TOD . As you can see, there are many ideas in this field, many of which exist only as sketches on paper. In many cases, the energy yield of the proposed systems is only a guess while, for those systems based on aerostats, the need of a non renewable resource (helium) is a considerable limit.

However, a few systems have been studied in depth and some tested in practical experiments. Systems based on rotors are possible and systems based on kites, in particular, do show a lot of promise. Saul Griffith of Makani Power has shown some images of a test done with a three rope kite. Wubbo Ockels, (Delft University of technology) has been also experimenting with a kite , this one using a single rope. In this field, the most advanced system seems to be the "kitegen"; a kite system created by Massimo Ippolito of Sequoia Automation , a company based in Italy. Tests on a prototype system have been completed and a first energy producing plant is being built in Northern Italy.

The Kitegen is a simple aerodynamic system: it uses state of the art kites which create lift dynamically by flying at 70-80 m/sec; this is the speed reached by the tips of the blades of a conventional wind turbine. In the simplest configuration (called "stem"), the system uses a single kite linked to a power generator located on the ground. The kite moves like a yo-yo: when it goes up, it generates energy that is transformed into electric power by the generator. When it reaches its maximum height, it is placed in an aerodynamically non-lifting configuration, so that it can be pulled down at a very small energy cost. Two coupled stems would work like a two-cylinder engine, although the "power" phase would last 90% of the time while the "pull back" phase would be much faster. A single stem could have a maximum power of a few MW. Larger plants could be operated in the "carousel" configuration. In this case, the kites fly at a constant height and at much higher altitudes, pulling a generator that moves on a circular rail. For a large carousel system, the maximum power obtained can be calculated as of the order of 1 GW or even higher.

Since the kitegen has been studied in detail, we can use it to make an estimate of the EROEI involved in high altitude wind generation. Before getting to that, however, let's summarize the known data for the current wind technology. A recent LCA study for a conventional 3 MW wind turbine was reported by Nalukowe et al, (2006). They estimate the total energy input for building and maintaining the turbine as ca. 8000 MWh for 20 years of lifetime. Since the total weight of the above ground part of the turbine is about 400 tons, we can estimate an embodied energy requirement of about 20 kWh/kg. The turbine will produce about 160,000 MWh during its lifetime and hence the final EROEI is ca. 20.

Now, let's see the results of a similar approach for the kitegen. According to Massimo Ippolito (data published on www.kitegen.com), the energy required to make a 3 MW rated power kitegen stem is of 40kWh/kg. The calculation that leads to this value takes into account all the requirements in terms of the materials needed: steel for the structure, copper for power lines, neodimium and boron for the magnets, machining, transportation, building, etcetera. This value includes also the energy costs involved with having workers at the plant and for the periodic substitution of cables and kites over a 30 year lifespan.

We see that the kitegen requires more energy per kg than a conventional wind turbine; this is expected because it is a more sophisticated machine. But the stem is much lighter: we are talking of about 30 tons in total for a 3MW plant. So, we can estimate the total energy requirement as 30*40= 1200 MWh. Assuming 5000 hours per year of operation at maximum power, the plant could produce approximately 15,000 MWh per year, or 450,000 MWh in 30 years. The final result is an EROEI = 375 (!!). If we assume a 20 year lifespan, the estimate should be reduced, but it remains large. For larger kitegen plants of the carousel type it would be possible to reach higher heights, tap into stronger winds and increase even more the EROEI. This calculation is valid for the specific case of the kitegen system, but other proposed systems based on kites or rotors would probably be able to attain similar large EROEIs. ...

From these data, we could be tempted to see high altitude wind power as a nearly limitless energy technology. But that would be a mistake. Energy production is not static - it goes with the economy and if the economy is powered by a source of cheap and abundant energy it tends to grow exponentially. Exponential growth is treacherously misleading: we could find ourselves bumping into the ceiling of high altitude winds much sooner than we would expect.

But there is a much more serious problem in the fact that energy is not the only parameter that affects the economy. Abundance of something is not abundance of everything. Abundant electric power doesn't necessarily translate into abundant food, although electricity can surely be used in agriculture in place of fossil fuels. That our problem is not just energy is confirmed by the models developed for the "Limits to Growth" series (Meadows 2004). The models can be run for scenarios that assume abundant (or even infinite) energy available, but the result is that the economic system collapses because of the strain on the environment and on agriculture generated by a combination of overpopulation and pollution. To avoid collapse, we need to stabilize both the economy and the population at a stationary level. Even so, the gradual depletion of mineral ores will make us depending on more and more energy if we want to keep the flux of mineral commodities at the present level (Diederen 2008, Bardi, 2008). So, even with abundant energy, we'll still need to recycle materials and reuse what we manufacture.

So, even with abundant energy we still need to come to terms with the fact that the earth is a limited system. However, high altitude wind power offers us a hope of a future of relative abundance, even of prosperity, if we'll be able to keep the economy and the population stable and avoid overexploiting our agricultural and mineral resources.

TED has a talk from Saul Griffith on his company Makani and their plans to generate large scale wind power using airborne wind turbines - Saul Griffith: Inventing a super-kite to tap the energy of high-altitude wind.

Alternative wind power experiments are all the rage lately - Inhabitat points to a new one from a company called Selsam.

The higher up in the air you go, the faster wind travels - so naturally the further from the ground a wind-turbines gets, the more efficient it can be. Thats why the idea of a flying wind-turbine is a such a win-win (or win-wind) proposition. Combining wind power with floating blimps, Selsam has been hard at work expanding the horizons of alternative energy with a revolutionary new breed of SuperTurbines that promise to take wind power to new heights.

Wind power is currently the fastest growing renewable energy source (in terms of capacity - solar has a faster percentage growth rate), and looks like remaining so into the next decade. While most attention is focussed on the mainstream approach of generating power using large wind turbines - both onshore and, as Jerome recently looked at, offshore - there are a wide range of alternatives being considered for harvesting energy from the winds. In this post I'll look at 2 approaches that have received some attention in the press recently - attaching kite sails to ships and airborne wind turbines.

SkySails and KiteShips - Using Wind Power To Make Shipping More Fuel Efficient

Shipping is the most energy efficient way of transporting goods around, and it therefore likely to be increasingly important for trade as the availability of fossil fuels decreases and the cost of these increases. According to the Financial Times, it often now costs more to ship a container by road 100km from a port to its final destination than it does to move the container by sea from China to Europe.

One way of increasing the energy efficiency of shipping is to increase the size of the vessel, which big shipping operators like Maersk are already pursuing. Another way to reduce fuel consumption for shipping is being promoted by German company SkySails - attaching a kite the size of a football field to a vessel and using wind power to help save fuel costs.

The SkySails apparatus consists of a towing kite with rope, a launch and recovery system and an automatic control system. Autopilot software monitors data from the sail and makes adjustments to ensure the sail is set at its optimal position. The company also offers a weather-based routing system to help ships sail in optimal wind conditions. The kites usually fly around 1000 feet above sea level where winds can be up to 50% stronger than at deck level.

The first ship using the system, the Beluga Skysail, is expected to set off from Germany this month (The Guardian says it is bound for Venezuela while The Times says it will, fittingly, be hauling windmills from Esbjerg, Denmark to Houston, Texas). Once it is well clear of the land, it will launch a 160 sq metre kite, which wind tunnel tests and sea trials suggest will tug it along and save 10-15% of the oil it would normally burn. In time it will be fitted with a larger kite, possibly saving 30-35% on fuel. One estimate predicts this could save around US$10 million over the lifetime of a vessel.

The Guardian quotes Christine Bornkessel from the Beluga shipping line, which has 52 merchant vessels, as saying "This is a serious attempt to reduce bunker [fuel] costs and polluting emissions. The kite will be used whenever it is possible on the voyage, and we are convinced it will revolutionise cargo shipping. We would consider fitting them to all our ships".

The SkySails company has ambitions to roll out the kites quite rapidly - “About 1100 of the 1900 newly built vessels joining the world’s merchant fleet each year are destined to be outfitted with SkySails propulsion”.

If the Beluga performs well on wind power and if the high-flying kites dramatically cut its fuel consumption, then the age of sail will be back.

“It marks the beginning of a revolution in the way that ships are powered,” said Stephan Wrage, the inventor of the SkySails idea. “We calculate that the sails can reduce fuel consumption by between 30 and 50 per cent, depending on the wind conditions. “The system could be applied to about 60,000 vessels out of the 100,000 or so listed in the Lloyd’s register. Bulk carriers, tankers — they could all benefit from the flying sails.”

Orders are already coming in, ranging from trawlers to a super-yacht. If he realises his dream of re-equipping the world fleet, Mr Wrage calculates that his sails could save 142 million tonnes of CO2 a year, equivalent to about 15 per cent of Germany’s total emissions.

The 35-year-old engineer, who came up with the idea as a teenager when he found himself being dragged by a kite along a blustery North Sea beach, is not the only person brooding about how exactly to reduce the gases released by the world’s shipping fleets.

The problem is huge. Merchant ships, which carry 90 per cent of the world’s merchandise, produce more sulphur dioxide than all the cars and lorries on the planet. And, according to the International Council on Clean Transportation, they also generate about 27 per cent of the world’s nitrogen oxide emissions.

Wallenius Wilhelmsen Logistics, the Norwegian fleet management company, has designed a green flagship, the Orcelle, which will use rigid, rotating sails to capture the strongest winds. When the wind drops the sails can be used as solar panels. But the company admits that it could take 15 years before the ship is developed.

The most straightforward way of reducing sea-borne pollution would be to impose a speed limit on the oceans. Carbon emissions would drop by 23 per cent if ships cut their speed by 10 per cent — but shipping experts say that slower cargo movements would lead to more vessels being deployed, and pollution would return to its former levels. ...

The sails, made of an ultralight synthetic fibre, are shot up as much as 300m (984ft) into the sky. ... Mr Wrage calculates that most ship owners should be able to recoup their £290,000 investment in the kite sails within three years.

Drawbacks associated with this scheme include the additional costs associated with the kites - both with buying, installing and maintaining them, and with training and rostering crew members to tend them.

Skysail isn't the only company investigating the kite sail idea - US company KiteShip is building large kites, mostly for yachts, and plans to expand into the larger cargo and cruise ship markets.

Flying Wind Farms

Airborne wind turbines are an idea that I've always found myself somewhat bemused by - "surely they can't be serious ?" I think to myself, whenever I come across another tale of a company trying to fly kites with wind turbines attached to them.

These sorts of stories have continued to appear with monotonous regularity however, and not just out on the alternative energy fringes - in recent months I've seen articles in places like The New York Times, The Economist, PBS' I, Cringely, The Energy Blog, TreeHugger and WorldChanging - many prompted by Google.org's investment in airborne wind company Makani Power (part of their program to make "Renewable Energy Cheaper Than Coal").

The driver for these schemes is that the intermittency issue which reduces the effectiveness of wind turbines operating at ground level is asserted to be much less of a problem at 1000 feet, where the winds tend to blow steadily. Proponents of airborne wind power like Ken Caldeira at the Carnegie Institution's department of global ecology at Stanford University say that if we could tap into 1 percent of the energy in high-altitude winds it would be sufficient to provide all our power needs.

The company that has garnered the most attention in this field is Ottawa-based Magenn Power. Magenn's system is a lighter-than-air wind turbine capable of powering a rural village - the 30 metre wide, helium-filled "Air Rotor System" contains a turbine that spins around a horizontal axis and can produce 10 kilowatts of energy as it floats above the ground while attached to a copper tether. Larger models — ones that might power a skyscraper — are also reportedly in the works. The company claims the governments of India and Pakistan have expressed interest in the first version. Magenn is planning to launch a 1kw prototype (costing around C$1 million) into the air above Ottawa this (northern) spring.



The entrepreneur behind Makani Power is Saul Griffith, originally from my hometown of Sydney but now based in San Francisco. This company is the least transparent of the airborne wind power companies, with very little known about what they are up to.

Another Californian high altitude wind power company is Sky WindPower. This company seems to be the farthest along in its efforts to commercialise their technology and is again directed by an Australian - ex-mechanical engineering professor Bryan Roberts. Jim Fraser at The Energy Blog has a fairly detailed look at what they are proposing:

Sky WindPower is proposing [to] use clusters of Flying Electric Generators (FEGs), on the end of a current carrying tether, in the jet stream at 15,000 to 30,000 feet. The company has done wind tunnel tests and low altitude tests to prove their idea. They are hoping to build a 200 kw model, flying at 15,000 ft, somewhere in a remote area of the U.S. They envision a commercial FEG will have four or eight rotors each generating 2.5 MW. Clusters of FEG's could provide as much power as need for a given site.

The FEG would fly up into the sky, with its rotors powered by electricity off the grid, pulling up its tether. Once it at its desired altitude it would change the pitch on its rotors and start generating power from the wind. GPS technology would be used to assure that the rotorcraft stays within a few feet both horizontally and vertically of where it is programed to be and a computer would control the rotorcraft's attitude, i.e. pitch, roll and yaw.

The amount of power that you can produce in a wind turbine varies as the cube of the velocity and linearly as the density. So although the density decreases with an increase in altitude, the increase in velocity that you gain with higher altitudes more than makes up for the decrease in density. This further explains the advantage that FEG's have flying at high altitudes and allows the rotors to be smaller in diameter. The wind speed, in addition to being higher, is more uniform.

It is much more steady, blowing at high, useful velocities a much greater percent of the time than do winds at ground level. This gives FEGs the advantage of having a higher capacity factor. Capacity factor is the percentage of energy actually captured relative to what would be captured if the wind turbines were operating at full capacity all the time. Ground based sites that can produce a capacity factor of 35% are hard to find. Capacity factors in the jet stream range from about 70% in the southern parts of the U.S to over 90% in the north. At a capacity factor of 90%, FEGs could become the nation's cheapest source of electricity, with an estimated cost per kilowatt hour of less than 2 cents, about half the price of coal.

Also the wind tends to blow from one direction and when it changes direction it changes slowly. The air is free from turbulence caused by friction and ground protuberances, but is subject to high altitude turbulence. This is moderated by the ability of the FEG to sway on its tether rather than being attached to a ridged tower. The FEG can move up and down on its tether the same way an airplane does to avoid turbulence.

The use of tethers to position objects in the sky is not unknown. Balloons tethered at altitudes up to 15,000 feet exist now at fifteen sites along the southern borders of the United States carrying radar equipment to detect illegal flights from the south trying to smuggle drugs. These sites are shown on all aeronautical charts, as restricted spaces, and are well known to pilots. By reserving less than one four hundredth (0.4%) of U.S. air space, located at relatively remote locations, not on airway routes, all the nations electrical energy needs could be met. The strength to weight ratio of new tether materials has improved over time so much that tethers now available are no longer too heavy to be held up by flying energy generating devices at the needed high altitudes. Tether technology is not simple, but a number of vendors now compete in this field selling primarily to the military and NASA. The FEG would be able to moved up or down on its tether to seek the best wind conditions or to move out of excessively turbulent air space.

WorldChanging has some more details on SkyWindPower, noting some of the main drawbacks with these schemes.

Output would also be less dependent on location than it is on the ground, simply because terrain doesn't matter much when you're at 35,000ft; however, since the jetstream and other "geostrophic" winds don't blow much at latitudes near the equator, it would be useful primarily for middle- and higher-latitudes.

They can't promise uninterrupted power all the time, however. In an electrical storm, the power-carrying tether becomes the biggest lightning rod you've ever seen. (Move over, Ben Franklin!) Their website says this problem is "frequently brought to our attention, and must be addressed." Their plan is to take the flyers down to land before a storm gets bad, and wait for it to end.

The flying windmills would initially get in position under their own power, using their motors to drive the propeller blades and helicopter upwards until they reached altitude. Then the motors would turn off and become generators as wind pushes the propeller blades, and the whirligig would float instead of fall because when tethered, the lift generated by the wind would overcome the craft's weight as it also generates power.

The obvious question is safety. What happens if one of these things falls out of the sky? The proposed design has quadruple-redundancy in the propellers used to hold it up and generate power, and the units could be located away from population centers, so that seems reasonable. What about planes running into them, or more likely, their tethers? They would fly in restricted airspace. Sky Windpower points out that there are already many high-altitude tethered balloons in the US that have not had problems, and that enough installations to generate 100% of the US's power needs could fit in 1/400th of the nation's airspace. What about birds getting killed? They say that the flying windmills could make noise that would keep birds away; this would be prohibitive for ground-based turbines, but at high altitude no people will be around to be annoyed by it.

The prototypes Roberts has been making for the last 25 years have gone from wind tunnel to field trial, and they apparently work, with no new technologies required. However, they have been stalled seeking funding for the last three years. The text on their website shows a woeful lack of marketing savvy, leaning towards crackpottyness, so they may not be getting funding anytime soon. But hopefully they will find someone willing to give them a shot, since they seem the most promising of the three [companies looked at here].

Robert Cringely of "I, Cringely" explains some of the history behind these schemes in "Oh, and We Also Saved the World: Google's Energy Plan".

What's cool about these tethered tensile wings, he explained, is that they can be designed in such a way that no aircraft fuselage is needed and yet they can lift (vertically, straight from the ground, no runway even required!) enormous weights. And I mean ENORMOUS weights, like a thousand tons. A fully loaded Boeing 747-400 weighs about 400 tons, so a THOUSAND tons would change the nature of airfreight.

But there's an even better application for this technology than airfreight, he explained, electric power generation. Build a gigantic tethered tension wing and power it with electric motors mounted in the leading edge of the wing. Send the electricity to run these motors up the tether, itself. The wing will take off vertically and once it is at the end of its rope, so to speak, can be made to circle thousands, or even tens of thousands, of feet off the ground without a pilot or any sort of crew.

Remember from your ground school days that wind tends to increase with altitude. Once aloft, circling in the stiff breeze a few thousand feet in the air, it should be possible during most daylight hours to just turn off the electric motors and get them running as generators, taking energy out of the wind. This would be regenerative air braking.

To my knowledge this idea of using a tethered kite to generate power was first put forth back in 2003 by Pete Lynn, a mechanical engineer and second-generation kite designer from New Zealand. He described his work back then in an extensive post on Google Groups as well as on his own web page. (That page is no longer directly available online, but in this week's links we've managed to recover that page thanks to the Internet Archive's WayBack Machine.)

Lynn's explanations in 2003 and 2004 were very clear and the implications of his work even clearer: this was probably the best way yet to extract energy from the wind — far better than more traditional windmills. ...

The problem with wind power is that much of the time there isn't enough of it available to even justify energizing the alternators attached to the large windmills used in wind farms. Unless the wind speed is over, say, 10 miles per hour, it isn't worth running the windmills at all. And above some speed on the order of 40 mph, it again isn't worth the effort, this time because of fear that high winds will damage the windmills — windmills sometimes costing hundreds of thousands of dollars each.

But power-generating tension kites are different, as Lynn so ably explained: "the numbers strongly infer that such a wind turbine system can produce power for around a fifth to a tenth the cost of current generation systems, depending on site costs. This is roughly US 0.5 cents a kilowatt hour, with the likelihood that this will reduce further with mass production."

Five tenths of a cent per kilowatt-hour is VASTLY cheaper than the average retail price was for electricity anywhere in the U.S. in 2005, where electricity costs ran as high as 12 cents per kilowatt-hour in California and 14 cents per kilowatt-hour in the state of New York. So even though the kits would have to be all new construction and the old coal, gas, and oil-fired power plants mothballed or dismantled, the payback period for doing so would be measured in months, not years or decades as most such capital expenses are today. Today, with energy costs even higher, the payback would be even quicker.

Getting the cost of wind-power production so low depends on a couple of factors — building kites that cost very little for the power they generate and allowing them to harvest energy from a larger slug of airspace than is used by the big ground-based windmills employed in most wind farms.

"The trick is that the propeller is operating at the speed of the airplane, which is many times greater than that of the true wind speed," wrote Lynn. "At an overall lift to drag ratio of ten the air plane speed is ten times that of the true wind, with power proportional to wind speed cubed, the propeller can have a thousandth the swept area of a comparable wind turbine for the same power. This makes for a very compact and effective unit, it is important to exploit this apparent wind directly as it allows for much higher specific speed of the propeller and generating unit, (no gearing)... Line length can actually scale with size, somewhat, a 100MW unit might optimally have around a 1000m line. "

Such wind kites would be cheaper to build than current windmills because their structural efficiency is so high, according to Lynn. In fact the weight of such a kite turbine might be only one percent that of a comparable windmill.

According to Lynn's figures, then, to completely replace the one million megawatts of electricity generated in the U.S. annually by a total of 16,000 generators of various types would require 10,000 of those 100-megawatt tethered flying wings. ...

Pete Lynn no longer works in New Zealand. Today he works in Emeryville, CA at a company called Makani Power, which is developing exactly the sort of power-generating kites Lynn envisioned six years ago. Go to the people section of Makani's website and you'll see the healthiest bunch of windsurfer/engineers imaginable, including Pete Lynn, who actually seems to play a minor role in the company.

The only other initiative in this area is a Dutch effort out of the Delft University of Technology, described by The Economist:

Wubbo Ockels of the Delft University of Technology in the Netherlands has been developing another approach to airborne wind generation at lower altitude, with backing from Royal Dutch Shell and Nederlandse Gasunie, a natural-gas company. Dr Ockels’s idea is that a kite (without rotor blades) be launched from a ground station, turning a generator as it rises to an altitude of several hundred metres. When it reaches its full height, it alters its shape to catch less wind, and can thus be reeled back in using much less power than it produced when it was being paid out.

An arrangement of two or more of these kites could act together to produce a steady supply of power. When one kite was being released, part of the electricity produced would reel the other kite back in, and vice versa. The whole system would thus remain in surplus, and if well designed could deliver a constant current. This system has the advantage that it requires only simple parts—generators, kites and cables—and should thus be much cheaper to build than a conventional turbine.

Controlling it, however, would be a different matter. Dr Ockels is working on kites with wings and rudders, which look much more like a plane than anything you might see flying in the park. The wings and rudders themselves would be under computer control—a technology already well established for flying aircraft without too much interference from a human pilot.

To test the idea, Dr Ockels’s team is building a 100kW prototype. He hopes to start testing a full-scale device, which would generate 10MW, within five years. That would be large enough to power around 10,000 homes. He believes the system should be capable of generating electricity at a cost of just 1 cent a kilowatt hour.

Any promise of such cheap energy has to be treated with scepticism, and all these projects are still a long way from the full-scale test rigs needed to prove they will succeed. No-one denies that it will be hard to build a flying generator that can make money. However, the political impetus behind renewable energy is growing and space is limited at ground level. Perhaps it is time for the wind power industry to reach for the sky.