Jacques Mattheij

Technology, Coding and Business

A Critique of Makani, the Google X Kite Based Power Project

My credentials in the wind industry are just about ‘0’, the only reason I’m writing this is because I’ve been a life-long wind power enthusiast, I’ve built (a prototype 2.5 KW 5 meter wind turbine and have a basic understanding of how wind power works and what the limitations and engineering elements are that go into a typical wind power setup. I also live in a country that has a very long history when it comes to using wind power and have a pretty good understanding of the kind of obstacles wind power deployments tend to run into. Makani is anything but typical, I’ve been following their saga as closely as the limited information allows and this is my view of the project, the website and the challenges they are dealing with.

Google has put quite a bit of money into a remarkable new renewable energy project that aims to power from a concept dubbed the energy kite.

The basic idea is that there is a small stub tower with a spool containing a very high strength tether that keeps a kite anchored to the ground and that is also used to transfer the power generated by the rotors mounted on the kite to the ground for further distribution. The kite continuously flies circles and the stub tower tracks the kite with every revolution.

It’s an impressive project for many reasons, for one, in defiance of the golden rule of alternative wind power projects it actually has a chance of working, for another, they’re building and testing their device at a relatively large scale (they mention that they have a 600 KW unit now).

The company claims that their machines are much lighter, can be sited in many places where conventional machines can not be sited (for instance because there are no roads) and that they generate 50% more power than conventional machines. All in all a pretty compelling story.

Wind Energy Power Generation Primer

Wind is moving air, and air has mass. When you interrupt that flow of air you can extract a certain percentage of energy from that moving air (this limit is called Betz’ law). A typical wind turbine has to deal with all kinds of real world limitations and so does not manage to extract more than 75 to 80% of the energy that is available in theory. When you look at the area of the rotor of a typical wind turbine the outer one third of the of rotor generates the bulk of the energy. The rest of the rotor is sort of along for the ride it merely exists to keep that outer third in place and to transfer the momentum from the outer portion to the hub. A bit like the spokes of a bicylcle wheel but a lot more resistant to torsion. Another annoying element is the fact that three times per revolution (in a 3 bladed rotor arrangement, which is not the most efficient but for many reasons the most practical compromise between efficiency and various other engineering parameters) the blades move in front of the relatively close tower that supports the Nacelle and thus indirectly the rotor itself, interrupting the flow of air and negatively impacting efficiency as well as stressing the rotor and the tower. Because of this and another factor called wind shear rotors are typically angled backwards a bit. This has the effect of reducing the advantage the top blade has over the lower one(s), it also increases the space between the tower and the rotor which reduces the chances of a tower strike and a phenomenon called ‘tower thump’ where the air between the blade and the tower is compressed as the blade passes in front of the tower forcing the blade outwards (in the direction of the wind).

Enter Makani, a (very!) clever solution to a whole bunch of limitations that these conventional wind turbines have. Because Makani does not have a tower beyond a small stub to attach the tether to they have a number of advantages: Tower thump is eliminated, there is no tower (though there is the equivalent of tower thump in that the smaller turbines still have to move in front of the kite leading edge these are higher frequency, the risk of impact with the kite is significantly reduced because the blades are much shorter). Wind shear is much less of an issue because the kites fly fairly high above the terrain in comparison with the height of a conventional turbine.

Wind power goes up as the cube of the speed of the wind. For one the energy in the moving mass of air is higher because of the increased speed (e=mv^2), but there is also more air moving through the turbine so you get another doubling for a total of a factor of 8 for a doubling of the wind speed.

This has some rather unfortunate consequences for wanna-be wind power generators: you have to take into account that the wind will per given amount of time for the most part be below the ‘rated’ speed of your turbine. That’s ok, you are simply generating (much) less power. Then, there are times when the wind speeds are higher than the rated wind speed of your machine. In those cases you can reduce the amount of energy extracted from the wind using a technique called furling. Depending on the design of the machine the rotor is angled out of the wind, or alternatively (and this is the most common method for large turbines) the blades are rotated around their long axis to a position of lower efficiency (that’s called ‘variable pitch’). All this to make sure that the rotor does not exceed the maximum RPM at which the machine would be damaged (to get a bit more technical: you really don’t want the rotortips to go supersonic and you want to maintain a rotorspeed where the centrifugal forces acting on the blade roots are not going to destroy the attachments or overheat the main bearing). And then there are those unfortunate times when the wind speeds are so high that the machine - in spite of being furled completely - would still overspeed. In these cases the best one can do is to shut down the machine and to ride out the storm. If that process does not work then the machine will fail, and this can be quite a spectacle and is one of the reasons you won’t see windmills in the middle of a town. (Technically you probably don’t want to live within throwing distance of one of the blades, which is rather more than you’d probably expect, a windmill blade travels 200+ kilometers per hour at the tip and weighs several tons).

Makani has a clear advantage here, they can - because of how their system works - vary the rotor diameter by making the kite fly tighter circles, and if the winds get too high for safe operation, simply reel the whole thing in and stow it until the storm passes. This probably gives Makani an edge when it comes to riding out storms.

So, in principle this is a very neat concept which has some clear advantages over a conventional tower based 3 bladed rotor.

The challenges Makani faces are pretty formidable. As you can see from the above the power in the wind is phenomenal. Makani’s 600 KW unit has an operational altitude of 140 to 310 meters and a circling radius of 145 meter.

Diameter of the rotor

For comparison, an Enercon E-126, the largest ground based turbine that is currently in regular production generates 7.6 MW out of a rotor diameter of 126 meters (slightly smaller than the Makani device) from a generator sitting in a nacelle at 135 meters. The total height of the machine does not go above 200 meters (up close that’s still a very impressive machine).

If the Makani would deliver on the promise of being able to extract more energy from a given amount of wind it would have to do about 12 times better than it does today to compete with a machine that is technically smaller (the rotor diameter is the key number).

Or you need 12 of them but this would require a very large amount of ground surface compared to an equivalent power ground based turbine.

So I fail to see how Makani can claim to generate ‘50% more energy’, if they did this would be a 15 MW machine, not a 600 KW machine. Maybe they mean this figure to be interpreted over the lifetime of the device but if that’s the case it is definitely not clear from their documentation.

Many more points of failure

The Makani device has a large number of ways in which it could fail because it is a far more complex setup than a regular wind turbine. The tether is an obvious weak point, ditto for the tether attachment points and the frequent movements of the tower. Material fatigue over the projected operating life-span of the machine will be a major challenge here. The movements are relatively frequent, highly variable and un-controlled compared to the steady movements associated with a regular wind turbine. This will stress the tether, the tower bearings and the frame of the kite repeatedly which may result in a reduced operating life compared to a machine that for the most part just sits there and has a much lower number of moving parts. The Enercon again deserves special mention here, it doesn’t even have a gear box (it is a direct drive concept). Simplicity is a very important element in wind turbine design, the simpler a device the longer its operating life and the more reliable it will be. The Makani concept is anything but simple and it is possible that they have a tether and associated hardware which will be able to go through 25 years of continuous abuse as shown in the video but the proof of the pudding is very much in the eating here. Wind technologies that need regular service or replacement of critical parts are pretty much a non-starter, it would increase the cost of operation tremendously offsetting advantages gained by having less material in the first place.

Because of the length of the tether if the tether should break the potentially affected area is much larger than with a regular turbine, the kite has a much higher starting point and is capable of much longer flight time than a broken off piece of a conventional machine. The good news is that the individual pieces will be lighter.

Wind is a fickle mistress

Conventional wind turbines have one very large disadvantage over every other power generation method: wind is fickle. It comes and goes and sometimes it comes and goes rather rapidly. The grid is not set up for such unstable power generation and dealing with these power surges from wind parks is a very complex problem that we’re making only slow headway with. Especially with older grids the possibillity of damage is very real (and this has been a long standing problem between Germany on the one and hand the Czech republic and Poland on the other where the power surges from the German windparks have caused damage to the older infrastructure on the other side of the border). The best techniques available revolve around superconducting coils functioning in the role of sources and sinks to reduce the variability in the output of a wind turbine (or even a whole park).

The Makani devices have a disadvantage here. Because they need to stay aloft if the wind should rapidly change either direction or intensity they switch the rotors from power generation to power consumption if the need arises. This will be worse than a regular wind turbine because those only generate more or less power, they don’t suddenly go and consume power. This increased instability will be a major factor against Makani when it comes to connecting to the grid. Another disadvantage is that because of the reduced weight of the rotating portion the Makani device has much less energy stored in the device itself which means the output can vary much more rapidly.

The state of the art

The Makani website lists a number of figures that are not representative of the state of the art in conventional wind turbines, for instance on their ‘challenge’ page they state that “Conventional turbines have grown taller, heavier, and more expensive in order to generate more power. ” which is true at first reading, but the effect of this development is that the price per KWh generated by these machines has gone down. So the total cost of power generated over the lifetime of these machines has gone down, their operational costs have gone down and total power has gone up tremendously (15% further than where the graphic on that page ends).

Just like a Makani device producing 6 MW (if such a device can be constructed in the first place, it would be a very large machine) would be heavier, more expensive and would have to fly higher in order to generate more power.

When the first 3MW machines were presented everybody thought that ‘this was as big as they were going to get’ and now we’re looking at 7.6MW machines with - compared to the Makani technology - a relatively modest footprint.

The environmental aspects

Getting a wind power park sited is a very long exercise in dealing with the general public. Especially in the developed world people don’t like wind power plants and there is a large amount of ‘NIMBY’ (not-in-my-backyard) behavior. Everybody would like to have green power and even better if it’s cheap but nobody wants a wind turbine in their backyard. There are visual aspects, there is noise pollution, there are shading effects when the rotor blades are intersecting the image of the sun and so on.

Makani has an uphill battle here because all of these aspects are considerably worse for their technology at a similar power level. The kites fly higher, are much more of an eye sore and occupy a much larger disc compared to a ground based turbine. And given the amount of backlash ground based machines get the Makani will likely not fare much better and probably worse. The noise from the many smaller turbines is an unknown factor but if my experience is any guide here the Makani device will produce quite a bit of it and from higher up which means the area the noise is radiated to will be larger. Turbines tend to generate noise at the tips and large turbines tend to be much more quiet than small ones.

Ground is expensive

Even though the footprint of the Makani base station is relatively small the total volume occupied by an operating unit is a very large multiple of what you’d need for a regular wind turbine. The volume a regular turbine occupies is a cylinder with a radius of one blade centered on the tower and with the maximum height of the blade tip at the apex as the height. You can truncate that cylinder at the top with a half sphere.

In contrast, the volume occupied by the Makani device is a half sphere with a radius determined by the maximum length of the tether + half the size of the kite. That’s a much larger volume, which translates into more complex siting arrangements and a much larger spacing between devices in a park than are required for regular turbines.

Because of the impact here and the fact that the size of the base stations is not that much different from the size of a ground based turbine and the variety of angles at which the tether can exit the stub tower it would be a fair assumption that a Makani setup would incur a premium for a given amount of power generated compared to a more conventional setup.

It's all economy

Wind power is not so much about which solution is more clever or generates more power than another. In the end it is all about economy: the total cost of a machine including all operational, maintenance, land, purchase and other costs divided by the total number of KWh generated over the life of that machine.

Makani has some clear advantages when it comes to raw material costs, their 600 KW unit (assuming it really does produce that 600 KW) looks decidely more efficient when just looking at the materials. But over the life-span of a machine (a typical wind power setup is designed with a 25 or 30 year life-span in mind) the initial materials cost is a relatively small fraction (in spite of the huge capital costs involved) of the total.

So the only interesting question is what the cost per KWh is when taking into account all the costs associated with a machine and this information is not yet available for the Makani project and it will take many years of operating a relatively large number of these units before we can begin to put a figure on this.

Conclusion

Makani is a super interesting concept, it is - unlike many other revolutionary wind technologies - not vapourware but there are tremendous technical, engineering, economical and environmental issues that they will need to overcome if this is to be a mainstream success. Depending on the niche they want to deploy to they will have an easier time with some of these challenges but conventional wind turbines will likely be hard to compete with in the niche in which they are already established.

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