by Erin Dahlstrom

Almost two years after its May 2013 purchase of alternative wind energy company Makani, Google X, Google’s semi-secret “moonshots” lab announced at SXSW that they will be starting to test full-scale models of Makani’s kite-like wind turbines in April 2015 [1,2].  While wind turbines have generally been trending bigger and more expensive in order to increase energy output, Makani has taken the opposite tact and developed wind turbines called “energy kites” which are more lightweight, cheaper, and can fly higher to catch stronger winds [3].  So far, Google X Makani has only tested 28-foot long models, but their website states that the full 84-foot long models (about a quarter of the length of a football field) whose testing will soon begin could “generate 50 percent more energy” by accessing stronger winds at higher altitudes and “eliminate 90 percent of the materials of conventional wind turbines” [3].  We’ll discuss these big claims Google X is making later, but first let’s take a look at how conventional wind turbines work and what Makani is doing differently.

How do conventional wind turbines generate energy?

A wind turbine uses energy from streams of air to produce electricity.  While there are many components to a wind turbine that help do this, there are four main parts we’ll discuss in this article: the base tower, the blades, a shaft, and a generator.

Figure 1: A typical wind turbine is made of many parts. We’ll focus on the tower, blades,
shaft, and generator. [4]

The base tower elevates the other components of a wind turbine above ground level to access wind with more energy.  100 feet or more above the ground, wind is both faster and less turbulent, allowing more energy to be produced more efficiently [4].  The blades (there are usually three of them) are connected to the front of the tower and are what harness the energy from the wind.  Similar to airplane wings, these blades are not symmetric – one side is flat, and the other is rounded – so when wind passes over them, it generates a difference in pressure above and below the blades.  The blades then move to try to equalize the pressure, causing lift, the same principle that allows airplanes to fly [5].  In a constant wind, this causes the blades to rotate continuously.  A shaft connects these blades to a generator.  Rotation of the blades causes the shaft to rotate, which in turn causes the generator to produce electricity.  You can think of a generator like a water pump – movement of the pump up and down (mechanical energy) causes the water to flow. In the case of a wind turbine generator, wind moving the blades and shaft (mechanical energy) causes electric charges to flow through a wire – electricity [6]!

Figure 2: The principle of lift is actualized in both wind turbines and airplanes [7].

What are the limits to current wind power technology?

The amount of electricity that a wind turbine can generate is directly related to how long the turbine blades are and how tall the tower is.  Longer blades have more mass, making them more difficult to move.  Because it takes more wind power to move them, they can consequently capture more energy from the wind once they are moving.  Doubling the length of the blades generates about four times as much energy. Winds are faster the higher up you go, which higher towers can take advantage of; doubling the height of the tower increases wind speeds by about an additional 12 percent [7].

Figure 3: Wind turbines are quickly reaching the limits of how big they can reasonably be [8].

 The main limiting factors in the production of wind turbines are the height of the turbines, the expense to build them as they get larger, and the amount of ground space they take up.  The wider and taller we make them in order to get bigger energy gains, the more expensive they are, and the more space they will take up.  “A typical large wind turbine can generate up to… 5.2 million KWh annually, under ideal conditions – enough to power nearly 600 households” [7].  However, conventional wind turbines “can only be installed where the winds routinely reach speeds of between 5-8 meters per second” – less than 15% of the world’s land has consistently high enough winds to meet this criteria and be able to power the 600 households [3].  It also takes 100 tons of material on average (think 20 elephants) to build a big enough wind farm to power just 500 households [3].  Not only does this make building a productive wind farm quite costly, it also limits the places wind farms can be built, as 100 tons of material take up quite a bit of space.  We are quickly reaching a plateau in terms of how much energy we can get from conventional onshore turbines.

How is Makani changing wind technology?

Makani’s wind turbine design is called an “energy kite” system.  The system consists of the energy kite itself, a ground station, and a tether that connects the kite and ground station.  Rotors and a generator are both mounted on the kite, which looks like a small plane.  The kite is launched from the ground station and travels in large circles at a high enough altitude where “the wind is strong and consistent.”  Once the kite reaches the proper altitude and begins circling, the rotors on the kite rotate in the wind, like the blades of a conventional turbine, and drive the production of electricity from the generator mounted on the kite.  The electricity travels through the tether to the ground station, where it can be introduced into an electricity grid [3].

Figure 4: The Makani wind energy system consists of a wind kite, a ground station,
and a tether that connects the two [3].

What makes this design better than conventional wind turbines?  There are three main advantages to the energy kite system: it reaches higher altitudes, uses less materials, and is more aerodynamically effective.  Because of the flexible tether connecting the kite and the ground station, energy kites can reach a maximum altitude of 350m compared to the 150m a typical onshore wind turbine can currently reach.  The winds are stronger higher up, producing more energy.  This means that, compared to conventional wind turbines, which are confined to the 15% of land economically feasible for energy production, energy kite wind farms have more options in the choice of physical location.  The ground station of an energy kite also takes up considerably less ground space than a conventional wind turbine.  Energy kites “eliminate 90% of the materials of conventional wind turbines,” which makes them both cheaper to produce and easier to install in more locations.  Google X Makani predicts that these two advantages combined with the improved aerodynamics will allow “each individual energy kite [to] generate 50% more energy” [3].

What’s the takeaway?

50% more energy may sound like a major step forward for the wind energy industry and for alternative energy technology in general, but a 50% gain compared to what? Google X doesn’t specify the particular conditions under which it expects to be able to achieve this 50% gain.  This claim as well as several of the other functionality and efficiency claims made on the Google X Makani website are somewhat vague and lacking in precise quantitative comparisons with either conventional wind turbines or other energy sources.

This project is still in an early stage of development and not close to introduction into the consumer market, so many details on its performance are unavailable outside of data from simulations.  It is also important to note, however, that Google X is a semi-secret lab, and much of the information about their experiments and developments is not available to the public.  As such, the information being released about the energy kites is fully controlled by Google X and is consequently one-sided, only referencing the benefits and none of the possible hazards or counterarguments for the technology, such as how to address varying wind speeds, harm to birds and bats, possible noise complaints, etc.

This is not to say that Google X Makani cannot revolutionize the wind energy industry — the 28 foot models they have tested so far have performed very well.  Despite having test runs in Pigeon Point in Pescadero, California where “the speed of the wind can change by 20 miles per hour in a second, and the direction of the wind can change by 90 degrees in a second,” none of the smaller model energy kites crashed during their test runs [1].  This is a promising start, but only time will tell if the full 84-foot models can consistently perform as well as expected and give the wind energy industry the purported “50% more energy.”

Figure 5: Google X’s latest video chronicling test flights of Makani energy kites so far [3].

Erin Dahlstrom is a Ph.D. candidate in the Physics Department.

References

[1] Google will fly a crazy, plane-like, 84-foot wind turbine next month: Astro Teller tells the crowd at SXSW that the 28-foot versions didn’t crash. Arielle Duhaime-Ross. The Verge. http://www.theverge.com/2015/3/17/8236723/google-x-makani-project-wind-turbine-planes
[2] Google X Announces Revolutionary Flying Wind Turbines at SXSW. Cole Mellino. EcoWatch. http://ecowatch.com/2015/03/18/google-x-makani-sxsw/
[3] Google X Project Makani. http://www.google.com/makani/
[4] How does a wind turbine work? Office of Energy Efficiency & Renewable Energy. http://energy.gov/eere/wind/inside-wind-turbine-0
[5] How Does A Wind Turbine Work? Center for Wind Energy at James Madison University. http://wind.jmu.edu/communityengagement/howturbineworks.html
[6] How Generators Work. Generator Source. https://www.generatorsource.com/How_Generators_Work.aspx
[7] How Wind Power Works Julia Layton. HowStuffWorks.com http://science.howstuffworks.com/environmental/green-science/wind-power3.htm
[8]Wind turbine size increase 1980-2010 Wikimedia Commons
[9] How Wind Energy Works. Union of Concerned Scientists. http://www.ucsusa.org/clean_energy/our-energy-choices/renewable-energy/how-wind-energy-works.html