Green Energy From the Drinking Bird

[Video courtesy of Wikipedia article: Drinking Bird, Drinkend vogeltje video.ogv]

I’m sure you’ve seen this classic science demo, either in the classroom, or as a toy sometimes marketed as Dippy Duck. It’s often used to teach the laws of thermodynamics, since its operation is powered by the energy absorbed as water changes from a liquid to the vapor state, aka latent heat of vaporization. For water, that heat amounts to 2260 kJ/kg.

To put that into perspective, 2260 kJ is equivalent to 0.63 KWh of electrical energy. That’s what it takes to vaporize 1 kg of liquid water (0.26 gallons). So one of these birds, properly scaled to “consume” 1 kg of water, could (at 100% efficiency) produce 0.63 KWh of electricity. Of course, the actual efficiency of the device depends on mechanics, friction, humidity, temperature, and altitude. The device is a variant of hygroelectricty but requires a liquid reservoir to tap into.

So what if we built a really large version of this toy, and attached a electric generator to its axis of rotation? Could it produce useful amounts of electricity? The answer seems to be “yes” according to new research paper titled “Drinking-bird-enabled triboelectric hydrovoltaic generator” published in the academic journal Device. If you follow that link you can see a graphic showing their invention built around the functional core of a drinking bird. The generators are not the standard electromagnetic generators used in, say, wind turbines or cars. They are triboelectric generators that build electrical charge from static electricity, much like what happens when you rub a balloon on a wool sweater.

The device built by this team is on a scale that could power small devices, like phones or tablets or clocks. But their design is scalable and larger versions could turn standard electrical generators that actually feed power into the grid. All that’s needed to power the device is a source of liquid water, and air that’s warm and dry enough to support rapid evaporation. Imagine a “flock” of these devices perched on the banks of the Colorado River where it runs through the desert Southwest.

The authors of the publication point out that the amount of solar energy absorbed by the oceans is about 50% of that absorbed by Earth’s solid surface. And water evaporation from the oceans drives a large part of our weather and climate cycles. In fact, the water cycle itself is a global version of the drinking bird. There’s potential here to tap into a huge renewable energy source.

So let’s take a closer look at the original Dippy Duck and see just how it runs:

That graphic shows the state of the system during 90% of its operating cycle. As you can see in the video, after a long climb up the tube, the shifting COM (center of mass) eventually tips the bird’s beak back into the water. When the beak hits the ambient temperature water there’s an immediate warming of the gas in the bird’s head. Internal pressure increases and the liquid inside the bird rapidly drops, shifting the COM again.

FYI, the liquid used inside the bird is typically methylene chloride, with a red dye added for visibility. This is a volatile compound that evaporates and condenses easily around room temperature. In other environments, with other ambient temperatures, other volatiles are available for use.

So are we ready to add Dippy Duck to the green energy mix? Wind, photovoltaic, hydro, and geothermal have a huge head start, but together contribute only 20% to the US demand (and 11% globally). We need more. Dippy Duck could be scaled up, but there are more promising sources available now — like wind and photovoltaic which are also scalable. You may see one of these Dippy Duck chargers available online in the not-too-distant future, but it’ll probably be marketed as a “novelty charger” for your phone.

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