Tikalon Blog by Dev Gualtieri

Raindrop Energy-Harvesting

October 5, 2020

I've maintained a huge collection of music CDs, started with the introduction of the CD format in the early 1980s. I had even heard a demonstration of a microwave oven sized prototype CD player at a meeting of the Audio Engineering Society in New York City a few years prior. While I still have a working CD player, it's now rare for me to listen to any of my collection, since presently there are easier ways to hear most of this music. For example, I have a CD of Steve Reich's Music for 18 Musicians, but I can now view the piece as a performance on YouTube.[1]

One of my other CDs is a soundtrack recording of The Magnificent Seven (John Sturges, Director, 1960),[2] composed by Elmer Bernstein. Bernstein's score for this Western incorporated many elements of the nostalgic Americana musical themes of Aaron Copland. Another popular western movie, produced nearly a decade later, was Butch Cassidy and the Sundance Kid (1969, George Roy Hill, Director),[3] which had an incongruous comedy relief sequence involving the Hal David and Burt Bacharach song, "Raindrops Keep Fallin' on My Head."

The terrestrial water cycle. (USGS image by John Evans and Howard Periman, via Wikimedia Commons. Click for larger and more detailed image.)

As can be expected for a planet whose surface is covered 71% by water, rainfall is a frequent occurrence in most areas. We should be thankful for our planet's water cycle, as illustrated in the above figure, that distributes water across Earth's land areas to create our plentiful vegetation. Our civilization was created by water through agriculture. In today's world, civilization is propelled by energy, and we get some of this energy through water by hydroelectric power.

Frequent power outages, like the week-long August, 2020, electrical power outage that Tikalon experienced as a consequence of Hurricane Isaias, reminds us that centralized power generation has its problems. Tikalon's power was maintained through this outage by local generation provided by a natural gas electrical generator installed years ago after frustration with repeated power outages in our suburban location. Since torrential rain is the cause of so many power outages, it's ironic that raindrops have been used as an energy source. I've written two earlier articles about some techniques for raindrop energy harvesting (Pyroelectric Energy Harvesting, October 15, 2010, and Power from Raindrops, March 23, 2020).

Just how much energy is contained in a raindrop? It's a simple calculation, since falling bodies in air achieve a terminal velocity V_t that depends on their mass and cross-sectional area.

where m is the mass of the raindrop (which can be calculated from the density of water and the diameter of the presumed spherical droplet), g is the gravitational acceleration, C_d is the drag coefficient (in this case, about 0.5), ρ is the density of air, and A is the projected area of the raindrop (also calculated from the droplet diameter). This equation ignores the small buoyancy of the droplets in air. Once we have the velocity, we can apply the (1/2)mv² kinetic energy equation to obtain the droplet energy. Statistics about droplet size and rainfall rates will give the maximum harvestable raindrop energy over an area.

Raindrop velocity as a function of diameter.

The curve is an aid for the eye, and it isn't a function fit.

(Graphed using Gnumeric from data from Wikimedia Commons image by Janusz Dorożyński. Click for larger image.)

Theory reveals the possibilities, but it takes actual experiments to see how much raindrop energy can be converted into useful electricity. There are two principal phenomena that accomplish the task to conversion of impact energy into electrical energy; namely, the piezoelectric effect, and the triboelectric effect. In the piezoelectric effect, certain crystalline and polycrystalline solids will produce electrical charges at their surface in response to an applied Mechanical stress. In the triboelectric effect, rubbing, and then separating, dissimilar materials produces a charge difference between them. Static electricity is an example of triboelectricity.

The Wimshurst machine was a device for harvesting triboelectricity. It appeared as a prop in classic Frankenstein films. Left image, a scan by Andy Dingley from the 1903, Electrical Installations (Volumes V) by Rankin Kennedy, Caxton Press, London. Right image, a modern replica. Images from Wikimedia Commons)

in 2008, scientists at the French Atomic Energy Commission (Commissariat à l'énergie atomique, Grenoble, France), extracted raindrop energy using a sheet of the piezoelectric material, polyvinylidene fluoride.[4-5] They showed that the raindrops found in drizzle (one millimeter diameter) produce an impact energy of two microjoules, and the five millimeter droplets of a heavy rain have a millijoule of energy.[4-5] Huge raindrops were detected that had more than ten millijoules of energy.[4-5] They calculated that rainfall in France would produce an average power of about one watt-hour per square meter.[4] The energy-harvesting was aided by the fact that most raindrops impact inelastically; that is, they don't bounce.[4-5]

Just this year, a team of scientists from China and the University of Nebraska-Lincoln (Lincoln, Nebraska) published in Nature details of a triboelectric raindrop energy harvester.[6-7] Their device,whose architecture is similar to that of a field-effect transistor, consists of a polytetrafluoroethylene (PTFE, Teflon) film on an indium tin oxide (ITO) substrate with an aluminum electrode (see figure).[6] A falling water drop will spread over the surface of the device to bridge the electrodes to release the developed electrical charge.[6]

Schematic diagram of the droplet-based electrical generator and temporal evolution of charge after droplet impact and subsequent spreading. Impact of water droplets on the PTFE generates surface charges that are conducted by the spreading water droplet to the aluminum electrode. (Left. a City University of Hong Kong image. Right, a still image from a City University of Hong Kong video.)

While the triboelectric raindrop energy harvester is more expensive to make than polyvinylidene fluoride piezoelectric devices, its performance is impressive. The instantaneous power density of this device is as high as 50.1 W/m².[7] As Zhong Lin Wang, an author of the paper and a member of the Chinese Academy of Sciences, explains, "Our research shows that a drop of 100 microliters (1 microliter = one-millionth liter) of water released from a height of 15 cm can generate a voltage of over 140V. And the power generated can light up 100 small LED light bulbs."[7] I published a short article on a triboelectric effect device in 2016.[8]

Members of this original research team from the South China Normal University (Guangzhou, PRC) have teamed with scientists from the University of Twente (Enschede, The Netherlands) in continued development of this energy harvesting concept and publishing their research in a recent issue of Physical Review Letters.[9-10]

This nanogenerator is based on the combined effects of triboelectricity and hydrophobicity has no moving parts, aside from water flow, and it's very efficient.[10] Water droplets impact an upper film of polytetrafluoroethylene (PTFE, Teflon) on a conducting plate. Impact of the droplet develops a negative charge on the PTFE film paired with a positive charge on the conductive plate, effectively forming a charged capacitor. Impact of each droplet adds to the capacitor charge that subsequently can be used as a source of electrical energy.[9-10]

Power is extracted from the capacitor by the droplets themselves, since a spreading droplet temporarily connects the charged surface to an electrode.[9-10] But that's not the end of the story. Hydrophobicity causes the impacted droplet to subsequently bead, and this automatically recharges the capacitor after each discharge (see figure).[9-10]

Evolution of water droplet shape on an hydrophobic surface

Evolution of water droplet shape on the hydrophobic surface of the triboelectric energy-harvester. (Created using Inkscape.)

As Siddharth Rajupet and Daniel J. Lacks of Case Western Reserve University (Cleveland, Ohio) Department of Chemical and Biomolecular Engineering observe in their commentary on this paper, the mechanism of triboelectricity is still unknown, as is the reason why some pairs of materials make better triboelectric couples than others.[10] In particular, it's not understood how rubbing is important. It might serve to increase the contact area, or it might serve to cleave charge carriers from one surface to the other.[10]

Raindrop Energy-Harvesting

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