Tikalon Blog by Dev Gualtieri

Power from Raindrops

March 23, 2020

A child's first encounter with the concept of unintended consequences is likely the nursery rhyme, "Rain, rain, go away. Come again some other day." The "come again another day" refrain is important, since we wouldn't want rainfall to cease entirely. I once read a short story in which a man who was granted wishes was annoyed at something, and shouted, "Stop everything." This caused the Earth's rotation to stop, throwing objects off our planet's surface. While stopping the Earth's rotation would have been far more disastrous than depicted in the story, if we go down a few levels in (classical) physics, when electrons would also stop their orbits around the nucleus, etc., this would indeed be the end of the universe, and a sudden end of the story.

In Tikalon's northern New Jersey home, we're blessed with frequent rains, and the occasionally unwelcome tropical storm. While all this rain is good for agriculture and replenishing our water table, it might also be used as an energy source. Many years ago, while unclogging a gutter downspout at my house, I noticed the forceful flow of water that occurred when I removed the clog. A small hydroelectric turbine at this downspout could harvest the rainwater's energy.

The ocean covers about 71% of Earth's surface. The Earth's water cycle starts with the evaporation of ocean and sea water by solar radiation. As this water rises into the atmosphere, the lower temperatures at high altitudes cause the water to condense into small, buoyant, liquid water droplets in clouds. These droplets collide and merge, finally becoming heavy enough to fall as precipitation.

About a half million square kilometers of water falls annually as precipitation, about 20% of which falls over land.

(Wikimedia Commons image by jansku136).

This energy arising from the gravitational potential difference between a roof and the Earth's surface starts with the solar energy that evaporates water into the atmosphere to allow the rain. Unfortunately, a one inch rainfall onto the area of my roof results in less than a tenth kilowatt hour of power; and, rainfall is not a continuous phenomenon. However, that amount of power would light a useful LED light bulb for an entire night when there was a electrical service disruption due to a storm.

Rain, of course, originates much higher than a roof level of 4-5 meters, so energy is lost in its impact with the roof. in 2008, scientists at the French Atomic Energy Commission (Commissariat à l'énergie atomique), Grenoble, France, decided to extract raindrop energy more directly. They harvested all the impact energy of raindrops using piezoelectric materials.[1-2]

Their research, published in Smart Materials and Structures, showed that a one millimeter diameter rain drop, a smaller type found in drizzle, has an impact energy of two microjoules, the five millimeter droplets of a heavy rain have a millijoule of energy, while some rain droplets have more than ten millijoules of energy. The energy-harvesting is aided by the fact that most rain drops impact inelastically; that is, they don't bounce.[1-2]

The French scientists used one mil (0.001 inch) thick polyvinylidene fluoride polymer sheets as a piezoelectric energy-harvester, and about 1 microwatts of power was obtained by impact of for the smallest droplets, and more than 10 milliwatts for the largest.[1] They calculated that rainfall in France would produce an average power of about one watt-hour per square meter.[1] This is still rather small, amounting to a little more than 200 watt-hours for my roof, but this might be a way to power remote environmental monitoring instrumentation in tropical rainforests.

There was an attempt in 2014 to replace piezoelectric energy harvesting of raindrops with triboelectric nanogenerators which were self-cleaning, robust, and easily fabricated at low cost.[3] These yielded a peak power density of less than one watt per square meter.[3] More recently, a team of scientists from the City University of Hong Kong (Hong Kong, China), the University of Science and Technology of China (Hefei, China), the University of Nebraska-Lincoln (Lincoln, Nebraska), the University of Electronic Science and Technology of China (Chengdu, China), and the Chinese Academy of Sciences (Beijing, China), did a further optimization of the triboelectric approach.[4-5] Their research is reported in a recent issue of Nature.[4]

Schematic diagram of the droplet-based electrical generator.

(City University of Hong Kong image.)

Their raindrop energy-harvesting device, as shown in the figure, consists of a polytetrafluoroethylene (PTFE, Teflon) film on an indium tin oxide (ITO) substrate with an aluminum electrode.[4] This device architecture is similar to that of a field-effect transistor, and it has a much higher high energy conversion efficiency and power density than other triboelectric effect energy-harvesting architectures.[5] A falling water drop will spread over the surface of the device to bridge the electrode elements to release the developed electrical charge.[4]

The instantaneous power density of this device is as high as 50.1 W/m², which is thousands times higher than other triboelectric raindrop energy-harvesters.[5] As water droplets continuously hit the PTFE, the generated surface charges accumulate to a saturation point (see figure).[5] The charge generation and storage are done by the PTFE and ITO parts of the device, and the spreading water droplet bridges the circuit to the aluminum electrode to create the electrical voltage that becomes an electrical current in an attached device.[5]

Temporal evolution of charge after droplet impact and subsequent spreading on the droplet-based electricity generator.

Temporal evolution of charge after droplet impact and subsequent spreading on the droplet-based electrical generator.

(Still image from a City University of Hong Kong video. Click for larger image.)

Says Zhong Lin Wang of the Chinese Academy of Sciences and an author of the paper describing this research, "Our research shows that a drop of 100 microlitres (1 microlitre = one-millionth litre) 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."[5] This research was funded by the National Natural Science Foundation of China and the United States National Science Foundation, among other sources.[5] patent applications for this invention have been filed in the US and China.[5]

I published a short article on a triboelectric effect device in 2016.[6] I also enjoy doing simple benchtop experiments using household items available to me in my high-tech household with an electronics workshop. Inspired by these raindrop energy-harvesters, I built a demonstrator that uses a piezoelectric loudspeaker element. This loudspeaker is just a 1-3/4 inch diameter circular brass diaphragm, 0.005 inch thick, onto which is deposited a one inch diameter circular patch of lead-zirconate titanate (PZT) with a silver electrode. The voltage generated is between the brass disk and the silver electrode.

My demonstration apparatus, as shown in the photograph (click for larger image), has part of an eye drop bottle connected to one liter water reservoir at a higher height to produce a series of droplets impacting on the piezoelectric element. The dropper can be varied in height, and the piezoelectric element is tilted at 45-degrees to allow the water to flow away from it. The piezoelectric element was coated with a thin oil film to promote drainage. The generated electrical voltages were sent to the microphone input of a desktop computer where data acquisition was done using the free and open source Audacity digital audio editor.

The droplet rate was about one every 1.5 seconds, with about 25 drops per milliliter, and a calculated drop diameter of 4 millimeters. Each drop caused the brass diaphragm to ring at its resonant frequency of about 750 Hz, and this pulse shape was easy to model (see figure). The model allowed an accurate calculation of the power generated by each drop by numerical integration, since the generated voltage was terminated in a 10,000 ohm resistance and the power P is equal to voltage E squared divided by the resistance R (P = E²/R).

The right image is the mathematical model of the electrical pulse produced by a water droplet striking a piezoelectric loudspeaker. The actual pulse is shown on the left. (Click for larger image.)

Of course, the peak voltage will increase as the drop height increases, as shown in the graph. There's some drop-to-drop variation, so the data are somewhat coarse. The drop velocity v at impact can be calculated from the simple equation, v = √(2 g h), where g is the gravitational acceleration, and h is the height. At the highest height measured (480 mm), this velocity was 3.07 meters/second. It's good that raindrops have a terminal velocity of about 9 meters/second, or life on Earth might not have been possible. At our 480 mm height, the calculated energy per drop is 0.7 nanojoule. Scaling to raindrop terminal velocity gives 2 nanojoules per drop for this simple apparatus.

Peak voltage as a function of drop height.

The peak voltage follows a roughly linear trend from zero height to the maximum height of 480 mm possible on the apparatus. Drop-to-drop variation is responsible for the scatter, since the largest peak in a pulse series was determined automatically in Audacity.

Power from Raindrops

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