A research team from Massachusetts Institute of Technology (MIT) recently found that water droplets can generate an electric charge when repelled from a superhydrophobic (ultra-waterproofed) metal surface. This technology may be used to develop portable devices that can harness the humidity from surrounding air, converting it into enough electricity to charge battery-powered electronics like cell phones.
How Is Humidity Harvested?
The process is achieved through condensation, when water vapor gathers onto a surface in the form of dew (as vapor is cooled, or as humidity reaches peak saturation); it therefore requires that an environment be both humid and contain a contrasting source of colder temperatures, like a river or cave, that can be used to cool the charging device to below the air temperature.
How Does Water Vapor Yield Electricity?
Published in Applied Physics Letters [citation below], the 2014 study is a follow-up to the MIT team’s earlier research. They had been testing surfaces that could improve the efficiency of heat transfer in industrial condensers (devices that convert vapor to liquid, usually through cooling) when they noticed some strange behavior in the droplets of condensed water. They already knew, as has been widely observed, that when water condenses on a superhydrophobic surface some of the droplets bounce up into the air. This happens when small beads of condensation are drawn together to become larger droplets (a process called coalescence); the resulting momentum can cause the droplets to be flung away from the waterproof surface. What the MIT team was surprised to find, when they viewed video footage of the droplets in slow-motion, was that these droplets were also being propelled away from one another mid-air.
As is observable in magnets, objects of the same electric charge (either positively-charged, or negatively-charged) repel each other; it therefore appeared that the droplets were carrying an electric charge. The scientists were able to test this by observing the reaction of the droplets to copper wire electrodes and noting that they reacted to positively-charged copper by moving away; in this way, they were able to confirm that the jumping water droplets were carrying a positive electric charge.
How Did Water Droplets Become Positively Charged?
Water, or H2O, is a polar molecule. What this means is that the distribution of electrons (which are negatively charged) between its atoms is uneven. Each water molecule consists of one oxygen atom and two hydrogen atoms. The hydrogen atoms each share a pair of electrons with the oxygen atom, but the oxygen atom also has two unshared pairs of electrons on its opposite end; electrons repel one another, and so the hydrogen atoms are pushed to one side of the oxygen atom by the four unshared electrons. The side of the oxygen molecule with unshared electrons is negatively charged, while the hydrogen side is positively charged. This polarity is what causes water molecules to join together, as each hydrogen atom bonds to the oxygen atoms of other molecules.
Occasionally an oxygen atom will attract an electron away from another H2O molecule’s hydrogen atom with enough force that the hydrogen atom detaches from its electron; this causes the hydrogen atom to become positively charged. The H2O molecule that has lost a hydrogen atom becomes hydroxide (OH-). The positively-charged hydrogen atom (H+) then attaches to another water molecule to form a new molecule called hydronium (H3O+). As a result, water contains a small amount of charged molecules and atoms (ions). All fluids contain both positive and negative ions.
When water comes into contact with an object, it causes what is called an “electrical double layer” or “double layer” (EDL, or DL). A DL is formed when any object comes into contact with a fluid; the first layer is a “surface charge” on the object (a layer of charged ions), and the second layer consists of accumulated ions with an opposing charge attracted by the surface charge.
In their 2014 study the MIT researchers concluded that an EDL on the superhydrophobic surface was responsible for the electric charge of leaping droplets. The droplets were leaping away from the surface quickly enough to separate the positive and negative layers of electric charge, leaving small electric charges on both the water droplet and the metal surface.
How Is This Electric Charge Harnessed?
To explore the potential of this phenomenon, the researchers have added another conductive metal plate parallel to the first; the second plate is set up to attract the leaping droplets both through a hydrophilic (water-attracting) coating and an opposing charge (i.e., magnetic) to that of the water droplets. As the droplets transfer their electric charge between plates electric pressure is produced, and that electric pressure can be harnessed by incorporating the plates into an electric circuit.
A circuit is constructed by linking the positive and negative sources of an active electrical charge (such as the poles of a battery) through a conductive material (such as a wire) in an uninterrupted loop through which the charge can travel; this loop can then be linked to the various conductive components of electronic devices. When a charge is active, it continuously travels through the open circuit until interrupted by a gap (such as an activated “off” switch) in the circuit.
How Much Electricity Is Produced?
The amount of electricity achieved by the initial experiments was limited to about 15 trillionths of a watt, or picowatts, per square centimeter (just under half an inch squared); however, the process can be adjusted to increase the amount of electricity to about one millionth of a watt, or microwatt, per square centimeter, which would be enough to produce a portable device that can charge some electronic devices.
A portable 50 centimeter-per-side cube (just under 1 ½ feet squared), consisting of layered plates of conductive metal such as copper or aluminum, would take about 12 hours to power a cellphone. The technology’s portable form could have useful applications during weather crises when public electricity might be down for several days, or in isolated areas where electricity is not available. On a larger scale, it could be an alternative or supplement to solar energy in regions where climate can accommodate the technology.
Nenad Miljkovic, Daniel J. Preston, Ryan Enright, Evelyn N. Wang. Jumping-droplet electrostatic energy harvesting. Applied Physics Letters, 2014; 105 (1): 013111 DOI: 10.1063/1.4886798
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