With the development of portable electronic devices there is a huge demand for electrical batteries. High-power mechanical energy harvesting can potentially provide a valuable alternative to the use of batteries, but, until now, a suitable mechanical-to-electrical energy conversion technology did not exist.
Existing methods of mechanical-to-electrical energy conversion such as electromagnetic, piezoelectric, or electrostatic are not well suited for effective direct coupling to the majority of high-power environmental mechanical energy sources suitable for portable applications, thus their energy output remains in the microwatt to hundreds of milliwatt range.Scientists have developed a good way to convert mechanical to electrical energy which can provide power upto tens of watts using Reverse Electrowetting Phenomenon
WHAT IS ELECTROWETTING?
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces.
Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field. The electrowetting effect has been defined as “the change in solid-electrolyte contact angle due to an applied potential difference between the solid and the electrolyte”. The phenomenon of electrowetting can be understood in terms of the forces that result from the applied electric field.The fringing field at the corners of the electrolyte droplet tend to pull the droplet down onto the electrode, lowering the macroscopic contact angle and increasing the droplet contact area.
The contact angle changes from > 90°to < 90° upon applying a potential to the droplet. The contact angle was measured as a function of applied potential, and the reversibility of wetting was also checked.
REVERSE ELECTROWETTING ON DIELECTRIC(REWOD)
In REWOD the reverse process is adapted in which mechanical energy is converted to electrical energy. n its essence, the REWOD process is conceptually straight forward. The droplet and the electrode are connected to the external electrical circuit that provides a constant bias voltage between the droplet and the electrode. External mechanical actuation is used to move the droplet in such a way as to force a decrease of its overlap with the dielectric-film-coated electrode. This results in the decrease of the total charge that can be maintained at the droplet liquid–solid interface. The excessive electrical charge then flows back through the electrical circuit that connects the droplet and the electrode, generating electrical current that can be used to power the external load. In the REWOD process, fluidic actuation can be accomplished in a number of different geometries, as shown in Figure 1, such as out-of-plane vibration, in-plane shear, and in-channel droplet motion.
These include (a) droplets between oscillating plates, (b) droplets between sliding plates, and (c) droplets in a microchannel. (d) Shows in greater detail schematics of reverse-electrowetting-based energy generation process in a microchannel geometry.
Successful high-power mechanical energy harvesting using the proposed approach is contingent on solving three major problems, namely achieving high level of electrical energy generation per unit area of the liquid–solid interface; attaining efficient mechanical actuation and electrical energy generation by the individual droplets; and demonstrating synchronous generation of electrical energy by a group of micro-droplets working in parallel
Energy harvesting from human locomotion using footwear-embedded harvesters is a long-recognized concept. Power can be produced by a footwear-embedded microfluidic harvester using the REWOD process.
(a) Footwear-embedded microfluidic energy harvester and (b) a REWOD-based vibration harvester.
While it may not solve the issue of always having a full battery, it certainly makes a lot of sense to reuse the energy from walking to at least partially recharge a battery. For those who commute to work on foot, or regularly go for walks or runs as exercise, it could mean a reduced power bill at least. The other good news is there is no extra energy required to walk while wearing the shoes harvesting the energy, so it really is a free power source you could work into your daily exercise and activity.