Let us consider the case of the film stack with the capacitance of 16 nF cm−2. The average generated power calculated using equation (2) is shown in Figure 3b. The average power per foot can exceed 2 W for bias voltages in excess of 35 V and 10 W for bias voltages in excess of 75 V. The bias voltage can be substantially reduced by increasing the capacitance of the dielectric film stack. However, it is important to mention, that even at its current level the bias voltage does not present a substantial practical issue. A wide range of commercially available DC–DC boost converter components can be used to convert the 3.7 V output of standard Li-ion batteries to the required bias voltage. Thus, this example clearly supports the use of footwear designed for high-power-energy harvesting based on reverse electrowetting.
The other common source for mechanical energy harvesting is vibration energy. It has been demonstrated that the energy of mechanical vibrations present in floors, stairs, vehicles and equipment housings can be used for electrical power generation. Currently, the majority of experimental vibration harvesters have output power in the range from 10^−6 to 10^−2 W. The reverse electrowetting (REWOD) process can enable the use of novel harvester architectures with greatly increased power output. One example of the REWOD-based vibration harvester device consists of an array of conductive droplets squeezed between two dielectric-coated electrodes, as shown in Figure 4b. The electrodes are separated by a millimeter-thick elastic spacer so that the resulting structure can be used as a mounting ‘pad’ for the load device. Mechanical vibration of the load device causes periodic change in the solid–liquid contact area and, thus, electrical current generation. For the film stack with a capacitance of 10^2 nF cm−2, the resulting power density can be scaled up to 10^−1 W cm^−2 at 50 Hz vibration frequency, thus enabling the fabrication of practical vibration harvesters with power output of several watts.
The above examples illustrate new possibilities in portable high-power energy harvesting that can be opened by utilizing the REWOD process. High-power energy harvesting can potentially provide a valuable alternative to the use of batteries. Even though energy harvesting is unlikely to completely replace batteries in the majority of mobile applications, it can have a very important role in reducing cost, pollution, and other problems associated with battery use. We believe that the REWOD-based mechanical to electrical energy conversion process, which we have developed, can go a long way in achieving this goal.
Schematics of two REWOD applications.
The next important consideration was to develop a dielectric film optimized for energy production. The electrode capacitance can be increased by using a higher-k material and by decreasing the dielectric layer thickness. Simple calculations indicate that if high capacitance on the order of 102 nF cm−2 is to be achieved at lower working voltages (<100 V), dielectric film thicknesses are limited to several 100 nm, but, as previously noted, field strengths are quite high causing the film to be susceptible to breakdown. We ultimately determined that Ta2O5, possessing a relatively high k of 25, demonstrated the best resistance to dielectric breakdown. The Ta2O5 films were produced by anodic oxidation at room temperature of sputtered deposited Ta films on quartz substrates. Precise Ta2O5 thickness could be controlled by adjusting the anodizing voltage. Although not completely understood theoretically, we found that charge trapping at the interface between the liquid and the dielectric surface had a dominant role in limiting energy production. The amount of charge trapped at the interface drastically reduces the amount of energy available for use. We investigated numerous dielectrics not only for their breakdown characteristics but also for their tendency to trap charge. As a result, we discovered that all oxide thin films, which we investigated, were poor at energy production. Inorganic fluorides were better, but fluoropolymers or Teflon-like films were by far the best. The fluoropolymer films were produced by spin coating with Cytop6 and then by curing in N2 at 250 °C for 2 h. Although the Cytop films showed the least amount of charge trapping, they demonstrated unreliable breakdown characteristics. Thus, our best results for energy production were obtained using a dielectric composite of Cytop deposited on Ta2O5.
(a) Shows energy generated per one oscillation cycle (in units of C0V2 ) as a function of dimensionless parameter ωRC0. The solid line represents equation (1). Experimental data for the droplets between sliding plates are shown in blue, and experimental data for the droplets in a channel are shown in green. The red dot represents the predicted energy for a train of 1,000 droplets. (b) Shows the predicted power for a train of 1,000 droplets as a function of the bias voltage. Calculations are for of a load impedance of 2 KΩ and a film-stack capacitance of 16 nF cm−2. The red curve corresponds to 1-Hertz stride (fast walk) and the blue curve to 0.5-Hertz stride (leisure walk).
How to use Shoe power ?
The power generated by the footwear-embedded harvester can be used in one of two ways. It can be used directly to power a broad range of devices, from smartphones and laptops to radios, GPS units, night-vision goggles and flashlights. In this case the power can be delivered to the device using a number of methods, ranging from simple wiring to conductive textiles to wireless inductive coupling.
Alternatively, a Wi-Fi hot spot can be integrated into the harvester to act as a “middleman” between mobile devices and a wireless network. Such an arrangement dramatically reduces power consumption of wireless mobile devices and allows them to operate for much longer time without battery recharge. No direct physical connection between the mobile devices and the harvester unit is required in this case.
Here is a more detailed technical description. The main idea is based on the fact that long-range radio (RF) communication requires much more power to operate as compared to their other functions or to short-range RF communications such as those conforming to Bluetooth standards. The high power required for long-range RF communications leads to accelerated battery discharge in many mobile electronic devices, such as smartphones, laptops with Wide Area Network (WAN) cards, cell phones, etc. Thus substantial decrease in power consumption by these devices can be achieved if their long-range RF transmission is minimized or even completely excluded. One of the ways to achieve this is to perform RF communications through an intermediate transceiver (Wi-Fi hot spot) that receives communications from the mobile devices in a low-power standard such as Bluetooth and then retransmits them through a long-range high-power RF link, such as G4, G3, or CDMA wide area cellular telephone network link. Since both short-range and long-range communications are performed wirelessly, the transceiver location can be anywhere in the vicinity of the mobile devices. In other words, the idea is to combine a human-motion-powered energy harvesting device with the wireless transceiver capable of receiving communications from personal mobile electronic devices in a low-power standard such as Bluetooth and then to retransmit them through a long-range high-power RF communication to a wide-area network. Thus a very substantial decrease in power consumption by mobile devices can be achieved since most of the energy consumed by long-range RF transmission will be provided by the energy-harvesting device rather than by the mobile electronic devices themselves.