The results described above establish the fundamentals of energy production using REWOD process and clearly demonstrate that the energy-generation process can be readily scaled upwards to achieve high power output in excess of 1 W in a relatively small package. Indeed, scaling up the REWOD-based energy-generation process entails increasing the number of droplets working in parallel to generate electrical current. In this work, we have experimentally demonstrated that this scaling can be readily achieved over two orders of magnitude, from a single droplet to 150 droplets. In terms of the liquid-substrate area, the scaling was demonstrated over almost three orders of magnitude, from 0.28 mm2 (single droplet in the sliding plates set-up) to 1.28 cm2 (22 droplets in a channel set-up, each droplet having 5.8 mm2 liquid-substrate overlap area). As the total capacitance of the system C linearly scales with the area, it was demonstrated to scale over three orders of magnitude as well, from 14 pF (single droplet in the sliding plates set-up, 5 nF cm−2 dielectric stack) to 20 nF (22 droplets in a channel set-up, 16 nF cm−2 dielectric stack).
Devices with even larger number of droplets can be readily fabricated by exploiting a natural synergy between the REWOD process and droplet-based microfuidics10
. Indeed, parallel actuation of a large number of micro-droplets required for scaling-up of the generated power is routinely performed in channel-based droplet macrofluidic devices, where thousands of droplets can be synchronously moved in microchannels with a great degree of control over their position and velocity10
. Combination of REWOD and droplet microfluids offers important advantages such as easy scaling, very flexible force–displacement relationship, and extremely simple device design with no moving solid parts.
Microfluidic power generators based on the REWOD process can take advantage of many previously inaccessible environmental mechanical energy sources. Two specific examples will be illustrated: energy harvesting from human locomotion and high-power harvesting of mechanical vibration energy.
Energy harvesting from human locomotion using footwear-embedded harvesters is a long-recognized concept1
. Data available in the literature indicate that up to 10 W per foot can be generated without adversely affecting a person's gait13
. For comparison, relatively high-power mobile electronic devices, such as cell phones and mobile computers, typically consume power on the order of 1 and 15 W respectively.
The following simple estimate illustrates the power that can be produced by a footwear-embedded microfluidic harvester using the REWOD process. Let us consider 2-m-long train of 1,000 conductive droplets, each 1 mm long separated by 1 mm spacers and positioned inside 1-mm-diameter circular cross-section channel with the total length of 4 m, . The total area covered by such channel is about 40 cm2 or less than ¼ of the area of a typical human footprint. The total volume of the liquid contained in the channel would be about 4 ml, which makes it readily compatible with footwear. Assuming that the heel area is about 20 cm2, we estimate that the total midsole compression required to achieve 4 ml volume displacement is around 2 mm. Such a displacement is well below the level that might affect the person's gait.
Schematics of two REWOD applications.
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 . 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 generation1
. Currently, the majority of experimental vibration harvesters have output power in the range from 10−6
W (refs 1
). The 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 . 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 102
, the resulting power density can be scaled up to 10−1
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.