I had the opportunity last week to sit down with Dr. Tom Krupenkin of the University of Wisconsin and learn about the energy harvesting technology he invented called Reverse Electrowetting (REWOD). 
REWOD is based on a matrix of capacitors whose capacitance varies with movement. The capacitor is formed by a conductive plate, a dielectric material, and a conducting liquid for the other plate. A voltage source provides a constant voltage. The capacitance changes when mechanical vibration or compression changes the geometry of the drops of conducting liquid. The amount of charge on the capacitor changes following the relationship Q=CV. Moving charge is current, and when it moves through a load it provides power. As with any circuit, the old rule that maximum power transfer occurs when the load impedance equals the source impedance is true for REWOD.
When I asked Dr. Krupenkin about how he came up with the idea of moving charge by varying a capacitor, he talked about why we use traditional generators. In a generator, a coil of wire is moved through a magnetic field. When charged particles, as in the electrons on the outer shell of copper atoms, move through a magnetic field, they experience the Lorenz force perpendicular to the direction of motion and to the direction of the magnetic field. Dr. Krupenkin points out that generating power by moving charge directly is actually more straightforward than a generator moving charge through a magnetic field causing a force to push the charge at a right angle to its motion. The physics work out such that if you have a modest force pushing something a large distance, as in the case of expanding gases from burning something moving a mechanical apparatus, traditional generators are the best choice. For harvesting energy from vibration or compression, electrostatic generators are better than magnetic generators.
Dr. Krupenkin has experimented with using REWOD to harvest energy from footsteps. Human footsteps can generate up to a few hundred milliwatts per sq. cm. The question is what to do with electric power in shoes. One idea is a Wi-Fi access point that connects the Internet through mobile data network. A typical transmission to the mobile network requires around 50mW. A transmission from handheld device to the user’s shoes would take only a fraction of a mW. Using the shoes as a repeater would extend the handheld device’s battery life. The power could also be stored in a battery and be made accessible by USB whenever the user needs extra power.
What this technology needs is a “killer app” that requires around 100 milliwatts in an object that experiences a large amount of vibration or compression.