A Charge-Depletion Study of an Electrostatic Generator With Adjustable Output Voltage

Micro-scale generators are becoming more popular for harvesting energy to power bio-implantable devices and sensor networks. Most electrostatic generators (ESGs) use constant capacitors as storage or reservoir components in conjunction with a variable capacitor. The main issue with some existing ESG topologies is that these capacitors deplete and discharge over time. This paper studies a typical ESG and derives the charge depletion problem mathematically. Subsequently, a new ESG capable of circumventing this problem is proposed. Closed-form formulas expressing the output voltage and generated power are derived and validated. The proposed ESG harvests 25% of the power that the mechanical energy source generates by actuating the variable capacitor when the maximum-to-minimum capacitance ratio of the variable capacitor is optimized. In the presented case study, the ESG generates 9.75 mW optimally when a variable capacitor with a maximum/minimum capacitance ratio of 39/ $9.75~\mu \text{F}$ is used for energy harvesting from a 1-Hz knee joint movement of a walking person. The overall volume of the ESG is estimated to be 125 mm3, and the variable capacitor is charged to 5 V at its maximum capacitance. A control mechanism and a self-starting circuit are presented for this ESG architecture, which allows it to generate any desired output voltage. This capability can be used to harvest the maximum available kinetic energy and compensate load variations.