Theoretical Comparison of Energy Harvesting Methods for Electret-free Variable-Capacitance Devices

Electret-free variable-capacitance energy harvesters are micro-electromechanical systems (MEMs) that generate electrical power from mechanical vibrations. In order to effectively harvest energy from these devices, the power electronic circuitry is extremely important, and can be the difference between generating or losing energy during a harvesting cycle. Four methods of harvesting energy from these devices are known: the Constant Voltage Method, the Constant Charge Method, the Charge Pump Method, and the Constant Charge With Parallel Capacitance Method. All four methods have been reported; however, the literature is lacking a formal comparison of these methods. This paper evaluates these four methods and the new Voltage and Charge Constrained Method while considering power electronic circuit efficiency as a key parameter. By including efficiency as a parameter, new fundamental properties of these devices are derived: a threshold efficiency necessary for energy harvesting, analytical solutions for optimal harvesting conditions, and a realistic comparison of the four methods. A case study demonstrates the advantage of using the Charge Pump method for MEMs applications, and illustrates the use of the new fundamental properties in the design of a circuit topology that is practical for MEMs vibration energy harvesting applications.