This chapter focuses on vibration energy harvesting using electrostatic converters. It synthesizes the various works carried out on electrostatic devices, from concepts, models and up to prototypes, and covers both standard (electret-free) and electret-based electrostatic vibration energy harvesters (VEH).After introducing the general concept of Vibration Energy Harvesting and the global advantages and drawbacks of electrostatic devices to convert mechanical power into electricity ( §1), we present in details the conversion principles of electret-free and electret-based electrostatic converters and equations that rule them in §2. An overview of electrostatic VEH, comparing the results from several laboratories (powers, sizes, concepts…) is provided in §3. In §4, we introduce several power management circuits dedicated to electrostatic VEH. These circuits are extremely important as they are the only way to turn VEH output powers into viable supply sources for electronic devices (sensors, microcontrollers, RF chips…). Assessments, limits and perspectives of electrostatic VEH are then presented in §5.

Integration of structures and functions has permitted to reduce electric consumptions of sensors, actuators and electronic devices. Therefore, it is now possible to imagine low-consumption devices able to harvest energy in their surrounding environment. One way to proceed is to develop converters able to turn mechanical energy, such as vibrations, into electricity: this paper focuses on electrostatic converters using electrets. We develop an accurate analytical model of a simple but efficient cantilever-based electret energy harvester. Therefore, we prove that with vibrations of 0.1g (~1m/s²), it is theoretically possible to harvest up to 30µW per gram of mobile mass. This power corresponds to the maximum output power of a resonant energy harvester according to the model of William and Yates. Simulations results are validated by experimental measurements, raising at the same time the large impact of parasitic capacitances on the output power. Therefore, we 'only' managed to harvest 10µW per gram of mobile mass, but according to our factor of merit, this puts us in the best results of the state of the art.

At present, the energy autonomy of a microsystem is limited by the lifespan of the battery. Hence the development of the energy harvesting concept, whereby the energy needed to power the sensor is taken from the operating environment. However, there is no single solution suitable for all types of environment. In this paper, we look at a still unexploited source of energy: rain. Our system scavenges the vibration energy from a piezoelectric flexible structure impacted by a water drop. We present an experimental device that validates the aforementioned theoretical results.

Materials that show large and reversible electrically driven thermal changes near phase transitions have been proposed for cooling applications, but energy efficiency has barely been explored. Here we reveal that most of the work done to drive representative electrocaloric cycles does not pump heat and may therefore be recovered. Initially, we recover 75–80% of the work done each time BaTiO3-based multilayer capacitors drive electrocaloric effects in each other via an inductor (diodes prevent electrical resonance while heat flows after each charge transfer). For a prototype refrigerator with 24 such capacitors, recovering 65% of the work done to drive electrocaloric effects increases the coefficient of performance by a factor of 2.9. The coefficient of performance is subsequently increased by reducing the pumped heat and recovering more work. Our strategy mitigates the advantage held by magnetocaloric prototypes that exploit automatic energy recovery, and should be mandatory in future electrocaloric cooling devices.

Thanks to miniaturisation, it is today possible to imagine self-powered systems that use vibrations to produce their own electrical energy. Many energy-harvesting systems already exist. Some of them are based on the use of electrets: electrically charged dielectrics that can keep charges for years. This paper presents an optimisation of an existing system and proves that electret-based electrostatic energy scavengers can be excellent solutions to power microsystems even with low-level ambient vibrations. Thereby, it is possible to harvest up to 200µW with vibrations lower than 1G of acceleration (typically 50µm pp at 50Hz) using thin SiO 2 electrets with an active surface of 1 cm² and a mobile mass of 1g. This paper optimises such a system (geometric, electrostatic and mechanical parameters), using FEM (Finite Element Method) software (Comsol Multiphysics) and Matlab to compute the parameters and proves the importance of such an optimisation to build efficient systems. Finally, it shows that the use of electrets with high surface potential is not always the best way to maximise output power.

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One of the main challenges in energy harvesting from ambient vibrations is to find efficient ways to scavenge the energy, not only at the mechanical system resonance, but on a wider frequency band. Instead of tuning the mechanical part of the system, as usually proposed in the state of the art, this paper develops extensively the possibility to tune the properties of the harvester using the electrical interface. Due to the progress in materials, piezoelectric harvesters can exhibit relatively high electromechanical coupling: hence, the electrical part can now have a substantial influence on the global parameters of the piezoelectric system. In order to harness the energy efficiently from this kind of generator on a wide frequency band, not only the electrical load's effect on the harvester's damping should be tuned, but also its effect on the harvester's stiffness. In this paper, we present an analytical analysis of the influences of the resistive and reactive behavior of the electrical interface on highly coupled piezoelectric harvesters. We develop a normalized study of the multiphysic interactions, reducing the number of parameters of the problem to a few physically meaningful variables. The respective influence of each of these variables on the harvesting power has been studied and led us to the optimal electrical damping expression and the influences of the damping and of the coupling on the equivalent admittance of the piezoelectric energy harvester (PEH). Finally, we linked these normalized variables with real reactive load expressions, in order to study how a resistive, capacitive and inductive behaviors could affect the global performances of the system. The theoretical analysis and results are supported by experimental tests on a highly coupled piezoelectric system (" = 23%). Using an adequate tuning of a RC load at each frequency, the maximum harvested power (11) under a small acceleration amplitude of 0.5. 0" is reach over a 14Hz large frequency band around 105Hz which has been predicted by the model with less than 5% error.

At present, the energy autonomy of a microsystem is limited by the lifespan of the battery. Hence the development of an energy harvesting concept, whereby the energy needed to power the sensor is taken from the operating environment. However, there is no single solution suitable for all types of environment. In this paper, we look at a still unexploited source of energy: rain. Our system recovers the vibration energy from a piezoelectric flexible structure impacted by a water drop. This paper describes in detail the theoretical study undertaken to optimize the mechanical system.