CEA-Leti researchers are developing an innovative energy-harvesting technology that collects vibrations from the environment and converts them into electricity to power a variety of sensors.
A unique aspect of Leti’s energy-harvesting microsystem is its ability to use vibrations of varying frequency and amplitude. Although it is easier to convert stable, highfrequency vibrations (over 1 kHz) into electrical energy than low frequency unstable ones, Leti has chosen to focus on harvesting low-frequency vibrations with varying frequencies and amplitudes. The reason for this choice is that many of the vibrations in natural and man-made environments are relatively low frequency (under 120 Hz), and often depend on energy sources of varying activity levels such as engine vibrations, vehicle speed, wind level, etc.
The output power of a vibration-driven energy harvester is directly proportional to the vibration amplitude and frequency of the energy source and to the size (seismic mass weight) of the harvester. Output power is inversely proportional to the harvester’s frequency bandwidth. Consequently, it is much harder to efficiently harvest power from low-frequency sources with a large frequency band response and with a very small system size than from a stabilized high frequency vibration source.
Seeking better harvesters
With those challenges in mind, Leti (Laboratory for Electronic and Information Technologies of the French Atomic and Alternative Energies Commission) set out to find a better way to maximize the output power of energy harvesters. The laboratory developed an electrostatic micro-electromechanical system (MEMS) structure capable of efficiently converting both low- and highamplitude vibrations into electrical energy, thanks to a unique patterned electrode structure – see figure 1.
Fig. 1: All three elements shown separately - The SiO2-based electret (purple slice) is to be mounted or patterned directly onto the MEMS seismic mass (shown standing). The whole assembly is then mounted a few micrometers away from the electrode glass plate (shown at the bottom with several electrode patterns) to form the vibration energy harvester.
This patented electrostatic structure translates the input vibration into multiple capacitance variations, which are used to convert the input vibration energy into electrical energy. When a constant charge is placed in the variable capacitor, the voltage varies in inverse proportion to the capacitance variation (capacitor voltage = charge/ capacitance) and the associated energy varies in proportion to this voltage:
Energy = ½ charge x voltage
In other words, a capacitance variation induces an energy variation, and that energy variation is used to supply the output load.
This type of structure differs from a piezoelectric structure in that the energy level converted per cycle by the electrostatic structure can be adjusted by adjusting the charge value and is not linked to a specific material property. If no charges are placed on the structure, no electrostatic forces will be applied to the structure. The structure is then free to move and no mechanical work is converted into electrical energy. However, if a large charge is placed on the structure, a large electrostatic force appears in the structure, preventing it from moving. As a result, the capacitance value remains constant and no electrical energy is delivered.
To maximize the output power, an optimum charge value is required that matches the mechanical impedance of the vibration source and the converter input.