*5.2. Kinetic Energy Harveters*

In contrast to solar or thermal energy, a kinetic energy source is not dependent on location or time. Kinetic harvesters are based on the extraction of vibration or motion and the conversion of the mechanical energy into electrical power through one or a combination of different transduction mechanisms. The most common ones are piezoelectric, electromagnetic, electrostatic, and triboelectric. These harvesters are classified related to their transduction mechanisms. Unlike other means of transduction, piezoelectric harvesters directly convert human motion changes into electrical signals without any requirements for further external input. Piezoelectric (PE) harvesters operate through the piezoelectric effect. When a force is applied to a PE element, a mechanical strain is developed, causing the material to exhibit changes in its polarization, causing the accumulation of electrical charges across the piezoelectric material. The changes in charge distribution produce an electric field depending on the applied force, frequency of oscillation, and geometry of the harvester.

Electromagnetic kinetic energy harvesters operate based on Faraday's law induction which states that once a conductor moves through an electric field, a current is induced. A system of springs, magnets and coils are used in electromagnet energy-harvesting systems. Coil number and magnetic mass are the main determinants of the output power of these energy harvesters. Therefore, reducing their size, weight and complexity is challenging. As example, the authors of [95] demonstrated the effectiveness of a frequencyconverted electromagnetic harvester which extracts energy mainly from human limb motion. A power density of 0.33 mW/cm<sup>3</sup> was achieved in this work using low-frequency human vibration to power wearable devices at extremely low frequencies.
