Energy harvesting (also known as power harvesting or energy scavenging or ambient power) is a method of generating electrical energy from normally unused energy sources available in the surrounding environment. In other words, it is the process by which energy, coming from alternative energy sources (commonly available in the environment: thermal energy, kinetic energy, chemical energy, potential or solar energy, etc.) is captured and accumulated. This process converts energy into a directly usable electrical current.
Energy harvesting holds great promise for both low-voltage and low-power applications in a wide range of portable or mobile markets such as medical equipment, consumer devices, transportation, industrial controls, and military. Energy conversion takes place in different ways depending on the environmental source. The energy can be captured from a variety of sources deemed wasted or otherwise unusable for any practical purpose.
- Mechanical sources: translational and rotational kinetic and potential energies, inertia, gravitational field, vibrations, elastic energy, piezoelectricity, triboelectric effect, acoustic waves, sea/ocean waves, and wind energy. For example, the conversion of mechanical motion can take place through piezoelectric crystals or particular polymers, which subjected to mechanical deformation stresses, generate small electrical potentials.
- Electromagnetic sources: radio waves, magnetic induction, electromagnetic radiation, photovoltaic, potential energy due to electric fields or magnetic fields. For example, energy from broadcasting or theoretically from any electromagnetic emission can be collected. A typical use of this technique is used to power the RFID (Radio Frequency Identification) identifiers.
- Thermal sources: temperature gradients, pyroelectricity, thermoelectrics. In the presence of thermal gradients, thermoelectric generators can be used.
- Chemical and biological sources: exothermic reactions, potential energy due to chemical bonds, ionization energy, levels of glucose in the blood, salinity gradients, tree-based metabolic energy.
Energy conversion takes place in different modes depending on the environmental source.
- Spurious energy from radio and television transmissions or theoretically from any electromagnetic emission can be harvested. A typical use of this technique is used to power Radio Frequency Identification (RFID) identifiers.
- Conversion of mechanical motion can take place using piezoelectric crystals or special polymers, which when subjected to stress from mechanical deformation, generate small electrical potentials. One mode uses the kinetic energy of the motion of a magnetic pendulum within an electromagnetic winding. The originating forces can come from other periodically applied pressures, such as in shoes during a walk, or from mechanical vibrations from motors, or any low-frequency ambient sound or noise. In the case of fibers embedded in clothing fabrics, the very movement of the person produces electrical energy. Where there are air currents, even if inconstant, micro wind turbines are used.
- Solar energy is converted through photovoltaic cells, which lend themselves easily to be scaled in small sizes, an example is that of pocket calculators, or that of the garden step lamps.
- In the presence of thermal gradients, thermoelectric generators can be used, with a typical voltage of 0.1-0.2 mV/K. Useful potentials can also be obtained for direct use, simply by arranging several generators in series. The powers drawn are typically on the order of mW, and human and animal heat can also be exploited. They are often coupled to heat sinks to improve (increase) the temperature gradient. Alternatively, one can exploit the pyroelectric effect, another ferroelectric behavior. An advantage of pyroelectric materials over thermoelectric ones is their resistance up to 1200 °C or more, allowing energy to be gleaned even from high-temperature sources and thus with greater thermodynamic efficiency.