Energy harvesting is the process of extracting small amounts of electrical energy from sources such as ambient temperature, vibrations, or air currents for low-power mobile devices. The structures used for this purpose are also referred to as nanogenerators. Energy harvesting avoids the limitations of wired power or batteries in wireless technologies. Energy harvesting is important for the energy supply of miniature devices in the digital world.
Micro-effects
- Piezoelectric Crystals generate electrical voltages when applied to forces, such as pressure or vibration
- Thermoelectric Generators and pyroelectric crystals extract electrical energy from temperature differences
- Antennas can be used to capture the energy of radio waves, a form of electromagnetic radiation, and use it energetically. One example of this is passive RFIDs.
- Photovoltaics, electrical energy from ambient lighting.
- Osmosis
Macro effects
---
- Some wristwatches are powered by the movement of the wrist. In most cases, a strong imbalance on one axis is usually involuntarily caused to rotate by the wearer and the rotational energy is mechanically or electromagnetically supplied to the core function of the watch. Variations with electric linear generators can be found, for example, in flashlights, whereby the acceleration of the moving core usually has to be very intense and thus arbitrary due to the rather high energy demand of a flashlight.
- Floor slabs generate energy from pedestrians walking over them. The weight of the people lowers the plate by up to 1 cm and thus absorbs up to 50 watts of power from the system. In Toulouse, France, there is the project named Trott-Elec. The energy generated will be used to illuminate the street. Something similar was done for dance floors and small electrical appliances by a company from Rotterdam.
- Backpacks produce electrical energy on their backs to charge a mobile phone.
Energy conversion through the piezoelectric effect
The direct piezoelectric effect converts mechanical pressure into electrical voltage. Piezo igniters are particularly well-known as energy generators, which convert the mechanical energy introduced into the button into an ignition spark. Newer applications in this area are radio switches, in which the energy required for the coded radio signal is also provided by hand-operated piezo elements.
In the field of self-sufficient sensors, piezo elements can generate the energy required for the measurement method and any radio transmission by converting the vibrational energy present in the “environment”, similar to the principle of self-winding clocks.
In 1821, Thomas Johann Seebeck discovered that an electrical voltage is generated between the ends of a metal rod when there is a temperature difference (temperature gradient) in the rod. With suitable materials, it is possible to generate electric current from temperature differences in Peltier elements.
Advantages include:
- No moving parts
- Maintenance-free due to the materials used
- Possible use of natural heat sources or residual heat
A disadvantage of thermoelectric generators is the low efficiency of less than 10%. Thermoelectric generators are available for kerosene lamps.
In the use of the photoelectric effect in photovoltaics, light energy is made directly usable for electrical consumers by means of solar cells. It is the most common method of self-sufficient energy supply and operates, for example, parking meters, traffic control devices, night lights and calculators.
The voltages and powers available from thermoelectric generators are usually very small and require appropriate electronics. For example, an integrated circuit designed for thermoelectricity requires only 20 mV input voltage. Amateurs also try their hand at simple means, such as photodiodes as a source of power supply.
Photovoltaic-powered facilities, on the other hand, are well established and often require only one accumulator in addition to a solar cell to operate. To avoid overcharging, excess current from the solar cell must be dissipated – unless the accumulator is sufficiently resistant to overcharging.
