Energy Harvesting (Glossary)

'Energy Harvesting' describes technologies for generating and storing energy from locally accessible sources of energy, for instance solar energy, thermal energy, wind or kinetic energy for standard, small, independent devices. A well-known example of this is the automatic watch, which has been manufactured since as early as 1923 and which secures its operating energy from the movement of the arms of the person wearing it.

Self-Generated Mobile Energy

New developments in electrical engineering and the constantly increasing number of mobile devices in everyday use are currently leading to a boom in devices that make use of energy harvesting. Some of these devices are future-oriented, while others are curious playthings used for PR purposes.

Piezoelectric

Piezoelectric can be used for generating power from kinetic energy (pressure, friction, vibration, centrifugal force, etc.). The most well-known device is the piezo cigarette lighter, in which an electric ignition spark is generated by applying pressure to a piezo crystal. Here the special characteristic of the piezo crystal is utilised, i.e. the fact that when it is deformed it transfers a charge which can be used for generating electricity. Currently intensive research is being carried out in piezo fibres in nano dimensions, for example made out of zinc oxide or barium titanate. Such fibres, which are processed into artificial lawn or textiles, can directly generate electricity from movement: here the kinetic energy can, for instance, come from ultrasound or from movements within the body, for example from the human heart beat.

Thermoelectric Voltage

By using thermoelectric generators (TEGs), an application of the Seebeck effect, electricity can be generated out of temperature differences (so-called thermoelectric voltage). The Seebeck effect is the reversal of the Peltier effect. Peltier elements are known as maintenance-free cooling elements for cooler boxes that operate without refrigerants: electricity is used to transfer heat from the inside to the outside, but the efficiency (the ratio between the refrigeration performance and the electrical performance) is very low. With TEG the Peltier principle is reversed and, for instance, electricity flows can be generated using the temperature difference between the warm skin surface and cold air in a room.

Solar Cells

Solar cells can be used for converting light into electricity. The photovoltaic effect on which this is based has been used since 1954 for generating electricity.

Special Energy Harvesting Technologies

In special cases it is possible to use other locally available sources of energy. For instance big differences in salt content, pH differentials or electrostatic charges can be used for generating electricity and electromagnetic waves can also be utilised for generating electricity through using reception aerials, as with RFID technology, for instance.

Usability

In the case of many of the above sensors only small flows of electricity, particularly small voltages, can be generated and the energy yield is often only small. These are suitable for consumers who require very limited electricity flows, in particular electronic circuits that manage with very small voltages. Furthermore the energy sources fluctuate greatly in terms of their strength and availability, which means that intermediate energy storage is necessary – for which rechargeable batteries or supercaps are normally suitable.

© Marc Stenzel

	See also:
		Glossary overview Battery-Packs Batteries - Accumulators - Cells

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