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Electrodynamic energy harvesting

This is sometimes called electromagnetic energy harvesting but this confuses it with harnessing infra red, visible light and ultraviolet. .Electrodynamic harvesting usually involves use of human power as with bicycle dynamos and wind-up devices but now it is a major focus for vibration harvesting and other sources. Like the above example, electrodynamic generation is caused by movement of a magnet across a coil or a coil across a coil, the field being induced. It is sometimes called electromagnetic but that term can be confused with use of electromagnetic radiation which is quite different. Thus at Southampton University Hospital in the UK, the human heart moving a magnet across a coil generates electricity for a pacemaker in one experiment. Many wind up lights, radios and the like employ dynamos to charge capacitors or batteries. Strong damping forces require rapid flux changes. These are difficult to achieve in a small device and at low frequency. The number of coil turns, and therefore the magnitude of the induction is limited in small devices such as MEMS resulting in small output voltages. Use of magnets or ferromagnetic materials may be called for, adding to weight and expense. In contrast to piezoelectrics, electromagnetic generators are easy to use with rotational energy or pendular swing of a mechanical load and less practicable with vibration, so the two approaches are complementary.
Some examples of electrodynamic energy harvesting are given below.
Examples of electrodynamic harvesting. Left, miniature right torch
Source: IDTechEx
Some wind up radios and lights for Africa have no battery, the clockwork mechanism being carefully designed to provide constant torque that drives the device directly through a dynamo. Another imaginative form of electrodynamics is the experiment at Southampton University Hospital in the UK where two small, liquid filled balloons are inserted in the human heart and connected by a silicone tube containing a moveable magnet. As the heart beats, it squeezes each balloon in turn, pushing the liquid through the tube and taking the magnet back and forth with it. Because there is a coil embedded in the tube, this generates electricity to charge the battery in an embedded pacemaker or defibrillator. The hope is that the system could be left in for the life of the patient or at least greatly reduce the need for intrusive surgery, even providing enough power to radio diagnostic and performance data. Faraday would have been proud. In a heart conference in late 2008, developer Paul Roberts said that the power taken does not significantly burden the heart. The output of 4.3 microjoules per heart beat is expected to double with use of better polymers.
Heat Harvester
Source: University of Southampton Hospital
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