The most popular EH technologies in numbers deployed are:
(PV) - harvesting light and sometimes infrared by a pn junction (almost all) or photoelectrochemical reaction (Dye Sensitised Solar Cells DSSC).
Electrodynamic ED - capturing movement, vibrational, rotational or linear by means of electromagnetic
machines such as dynamos, alternators and electric vehicle traction motors working backwards. To be precise, EDEH derives from interactions of electric currents with magnets, with other currents, or with themselves.
A long way behind: Piezoelectric
- capturing movement by means of materials that generate electricity when acted on by a mechanical force and Thermoelectric - capturing temperature difference via a pn junction - Seebeck effect. Thermoelectric will now pull ahead but not rival 1 and 2.
Photovoltaic and electrodynamic options are and will remain the most successful not because the transducers (electricity production part) are changing radically but because they are exceptionally versatile. They keep being reinvented in new forms, recent examples being solar roads and electrodynamic Airborne Wind Energy AWE using autonomous tethered multicopters. These and other new examples almost always use the existing physics of single and polycrystalline silicon photovoltaics and asynchronous and synchronous rotating machines for electrodynamics.
It is not particularly useful to debate whether PV or ED is the biggest market. In numbers it is PV if we include toys and novelties and on value EH wins on some calculations. Both will remain very important in future.
Although the harvesting for microgrids is typically from wind turbines for bad weather and photovoltaics for good weather, a host of additions and alternatives promise better. For example, autonomous tethered multicopters can fly higher where wind is stronger and more consistent. Stretchable piezo, capacitive or other elastomer has been proposed for oscillation in even small streams to produce power though currently their power output is far too small. Electric vehicles have been demonstrated that harvest power beyond their operational needs just with photovoltaics. Multiple energy harvesting
by vehicles is coming. These vehicles, when parked, can form part of microgrids. Communities near the ocean will be able to harvest tide and waves more economically with planned new technology.
Top image: Airborne Wind Energy