Flow-induced energy harvesting has attracted more and more attention among researchers in both fields of the wind and the fluid. Piezoelectric energy harvesters and triboelectric nanogenerators are exploited to obtain superior performance and sustainability, and the electromagnetic conversion has been continuously improved in the meantime. Aiming at different circumstances, researchers have designed, manufactured, and tested a variety of energy harvesters. In this paper, we analyze the state-of-the-art energy harvesting techniques and categorize them based on the working environment, application targets, and energy conversion mechanisms. The trend of research endeavors is analyzed, and the advantages, existing problems of energy harvesters, and corresponding solutions of energy harvesters are assessed.Energies 2019, 12, 2184 2 of 22 pose a potential threat to human health. The environmental threat is more obvious for one-off batteries, which are not intended for replacement and recycling, resulting in appreciable pollution problems [13]. Harvesting energy from the environment instead of carrying a chemical battery around is safer, undoubtedly, for both humans and the nature. Safety and universality make environmental energy harvesting an attractive topic. Relatively speaking, energy harvesting has advantages, as follows: (a) Battery-less operations are feasible, (b) embedded systems are easy to wire, (c) the infrastructure is easy to retrofit, (d) labor is saved, or "fit and forget" [14], and (e) the equipment is lightweight. Some long-term monitor systems with low power consumption, such as sensors for building structure health detection, body information collection, and low-energy self-maintenance systems [15] usually need long-term stable energy supply. In practical applications, ocean climate detectors have an option to take advantage of the ocean by using tidal energy or wave energy, and bridge safety monitors can be recharged by wind flow to suit local conditions. Thus, ubiquitous flow energy provides a good energy harvesting option toward self-powered systems.Energy harvesting from flow environments flourishes with the development of the energy industry along the general trend. Various forms of flow energy, such as wave, tidal, water current, wind flow have been exploited and utilized [16]. The mass level of large flow energy harvester can reach 2 × 10 5 kg, where the energy harvesting magnitude can reach 18.2 KW [17]. Flow energy harvesters can also be minimized as small as a few centimeters and indicate energy harvesting density as 1-10 mW [18,19]. A variety of energy harvesters have been developed based on different energy conversion mechanisms. Among them, the large-scale electromagnetic energy harvester relies on large generators, suffering from complex structures [20]. The approach can produce energy in large magnitudes and can generate electricity feeding the grid for the industrial production process and daily life use. As for triboelectric nanogenerators (TENG) and piezoelectric energy harvesters ...
