Energy harvesting using piezoelectric inverted flags

A flexible foil aligned to the incoming fluid flow with a free leading-edge and fixed trailing-edge, known as an ‘inverted flag’, under the correct operating conditions performs high amplitude limit-cycle oscillations which cause high amounts of cyclic strain in the flag. This cyclic strain, when coupled with piezoelectric elements, can be used as a source of electric power. Since their invention in 2013, piezoelectric inverted flags have gradually matured into energy harvesting prototypes suitable for small-scale applications, particularly for microsensors, either in stand-alone installations or distributed network configurations. This review provides a critical assessment of piezoelectric inverted flags, devoting special attention to important aspects that have been overlooked in previous investigations, including how the flapping dynamics is affected by changes in the flag geometry and by the inclusion of piezoelectric elements, the design of suitable electric power conditioning circuits to couple flag harvesters with electronic devices, and a critical assessment of the electric power generation capability of inverted flags in comparison with other, more mature, wind energy harvesting solutions. Research gaps and topics that necessitate further investigation to move inverted flag harvesters forward from the prototyping stage to field applications are identified and discussed.