Piezoelectric harvesters were proposed and studied by many researchers because of their low cost, light construction, simple production process, and high energy density [156]. Different modes of piezoelectric material are usually used to collect energy from the piezoelectric material depending on the (d₃₃), axial (d₃₁), and shear (d₁₅) modes. A compressive force is applied to the piezoelectric material in the d₃₃ mode, while in the d₃₁ mode the piezoelectric patch is fixed on the console, and vibrations are created in the beam in order to create a bending strain in the piezoelectric patch. In the shear mode, the corresponding force is applied along the y-axis in the yz-plane, while the polarization direction remains along the x-axis [157].

In [157], an PZT energy harvester operating in a shear mode was developed and manufactured to collect the energy of water flowing at varying pressure. The open circuit voltage of 72 mV and the power of 0.45 nW were obtained experimentally at a pressure amplitude of 20.8 kPa and a frequency of about 45 Hz. An analytical mathematical model of voltage generation by a unimorphic piezoelectric cantilever beam based on the classical beam theory was proposed in [158]. The obtained numerical results of the model are consistent with the experimental results. At the same time, about 24.5 μW of power was obtained in testing at a resonant frequency. It was shown in [159] that by changing the value and orientation of the magnetic force, the frequency response of a piezoelectric cantilever can be changed in such a way as to provide a useful method for collecting out-of-resonance vibrations.

In [160], the topology of the PZT energy harvester was presented, consisting of a wind turbine based on an elevator with a PZT beam without a contact vibration mechanism. It has been demonstrated that a power density of 2 mW/cm³ can be achieved at a voltage of 3.8 V at a wind speed of 0.9 m/s. In [161], a windmill was constructed having a wind turbine with a horizontal axis and 12 magnets of variable polarity along the periphery. At the same time, a bimorphic PZT element with a size of 60 × 20 × 0.7 mm³ had a magnet at its end. In this design, a peak electrical power of 450 µW can be achieved with a nominal wind speed of 4.2 miles per hour. In [162], a rotary harvester was proposed based on the vibration caused by the wind impact. A PVDF piezoelectric beam was used to maintain high deflection during impact. Analytical modeling was developed followed by the use of FE calculations. The maximum power of 2566.4 μW was achieved at a wind speed of 14 m/s.

In [163], a galloping cantilever beam was developed to collect energy. The beam had a D-shaped cross-section of a piezoelectric sheet bonded in the form of a bimorph. Continuous air supply to the D-shaped section led to abrupt oscillatory movements and, consequently, was converted into electrical energy through a piezoelectric sheet. Experimentally and analytically, a maximum power of 1.14 mW was obtained for the prototype at a wind speed of 4.69 m/s with galloping vibration. In [164], the PZT generator was modeled as a bending mode using the Euler–Bernoulli beam theory. With a load of 29 kΩ, the maximum power of 6.5 mW has been achieved. In [165], a cantilever harvester for crosswind was developed. The phenomenon of dropping the tops was used to create vibration of the cantilever beam. This allowed generating 2 W of output power at a resonant frequency.

The energy received from rotating magnets can be increased by expanding the broadband frequency range [166]. It was shown in [167] that by using the dynamic magnetic force method, the harvesting power of multi-cantilever beam harvesters can be increased to 55.6%. In [168], the strain of a prestressed piezoelectric beam caused by interaction with a magnet was used. This harvester was used in the tire pressure monitoring system. A hybrid energy harvester combining piezoelectric and electromagnetic generators has been developed for electronic devices and a wireless system. In [169], the piezoelectric and electromagnetic methods were combined to make a hybrid energy harvester appropriate for the power requirements of electronic devices, wireless sensors, and nodes. The oscillation of the bimorphic cantilever created an electric potential on piezoelectric surfaces as the relative motion of the magnet disposed on the cantilever end. It induced an electromotive force in the coils in addition to the magnet. About 10.7 mW of power was collected, which is 81.4% higher than that of one electromagnetic harvester. In [170], a cantilever-type piezoelectric harvester on a shear mode (d₁₅) was present for collecting energy. The estimated power was approximately 16 mW with an external load of 3 MΩ.

In [166], rotating magnets were used to increase the broad band frequency range of the energy harvester and increase productivity. In [171], a vibration-based energy harvester was developed for applications with rotational motion. Beams with proof masses and piezoelectric pads fixed to the beams were installed on the cantilever of rotating hub. Vibration was induced in the beams due to rotation and gravitational influence of the proof masses. Both polyvinylidene fluoride (PVDF) and PZT were used to collect energy at optimal load resistance. The maximum power of 6.4 mW was achieved at a shaft rotation speed of 138 rad/s when using PZT material. In [172], a cantilever-type harvester was designed, excited by a flow created for heating, ventilation, and air conditioning. Blowing air by hitting the aerodynamic keel fixed at the end of the beam caused vibrations in the beam. As a result, a power of about 3 mW was generated at a flow rate of 5 m/s.

In [173], a piezoelectric harvester based on a scotch yoke mechanism was developed to convert rotational motion into linear vibrations of two piezoelectric levers through springs. The simulation results showed that for a piezoelectric wind turbine with a blade radius of 1 m at a wind velocity of 7.2 m/s and an estimated angular speed of 50 rad/s, a power of about 150 W can be obtained. In [174], an octogenerator was proposed for collecting wind energy by using a piezoelectric material. The analytical model made it possible to calculate the power of 5 kW for the device at a tide speed of 1.75 m/s in the ocean.

In [175], an annular piezoelectric harvester with internal and external concentric rings was developed. The inner ring rotated with some frequency, and the outer ring was stationary to create relative movements between these rings. Piezoelectric pads were installed inside the stationary ring, and each piezoelectric plate magnet was mounted on PZT pads; a magnetic sinusoidal repulsive force was created on the outer periphery of the inner ring during rotation. At an inner ring speed of 30 m/s, the maximum power of 5274.8 W was collected using this device with a radius of 0.5 m.

Thus, the above values of output power in practice may differ by several orders of magnitude. Obviously, full-scale energy-harvesting facilities (in particular, rotary devices) make it possible to significantly expand the output power range observed for low-power electronics, which usually varies from a few microwatts to milliwatts. It could be explained that such an expansion of the range occurred due to fundamentally new design features of macro objects, their complex design, and additional electromagnetic elements of considerable sizes manufactured on the basis of promising materials. The scale factor is characteristic not only for fracture mechanics but also for energy harvesting.

In [176], the shear mode (d₁₅) of a cantilever-type piezoelectric bimorphic energy harvester was used to satisfy the Timoshenko beam theory. The developed analytical model gave results close to finite-element modeling. Open circuit peak voltage and power obtained at optimal electric load resistance and operating resonant frequency were observed. Based on the Timoshenko beam theory, an energy harvester, operating in a shear mode (d₁₅), is presented in [2]. The cantilever PZT sandwich beam was excited by the vibration of the base, causing shear deformation. It was found that the energy obtained in the d₁₅ mode was approximately 50% higher than the energy obtained in the d₃₃ mode. The data [177] showed that the value of d₁₅ is greater than the values of d₃₁ and d₃₃. Therefore, a shear excitation mode of piezoelectric material should be used to capture power during rotational motion with a simple design of the structure.

In the last decade, some author’s solutions for rotating harvesters have been researched and developed.