Novel Piezoelectric Vibration Energy Harvesters Using Liquid As The Energy-capturing Medium

Piezoelectric energy harvesting technology has promising application prospects in solving the self-powered problem of trillions of wireless sensors deployed around the world in the Internet of Things era.Most of the proposed piezoelectric vibration energy harvesters adopt a rigid energy trapping structure,which has the characteristics of high resonance frequency,large starting potential well,and single vibration direction.It is difficult to harvest the ultralow frequency,low intensity and multi-directional vibration energy effectively that are common in the surrounding environment.For this,we innovatively proposed a piezoelectric vibration energy harvester using liquid as the energy-capturing medium.In addition,in view of the problems that most piezoelectric rotation energy harvesters are hard to harvest the ultralow speed rotation energy and unable to be installed off the center of rotation,we proposed a piezoelectric rotation energy harvester using liquid as the energy-capturing medium.In order to achieve ultralow frequency,low intensity,and multidirectional vibration energy harvesting,a prototype which is mainly composed of a cylindrical container with a certain amount of water,a piezoelectric cantilever beam,and an array of floater-levers was proposed,and the large-scale prototypes was fabricated for experimental verification:(1)The dynamic behavior of forced linear sloshing of liquid in a three-dimensional vertical cylinder was analyzed,and the reason of using liquid as ultralow frequency vibration energy-capturing was given.(2)Several prototypes with single and multiple floaters were fabricated,and the influence of single parameter changes such as the shape of the floater,rope margin,acceleration,and liquid height as well as the effects of multi-parameter changes on the output performance were studied,which shows that the prototype with three or four floaters can real ize ultralow frequency and low intensity vibration energy harvesting within 360° in the horizontal plane;(3)The output power of the prototypes were measured,under the vibration excitation with frequency of 2.7 Hz and acceleration of 0.03 g,the peak output power of the prototype with a single triangular with baffle shaped floater was 1.61 mW,the effective output power was 60.98 μW,and the normalized effective power density was 53.25μW/(cm3·g2·Hz);the peak output power of the prototype with three triangular with baffle shaped floaters was 351.07μW,the effective output power was 20.56 μW,and the normalized effective power density was 17.96 μW/(cm3 · g2·Hz);(4)The energy supply effect of the prototype is demonstrated,under the vibration excitation with frequency of 2.7 Hz and acceleration of 0.03 g,a prototype with single triangular with baffle shaped floater can charge a 33 μF capacitor to 5.24 V in 60 s,and 333 parallel LEDs were be successfully lit;assembling the prototype with three triangular with baffle shaped floaters under vibration excitation in different directions in the horizontal plane,the effect of charging the 33 μF capacitor within 60 s is not much different.In response to the actual application requirements of ultralow speed rotation energy harvesters that have to be installed off the center of rotation,a piezoelectric rotation energy harvester which is mainly composed of a hollow cylinder,liquid,baffle and piezoelectric cantilever beam was proposed,and the large-scale prototypes was fabricated for experimental verification:(1)The mechanical equilibrium conditions of the liquid in the hollow cylinder during the eccentric rotation was analyzed,and the working principle of the device was clarified;(2)Several large-scale prototypes with different baffle structures were fabricated,and the influence of parameters such as baffle plate,offset distance,liquid height,speed and other parameters on the output performance of the device has been experimentally studied,finally an optimization scheme of a large-scale prototype with rectangular groove baffle plate was obtained.(3)The output power of the prototype was measured,under the offset distance of 120 mm and the rotation speed of 75 rpm,the peak-to-peak output power of the prototype was measured as 13.90 mW,the peak output power reached 4.90 mW,the effective output power was 140.90 μW.In additional,the peak power density and the effective power density is calculated as 10.81 μW/cm3 and 0.31 μW/cm3,respectively.(4)Demonstrated the energy supply effect of the prototype in the actual application environment,the prototype was installed 120mm away from the rotation center on the side of the car hub,approximately constant speed driving on the internal road of the campus(15～20 km/h,average speed 17.5 km/h)about 30 minutes can store 36.00 mJ of energy in the energy management circuit,which can be used for the wireless sensor with Bluetooth transmission signal function(Xiaomi Thermo-Hygrometer 2)to work for about 5 minutes and 20 seconds,and send 38 data signals to the mobile phone in total;During the peak period of commuting to get off work on congested roads(0-50 km/h,average speed 18.4 km/h),driving at a non-uniform speed for about 27 minutes can store 28.50 mJ of energy in the energy management circuit.