TFG: Analysis and design of an energy harvesting system for human body

shoe sole

The aim of this project, was to design a functional prototype for the transformation of energy based on the principle of piezoelectricity, in order to harvest the energy produced. After some research, this is determined to be the best postulate to generate electrical power at a low scale for applications in electrical systems that require low voltage power supply, working as a stand-alone power to charge both, medical and electronic devices.

When a piezoelectric material is exposed to mechanical deformation, a voltage is produced. The theoretical behaviour can be appreciated in the following image:

piezoelectricity

Therefore, the energy that can be harvested depends on two factors: the properties of the piezoelectric material and the amount of deformation applied to the material.

Some of the materials that show piezoelectricity are: quartz, lead zirconate titanate (PZT), aluminum nitride (AlN), zinc oxide (ZnO) and polyvinylidene fluoride (PVDF).

The special property of these piezoelectrics is that it allows them to convert physical energy into electricity, AC. However, we need DC, not AC to power devices. This problem can be solved creating a rectifier bridge with diodes to convert the power from AC to DC, and thus be able to use it.

Although piezoelectric elements generate a lot of voltage, they do not generate many amps. We can solve this problem by wiring all the piezoelectric elements in parallel

Taking into account all the mentioned above, the prototype that has been created is formed by 7 PZT piezoelectrics of 35 mm diameter, as shown in the picture at the top pf the page.

Finally, it has been proved, when charging some capacitors, that it is better to have the shoe sole outside, placed on a smooth surface (as a carpet) and then making pressure on them. In such a way, the most relevant results were obtained. The capacitors were charged more quickly than while walking with the shoe sole inside. The order of magnitude of the power generated by this assembly was mW, and the energy generated was in the order of mJ.