Energy harvesting from a backpack instrumented with piezoelectric shoulderstraps

Abstract

Over the past few decades the use of portable and wearable electronics has grown steadily.These devices are becoming increasingly more powerful. However, the gains that have beenmade in the device performance have resulted in the need for significantly higherpower to operate the electronics. This issue has been further complicated dueto the stagnant growth of battery technology over the past decade. In order toincrease the life of these electronics, researchers have begun investigating methodsof generating energy from ambient sources such that the life of the electronicscan be prolonged. Recent developments in the field have led to the design of anumber of mechanisms that can be used to generate electrical energy, from avariety of sources including thermal, solar, strain, inertia, etc. Many of theseenergy sources are available for use with humans, but their use must be carefullyconsidered such that parasitic effects that could disrupt the user’s gait or enduranceare avoided. These issues have arisen from previous attempts to integrate powerharvesting mechanisms into a shoe such that the energy released during a heal strikecould be harvested. This study develops a novel energy harvesting backpack thatcan generate electrical energy from the differential forces between the wearerand the pack. The goal of this system is to make the energy harvesting devicetransparent to the wearer such that his or her endurance and dexterity is notcompromised. This will be accomplished by replacing the traditional strap of thebackpack with one made of the piezoelectric polymer polyvinylidene fluoride (PVDF).Piezoelectric materials have a structure such that an applied electrical potential resultsin a mechanical strain. Conversely, an applied stress results in the generationof an electrical charge, which makes the material useful for power harvestingapplications. PVDF is highly flexible and has a high strength, allowing it to effectivelyact as the load bearing member. In order to preserve the performance of thebackpack and user, the design of the pack will be held as close to existing systemsas possible. This paper develops a theoretical model of the piezoelectric strapand uses experimental testing to identify its performance in this application.