An Investigation of Energy Harvesting Using Electrostrictive Polymers
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An Investigation of Energy Harvesting Using Electrostrictive Polymers Kailiang Rena,b, Yiming Liua, Heath Hofmanna, Qiming Zhanga,b a Department of Electrical Engineering, Penn State Univeristy, University Park, PA 16802 b Material Research Institute, Penn State Univeristy, University Park, PA 16802 ABSTRACT Owing to their low acoustic impedance, high elastic energy density, and relatively high electromechanical conversion efficiency, the electroactive polymers have begun to show the potential for energy harvesting or mechanical to electrical energy conversion. In addition, due to the electromechanical coupling in these materials the electric and mechanical properties of these polymers will depend on the imposed electrical and mechanical conditions. This paper discusses how to utilizing this unique property to maximum the energy conversion efficiency and the harvested electrical energy density in the electrostrictive polymers. As an example, we demonstrate that when a properly phased and externally applied electric AC field is superimposed on the mechanical cycle, an output electrical energy density of 39mJ/cm3 and mechanical-to-electrical conversion efficiency of about 10% can be obtained from the electrostrictive P(VDF-TrFE) based polymers. I. INTRODUCTION: Compared with piezoceramics, which have been investigated for the mechanical-to-electrical energy harvesting, polymers offer many inherent advantages such as much lower elastic modulus, high strain level, and high reliability [4-6]. However, the traditional electroactive polymers such as the piezoelectric PVDF and its related copolymers suffer low electromechanical conversion efficiency and low mechanical energy density associated with electromechanical strain. These features make them not suitable for practical energy harvesting applications. Recently, it was shown that in several electroactive polymers (EAPs), very high electric field induced strain and high elastic energy density (~ 1 J/cm3) can be achieved [1-3]. Furthermore, these EAPS also exhibit much higher electromechanical energy conversion efficiency. In this paper, we investigate the energy harvesting using an electrostrictive polymer. We will show that in addition to the intrinsic materials energy conversion efficiency, one can also make use of the external electrical conditions to further raise the energy conversion efficiency and harvested electric energy density. That is, the efficiency of an energy harvesting system can be much higher than that of the electroactive material used. II. ENERGY HARVESTING CYCLE ANALYSE Presented in figure 1 is a typical energy harvesting cycle used in many publications to illustrate the energy conversion process in a piezoelectric material. [9] In this energy harvesting cycle, the total input mechanical energy density from the external mechanical source to the piezoelectric material is W1+W2 and the harvested electric energy density is W1. The coupling factor or the energy conversion efficiency, therefore, is [9] W1 = k2 (1) W1 + W2
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