Piezoelectric polymer films as power converters for human powered electronics

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Abstract

Piezoelectric polymer film material allows for conversion of mechanical energy into electrical energy that can be used for supplying electronic devices. While this method does not allow obtaining large useful power, recent advances in electronic technology, in particular wide availability of submicron low-power CMOS processes, have made feasible the idea of using piezoelectric polymers as power converters for human powered electronics. This concept allows to overcome the necessity of using battery as a power source, which is one of the main obstacles to widespread adoption of wearable computing devices. Of particular interest is harvesting energy from walking, which can be achieved by using piezoelectric polymers. In this paper maximum power has been calculated that can be drawn from walking energy owing to application of the copolymer polyethylene–polypropylene (PE–PP) shoe insole. The amount of electric energy obtained from a PE–PP foil of a thickness of 11 μm for a single step of a duration of 1 s – that is equivalent to a frequency of 1 Hz – amounts to 340 nJ.

Introduction

Some polymer materials exhibit piezoelectric properties, i.e. generation of surface electric charge when subjected to a mechanical stress, as well as a reciprocal effect, that is a change of dimensions when subjected to an electric field. The prerequisite for the piezoelectric properties of electroactive polymers (EAP) is their permanent polarization that results from stable arrangement of molecular dipoles or an uncompensated surface or volume electric charge. Piezoelectric film material allows for the conversion of mechanical energy applied to the material into electrical energy that can be used for supplying electronic devices.

For the last several decades, the computer industry has been driven by Moore’s law. Although it has originally referred to transistor density in an integrated circuit, it has also been found applicable to other parameters of the computer system, such as memory size, CPU speed, RAM, and hard drive capacity. These advances have enabled the development of first portable, and later wearable, computing devices. However, a certain critical element of a wearable computing system does not scale according to Moore’s law; i.e. the power source. For the last several years, there has been no significant improvement in battery energy density. The inadequateness of battery power sources warrants research of alternative power sources, such as those harvesting energy from a human body movement, e.g. from walking [1], [2], [3], [4], [5]. This concept allows to overcome the necessity of using battery as a power source, which is one of the main obstacles to widespread adoption of wearable computing devices [6], [7]. Multiple approaches to harvesting energy from walking have already been presented [1], [2]. Most notable ones are: use of electromechanical generators, in particular electromagnetic converters based on rare earth magnets, use of piezoceramics and piezopolymers. The possibility to recover some of the energy dissipated during walking or running owing to storing the charge produced by the piezoelectric materials embedded into shoes, as well as new devices powered in such a way have been investigated at the MIT Media Laboratory [1]. Kymissis et al. [2] examined three different devices built into a shoe and used for generating electrical power while walking: piezoceramic PZT strip, multilayer PVDF (polyvinylidinefluoride) foil stave and rotary magnetic generator.

Improvement of the composition of organic piezoelectric materials and their production methods aimed at an increase in the generated electrical signals and foil durability have been subject of many publications. In this paper we focus on using new piezoelectric polymers, in particular PE–PP foil. Compared to the competing technologies, this method has several advantages, namely: low mass, lack of moving parts and flexibility. The chief disadvantage, compared to the other solutions, is low-power output; however recent advances in electronic technology, in particular wide availability of submicron low-power CMOS processes, have made feasible the idea of using EAPs as power converters for human powered electronics.

The aim of this paper is to investigate feasibility of using a polypropylene–polyethylene copolymer PE–PP as a power source for human powered electronics. We focus on the application in form of a shoe insole, made of PE–PP. In particular, we are interested in determining these properties of the piezomaterial, which are useful in designing power converters, i.e. piezoelectric constants and internal resistance. The results of laboratory measurements of power generated by a PE–PP foil are compared with the signals generated by a shoe insole in the natural conditions, while walking. Finally, we discuss feasibility of using a PE–PP shoe insole for powering an electronic device.

Section snippets

Materials

The investigated polymer material was prepared in the form of a foil 11 μm thick. This amorphous piezoelectric foil is a self-made new product based on polypropylene–polyethylene (PE–PP) copolymer. Relevant physical properties of the foil have been summarized in Table 1. The application of this foil enabled us to attain a voltage signal three times higher than that for PVDF foil.

The test samples with a surface area of 10 cm2 (the surface of measuring electrode) were placed between metal capacitor

Conversion of mechanical energy into electrical energy of PE–PP foil piezoelectric element

Conversion of mechanical energy into electrical energy and maximum power attainable by the use of a PE–PP foil were investigated experimentally under laboratory conditions and under walking conditions. The piezoelectric elements were loaded with a resistance ranging from 2 to 165 MΩ and from 2 to 40 MΩ under laboratory conditions and under walking conditions, respectively. In both cases, the dependence of r.m.s. voltage value on the load resistance was determined and the power was calculated for

Discussion

The results of the investigations on the maximum power that can be generated in the PE–PE foil under laboratory conditions and under natural conditions are collected in Table 2. The results of the laboratory investigations allow us to predict only very approximately the behaviour of the piezoelectric foil under natural conditions. The major reason of it is that in the laboratory investigations, the stresses were applied mainly in the direction 33, while under natural conditions, the stresses

Conclusion

Our results show that at present it is possible to use a PE–PP piezoelectric polymer foil placed in a typical insole of trainer shoes as a micropower generator, producing during normal walking for a single layer about 0.3 μW. Application of multilayer foil in future works should increase the generated power by one order of magnitude.

Acknowledgements

The authors wish to express their gratitude to Mr. Andrzej Cichocki and Mr. Wiesław Prochwicz, M.Sc. Eng. for their assistance with electric measurements and editing of this paper.

The investigated material was fabricated in the framework of an EU FP6 Project No. 507378 “CEC-MADE-SHOE – Custom, Environment, and Comfort made shoe”.

This work was funded by Polish Ministry of Science and Higher Education in range of the Research Project No. N507 4634 33.

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