Dynamic Model of Dielectric Elastomer Diaphragm Generators for Oscillating Water Column Wave Energy Converters

Dielectric Elastomers (DEs) are incompressible rubber-like solids whose electrical and structural responses are highly nonlinear and strongly coupled. Thanks to their coupled electromechanical response, intrinsic lightness, easy-manufacturability and low-cost, DEs are perfectly suited for the development of novel solid-state polymeric energy conversion units with capacitive nature and high-voltage operation, which are more resilient, lightweight, integrated, economic and disposable than traditional generators based on conventional electromagnetic technology. Inflated Circular Diaphragm DE Generators (ICDDEGs) are a special embodiment of polymeric transducer which can be used to convert pneumatic energy into usable electricity. Potential application of ICD-DEGs is as Power Take-Off (PTO) system for wave energy converters based on the Oscillating Water Column (OWC) principle. This paper presents a reduced, yet accurate, dynamic model for ICD-DEGs which features one degree of freedom and which accounts for DE visco-elasticity. The model is computationally simple and can be easily integrated into existing wave-to-wire models of OWCs to be used for fast analysis and real-time applications. For demonstration purposes, integration of the considered ICDDEG model with a lumped-parameter hydrodynamic model of a realistic OWC is also presented along with a simulation case study. INTRODUCTION Dielectric Elastomer (DE) transducers are a promising technology for the development of solid-state actuators, sensors and generators [1]. DE transducers comprise one or more sheets of incompressible dielectric rubber which are sandwiched between compliant electrodes to form a deformable capacitor. In actuator mode, electrostatic attraction between oppositely charged electrodes is used to convert electricity into mechanical energy. In sensor mode, measurements of the deformable capacitance are used to infer transducer strains or stresses (that is, displacements or forces). In generator mode, mechanical energy is converted into direct electricity via the variablecapacitance electrostatic generator principle. Properties of DEs which make them suited for transduction applications are: low mass density; large deformability; high energy density; rather good electro-mechanical conversion efficiency; moderate or low cost; solid-state monolithic embodiment with no sliding parts; easy to manufacture, assemble and recycle; good chemical resistance to corrosive environments; silent operation. Initially proposed as muscle-like actuators for robots, DEs are now receiving significant attention in energy scavenging applications [2-6], in particular for the development of Wave Energy Converters (WECs) [7-11]. Specifically, DE Generators (DEGs) are foreseen to replace the Power-Take-Off (PTO) systems of traditional WECs, which are currently made of stiff, heavy, shock-sensitive, corrosion-sensitive and costly (metallic and rare-earth) materials. Expected advantages of DEG based PTOs are: ease of installation and maintenance, low capital and operating costs, shock and corrosion insensitivity, noise and vibration free operation, high energy conversion efficiency that is independent of sea-wave period. A very interesting concept of a DEG-based WEC is the Polymeric Oscillating Water Column (Poly-OWC) [9] that is shown in Figure 1. A Poly-OWC is a partially submerged hollow structure featuring an immersed part opened to the sea action, and an upper part closed by a DEG membrane and forming an air chamber. The structure partially encloses a column of water which is exposed to the incident wave field at the bottom and to the chamber air pressure at the top. As the waves break on the Poly-OWC structure, wave-induced pressure oscillations at the underwater interface cause the reciprocating motion of the water column, with a concomitant 1 Copyright © 2013 by ASME Proceedings of the ASME 2013 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2013 September 16-18, 2013, Snowbird, Utah, USA