Stack design for portable artificial muscle generators: is it dangerous to be short and fat?

Dielectric elastomer generators (DEG) are suited for harvesting energy from low frequency and high strain natural sources including wind, wave and human movement. The stack configuration, for instance, in which a number of layers of DE membrane are placed one atop the other, offers a robust, compact and solid-state way for arranging the DE material for energy harvesting during heel strike. But the end conditions at top and bottom of a stack can substantially limit its ability to strain. Using an analytical model for compression of the stack, we have calculated thickness changes in capacitive membranes along the stack for several cylindrical shapes. DE generators that are short and fat will have approximately parabolic profiles with continuous reduction in layer thickness towards the middle. This will result in higher electrical fields at the middle with greater susceptibility to breakdown. For long, thin DEG stacks, the outward bulging will be confined to zones at the two ends with a more uniform cylindrical profile in between. The placing of inexpensive compliant end-caps between the DEG and a rigid structure will promote more homogeneous deformation across the active layers so that the efficacy of these layers for energy harvesting will improve.