Characterization of a Pneumatic Strain Energy Accumulator: Efficiency and First Principles Models with Uncertainty Analysis

Abstract

Several technical needs were identified and addressed for advancing the Strain Energy Accumulator (SEA), which is an energy storage device consisting of an expandable rubber bladder inside of a rigid shroud that stores energy in the form of pressure and strain. First, multiscale modeling methods were investigated to estimate the homogenized elastic modulus of carbon nanotube (CNT) rubber. The result is homogenized modulus estimates ranging from a few times to almost 80 times the elastic modulus of rubber, indicating the need for validation of existing models or development of new models to estimate the modulus for matrix and inclusion materials having drastically dissimilar moduli. Second, an analytical methodology was developed for simultaneously characterizing the energy storage in pneumatic and strain energy systems including component efficiency. By incorporating uncertainty analysis, the efficiencies of the strain energy accumulator are measured in over 2500 cycles of testing to be consistently over 93 %.

Third, system state efficiency models were developed and expanded. Through experimentation, the model was determined to be favorably conservative with system efficiency projections ranging from 31 % to over 60 % depending on the system configuration. In addition, materials challenges in high pressure applications led to the conceptual investigation of CNT elastomers offering improved material strength properties and the potential for self-sensing. In previous research, carbon nanotube sensor thread was tested as a distributed sensor on carbon fiber reinforced composites and was able to monitor strain and detect damage in composite panels. The use of nanomaterials for self-sensing was extended in the current work with proof of concept tests performed on electrically conductive elastomers that exhibited the ability to monitor load and detect damage in specific directions.

Each of these contributions in the areas of materials modeling, uncertainty analysis, and component and system efficiency quantification techniques has helped to advance the Strain Energy Accumulator technology.