16HS3: Controlled Stirling Power Unit

Project Leader: Eric Barth, Professor of Mechanical Engineering

Institution: Vanderbilt University

This project will develop a completely silent, high energy dense, and portable fluid power supply using a stirling device. Using a high energy density fuel source, the Controlled Stirling Power Unit will address the energetic deficiencies of the state-of-the-art power supply and actuation systems for mobile robots.

Broadening the market’s scope to military, medical, manufacturing, and construction applications, mobile robots and exoskeletons face an increasing need for longer lasting, lighter weight power sources to facilitate autonomous operation. Current power supply systems, such as battery-powered servomotors, IC engines, and monopropellants, are limited by energy density, efficiency, weight, mechanical output, and/or operating time, making them a less viable option for portable, untethered robots. Battery-powered servomotors offer a relatively cheap and easily controlled system, but however are limited by their low energy density and low power capacity. IC engines utilize energy dense fuels and are capable of driving both fluid-powered and electrically powered systems but operate loudly and carry additional weight which further degrades the effective energy density. Monopropellants are a third alternative which offer both the high energy density and low system weight desired for a portable power source. While they produce power on demand and convert directly to mechanical work, monopropellants are not readily available and contribute possible safety hazards in their use.

Due to the limitations of each power supply option, a small, lightweight, quiet, energy dense power source with good conversion efficiency is essential for future autonomous robots and exoskeletons. The Stirling engine is a novel, superior system which successfully satisfies these needs by utilizing a flexible fuel/heat source within its small, easily scaled-down design. The controlled Stirling concept discussed here can use a number of high energy density working fluids such as butane, methane, natural gas, and propane, among others. This hydrocarbon fuel drives a power extraction unit by converting the absorbed heat to oscillatory pressure energy. Due to the high energy density of the fuel, the Stirling engine needs an average efficiency of only 1.4% to surpass the fuel specific work output of a battery-powered system. With this goal, a compact, portable, fluid-powered actuation system has been developed to achieve power and energy densities an order of magnitude greater than state-of-the-art batteries. Additionally, the controlled Stirling power unit is silent (whisper level dBA).

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