AN ACTIVE STIRLING THERMOCOMPRESSOR: DYNAMIC MODELING AND CONTROL

Abstract

Much of the energy consumed by the world’s industrial sectors is lost as waste heat. Concurrently, manufacturing facilities also require vast, valuable quantities of compressed air, air that is most often produced from noisy, inefficient air compressors. Due to its noiseless operation and fuel-flexibility, the Stirling thermocompressor could increase overall production efficiency by converting waste heat directly into pneumatic power. As a little-known class of Stirling devices whose work output is pneumatic rather than mechanical, the Stirling thermocompressor presents new challenges for modeling and experimental validation because few devices have actually been built and tested. This work showcases a simple, first-principles model for predicting the dynamics of a thermocompressor’s power output under a variety of operating conditions and working fluids and validates that model with experimental evidence using one of the few extant thermocompressor platforms in the world. The work also builds on recent findings that suggest that Stirling device performance can be improved using a decoupled, controlled displacer piston to directly shape the device’s thermodynamic behavior, and uses the model to show the mechanism behind the experimentally-demonstrated advantages of a square-wave displacer piston motion profile over a traditional sinusoidal input. Finally, while helium is a popular choice for maximizing Stirling efficiency due to its desirable heat transfer properties, this work not only characterizes a Stirling thermocompressor’s performance using helium at high charge pressures but also using air near atmospheric conditions up to 80 psig, a standard operational range for industrial air compressors. The experimental results together with the model are used to predict the projected performance of a multi-stage thermocompressor, to determine its configuration requirements, and to compare the motor energy consumption of such devices to existing machines. The results of this work conclude that 1) the model can accurately represent a Stirling thermocompressor’s dynamics under a variety of operating conditions; 2) the controlled displacer concept is validated within a low-frequency context, proving to be able to improve peak power output and overall potential work output; and 3) a multi-stage thermocompressor can plausibly increase overall production efficiency in certain industrial contexts.