Investigation of a MEMS-Based Boiler and Free Piston Expander for Energy Harvesting

Abstract

Initial investigations of a small-scale Free Piston Expander (FPE) are presented. In final form, the FPE will be a MEMS-based device capable of operation from low temperature waste heat sources. In this present study, a millimeter scale device is constructed and tested to yield insight into critical operational parameters. Different constructions and operating conditions are considered as are the effects on basic piston motion and performance. These include piston length and mass. In addition, different sealing and lubricating fluids are considered. Construction of this testbed device is via concentric copper tubing, allowing an effective baseline study of these determining parameters. Results show that, while thick lubricants seal well in a static test, piston motion is decreased in a dynamic test indicating leakages. By contrast, reduced viscosity lubricants dont seal as well in a static test, but yield increased piston motion in dynamic testing. This indicates effective sealing. The trends established by the study of varying viscosity lubricants hold true for pistons of increasing mass and length as well. A mixture of isopropanol and water performed well in these tests, and represented a low viscosity sealing fluid. Compared to conditions where no lubricant was used, maximum velocity was increased up to 50%. These results indicate that a thin, wetting fluid will be the best lubricant for the FPE, due to increased sealing and performance when in dynamic operation.