Performance analysis of a miniature free piston expander for waste heat energy harvesting

• A novel free piston expander is experimentally analyzed for waste heat retrieval.
• A variety of variables are analyzed using three separate experiments.
• Lubrication of device is optimized with lower viscosity lubricants.
• Circular cross sectional pistons show increased repeatability and sealing.

Initial experimental analysis of a small-scale Free Piston Expander (FPE) is presented. In final form, the FPE will be a MEMS-based device capable of operation from low temperature waste heat sources. Currently, a millimeter scale device is constructed and tested to yield insight into critical operational parameters for use in later design and testing.Operating conditions are examined to increase operational performance. Piston stroke length and repeatability are considered. Optimized variables include piston length and mass, FPE shape and size, input pressure, and lubrication. 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 static configurations, piston motion is decreased in dynamic testing, indicating leakage. By contrast, reduced viscosity lubricants prove ineffective as sealing agents during static conditions, however, yield increased piston motion in dynamic testing with little leakage around critical piston sealing surfaces. 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, which was used in later repeatability tests. Repeatability tests were performed in a closed dynamic environment on FPE designs with multiple cross sectional shapes and areas. Results from these tests show that circular FPE’s are more precise than square FPE’s. The final closed system tests yield a pressure–volume curve indicative of an engine cycle as a first look at thermodynamic cycle operation from the FPE.