Study of a MEMS-Based Free Piston Expander for Energy Sustainability

A novel free piston expander (FPE) has been designed and modeled for energy sustainability applications. Specifically, the device has been designed to produce power from low-temperature energy sources as part of a larger low-temperature steam system. Due to the needs imposed by sustainability applications, the FPE was designed with two membranes: the first, a primary power output membrane and, the second, a regenerative membrane capable of mechanical energy recapture during the FPE cycle. The FPE model was studied under a variety of conditions. Different membrane sizes were shown to alter FPE performance considerably. Using 10 mm side length membranes, modeling showed that up to 25.6 mW of FPE power output was possible. The gap between the sliding free piston and its surrounding bore was examined using various fluids and gap geometries to simulate friction losses. By reducing fluid viscosity or increasing piston/bore gap, the energy lost to fluid shear was minimized. This resulted in improved energy recapture via the regenerative membrane. Various piston masses and materials were also considered. Decreasing piston mass reduced piston stroke length and increased frequency of operation. This resulted in an increased power output. The energy recapture capability of the FPE showed approximately 88% to 90% effectiveness for most of the scenarios considered in this work.