|M.Sc Student||Fuchs Jonathan|
|Subject||Phase Change Material Engine for Micro Air Vehicle|
|Department||Department of Mechanical Engineering||Supervisors||Professor Eran Sher|
|Dr. Leonid Tartakovsky|
|Full Thesis text|
In the past several years as the use of unmanned flying vehicles has increased dramatically, miniaturization has become a main line of developing systems. As vehicles have become smaller and smaller, the demand for energy and power has made the energy system a limiting factor by its size and weight, conventional systems such as internal combustion engines (ICE) and battery operated electrical motors are cutting it close to meet the specific energy and power requirements for a miniature vehicle.
Lidor et al.  examined several alternatives: springs, flywheels, thermonuclear devices, artificial muscles, carbon nanotubes, pneumatic systems, fuel cells and phase changing materials (PCM) based systems which found to be a promising candidate with satisfactory levels of specific energy and power. The PCM based engine offers many advantages compared to ICEs: it is a non-complex system with very few parts which makes it very lightweight and reliable, it offers very low sound signature and practically no heat signature, making it a strong candidate for military use, it is an anaerobic engine that requires no oxidizer to operate so it may also be used underwater. Although it carries less energy per weight than the standard ICE, the latter is very limited regarding miniaturization due to its steep efficiency degradation with size reduction . PCM based engine uses the energy stored in a cryogenic fluid. The super cooled fluid draws heat from the environment which causes it to change phase, build pressure and release its energy by expending to the environment through a pneumatic motor.
In our current work, we propose a design for an open system PCM engine that could be built in a miniature scale. Its principles and behavior are analyzed using a fundamental thermodynamic analysis and numerical tools and its advantages and possible drawbacks that might be overtaken or avoided are carefully examined. Where possible, modules are optimized and design tools are provided, it is accomplished through parametric studies to the defining parameters such as temperatures, pressures, volume, and power. Results show that a nitrogen based PCM engine producing 100[W] and running for 20[min] could provide specific power levels of about 30-35[W/kg], specific energy of 10-12[W-h/kg] and efficiency levels of 20-25%.
Different cryogenic working fluids are examined, finding behavior advantages and pointing to the best alternatives - Hydrogen, Neon, Methane and Nitrogen and what are the thermodynamic qualities that makes them optimal. The proposed system is analyzed for stability - proving that stabilization actions must be taken, thus a feedback control system for process stabilization including two alternative control methods are offered.
Finally, a large-scale system is built which serves two goals - a proof of concept and comprehending the notion in where a theoretical design is limited and serving as a test bench, comparing empirical and theoretical results. The empirical phase of the research proved that a PCM engine is feasible and practical, an accumulated period of hours of operation revealed a reliable system with a steady power output and offers an insight to a miniature scale design of an operational system.