Energy loss characterization of the P3 MEMS heat engine

Kirsten Elizabeth McNeil

The P3 MEMS heat engine has been developed to convert waste heat into useful electrical energy. Such energy conversion devices are complex systems. Other micro engines under development around the world are facing similar challenges, but the P3 micro heat engine has a special advantage, building from the principles which govern the scale on which it is designed. The engine system has many parameters, including membrane size, cavity thickness, bubble size and initial deflection, each of which influences the operation of the system. The free vibration response of the engine has been studied to characterize each aspect of the system and determine the dominant parameters. Governing equation analysis details the relationship between the mass, stiffness and damping of each component of the system. Two main figures of merit are currently used to study the engine system: resonant frequency and quality factor (Q). These two parameters have been extensively characterized for many different engine configurations. Trends of these figures of merit with the engine parameters have been determined. Experiments and modeling of the engine show that a higher upper membrane stiffness and lower liquid damping yield higher resonant frequency and Q. The model of the engine as a linearly coupled oscillator yielded trends of Q with upper membrane stiffness, liquid stiffness, and liquid mass. A decrease of several orders of magnitude of the upper membrane stiffness results in a much greater change in Q than a similar variation in liquid stiffness. If the generator stiffness is increased by a factor of 100, resonance is about 4700Hz, with a Q of 70; whereas if the stiffness of the liquid was increased by a factor of 100, resonance is about 1570Hz with a Q of 23. The model reflects experimental trends seen with Q and bubble size as well. Varying the bubble size from 1 to 4mm yields Q values from 22.8 to 6.5, reflecting the trend seen experimentally of Q decreasing with increasing bubble size. Changing the liquid mass in the model yielded no significant changes in Q, showing that this parameter has little effect on the system. These trends along with details discovered through the model lead to the conclusion that the single most influential engine parameter is the liquid damping. Trends set the foundation for optimization, where engine performance can meet the resonant frequency goals while losing the least amount of energy to internal and external forms of friction and energy loss mechanisms. Experimental data and the linearly coupled oscillator model combine to facilitate optimization.

Energy loss characterization of the P3 MEMS heat engine

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