| A innovative micro heat engine design has been developed at the MEMS laboratory of Washington State University. The micro heat engine, dubbed the P3 micro engine, is an attractive candidate for MEMS power. The mass and heat transfer are the controlling processes of this micro heat engine in the conversion of thermal energy into mechanical and then electrical energy. In this study, two-dimensional axisymmetric models for the mass and heat transfer processes in the micro heat engine are presented. A computer code based on the numerical models and the control volume method has been developed and used to investigate the effects of both the geometric and operating parameters on the cycle characteristics and cycle efficiency. A typical set of parameters is chosen first, and the results are then used as a basis for comparison to investigate each of the operating and geometric parameters on the cycle characteristics and cycle efficiency by giving one parameter a small change around its value in the base case and keeping other parameters fixed.; Detailed numerical results are presented, and the effects of each of the operating and geometric parameters on the mass and heat transfer processes and the cycle efficiency are discussed. For the base case, a single engine delivers a first law efficiency of 0.622% and a second law efficiency 25.9%. The parametric analysis shows that the thermal efficiency of the micro heat engine is very sensitive to the geometric parameters, especially the radii of source ring and the PZT membrane. Increasing the radius of the PZT membrane while decreasing the radius of the source ring is an effective way to improve the thermal efficiency of the micro heat engine. Given the geometric parameter, the compression deflection has the most significant effect on the thermal efficiency. Numerical results also shows that the introduction of an insulation coating on the membranes leads to more than 50% improvement in the thermal efficiency. By cascading the unit micro heat engines substantial efficiency gains may be realized. A ten-engine cascade results a thermal efficiency increase from 1.0% to 9.6%. |
