DESIGN OF A PIEZOELECTRICALLY ACTUATED MICROVALVE FOR FLOW CONTROL IN FUEL CELLS

This thesis presents a novel piezoelectrically actuated microvalve for flow control in fuel cells. A fuel cell is an electrochemical device, which directly converts chemical energy stored in a fuel (e.g. hydrogen) and an oxidizer (e.g. oxygen) directly into electrical energy. Poor flow distributions within the cell have been attributed to degraded performance and even damage. In this study, it is proposed to embed microvalves directly into the fuel cells to manage the gas flows and improve efficiency, performance, and reliability. The microvalve has four parts. The actuator is a piezoelectric trimorph which has two piezoelectric layers and one brass layer sandwiched between them and has dimensions of 20000 x 4000 x 290 microns. It also has a valve gate placed on the tip. For a 5-volt input, a deflection of 32 microns can be achieved in the trimorph tip, which is what modulates the flow through the valve.The valve design and analysis are complete. Maximum stress on the bender reaches up to 60 Mpa when the the fluidic and thermal forces are at their maximum. This maximum stress is below the tensile dynamic strength values of piezoelectric and brass layers used. A minimum factor of safety of 1.5 is obtained at 20 degrees C. At the operating temperature, which is about 100 degrees C the factor of safety is higher since the stresses are much lower. The drag and pressure forces are found to reduce the free deflection by only 0.2 microns whereas the thermal expansion forces increases the deflection almost by the same amount. Finally detailed fabrication plan and drawings were completed