A New Design of Mini Heat Exchanger for PEMFC Bipolar Plates

Fuel cell is a power generator which directly converts chemical energy to electric energy through an electrochemical reaction instead of thermal combustion. The electrochemical reaction produces as much heat as electric energy. In transport applications where the specific power density of the fuel cell is relatively high a large amount of heat should be evacuated from the system. Therefore, heat exchangers integrated in the bipolar plates play an important role in the operation and life time of fuel cells. Conventional designs of the cooling system of the current bipolar plates consist of a net work of parallel straight channels of half of the bipolar plate’s thickness. These designs reach rapidly their limit of efficacy and new designs are needed. Reynolds number in the bipolar channel flow is around 200; therefore the flow regime is laminar which is known for its weak heat transfer efficiency. In this work we present a new geometry of the cooling channels of bipolar plates in which heat transfer efficiency in laminar regime is enhanced by generating chaotic trajectories. Here we characterize the heat transfer characteristics of a single channel. Firstly, hydrodynamic and heat transfer characteristics of several channel geometries are characterized by using the CFD code Fluent. Thermo-physical properties of the working fluid are those of water and the velocity profile at the channel entrance is that of a fully developed Poiseuile flow. Secondly, fluid mixing along the channels is evaluated using two different criteria. For these calculations, thermal boundary conditions on the channel walls are adiabatic and the entrance of the channel is divided in two (horizontal or vertical) parts. In one part water flows at 300K while in the other part water is at 320K. The first criteria is the ratio of (Tmin/Tmax-Ro)/(1-Ro) over cross-section surfaces calculated for Reynolds numbers 100 and 200 and for both horizontal and vertical positions of the dividing surface at the entrance.