Thermal Management of Edge-Cooled 1 kW Portable Proton Exchange Membrane Fuel Cell Stack

Abstract

Air-cooled proton exchange membrane (PEM) fuel cell stacks are gaining momentum as power sources for portable applications such as electric bikes, unmanned aerial vehicles (UAVs), forklifts, etc. Usually the power of portable stacks (PS) is up to several kW [1,2]. Most of the heat generated during PS operation is dissipated into the surrounding atmosphere via forced air convection along the cooling channels or edge-cooling fins, while the higher power stacks require liquid cooling. The emphasis during the design phase of PS is primarily directed towards determining the most suitable material for bipolar plates and the accompanying thermal management. Some of the thermal management objectives are: (i) ensuring the appropriate heat transfer from the stack during startup, i.e. accurately determining the required convection flow rate, to achieve sufficiently high operating temperature from startup at ambient temperature, thus preventing flooding (occurring if the flow rate is high and temperature of the stack is low) or membrane dehydration (occurring if the flow rate is low and temperature inside the stack is high); (ii) ensuring sufficiently high heat removal rates via forced air convection during the operation characterized by stable temperature of the stack, i.e. loosely termed steady-state operation; (iii) ensuring as low as possible temperature difference between the inside and outside portions of the stack by choosing the materials with favorable thermal properties; (iv) properly design the stack and edge-cooling fins to ensure sufficiently high heat removal rates; (v) design the frame for the stack which will ensure that the cooling air is directed to uniformly flow along the edge-cooling fins. In previous studies [3,4] it was shown that the operating temperature inside PEM fuel cell is highly non-uniform and it is not accurate to represent the operating temperature with just one parameter, while at the same time if the temperature profile is controlled it can be exploited to result in high performance of the cell by manipulating the water vapor saturation profile, i.e. relative humidity profile inside the cell. The main contributions of this work to the research field are evident in outlining that the commonly used lumped models can be misleading for dynamic analysis of PEM fuel cell performance due to inability to accurately calculate the maximal temperature inside the stack and this work also outlines under which circumstances the lumped models can be used. This work also presents a novel transient CFD model for analysis of thermal management of PS which gives insight in mass and heat transfer both inside and outside of the cell (internal heat and mass transfer and forced air convection edge-cooling), while other works focus on only one aspect and simplify the other to a significant extent.In order to study thermal management influence on the performance od 1kW edge-cooled proton exchange membrane fuel cell stack without external humidification comprehensive numerical analyses are conducted. Two numerical approaches are considered and compared for a prescribed load profile: (i) lumped model and novel (ii) real-time transient computational fluid dynamics model incorporating realistic modeling of forced air convection on the edge-cooling of the stack. The developed computational fluid dynamics model is used to study the influence of (i) bipolar plate materials (ii) operating delta pressure along the flow field and (iii) different cooling fin configurations on the water and heat balance inside the stack. The results indicate that (i) maximal and average temperatures of the stack are almost linearly correlated to the thermal conductivity of bipolar plate materials and maximal temperatures can be significantly higher (ii) the operating delta pressure can be manipulated to increase the performance of the stack and (iii) the cooling fin redesign has major influence on the overall temperature uniformity across the stack. Additionally, the heat transfer between the stack and metal hydride tank is studied.