A Model Based Analysis of Evaporative Cooling for Polymer Electrolyte Fuel Cells

Abstract

Even though state-of-the-art polymer electrolyte fuel cells (PEFC) reach system efficiencies above 60 % [1], a substantial amount of heat has to be rejected to the environment. In order to facilitate the heat transfer, higher operating temperatures of PEFCs are favorable. However, the ionic conductivity of the proton exchange membrane is limiting the operating temperature substantially to about 90 °C, since it is highly dependent on a proper humidification [2]. In order to control the temperature, a dedicated cooling system is required. The state-of-the-art solution is liquid cooling which aims at transferring the waste heat to a liquid coolant that flows thru separate cooling channels in the bipolar plates. This results in high heat fluxes from the cell to the coolant and ensures a uniform temperature distribution. However, a thick and complex multi-layer design of bipolar plates is required [3] which contributes to about 75 % of stack volume and approximately 80 % of its mass [4]. The coolant is subsequently chilled in an external heat exchanger which requires a high surface area due to the small temperature gradient and thus shows a high volume demand of about 25 % of stack volume. One auspicious alternative to overcome these hindrances is evaporative cooling. In our concept, liquid water is fed to the cell through dedicated water channels in the flow field parallel to the gas channels. Consequently, the water is distributed in a specially designed gas diffusion layer (GDL) with a mixed hydrophilic and hydrophobic pattern (see Figure 1) [5]. Once in contact with the gas flow, the water in the hydrophilic lines evaporates, cools the cell by taking up the heat of evaporation and is eventually released as vapor with the exhaust gases. Concurrently, the water vapor contributes to a better humidification and thus higher ionic conductivity of the membrane, which enables higher operating temperatures. Additionally, evaporative cooling shows the potential to reduce the system volume, mass, complexity and cost up to 30 % by simplifying the design of bipolar plates and balance of plant [6]. In order to quantify the potentials and limits of evaporative cooling, a combined numerical and experimental study has been conducted in the present work. A semi-empirical zero dimensional fuel cell system model has been developed, taking into account mass and energy balances as well as electrochemistry (i.e. Nernst potential, kinetics, ohmic losses and mass transport losses). Relevant electrochemical parameters and evaporation rates have been determined experimentally from an actual evaporatively cooled differential fuel cell with an active area of 4.4 cm2 [5]. The impact of different operating conditions (i.e. temperature, pressure and stoichiometry) on the evaporation rate and thus cooling performance has been studied. Main findings show, that an increased operating temperature and stoichiometry as well as a decreased system pressure are increasing the evaporation rate and are therefore beneficial for the cooling performance. Furthermore, it is shown, that the steady state temperature which is reached when the heat of evaporation is equal to the waste heat (i.e. thermal neutrality) is dependent on system pressure, stoichiometry and current density (see Figure 2). However, the electrochemical performance is slightly decreased (-25 mV at 1 A/cm2) at operating conditions which are favorable for evaporative cooling (1.5 bara, dry gases), in comparison to conventional operating conditions. (2 bara, 80 °C, fully humidified), see Figure 3. In addition, the water balance of an evaporatively cooled fuel cell system has been investigated and it is shown, that a sufficient amount of water can be retained from the exhaust gas at condenser outlet temperatures below 60 °C to supply enough water to the stack (see Figure 4). Finally, a feasible operating window for evaporative cooling is proposed which is shifted towards higher temperatures (approx. 85 to 100 °C) and slightly lower pressures (approx. 1.5 to 2.5 bara) in comparison to conventional fuel cells.