A Feasibility Study of Placing a Heated Turbulence Grid in Front of an Air-Cooled Fuel Cell Stack in Freezing Conditions
Authors: Martinho Diogo , Hansen Jóhannes , Yin Chungen , Berning Torsten 
Published: July 2022

The telecommunications industry is growing fast, and the air-cooled fuel cell is a promising alternative to the diesel-gensets as a backup power system. These systems are required to operate in freezing conditions where temperatures can be as low as -40°C, and they are usually located in remote areas where they can not be easily accessed. Berning proposed the placement of a turbulence grid in front of an air-cooled fuel cell stack to improve the heat transfer and thereby the performance, since the performance of the fuel cells was restricted due to the membrane overheating[1]. In an experimental study, Al Shakhshir et al. measured a performance increase of over 30% by placing a turbulence grid before an air-coiled fuel cell stack [2]. Then, Berning and Knudsen studied how the performance of the fuel cell would be affected while operating in different climate regions [3]. Additionally, previous studies allocated a turbulence inducing grid in front of the cathode inlet of the fuel cell where the main goal was to improve the heat transfer between the air and the components to achieve higher performances of the fuel cell while keeping the operating temperature in a good range [4] and how the design of this turbulence inducing grid would affect the cell [5]. Besides the use of a turbulence grid, this paper aims to analyze how the effect of the freezing ambient temperature can be reduced by pre-heating the incoming fuel cell air. A feasibility study is developed where different temperature differences and four different grid designs where the number of grid pores and its thickness are varied. The aims of this project were to identify feasible surface temperatures of the grid while having a reasonable consumption of fuel cell power to be provided to the grid and heated it up. A three-dimensional, steady state numerical analysis has been conducted on these grid dimensions where the computational domain considered was a single cathode channel where the gas diffusion layer would be the bottom surface, and the bipolar plate is to remain walls. Four different grid dimensions, namely grids with one and two millimeters of thickness and six 1 mm2 pores and grids with one and two millimeters of thickness and twenty-four 0.17 mm2 pores, have been considered. The outside temperature has been varied from -20 °C to 0 °C and heated up to 5 °C before air flow enters the cathode. The goal of this study is to better understand the temperature of the turbulence grid that is required to pre-heat the incoming air and thus conclude, which materials are suited. A parametric study was conducted, and with the last designed grid the best result was achieved. Preheating the air by 10°C, a surface temperature around 236°C was predicted. The fuel cell power usage would be approximately 30% of the total provided by fuel cell. It is possible to conclude the surface area of a single grid is not sufficient, leading to too high temperatures. A possibility could be having two staggered grids. Finally, an energy storage system could be used to provide this additional power required, avoiding such consumption of hydrogen and its slow dynamic response.
[1] Berning T. A Numerical Investigation of Heat and Mass Transfer in Air-Cooled Proton Exchange Membrane Fuel Cells. InFluids Engineering Division Summer Meeting 2019 Jul 28 (Vol. 59032, p. V002T02A030). American Society of Mechanical Engineers.
[2] Shakhshir SA, Gao X, Berning T. An experimental study of the effect of a turbulence grid on the stack performance of an air-cooled proton exchange membrane fuel cell. Journal of Electrochemical Energy Conversion and Storage. 2020 Feb 1;17(1):011006.
[3] Berning T, Knudsen Kær S. A thermodynamic analysis of an air-cooled proton exchange membrane fuel cell operated in different climate regions. Energies. 2020 Jan;13(10):2611.
[4] Lind A, Yin C, Berning T. A Computational Fluid Dynamics Analysis of Heat Transfer in an Air-Cooled Proton Exchange Membrane Fuel Cell with Transient Boundary Conditions. ECS Transactions. 2020 Sep 8;98(9):255.
[5] Pløger LJ, Fallah R, Al Shakhshir S, Berning T, Gao X. Improving the Performance of an Air-Cooled Fuel Cell Stack by a Turbulence Inducing Grid. ECS Transactions. 2018 Jul 23;86(13):77.

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