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PEM water electrolysis is a clean technology for hydrogen production. In spite of its many advantages, the costs of the conventional PEM electrolysis cell makes it commercially less competitive vis-à-vis its peers. An alternative cell design has been proposed which has up to a 25 % costs advantage over the conventional cell. In this alternative cell design, the flow channel plate which bears the most costs in the conventional cell design has been replaced with a 3-D Porous Transport Layer (PTL) structure. It has however, been observed that the conventional cell by far out performs the low cost cell at high current density operations, due to increased mass transport limitation in the later. Industrial and commercial hydrogen production efforts are focused towards high current density operation (> 3 A/cm²), so the alternative cell design must be optimized for mass transport limitation.
PEM water electrolysis is a clean technology for hydrogen production. In spite of its many advantages, the costs of the conventional PEM electrolysis cell makes it commercially less competitive vis-à-vis its peers. An alternative cell design has been proposed which has up to a 25 % costs advantage over the conventional cell. In this alternative cell design, the flow channel plate which bears the most costs in the conventional cell design has been replaced with a 3-D Porous Transport Layer (PTL) structure. It has however, been observed that the conventional cell by far out performs the low cost cell at high current density operations, due to increased mass transport limitation in the later. Industrial and commercial hydrogen production efforts are focused towards high current density operation (> 3 A/cm²), so the alternative cell design must be optimized for mass transport limitation.
This work seeks to understand the source of, and to eliminate the mass transport losses in the alternative cell design to get it performing at least as good as the conventional cell at current densities up to 5 A/cm². A 2-D non-isothermal semi-empirical fully-coupled models of both cell designs have been developed and experimentally validated. The developed validated models were then used as tools to simulate and predict the best operating conditions, design parameters and micro-structural properties of the PTL at which the mass transport issues in the alternate cell will be at its minimum, at high current densities. The models are based on a multi-physics approach in which thermodynamic, electrochemical, thermal and mass transport sub-models are coupled and solved numerically, to predict the cell polarization and individual overpotentials, as well as address heat and water management issues. The most unique aspect of this work however, is the development of own semi-empirical equations for predicting the mass transport overpotential imposed by the gas phase (bubbles) at high current densities. For the very first time, calculated polarization curves up to 5 A/cm² have been validated by own experimental data. The results show that, the temperature and pressure, water flowrate and thickness of the PTL are the critical parameters for mitigating mass transport limitation. It was found that, for the size of the cells studied (25 cm² active area each), when both cells are operating at the same temperature of 60 °C, alternative design will have a comparable performance to the conventional designed cell even at 5 A/cm² current density when; the operating pressure is ≥ 5 bar, the feed water flowrate is ≥ 0.024l/min∙cm², PTL porosity is 50 %, PTL pore size is ≥ 11 µm and PTL thickness is 0.5 mm. At these operating, design and micro-structural conditions, the predicted difference between the polarizations of both cells will be only ~10 mV at 5 A/cm² operating current density.
The continual optimization process for more efficiency of industrial flows has raised the need for providing deeper understanding of turbulence. These details can be provided by direct numerical simulation (DNS), which is impossible for most flows with current computers. Therefore, progress in optimizing Reynolds averaged Navier-Stokes (RANS) and large eddy simulation (LES) modeling strategies will need to continue. Another ansatz is the reduction to 2D or 1D models to reduce the numerical cost.
One dimensional turbulence (ODT) as presented by A. R. Kerstein is a new modeling strategy that reduces the 3D simulation to a 1D line of sight through the flow region. Due to the higher resolution afforded by the 1D model, it is possible to simulate even the smallest scales and to provide insight into turbulence statistics.
To assess the advantages and disadvantages of the model, ODT has to be validated against several flows. Within this thesis, ODT is validated against the channel flow, the passive scalar transport and the channel flow with a fluctuating pressure gradient. These flows are simplified test cases for the phenomena present in single-phase industrial flows. ODT produces meaningful results for friction Reynolds numbers up to Reτ = 6·10⁵ and for Prandtl numbers from Pr = 0.025 to 50. Statistics of the wall shear stress are presented and the influence of pressure fluctuations is discussed.
Based on these channel results, the non-breaking and breaking jet are simulated. While the former is a simplified case of a free-surface flow, the latter is of primary interest for spray formation and fuel injection. Detailed statistics of the TKE budgets and the breakup are presented. As the last case, the cloud top of a stratocumulus-topped boundary layer (STBL) was simulated. The case combines the interaction of an active and a passive scalar. It further combines the simulation of a stable and an unstable stratified region that suppresses and enhances turbulence respectively. The simulations reproduce the entrainment velocity and generate comparable mean and flux profiles compared to DNSs.