Objectives and Scope: Broadly water electrolysis can be divided into alkaline electrolysers (AEL), proton exchange membrane (PEM) electrolysers, and solid oxide electrolysers (SOE). All water electrolysers are exothermic and consume water and electricity. As unit costs are high it is imperative that cells operate as efficiently as possible. Thermal management is usually over-designed as thermal degradation will rapidly reduce unit performance. Due to this flow field design for PEM electrolysers in particular focus on achieving low pressure drops. Due to constraints such as e.g., material properties, material consumption, size and geometry, manufacturing tolerances to name only a few challenges, little attention has historically been focused on optimal flow field design. Popular designs found in the literature can have twice as much flow [in ml/s] in certain parts of the cell compared to other parts of the cell.
PEM cell sizes are typically of the order 10 cm. Thus, flow paths are relatively short, but have very large aspect ratios with channel width and height being of the order 1 mm. At the entrance to these channels pure water is pumped in, and reactants (hydrogen or oxygen gas) enter the flow channel through the gas diffusion layer (GDL. Thus, the holdup in the channel is not constant and the resulting flow is an accelerating and developing multiphase flow. Consequently, pressure drop correlations for fully developed multiphase flow may not be suitable. This makes it questionable to apply single phase design principles when optimizing flow fields for PEM cells. We investigate experimentally and numerically the multiphase flow in mini channels with continuous gas injection along the channel length.
Methods: To experimental investigating multiphase pressure drop and phase distribution in a relevant electrolyser cell geometry, a flow cell with a single mini channel was fabricated using acrylic pressure lamination technique. The flow channel (0.5 x 1.5 mm cross section) had small holes at regular intervals along the bottom wall of the channel to simulate the porous structure of a GDL. Four pressure sensors were used to measure the pressure drop from inlet to the start of the flow channel, across it and in the outlet section. High speed video recordings were obtained and analysed using a novel PIV algorithm. Simulations were performed using a Eulerian-Eulerian multiphase model in a commercial CFD code.
Results and Observations: A matrix of test points for the air and water flow velocity was chosen based on relevant scenarios for electrolysers, i.e. superficial velocities (air and liquid) from ~0.1 to 1m/s and stochiometric ratios 125, 250 and 500. In total 18 flow conditions were tested, with multiple repeated experiments. Accelerating flow was observed in the flow channel and mainly two flow remiges, bubbly, and Taylor bubble flow. In the Taylor bubble flow interesting bubble coalescence dynamics was observed. The pressure drops were compared to available simplified known models in the literature.Simulation showed good comparison with selected experiments both with respect to predicting pressure drop and void distribution.