More power with STAR-CD Optimizing flow in SOFC fuel cells
Volker Schaika, Webasto AG, Germany

Due to the increasing number of safety and comfort devices in modern vehicles, their consumption of electrical energy is on the rise. In order to fulfil this demand, Webasto AG is developing an Auxiliary Power Unit (APU) based on solid oxide fuel cells (SOFC). The APU generates electrical energy and heat using the vehicle’s fuel, while operating independently of the engine. This obviates the limitations of traditional systems, such as the amount of energy available when the engine of the vehicle is not running. By using STAR-CD to optimise the geometry, a great improvement in the performance of the APU was achieved.

CFD has been extensively used in the development of the APU components, such as the reformer, after-burner and heat exchangers. This article describes the optimization of one of the APU’s key components: the anode gas flow in the SOFC plate. The performance of a single fuel cell, and therefore the whole stack, strongly depends on the even distribution of the fuel gas in the anode plate, thus maximizing the reacting area.

The anode plate consists of inlet and outlet manifolds and an active area connected to the solid oxide electrolyte. Approximately forty variations of the plate were examined in STAR-CD. The inlet and outlet manifolds were designed with various baffles. Different numbers of these baffles, as well as the arrangement and height of the inlets and outlets, were considered. The active area consists of a channel structure made of porous metal foam. Figure 1 shows the basic structure of the anode plate.

pro-STAR’s trimmed cell automatic meshing was used to build the plate models. The manifolds and the active area were meshed separately. The different combinations were then connected using the coupling capabilities in STAR-CD. The resulting grids consisted of approximately 600k cells.

The CFD simulations were run using the AMG solver and the MARS advection scheme. As the flow in the plates is laminar, no turbulence model was necessary. The density and viscosity in the fuel gas were calculated for a given mixture at the temperature of 850°C and kept constant throughout the simulation. This set-up allowed short calculation times of approximately 60 minutes, running on a 6 CPU AMD Opteron Linux cluster.

Figure 2 shows the flow distribution in the active area of one of the investigated geometries. The different velocities in the channel structure indicate the uneven distribution of the fuel gas and therefore a poor performance of the fuel cell. The flow distribution through the optimized model is given in figure 3. The uniform velocities show the improvement of the flow field.

This analysis demonstrates the use of STAR-CD to investigate a large number of design variations of a SOFC fuel cell in a short time. The resulting geometry shows a great improvement in a crucial component of the auxiliary power unit.