This post presents an overview of the initial considerations for fuel cell design in room-temperature fuel cells.
The first step in engineering a fuel cell stack is to obtain the power requirements. The stack is then designed to meet those requirements, and the maximum power, voltage, and current are often known. The power output of a fuel cell stack is a product of stack voltage and current:
The maximum power and voltage requirements are dependent upon the application. The engineer must understand these specifications to build an appropriately-sized fuel cell stack. It is helpful to know the current and power density when designing a fuel cell stack. These are often unavailable initially but can be calculated from the desired power output, stack voltage, efficiency, and volume and weight limitations. The current is a product of the current density and the cell active area:
The cell potential and the current density are related by the polarization curve:
An example of a polarization curve is shown in Figure 1. The polarization curve can be used to help initially design the fuel cell stack.
Figure 1. Typical polarization curve for a PEM fuel cell stack.
Most fuel cell developers use a nominal voltage of 0.6 to 0.7 V at nominal power. Fuel cell systems can be designed at nominal voltages of 0.8 V per cell or higher if the correct design, materials, operating conditions, balance-of-plant, and electronics are selected.
The actual fuel cell performance is determined by the pressure, temperature, and humidity based on the application requirements, and can often be improved (depending upon fuel cell type) by increasing the temperature, pressure, and humidity, and optimizing other important fuel cell variables. The ability to increase these variables is application-dependent because system issues, weight, and cost play important factors when optimizing certain parameters.
The number of cells in the stack is often determined by the maximum voltage requirement and the desired operating voltage. The total stack potential is the sum of the stack voltages or the product of the average cell potential and number of cells in the stack:
The cell area must be designed to obtain the required current for the stack. When this is multiplied by the total stack voltage, the maximum power requirement for the stack must be obtained. Most fuel cell stacks have the cells connected in series, but stacks can be designed in parallel to increase the total output current. When considering the stack design, it is preferable to not have cells with a small or very large active area because the cells can result in resistive losses. With fuel cells that have large active areas, it can be difficult to achieve uniform temperature, humidity and water management conditions.
The cell voltage and current density is the operating point at nominal power output and can be selected at any point on the polarization curve. The average voltage and corresponding current density selected can have a large impact on stack size and efficiency. A higher cell voltage means better cell efficiency, and this can result from the MEA materials, flow channel design, and optimization of system temperature, heat, humidity, pressure, and reactant flow rates. The fuel cell stack efficiency can be approximated with the following equation:
In the traditional bipolar stack design, the fuel cell stack has many cells stacked together so that the cathode of one cell is connected to the anode of the next cell. The main components of the fuel cell stack are the membrane electrode assemblies (MEAs), gaskets, bipolar plates with electrical connections and end plates. The stack is connected by bolts, rods, or other methods to clamp the cells together.
When contemplating the appropriate fuel cell design, the following should be considered:
• Fuel and oxidant should be uniformly distributed through each cell, and across their surface area.
• The temperature must be uniform throughout the stack.
• If designing a fuel cell with a polymer electrolyte, the membrane must not dry out or become flooded with water.
• The resistive losses should be kept to a minimum.
• The stack must be properly sealed to ensure no gas leakage.
• The stack must be sturdy and able to withstand the necessary environments.
The most common fuel cell configuration is shown in Figure 2. Each cell (MEA) is separated by a plate with flow fields on both sides to distribute the fuel and oxidant. The fuel cell stack end plates have only a single-sided flow field. Most fuel cell stacks, regardless of fuel cell type, size and fuels used are of this configuration.
Figure 2. Typical fuel cell stack configuration (a two-cell stack)
Many parameters must be considered when designing fuel cells. Some of the most basic design considerations include the power required, size, weight, transient response and operating conditions. From these initial requirements, the more detailed design requirements (such as the number of cells, material and component selections, flow field design, gas diffusion design, etc.) can be chosen.