Small plant components are required to deliver the reactants to the fuel cell with the required conditions. Examples of these components are blowers, compressors, pumps, and humidification systems used to deliver the gases to the fuel cell with the proper temperature, humidity, flow rate and pressure. This blog will describe these components and provide some relevant equations needed for producing quick models for the fuel cell subsystem.
In PEM fuel cells, a hydrogen humidification system may be required to prevent the fuel cell PEM from dehydrating under load. Water management can be challenging for certain fuel cell types because there is ohmic heating under high current flow, which will dry out the polymer membrane and slow ionic transport. Some fuel cell stacks may not require any humidification due to water generation at the cathode. In larger fuel cell systems, either the air or the hydrogen or both the air and hydrogen must be humidified at the fuel inlets. The gases can be humidified by bubbling the gases through water, water or steam injection, flash evaporation, or through a water/heat exchanger device. Examples of these humidification methods are shown in Figure 1.
Figure 1. Conventional humidification methods: (a) dewpoint humidification, (b) evaporation humidification, (c) steam injection humidification, and (d) flash evaporation humidification.
When the total pressure is constant, the humidity depends upon the partial pressure of the vapor in the mixture. For a vapor–gas system where the vapor is component A, and the fixed phase is component B:
The wet bulb temperature is the dynamic equilibrium temperature attained by the liquid surface when the rate of heat transfer to the surface by convection equals the rate of heat required for evaporation away from the surface. The partial pressure and the vapor pressure are usually small relative to the total pressure; therefore, the wet bulb equation can be expressed in terms of humidity conditions:
A humidity (psychrometric) chart provides a way to determine the properties of a gas–vapor mixture. Figure 2 shows an example of a psychrometric chart for a mixture of air and water. Any point on the chart represents a specific mixture of air and water. Points above and below the saturation lines represent a mixture of saturated air as a function of air temperature. The curved lines between the saturation line and the temperature axis represent mixtures of air and water at a specific percentage humidity.
Figure 2. Example psychrometric chart.
A commonly used method of providing air to a fuel cell is using fans or blowers. The fan or blower is driven by an electric motor, which requires power from the fuel cell or another source to run. One of the most commonly used fans is the axial fan, which is useful in moving air over parts, but not effective across large pressure differentials. The back pressure of this fan type is very low at 0.5 cm of water. These fans are well suited for many hydrogen–air PEM fuel cell designs. The following equation gives the actual fan power:
The ideal power can be calculated by:
where cp,avg is the specific heat at the average temperature of the inlet and outlet. The ideal exit temperature can be calculated from the equation:
where T2 is the isentropic temperature, and γ is the ratio of the specific heat capacities of the gas, Cp/Cv.
The operating information for an actual fan can be obtained from manufacturer’s data. The actual speed and power required can be found from the manufacturer’s table once the inlet volume rate and pressure boost are specified. Fan data can sometimes be represented in terms of dimensionless parameters. These are defined as:
where V is the volumetric flow rate, ρ is the density of the fluid, D is the wheel blade diameter, N is the fan speed, ⌂P is the fan pressure boost, and W is the fan power.
More substantial pressure differences can be obtained by using centrifugal fans. Centrifugal fans have air or gases entering in the axial direction and discharge air or gases in the radial direction. These are used for circulating cooling air through small to medium-sized fuel cells. The pressure created by these fans is from 3 to 10 cm of water.
Blowers are also used in atmospheric systems to draw air into the fuel cell. A battery is used for starting the blower, and then some of the power output of the fuel cell is used to keep the blower running (like other plant components). The blower power required is:
where nblower is the blower efficiency.
Compressors are used to compress air, which allows a higher concentration of oxygen per volume per time, and thus increases the fuel cell efficiency. This enables the drop-off in voltage due to mass transport to be delayed until higher current densities. If the pressure is higher, a lower volumetric flow rate can be used for the same molar flow rate, and the humidification will require less water for saturation (per mole of air). The compression can be isothermal or adiabatic. Isothermal compression allows temperature equilibration with the environment, and adiabatic uses compression without any heat exchange with the environment.
The most common type of compressor is the centrifugal compressor. It uses kinetic energy to create a pressure increase. The centrifugal compressor can be operated with high efficiencies through a high range of flow rates by changing both the flow rate and the pressure. This compressor type is commonly found on engine turbocharging systems. Figure 3 shows an example of a motor-driven turbocompressor for PEM fuel cells.
Figure 3. Example of a motor-driven turbocompressor for PEM fuel cells.
The efficiency of the compressor is important for the overall efficiency of the fuel cell system. The efficiency is found by using the ratio of actual work done to raise the pressure from P1 to P2:
where T2 is the isentropic temperature, and γ is the ratio of the specific heat capacities of the gas, Cp/Cv. Many compressors are manufactured commercially, and when designing the fuel cell system, the essential factors to consider are the temperature, pressure, type of gas handled, reliability, efficiency, and corrosion-free materials.
Pumps, like blowers, compressors, and fans, are essential components in the fuel cell plant system. These components are required to move fuels, gases, and condensate through the system and are important factors in the fuel cell system efficiency. Small to medium-sized PEM fuel cells for portable applications have a back pressure of about 10 kPa or 1 m of water. This is too high for most axial or centrifugal fans, as discussed earlier.
Choosing the correct pump for the fuel cell application is important. As in fans, blowers, and compressors, factors to consider are efficiency, reliability, corrosion-free materials, and the ability to work with the required temperatures, pressures, and flow rates for the specific fuel cell system. The matching of a high-efficiency pump with the appropriate motor speed/torque curve may allow for a more efficient fuel cell stack and system. The equations that describe pump performance characteristics are the same as the fan performance characteristics equations 6 – 9.
A fuel cell system can be very efficient with a few simple plant components -- or a very complex integrated system. Typically, the larger the fuel cell stack, the more complex the fuel cell plant subsystem will be. The number of ways to design and optimize the fuel cell plant subsystems is endless. The plant components reviewed in this post include humidifiers, fans, blowers, compressors, and pumps. A series of quick models can help the fuel cell stack designer to make robust and simple design decisions.