Fuel-cell testing
Testing a fuel cell
Because fuel cells are still in the development stage, the automotive industry has not settled on standard testing equipment or test-equipment vendors. Many companies are stepping up to the challenge of developing both modular and turnkey solutions to accurately monitor and control fuel cells. Notable among these companies are Hydrogenics and National Instruments, who are creating hardware and software that permit more expedient development of fuel-cell technology. Hydrogenics has developed three test systems that permit characterization of either single cells or stacks of cells. By using National Instruments data-acquisition and control hardware in its systems, Hydrogenics is able to incorporate most of the desired measurement and safety options required by scientists.
 Virginia Tech engineering students prepare a PEM fuel cell for use in their hybrid-electric Chevrolet Lumina for the 1999 Future Car Challenge. |
Although the overall goals of research and development, manufacturing, and operations vary, their need to monitor and control fuel cells is similar. For R&D, testing is done to characterize and optimize energy output as well as extend the life and robustness of the stacks. In design validation, the main goal is to optimize the design in preparation for mass production and to reduce the overall cost of the stack without reducing the efficiency. For manufacturing applications, the stacks are monitored to ensure they meet the engineer's specifications. In actual use, monitoring is essential to a stack's life and operation. Fortunately, different stages of fuel-cell implementation have similar needs of a well-designed tester to accommodate the applications.
PEM fuel cells share the characteristics of requiring humidified hydrogen and oxygen and generating electricity with a byproduct of water. Although water is a desired output in the space program, the only output automotive scientists are truly interested in is electrical (current and voltage). Parameters that control the production of this power include gas-stream temperature, pressure, humidity, and flow rates. The stack's individual cell voltages are measured and the overall stack temperature is monitored and controlled using active cooling. In many applications, the load resistance is variable, allowing engineers to develop tafel plots (voltage/current density plots that indicate the power and efficiency of a stack or cell). A fuel-cell tester should be able to monitor and control all of these parameters as well as measure and log the voltage and current outputs of the stack.
Consider the output of fuel cells: voltage and current. In a typical fuel-cell application, a known load is applied to the fuel cell to control output voltage and current. When the voltage output of a fuel cell increases, the output current decreases. The operating load of a fuel cell is a balance between the maximum power output and the maximum efficiency. For example, a PEM was used by Virginia Tech's hybrid-electric Chevy Lumina for the 1999 Future Car Challenge. An Energy Partners fuel-cell stack was used, which created a range of 60-100 V dc. Under load with current flowing, the output per cell would drop from 1 V to as low as 0.6 V per cell. Knowing the voltage of each individual membrane allowed Virginia Tech to closely monitor the health of its stack.
If one cell exhibits a different potential, it is an indication of a problem with the cell, including incorrect temperature, under hydration, or flooding. The voltage from each cell or group of cells is monitored to operate, test, or design a fuel cell properly. By measuring a group of cells, the channel count and wiring requirements can be reduced while still monitoring the health of the cells. While each group of cells may reach up to 10 V in a PEM fuel cell, the membranes are stacked together to yield higher voltages. Because the stack can reach over 100 V, the tester must not only have many channels that are capable of reading 10 V per channel, but also maintain isolation of hundreds of volts between the first and last cell in the stack.
Obviously, simply monitoring the voltage is not sufficient to characterize and control a fuel cell. Current output is another item that is monitored. Because the current output can be very high, it is usually monitored using the Gaussian effect. This method allows engineers to unobtrusively monitor the current flowing through a wire; it requires signal conditioning and scaling to convert the data back into a current reading. PEM fuel cells typically require temperatures in the range of 60-80°C (140-175°F) to produce energy efficiently. This temperature is monitored for goals such as variation and correlation to power output. Thermocouples and thermistors are good sensors for monitoring both the stack temperature and the temperature of the incoming reactant gas streams. In many applications, the gas streams are at elevated pressures, which are monitored and managed. Pressure is measured with a pressure transducer and signal conditioning, and the flow rate is measured with a flowmeter that outputs pulses at a rate proportional to the gas flow rate. These pulses are then monitored by a counter timer board and scaled by software into a flow rate. Electronic regulators can control the pressure and flow via 4-20 mA inputs that are supplied by the test stand.
One of the final challenges in a fuel-cell test stand is the measurement and control of gas-stream humidities. The water flow in a cell is critical to its operation, and each membrane must remain hydrated to maintain its protonic conductivity. If a cell becomes too dry, the membranes are prone to damage. If the membrane floods, the transport of reactants is reduced and a dramatic drop in overall system performance occurs. Therefore, proper humidification control and monitoring is essential to the operation of a PEM fuel cell. One way to monitor the humidity is through an electric humidity sensor that outputs 4-20 mA current at a level proportional to the humidity. A voltage input channel of the tester can then read this signal.
Along with monitoring, control is also required to conduct fuel-cell testing. Almost all of the monitored items need to be controlled for repeatable tests. To control gas-stream pressures, analog output channels from the tester set the electrically variable pressure valves. Digital output lines provide the control for emergency shutoff, purge output, and bypass valves. General Purpose Interface Bus (GPIB) or analog output is used to control the heaters and fans used for temperature control. In addition, a programmable load is used to change the resistance seen by the fuel cell. One way a tester can accomplish this change is with a GPIB-controlled load device or by using digital relays to connect various resistors in parallel. In the first method, a stand-alone box is instructed, via GPIB, to change the loads placed on the fuel cell. The second option uses relays and switches different loads in and out. To vary the humidity, the water flow rate for the humidifier is adjusted.
