Designing and manufacturing quality fuel cells requires characterizing the device and testing device performance. For standby power stations and automotive vehicles, fuel cells are assembled in stacks to meet the necessary power requirements for the application. These stacks can generate high power; thus, high wattage power supplies and electronic loads are needed for fuel cell characterization and test. Fuel cell stacks can deliver well over 10 kW of power. Elektro Automatik not only manufactures high-power DC supplies and loads, but these products have a number of features that simplify the task of simulating, characterizing, and testing fuel cells and make the testing more efficient.
Measuring Fuel Cell Resistance
Figure 1 shows a simplified model of a fuel cell. One of the most important parameters of fuel cell efficiency is its ohmic resistance which determines power loss in the fuel cell’s output. The recognized techniques to measure the fuel cell’s resistance involve a perturbation in which a ΔI in the load on the fuel cell is created; and the resulting ΔV (or ΔU) is measured. The fuel cell resistance is then ΔV/ΔI. The ΔI can be large, as in the current interrupt method, in which the current is momentarily turned off so that the voltage drop across the fuel cell resistance is 0 V. The other method induces a small AC perturbation on the cell and makes voltage measurements at one or multiple frequencies. The two methods produce different results. The current interrupt method produces a higher resistance than the high frequency technique resulting from an additional voltage generated in the porous electrodes due to the large perturbation. The AC perturbation technique minimally disturbs the fuel cell. We feel this method for determining fuel cell resistance gives results that more closely determine the actual fuel cell resistance.
Figure 1: Simplified model of a fuel cell.
So how do you create the ΔI needed to characterize the fuel cell? You need an electronic load and the ability to vary its constant current output with an AC signal of varying frequency. You could connect the load and a waveform generator together. Waveform generators are generally low power devices, so you would have a problem testing a fuel cell stack containing a number of fuel cells. You could connect the waveform generator through a bias-T to the electronic load; but, bias-T’s are low power components used primarily for RF applications. Connecting a low power waveform generator to a high-power electronic load is a challenge. We solve that challenge with our ELR electronic loads by integrating the waveform generator into the load. You do not have to worry about external connections and protecting a waveform generator from damaging high power. The waveform generator outputs sine waves, triangle waves, square waves, trapezoidal waves, ramps, and arbitrary waveforms. With the ELR load, you can create any type of dynamic load including a sinusoidal perturbation on a DC current sink for fuel cell resistance characterization.
In addition, the ELR load, with its internal waveform generator can subject the fuel-cell-under-test to a wide range of dynamic load variations for both performance and durability testing. The load can stress a fuel cell with large step load changes at varying duty cycles.
Simulating a Fuel Cell for Realistic Inverter or DC-DC Converter Testing
Now that the fuel cell is characterized, the PSB-series power supplies, which also have a built-in waveform function generator, can simulate the output of a fuel cell. Using the simulated fuel cell, an inverter for a device such as a standby power source or a DC-DC converter for an automotive vehicle can be tested under the most realistic conditions. Just use the Function Generator Application in the Elektro Automatik Power Control Software. Enter the key voltage and current parameters, and the Function Generator App enables the PSB power supply to emulate the output of your fuel cell stack. Figure 2 shows the window for the Fuel Cell table which shows the fuel cell characteristic V-I curve and defines the fuel cell output. At maximum voltage, fuel cell output is dominated by electro-kinetic effects. In the central part of the curve, the linear, ohmic resistance of the fuel cell determines the output characteristics. At the high current-low voltage portion of the curve, the exponential characteristic is defined by energy being consumed at a faster rate than hydrogen and oxygen can diffuse to the anode and cathode to supply energy.
The simulated fuel cell output can test inverter or DC-DC performance as these loads draw both low current and high current. The results indicate how well the loads can maintain their output under the varying voltage of the fuel cell. Fortunately, you do not need a complex test setup with an external variable resistance for testing inverters and DC-DC converters. All you need is a PSB-series power supply.
Figure 2. Function Generator application with the Fuel Cell table displayed
Save on extra and higher power supplies and loads
Both the PSB power supplies and the ELR electronic loads autorange to offer a wide span of voltage outputs/inputs and current outputs/inputs. Autoranging allows the supply or the load to output (consume) a wide output span of voltage and current at full power. Figure 3 shows the output characteristic of an autoranging power supply compared with the characteristic output of a fixed range power supply. For example, a fixed range, 1 kW supply could have a maximum output of 100 V at 10 A; while, a 1kW autoranging power supply could have a 200 V output at 5 A and a 10 V output at 100 A. Thus the autoranging supply can provide a much wider range of voltage and current. If you need more than 100 V or 10 A, you would need a higher wattage, fixed range power supply. That’s more expense and a larger instrument. With autoranging, you have the flexibility to address more test applications with a single supply. Using fixed range supplies could require a different power supply for different tests. Think about autoranging when you need a new power supply or load.
Figure 3. An autoranging power supply with wider voltage and current output compared with a fixed range output power supply
Save energy and run cooler
Both the ELR series electronic loads and the PSB series bidirectional (source and sink) power supplies can absorb power and deliver it back to the power grid with an outstanding 96% efficiency. When you use the ELR loads or the sinking function of the PSB supplies, efficient inverters in the two instruments provide the regenerative energy recovery to substantially reduce your electric utility costs. Returning the absorbed energy to the grid saves on cooling requirements for these instruments which can output and sink up to 30 kW. The instruments need lower capacity fans which run more quietly and less cooling infrastructure to maintain them at a safe operating temperature. Benefit from lower utility costs and the knowledge that you are helping the environment.
Work in any automated test environment
The Elektro Automatik PSB power supplies and ELR loads offer a number of interfaces to allow easy communication and control in a number of test environments The PSB Series supplies and the ELR loads have USB and ethernet as standard interfaces for simplified connection to a PC. With optional ModBus and Profibus interfaces, the instruments conveniently allow control by a programmable logic controller (PLC). With the CAN interface, the instruments can interface to an automotive control system. That is more flexibility than you get from other power supply and electronic load manufacturers.
Or operate manually
The supplies and the loads have a multi-colored touchscreen display that shows all programmed and measured values. In addition, you only have to learn to operate two control knobs. For worldwide use, you can select your display to work in one of four languages: English, German, Russian, or Chinese. Working in your native language shortens the learning curve for operating the instrument and enables you to be more comfortable with the instrument.
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