By Eric Turner, Intepro Systems (www.inteproate.com)
Proper simulation ensures regulatory and safety compliance
Simulating a fuel-cell-based power system is more complex than just attaching a dc source to the circuit. Polymer electrolytic membrane fuel cells (Figure 1) and related chemical energy harvesting systems, like reflow batteries and such, don’t put out with a perfect consistency, and the related power systems must be able to handle variations efficiently and effectively. Individual cells in a stack can receive fuel inconsistently, and thermal issues also impact fuel-cell performance.
Proper system emulation addresses several issues compared to running an actual fuel cell in the test environment, as one of the biggest challenges in testing fuel-cell power systems is having a lab that meets all the proper regulations and safety concerns with a device that uses hazardous materials. In addition, using a proper testing setup means devices and components can easily be tested under a wide range of parameters quickly when compared to using a live fuel cell.
Another advantage to having a dynamic testing setup is being able to test the system beyond the limits of the fuel cell, to create simulation profiles for circumstances when the fuel cell is not working properly, and to test the system’s end-of-life characteristics (Figure 2). Emulation greatly reduces time and cost of research and development testing, production testing, and certification testing. Highly advanced, programmable power sources should have the programming capability to perform this type of emulation for testing DC/DC convertors or AC inverters.
For example, the Intepro Systems’ PSI 9000 series of fast-response DC sources is equipped with just such a fuel-cell emulator — creating a non-linear voltage output that simulates a fuel cell or fuel cell stack output voltage. The fuel cell table function is used to prescribe the characteristics of voltage and current of a fuel cell. This is achieved by setting up the parameters that define points on a typical fuel cell curve, which is then calculated as voltage-current table (Table 1) and passed to the internal function generator. The emulator function includes a set-up feature that walks the user through the process of entering four V-I support points. When finished, these points will be used to calculate the curve shown in Figure 2.
The fuel cell emulator is an application-specific implementation of an FPGA-based function generator that uses a table-based regulation circuit for the simulation of non-linear internal resistances. By linking together several differently configured sequences, complex progressions can be created. Smart configuration of the arbitrary generator can be used to match triangular, sine, rectangular or trapezoidal wave functions to create, for example, a sequence of rectangular waves with differing amplitudes or duty cycles.
Closing the Loop with a Regenerative Load
The programmable source used to emulate the output of fuel cells, in our example, draws its power directly from the ac line. That output is applied as the input to the device under test. By using a programmable electronic regenerative load, a high percentage of the power output from the DUT can be regenerated and returned to the ac line. This has the exceptional benefit of dramatically reducing the direct energy costs plus reducing the need to expensive, noisy and energy-consuming cooling systems. (Figure 3)
Emulation Avoids Error
Proper system setup when testing power systems is more critical than ever lately, as energy densities rise and power requirements increase. The performance of the power system is directly linked to effective thermal management as well as safety and reliability, so it is not an area to indulge in half measures like ineffective test protocols. Use the right emulation system to get the best test results and your products will reflect the effort.