It can be easier to use programmable auto-ranging power supplies as alternatives to fuel-cell power sources that would otherwise force the installation of safety and compliance equipment just to run a few characterization tests.
ONE of the most challenging aspects of configuring a test system is that the test rig must be “better” than the situation being simulated. If a device is rated to a certain temperature level or vibration standard or moisture resistance, the test systems involved must exceed the test parameters or the test may not effectively represent real-world performance.
This relationship between test systems and the things they test is extremely important when it comes to characterizing power systems. Improper characterization of power system performance can result in a product that fails to perform at the edges of its envelope of operational tolerances, often with catastrophic results. This failure to perform is especially an issue with power systems driven by alternate energy sources like solar panels and fuel cells.
In particular, the simulation of a fuel-cell-based power system is more complex than just attaching a dc source to the circuit. Polymer electrolytic membrane fuel cells and related chemical energy harvesting systems, like reflow batteries and such, don’t generate power with a perfect consistency. The loads that the fuel cell powers 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.
Several issues arise when running a real fuel cell in a real test environment. One of the biggest challenges is instrumenting a lab that meets all the regulations and safety concerns that arise when working with devices that use hazardous materials. Fuel cells commonly used in transportation systems, for example, normally have pressurized hydrogen as a fuel source. Labs working with pressurized hydrogen must adhere to safety standards associated with the handling of these pressurized tanks.
Consequently, it is often advantageous to emulate the fuel cell with a programmable power source for testing purposes, rather than use real cells. In addition, a testing setup should enable devices and components to be easily tested under a wide range of parameters. And tests can proceed more quickly than when 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 qualities. 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 converters or ac inverters.
For example, the Intepro Systems PSI 9000 series of fast-response dc sources includes 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 voltage and current qualities of a fuel cell. The procedure consists of first setting up the parameters that define points on a typical fuel cell curve. This information is used to calculate a voltage-current table that is 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.
The fuel cell emulator is an application-specific implementation of an FPGA-based function generator. The function generator uses a table-based regulation circuit for the simulation of non-linear internal resistances. Complex progressions can be created by linking together several differently configured sequences. 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.
Programmable Power Sources
Fuel cell simulation is just one of myriad scenarios needed in both design and production testing of power conversion devices. Tools like the Intepro PSI Series specifically target difficult test situations resembling those involving fuel cells. They also are important in similar applications demanding fast response times, advanced power simulation modes, and precision control. Advanced programmable supplies should always include an integrated function generator that allows creation of arbitrary disturbances for complex testing. They should also include a galvanically-isolated analog interface for voltage, current, and power programming and monitoring.
Another useful feature in some programmable supplies (such as the Intepro PSI 9000) is auto-ranging, which allows for more voltage and current combinations in the output. This feature allows you to put out maximum power in more ways than a traditional power supply. A traditional supply generally puts out its rated wattage only when it simultaneously generates both its rated maximum voltage and rated maximum current.
Auto-ranging dc power systems are typically a bit more costly than conventional supplies with the same power rating. The price difference is mostly because the output stages of auto-ranging systems must be designed to operate reliably over a wider range of output voltages and currents. But the real cost is lower because one auto-ranging unit can be used to replace multiple conventional units. Each chassis features a controller which brings the flexibility of separating the instrument into individual sources or sources that are paralleled for high-power applications.
There are several features that are increasingly considered ‘must haves’ for advanced power sources. For example, they should be equipped with common communications protocols enabling remote programming and operation. In nearly all testing applications, users should also look for remote sensing to compensate for voltage drops along the load cables. The power supply automatically detects whether the sensing input is connected and will stabilize the voltage directly at the load.
Using 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. The power source 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.
As a quick review, an electronic regenerative load redirects the power it receives back to the utility by using an internal micro-inverter stage that is synchronized to the power line input. Use of a regenerative load can reduce the amount of energy that would otherwise be dissipated by up to 93%. And it doesn’t take much to cool a regenerative load. A 10-kW regenerative load dissipates just 700 W of heat, about as much as a typical hair dryer.
Internally, the dc energy that goes to the regenerative load flows into a dc-dc converter which is tied into a dc-ac inverter. The output of the inverter synchronizes with the utility grid to recycle the energy. The regenerative load must also include an automated grid monitoring system that detects the phase voltage and frequency that is used for grid synchronization. If the grid drops out, so does the regenerative load. The unit simply shuts down and waits for the operator to turn it back on. In general, savings through reduced energy costs will pay for a regenerative load in about three years.
All in all, proper system setup when testing power systems is more critical than ever, as energy densities rise and power requirements rise. 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. The right emulation system will give the best test results, and your products will reflect the effort.