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Fuel cells are one of themost promisingtechnologies for delivering

clean and efficient power forautomotive and residentialapplications. A fuel cell directlyconverts the chemical energy of hydrogen and oxygen into electricitywith a byproduct of pure water. Until recently, fuel cells have largelybeen restricted to NASA space missions and a few research labs aroundthe world. However, with increased urgency in reducing pollution andgreenhouse gas emissions, a resurgence of interest in fuel cells hasoccurred in the scientific community. Today, governments and largecorporations are making massive investments into the development ofthese clean power sources. Although fuel cells hold great promise forclean, inexpensive power, they are still in their developmental infancy,and a great deal of research is necessary before they are consideredviable power systems. Test capabilities that deliver reliable monitoringand control, and offer the benefit of a flexible configuration, are criticalto these advances. The capabilities will permit scientists to easily tailortheir systems to keep pace with evolving fuel-cell technology.

Capabilities that deliverreliable monitoring andcontrol, as well as offer thebenefit of a flexibleconfiguration, are critical tokeep pace with evolvingfuel-cell technology, accordingto National Instruments.

A Ballard fuel-cell stack beingtested prior to shipment.

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Even though several types of fuel cells exist, they all workunder the same basic premise of converting hydrogen and oxy-gen into electrical power. Of the fuel-cell technologies, whichinclude alkaline (AFC), molten carbonate (MC), phosphoric acid(PAFC), proton exchange membrane (PEM), and solid oxide(SOFC), PEM is gaining most of the attention in automotiveapplications. PEMs are popular due to their relatively low op-erating temperature and high efficiency. The PEM fuel cell op-erates by using platinum-coated membranes as a catalyst tobreak a hydrogen atom into a proton and an electron. The mem-brane is permeable to protons, but impenetrable to free elec-trons. These electrons are forced to travel through an electriccircuit before they rejoin with free protons and oxygen mol-ecules to form water. In this way, the anode of the fuel cell pro-duces electricity, and the cathode creates heat and water. How-ever, just as it took years of tests and improvements to achievethe efficiencies currently realized by internal combustion en-gines, many improvements are necessary before fuel cells areviable for automotive use.

The introduction of computer control revolutionized the in-ternal combustion engine. It allowed engineers to monitor andcontrol fuel rate, timing, and cooling. With the adoption of moni-toring and controlling techniques such as fuel injection, oxygensensors, knock detectors, and mass flow sensors, engine efficien-cies have reached an all-time high, while pollution per enginehas been greatly reduced. Engineers have learned that throughcomputer control and careful monitoring of important variables,vehicle powerplants can be greatly improved. To develop a vi-able fuel cell, engineers need to accurately monitor the condi-tion of the hydrogen stream, oxygen stream, output voltage, andcurrent. To optimize a fuel cell, not only are the flow and pres-sure of the hydrogen and oxygen monitored, but also the hu-midity and temperature of the gas streams. Knowing the volt-ages of the individual membranes can enable an engineer toread the health of a fuel-cell stack and control the output resis-tance to map the power densities of the stacks. To improve the

efficiencies of next-generation fuel cells, engineers are con-stantly incorporating new measurements into their tests anddemanding reliable, accurate, and flexible test systems.

Testing a fuel cellBecause fuel cells are still in the development stage, the auto-motive industry has not settled on standard testing equipmentor test-equipment vendors. Many companies are stepping upto the challenge of developing both modular and turnkey so-lutions to accurately monitor and control fuel cells. Notableamong these companies are Hydrogenics and National Instru-

ments, who are creating hardware and software that permitmore expedient development of fuel-cell technology.Hydrogenics has developed three test systems that permit char-acterization of either single cells or stacks of cells. By usingNational Instruments data-acquisition and control hardwarein its systems, Hydrogenics is able to incorporate most of thedesired measurement and safety options required by scientists.

Although the overall goals of research and development,manufacturing, and operations vary, their need to monitor andcontrol fuel cells is similar. For R&D, testing is done to charac-terize and optimize energy output as well as extend the lifeand robustness of the stacks. In design validation, the maingoal is to optimize the design in preparation for mass produc-tion and to reduce the overall cost of the stack without reduc-ing the efficiency. For manufacturing applications, the stacksare monitored to ensure they meet the engineer’s specifications.In actual use, monitoring is essential to a stack’s life and op-eration. Fortunately, different stages of fuel-cell implementa-tion have similar needs of a well-designed tester to accommo-date the applications.

PEM fuel cells share the characteristics of requiring humidi-fied hydrogen and oxygen and generating electricity with abyproduct of water. Although water is a desired output in thespace program, the only output automotive scientists are trulyinterested in is electrical (current and voltage). Parameters thatcontrol the production of this power include gas-stream tem-perature, pressure, humidity, and flow rates. The stack’s indi-vidual cell voltages are measured and the overall stack tem-perature is monitored and controlled using active cooling. In

Figure 1. Electricity generated in a ProtonExchange Membrane (PEM) fuel cell.

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Figure 2. Virginia Tech engineering students prepare aPEM fuel cell for use in their hybrid-electric ChevroletLumina for the 1999 Future Car Challenge.

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Fuel-cell testing

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Fuel-cell testing

many applications, the load resistance is variable, allowing en-gineers to develop tafel plots (voltage/current density plotsthat indicate the power and efficiency of a stack or cell). A fuel-cell tester should be able to monitor and control all of theseparameters as well as measure and log the voltage and currentoutputs of the stack.

Consider the output of fuel cells: voltage and current. In atypical fuel-cell application, a known load is applied to the fuelcell to control output voltage and current. When the voltageoutput of a fuel cell increases, the output current decreases.The operating load of a fuel cell is a balance between the maxi-mum power output and the maximum efficiency. For example,a PEM was used by Virginia Tech’s hybrid-electric Chevy Lu-mina for the 1999 Future Car Challenge. An Energy Partnersfuel-cell stack was used, which created arange of 60-100 V dc. Under load withcurrent flowing, the output per cell woulddrop from 1 V to as low as 0.6 V per cell.Knowing the voltage of each individualmembrane allowed Virginia Tech toclosely monitor the health of its stack.

If one cell exhibits a different poten-tial, it is an indication of a problem withthe cell, including incorrect temperature,under hydration, or flooding. The voltagefrom each cell or group of cells is moni-tored to operate, test, or design a fuel cellproperly. By measuring a group of cells,the channel count and wiring require-ments can be reduced while still monitor-ing the health of the cells. While eachgroup of cells may reach up to 10 V in a PEM fuel cell, themembranes are stacked together to yield higher voltages. Be-cause the stack can reach over 100 V, the tester must not onlyhave many channels that are capable of reading 10 V per chan-nel, but also maintain isolation of hundreds of volts betweenthe first and last cell in the stack.

Obviously, simply monitoring the voltage is not sufficientto characterize and control a fuel cell. Current output is an-other item that is monitored. Because the current output canbe very high, it is usually monitored using the Gaussian effect.This method allows engineers to unobtrusively monitor the cur-rent flowing through a wire; it requires signal conditioning andscaling to convert the data back into a current reading. PEMfuel cells typically require temperatures in the range of 60-80°C

(140-175°F) to produce energy efficiently. This tem-perature is monitored for goals such as variationand correlation to power output. Thermocouplesand thermistors are good sensors for monitoringboth the stack temperature and the temperatureof the incoming reactant gas streams. In many ap-plications, the gas streams are at elevated pres-sures, which are monitored and managed. Pres-sure is measured with a pressure transducer andsignal conditioning, and the flow rate is measuredwith a flowmeter that outputs pulses at a rate pro-portional to the gas flow rate. These pulses are thenmonitored by a counter timer board and scaled bysoftware into a flow rate. Electronic regulators cancontrol the pressure and flow via 4-20 mA inputsthat are supplied by the test stand.

One of the final challenges in a fuel-cell test stand is themeasurement and control of gas-stream humidities. The waterflow in a cell is critical to its operation, and each membranemust remain hydrated to maintain its protonic conductivity. Ifa cell becomes too dry, the membranes are prone to damage. Ifthe membrane floods, the transport of reactants is reduced anda dramatic drop in overall system performance occurs. There-fore, proper humidification control and monitoring is essen-tial to the operation of a PEM fuel cell. One way to monitor thehumidity is through an electric humidity sensor that outputs4-20 mA current at a level proportional to the humidity. A volt-age input channel of the tester can then read this signal.

Along with monitoring, control is also required to conductfuel-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 vari-able pressure valves. Digital output lines provide the controlfor emergency shutoff, purge output, and bypass valves. Gen-eral Purpose Interface Bus (GPIB) or analog output is used tocontrol the heaters and fans used for temperature control. Inaddition, a programmable load is used to change the resistanceseen by the fuel cell. One way a tester can accomplish thischange is with a GPIB-controlled load device or by using digi-tal relays to connect various resistors in parallel. In the firstmethod, a stand-alone box is instructed, via GPIB, to changethe loads placed on the fuel cell. The second option uses relaysand switches different loads in and out. To vary the humidity,the water flow rate for the humidifier is adjusted.

Table 1Fuel-cell Parameters to be Monitored and Controlled

Item Channel type Signal conditioning

Voltage Analog input Isolation, attenuationCurrent Analog input Scaling attenuationPressure Analog input ScalingHumidity Analog input ScalingFlow rate Counter input ScalingTemperature Analog input Scaling, amplification, excitationEmergency shutoff Digital output SwitchingNitrogen purge Digital output SwitchingPressure valves Analog output AmplificationHeater and fans GPIB or digital output Switching (with digital output)Load GPIB or digital output Switching (with digital output)

Table 2Fuel-cell Testing Hardware Components

Item Description

PC/PXI controller Performs the test execution and data storagePC/PXI chassis Houses controller and I/O componentsMultifunction I/O Performs the analog to digital conversion and controls the conditioningRelays Routes power outputAnalog output Controls pressure valvesIsolated/amplified analog input Monitors voltage and currentIsolated thermocouple input Monitors temperature from thermocouplesIsolated digital output Controls shutoff, bypass, and purge valvesProgrammable load Absorbs power output of fuel cellMass flow controller Controls gas flowFixturing/piping Routes gases into and out of fuel cell

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Components of a fuel-cell testerThe major complication in the development of a true “turnkey”solution to fuel-cell testing has always been the mercurial needsof scientists due to the rapid evolution of fuel cells. As new ad-vances are made in fuel-cell development, researchers need ad-ditional measurement data that was not always anticipated inthe original design. One very clear example is in the trend to-ward higher stack voltages and more cells. For accurate moni-toring of a cell stack, it is important to track the voltage of theindividual cells. With an output potential of 0.6-1.0 V per cell, a100-V stack will need up to 100 isolated analog inputs. Instead

of building a black-box tester, many companies are working todevelop modular systems that will allow researchers to modifythe design as their testing needs change. Central to this flex-ible design is a virtual interface and virtual instrumentationthat will allow the addition and modification of input param-eters and of stored data. Almost all testers today use a com-puter interface to collect, analyze, display, and store data. Ro-bust testers, such as Hydrogenics’ systems, also incorporate astable, real-time operating system for the data collection andfor the nitrogen purge safety systems.

Hardware and softwareAlthough the needs of PEM test engineers are challenging,many components from the test and measurement industryare equipped to handle the task. The hardware componentsof a test stand include the controller, the fixturing, and thetransducers. A popular choice for the controller is a PC-basedone. This method offers the advantage of leveraging PCadvancements such as speed, memory, and upgradability. A

more robust form factor and operating system (OS) offers ben-efits for the demands of fuel-cell design testing.

A choice that has gained support in recent years is a PXI orCompact PCI, which offers PC capabilities in a rugged and modu-lar form factor. These can be outfitted with a real-time OS thatcontrols data-acquisition and safety features of the test stand.

The highest level of the control software is the test execu-tive. This supervisory level piece of software calls individualtest routines, indicates pass/fail, and generates results.

The next level is the test routine software. For reliabilitypurposes, the test routine would run on the non-Windows real-time operating system. An ideal architecture for the routinesoftware would promote modularity and ease of modification,which is important because the procedures for testing fuel cellsare evolving along with the technology itself. Test systems builtaround test executive and graphical programming software areunder development and will retain the ability for future modi-fication and run out of the box.

In the testing hardware, I/O components that can digitizesignals for the PC are needed. Testers equipped with a multi-function I/O board can scan many channels at both low andhigh rates. This ability allows engineers to monitor steady-stateand transient voltage, current levels produced by the fuel cell,and stack operating parameters. Signal conditioning handlesthe conversion of current, pressure, and temperature to volt-ages. In addition to computer and data-acquisition cards, thereare a programmable load, a humidification system, gas flowcontrollers, and a stack temperature controller. The last hard-ware element is the fixturing. To avoid ionic contamination ofthe cell membranes, 316 SS, Teflon, or titanium is often used inthe water, hydrogen, and oxygen piping. For the same reason,all the water used for cooling and humidification must be deion-ized before introduction into the stack.

Continued evolutionFuel cells, as a developing technology, show promise to becomeone of the most efficient and clean energy-producing sourcesavailable. In addition to providing on-demand energy withoutthe CO and NOx typically associated with combustion, theyalso promise to reduce greenhouse gas emissions with theirCO2-free exhaust. However, before they are practical for wide-spread use, great developmental strides need to be accom-plished to reduce size and increase energy yield. Test systemsthat have the capability to make all relevant measurementswhile providing the flexibility to incorporate new proceduresand calculations are critical to this development. A test plat-form based on PC technology with the open architecture of thePXI/Compact PCI form factor provides a good test foundationby blending mainstream PC technologies and a rugged reli-ability while delivering a high degree of modularity. With mil-lions of dollars being invested each year and interest in fuel-cell development being propelled by environmental, govern-mental, and consumer pressures, the fuel cell will continue toevolve at a rapid pace, and virtual-instrumentation-based con-trollers will test it every changing step of the way.

Information for this article was supplied by Dave Wilson, Director, Automotive MarketDevelopment, and Todd Walter, Application Engineer, National Instruments.

Figure 3. Hydrogenics’ fuel-cell test system usesNational Instruments’ FieldPoint distributed I/O tomonitor and control fuel-cell testing.

Fuel-cell testing