Download Fuel cells are one of the most promising technologies for delivering

Capabilities that deliver
reliable monitoring and
control, as well as offer the
benefit of a flexible
configuration, are critical to
keep pace with evolving
fuel-cell technology, according
to National Instruments.
F
uel cells are one of the
most promising
technologies for delivering
clean and efficient power for
A Ballard fuel-cell stack being
tested prior to shipment.
automotive and residential
applications. A fuel cell directly
converts the chemical energy of hydrogen and oxygen into electricity
with a byproduct of pure water. Until recently, fuel cells have largely
been restricted to NASA space missions and a few research labs around
the world. However, with increased urgency in reducing pollution and
greenhouse gas emissions, a resurgence of interest in fuel cells has
occurred in the scientific community. Today, governments and large
corporations are making massive investments into the development of
these clean power sources. Although fuel cells hold great promise for
clean, inexpensive power, they are still in their developmental infancy,
and a great deal of research is necessary before they are considered
viable power systems. Test capabilities that deliver reliable monitoring
and control, and offer the benefit of a flexible configuration, are critical
to these advances. The capabilities will permit scientists to easily tailor
their systems to keep pace with evolving fuel-cell technology.
aei
MARCH 2001 85
as seen in
Fuel-cell testing
Even though several types of fuel cells exist, they all work
under the same basic premise of converting hydrogen and oxygen into electrical power. Of the fuel-cell technologies, which
include alkaline (AFC), molten carbonate (MC), phosphoric acid
(PAFC), proton exchange membrane (PEM), and solid oxide
(SOFC), PEM is gaining most of the attention in automotive
applications. PEMs are popular due to their relatively low operating temperature and high efficiency. The PEM fuel cell operates by using platinum-coated membranes as a catalyst to
break a hydrogen atom into a proton and an electron. The membrane is permeable to protons, but impenetrable to free electrons. These electrons are forced to travel through an electric
circuit before they rejoin with free protons and oxygen molecules to form water. In this way, the anode of the fuel cell produces electricity, and the cathode creates heat and water. However, just as it took years of tests and improvements to achieve
the efficiencies currently realized by internal combustion engines, many improvements are necessary before fuel cells are
viable for automotive use.
efficiencies of next-generation fuel cells, engineers are constantly incorporating new measurements into their tests and
demanding reliable, accurate, and flexible test systems.
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 Instru-
Electricity
Oxygen (O2)
Anode (-)
Cathode (+)
Figure 2. Virginia Tech engineering students prepare a
PEM fuel cell for use in their hybrid-electric Chevrolet
Lumina for the 1999 Future Car Challenge.
Hydrogen (H2)
Courtesy of Hydrogenics
Water
Heat
Proton exchange
membrane
Figure 1. Electricity generated in a Proton
Exchange Membrane (PEM) fuel cell.
The introduction of computer control revolutionized the internal combustion engine. It allowed engineers to monitor and
control fuel rate, timing, and cooling. With the adoption of monitoring and controlling techniques such as fuel injection, oxygen
sensors, knock detectors, and mass flow sensors, engine efficiencies have reached an all-time high, while pollution per engine
has been greatly reduced. Engineers have learned that through
computer control and careful monitoring of important variables,
vehicle powerplants can be greatly improved. To develop a viable fuel cell, engineers need to accurately monitor the condition of the hydrogen stream, oxygen stream, output voltage, and
current. To optimize a fuel cell, not only are the flow and pressure of the hydrogen and oxygen monitored, but also the humidity and temperature of the gas streams. Knowing the voltages of the individual membranes can enable an engineer to
read the health of a fuel-cell stack and control the output resistance to map the power densities of the stacks. To improve the
86 MARCH 2001
aei
ments, 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.
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
as seen in
Fuel-cell testing
(140-175°F) to produce energy efficiently. This temperature is monitored for goals such as variation
and correlation to power output. Thermocouples
Item
Channel type
Signal conditioning
and thermistors are good sensors for monitoring
Voltage
Analog input
Isolation, attenuation
both the stack temperature and the temperature
Current
Analog input
Scaling attenuation
of the incoming reactant gas streams. In many apPressure
Analog input
Scaling
plications, the gas streams are at elevated presHumidity
Analog input
Scaling
sures, which are monitored and managed. PresFlow rate
Counter input
Scaling
sure is measured with a pressure transducer and
Temperature
Analog input
Scaling, amplification, excitation
signal conditioning, and the flow rate is measured
Emergency shutoff
Digital output
Switching
with a flowmeter that outputs pulses at a rate proNitrogen purge
Digital output
Switching
portional to the gas flow rate. These pulses are then
Pressure valves
Analog output
Amplification
monitored by a counter timer board and scaled by
Heater and fans
GPIB or digital output
Switching (with digital output)
software into a flow rate. Electronic regulators can
Load
GPIB or digital output
Switching (with digital output)
control the pressure and flow via 4-20 mA inputs
that are supplied by the test stand.
many applications, the load resistance is variable, allowing enOne of the final challenges in a fuel-cell test stand is the
gineers to develop tafel plots (voltage/current density plots
measurement and control of gas-stream humidities. The water
that indicate the power and efficiency of a stack or cell). A fuelflow in a cell is critical to its operation, and each membrane
cell tester should be able to monitor and control all of these
must remain hydrated to maintain its protonic conductivity. If
parameters as well as measure and log the voltage and current
a cell becomes too dry, the membranes are prone to damage. If
outputs of the stack.
the membrane floods, the transport of reactants is reduced and
Consider the output of fuel cells: voltage and current. In a
a dramatic drop in overall system performance occurs. Theretypical fuel-cell application, a known load is applied to the fuel
fore, proper humidification control and monitoring is essencell to control output voltage and current. When the voltage
tial to the operation of a PEM fuel cell. One way to monitor the
output of a fuel cell increases, the output current decreases.
humidity is through an electric humidity sensor that outputs
The operating load of a fuel cell is a balance between the maxi4-20 mA current at a level proportional to the humidity. A voltmum power output and the maximum efficiency. For example,
age input channel of the tester can then read this signal.
a PEM was used by Virginia Tech’s hybrid-electric Chevy LuAlong with monitoring, control is also required to conduct
mina for the 1999 Future Car Challenge. An Energy Partners
fuel-cell testing. Almost all of the monitored items need to be
fuel-cell stack was used, which created a
range of 60-100 V dc. Under load with
Table 2
current flowing, the output per cell would
Fuel-cell Testing Hardware Components
drop from 1 V to as low as 0.6 V per cell.
Item
Description
Knowing the voltage of each individual
PC/PXI
controller
Performs the test execution and data storage
membrane allowed Virginia Tech to
PC/PXI chassis
Houses controller and I/O components
closely monitor the health of its stack.
Multifunction I/O
Performs the analog to digital conversion and controls the conditioning
If one cell exhibits a different potenRelays
Routes power output
tial, it is an indication of a problem with
Analog
output
Controls
pressure valves
the cell, including incorrect temperature,
Isolated/amplified analog input
Monitors voltage and current
under hydration, or flooding. The voltage
Isolated thermocouple input
Monitors temperature from thermocouples
from each cell or group of cells is moniIsolated digital output
Controls shutoff, bypass, and purge valves
tored to operate, test, or design a fuel cell
Programmable load
Absorbs power output of fuel cell
properly. By measuring a group of cells,
Mass flow controller
Controls gas flow
the channel count and wiring requireFixturing/piping
Routes gases into and out of fuel cell
ments 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
controlled for repeatable tests. To control gas-stream pressures,
membranes are stacked together to yield higher voltages. Beanalog output channels from the tester set the electrically varicause the stack can reach over 100 V, the tester must not only
able pressure valves. Digital output lines provide the control
have many channels that are capable of reading 10 V per chanfor emergency shutoff, purge output, and bypass valves. Gennel, but also maintain isolation of hundreds of volts between
eral Purpose Interface Bus (GPIB) or analog output is used to
the first and last cell in the stack.
control the heaters and fans used for temperature control. In
Obviously, simply monitoring the voltage is not sufficient
addition, a programmable load is used to change the resistance
to characterize and control a fuel cell. Current output is anseen by the fuel cell. One way a tester can accomplish this
other item that is monitored. Because the current output can
change is with a GPIB-controlled load device or by using digibe very high, it is usually monitored using the Gaussian effect.
tal relays to connect various resistors in parallel. In the first
This method allows engineers to unobtrusively monitor the curmethod, a stand-alone box is instructed, via GPIB, to change
rent flowing through a wire; it requires signal conditioning and
the loads placed on the fuel cell. The second option uses relays
scaling to convert the data back into a current reading. PEM
and switches different loads in and out. To vary the humidity,
fuel cells typically require temperatures in the range of 60-80°C
the water flow rate for the humidifier is adjusted.
Table 1
Fuel-cell Parameters to be Monitored and Controlled
aei
MARCH 2001 87
as seen in
Fuel-cell testing
Components of a fuel-cell tester
The major complication in the development of a true “turnkey”
solution to fuel-cell testing has always been the mercurial needs
of scientists due to the rapid evolution of fuel cells. As new advances are made in fuel-cell development, researchers need additional measurement data that was not always anticipated in
the original design. One very clear example is in the trend toward higher stack voltages and more cells. For accurate monitoring of a cell stack, it is important to track the voltage of the
individual cells. With an output potential of 0.6-1.0 V per cell, a
100-V stack will need up to 100 isolated analog inputs. Instead
more robust form factor and operating system (OS) offers benefits for the demands of fuel-cell design testing.
A choice that has gained support in recent years is a PXI or
Compact PCI, which offers PC capabilities in a rugged and modular form factor. These can be outfitted with a real-time OS that
controls data-acquisition and safety features of the test stand.
The highest level of the control software is the test executive. This supervisory level piece of software calls individual
test routines, indicates pass/fail, and generates results.
The next level is the test routine software. For reliability
purposes, the test routine would run on the non-Windows realtime operating system. An ideal architecture for the routine
software would promote modularity and ease of modification,
which is important because the procedures for testing fuel cells
are evolving along with the technology itself. Test systems built
around test executive and graphical programming software are
under development and will retain the ability for future modification and run out of the box.
In the testing hardware, I/O components that can digitize
signals for the PC are needed. Testers equipped with a multifunction I/O board can scan many channels at both low and
high rates. This ability allows engineers to monitor steady-state
and transient voltage, current levels produced by the fuel cell,
and stack operating parameters. Signal conditioning handles
the conversion of current, pressure, and temperature to voltages. In addition to computer and data-acquisition cards, there
are a programmable load, a humidification system, gas flow
controllers, and a stack temperature controller. The last hardware element is the fixturing. To avoid ionic contamination of
the cell membranes, 316 SS, Teflon, or titanium is often used in
the water, hydrogen, and oxygen piping. For the same reason,
all the water used for cooling and humidification must be deionized before introduction into the stack.
Continued evolution
Figure 3. Hydrogenics’ fuel-cell test system uses
National Instruments’ FieldPoint distributed I/O to
monitor and control fuel-cell testing.
of building a black-box tester, many companies are working to
develop modular systems that will allow researchers to modify
the design as their testing needs change. Central to this flexible design is a virtual interface and virtual instrumentation
that will allow the addition and modification of input parameters and of stored data. Almost all testers today use a computer interface to collect, analyze, display, and store data. Robust testers, such as Hydrogenics’ systems, also incorporate a
stable, real-time operating system for the data collection and
for the nitrogen purge safety systems.
Hardware and software
Although the needs of PEM test engineers are challenging,
many components from the test and measurement industry
are equipped to handle the task. The hardware components
of a test stand include the controller, the fixturing, and the
transducers. A popular choice for the controller is a PC-based
one. This method offers the advantage of leveraging PC
advancements such as speed, memory, and upgradability. A
88 MARCH 2001
aei
Fuel cells, as a developing technology, show promise to become
one of the most efficient and clean energy-producing sources
available. In addition to providing on-demand energy without
the CO and NOx typically associated with combustion, they
also promise to reduce greenhouse gas emissions with their
CO2-free exhaust. However, before they are practical for widespread use, great developmental strides need to be accomplished to reduce size and increase energy yield. Test systems
that have the capability to make all relevant measurements
while providing the flexibility to incorporate new procedures
and calculations are critical to this development. A test platform based on PC technology with the open architecture of the
PXI/Compact PCI form factor provides a good test foundation
by blending mainstream PC technologies and a rugged reliability while delivering a high degree of modularity. With millions of dollars being invested each year and interest in fuelcell development being propelled by environmental, governmental, and consumer pressures, the fuel cell will continue to
evolve at a rapid pace, and virtual-instrumentation-based controllers will test it every changing step of the way.
Information for this article was supplied by Dave Wilson, Director, Automotive Market
Development, and Todd Walter, Application Engineer, National Instruments.