Fuel Cell Testing - The NI Way

Publish Date: Jan 19, 2012 | 110 Ratings | 3.90 out of 5 |  PDF

Table of Contents

  1. Introduction
  2. What Is a Fuel Cell?
  3. Fuel Cell Test Systems
  4. Measurement Challenges
  5. National Instruments Fuel Cell Testing Tools
  6. National Instruments Software
  7. Summary

1. Introduction

Fuel cells are becoming a very efficient and clean source of electrical energy for the future. Fuel cells boast many advantages when compared with conventional energy sources of today. The power from a fuel cell’s is derived from hydrogen, an element that is extracted from many renewable resources. This conversion of hydrogen to electricity has no emissions. Conventional energy production requires nonrenewable fuels and produces pollutants as well. These properties are two of the many reasons that the fuel cell is a viable energy source for the future in automotive, commercial, residential, portable, and many other electrical power applications.

National Instruments currently offers many computer-based measurement products that aid scientists and engineers in research, design, validation, and production testing of fuel cells. NI is a world leader in computer-based measurement and many leading fuel cell manufacturers are using NI hardware and software tools for testing fuel cells in all phases of development. These test tools have been essential in the production and testing of many types of fuel cells including proton exchange membrane (PEM), phosphoric acid fuel cells (PAFCs), and solid oxide fuel cells (SOFCs).

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2. What Is a Fuel Cell?

Fuel cells all work under the same basic premise of converting the chemical energy of hydrogen into electricity. There are a number of different types of fuel cell technologies. In addition to PEMs, PAFCs, and SOFCs, there are alkaline fuel cells (AFCs) and molten carbonate (MC) fuel cells. PEMs are popular for automotive applications due to their relatively low operating temperature and high efficiency. Figure 1 shows how a PEM fuel cell operates.


Figure 1. Basic Concepts of a Fuel Cell


Because a single cell generates about 1 V, most applications require more than a single cell. A fuel cell stack, composed of individual fuel cells wired in series similar to batteries, offers increased power output. Some fuel cell stacks containing thousands of individual cells are capable of generating the high voltages and currents needed for many transportation, commercial and industrial power applications.

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3. Fuel Cell Test Systems


In research and development (R&D) applications, fuel cell testing is performed to characterize and optimize energy output as well as to extend the life and robustness of the stacks. In 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 that they pass design specifications. Scientists and engineers researching, developing, or manufacturing fuel cells need a variety of measurement, control, analysis and visualization tools to evaluate and validate fuel cell technology.

Fuel cell test systems need accurate monitoring and control of hundreds of measurements ranging from the flow, temperature, pressure, and humidity of the hydrogen fuel to the output voltage and current of the fuel cell stack. Test systems must be able to monitor and control fuel cell operation under widely varying conditions and accurately capture information relating to real-time performance and operational characteristics. Test systems must also provide flexible data acquisition, monitoring, and control capability to precisely control fuel cell operation and experiments. A system developed by Advanced Measurements for solid oxide fuel cell testing is shown in Figure 2 below.


Figure 2. SOFC Fuel Cell Test System


Engineers are constantly incorporating new measurements into their tests, demanding reliable, accurate, and flexible test systems to help shorten development cycles, increase quality, and lower costs to develop next generation fuel cells.

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4. Measurement Challenges


Fuel cell test systems make a variety of measurements that require signal conditioning before the raw signal can be digitized by the data acquisition system. An important feature for the testing of fuel cell stacks is isolation. Each individual cell may generate about 1 V, and a group of cells may reach up to 10 V because in a PEM the membranes are stacked together to yield higher voltages. High-performance stacks have hundreds of cells that result in voltage measurements that require common-mode rejection of several hundred volts. Because of this, the tester must not only have many channels that are capable of reading 1 to 10 V per channel, but also maintain isolation of hundreds of volts between the first and last cell in the stack. Because fuel cell test systems also include channel counts that can range anywhere from 100 to more than 1000 channels, data acquisition system capable of expansion is a must. These systems also have signals that require attenuation and amplification. Modularity is also a must for test systems today; your system must be able to change with production and validation technologies. Any test system should have calibration as well to ensure valid and accurate measurements.

Based on the above criteria, it is obvious that simply monitoring the voltage is not sufficient to characterize and control a fuel cell. Monitoring current output is also necessary. Because the current output can be very high, it is usually monitored using the Hall Effect, whereby engineers unobtrusively monitor the current flowing through a wire. This method requires signal conditioning and scaling to convert the data back into a current reading. Another vital parameter for PEM fuel cells is temperature. To produce energy efficiently PEM fuel cells must operate in the range of 60 to 80 °C (140 to 175 °F). Temperature is monitored to optimize variables such as variation and correlation to increase power output. Thermocouples and resistance temperature detectors (RTDs) are good sensors for monitoring both the stack temperature and the temperature of the incoming reactant gas streams. In many applications, the gas streams are at elevated pressures, which must be monitored and managed. Pressure is measured with a pressure transducer and signal conditioning; hydrogen and airflow rates are typically measured with mass flow meters that generate pulses at a rate proportional to the gas flow rate. These pulses are then monitored by a counter/timer board and scaled by software into a flow rate. Electronic regulators can control the pressure and flow via voltage or current outputs that are supplied by the test stand.

National Instruments offers many solutions for testing fuel cells that provide the features previously listed plus many more. The key for this test platform is ease of use, modularity, and flexibility, all of which are strengths that NI products offer.

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5. National Instruments Fuel Cell Testing Tools


SC Express for Signal Conditioning on PXI

SC Express combines signal conditioning and data acquisition onto a single module and is ideal for fuel cell testing systems that necessitate high channel counts and synchronization.  There is a variety of I/O available on PXI that can be used to design your system. 

  • Modularity - PXI platform offers wide variety of I/O from many different vendors
  • Isolation - channel-to-channel and channel-to-earth isolation available on certain modules
  • Throughput - Bandwidth capabilities of PXI Express ensure high data acquisition rates
  • Channel Expansion- Dedicated bandwidth and ADC's allow you to expand your system without sacrificing acquisition rates
  • Per-channel software programmable conditioning for easy system customization 
  • Screw terminal connectivity via front mount terminal blocks

PXI Based Test System

Measurement Channel Type Signal Conditioning Modules
Voltage Analog Input Isolation, Attenuation PXIe-4300
Current Analog Input Scaling, Attenuation PXI-6236
Pressure Analog Input Scaling PXIe-4330 or PXIe-4331
Humidity Analog Input Scaling PXIe-4330 or PXIe-4331
Flow Rate Analog Input Scaling PXI-6602
Temperature Analog Input Scaling, Amplification PXIe-4353
Emergency Shut Off Digital Output Switching PXIe-2566
Nitrogen Purge Digital Output Switching PXIe-2566
Pressure Valves Analog Output Amplification PXI-6704
Heaters and Fans Digital Output Switching (w/dig out) PXI-6602

 

SCXI Signal Conditioning

Fuel cell test stands often use SCXI for testing fuel cells. SCXI offers a wide range of signal conditioning features for both input and output. SCXI provides the critical technologies you need for measurement-ready conditioning, such as:

  • Modularity – SCXI products offers robust architecture from which to select your I/O modules
  • Isolation – protects your system from transient signals; may also be required when the sensor is on a different ground plane than the measurement sensor
  • SCXI-1125 – Up to 160 dB common mode rejection (300 Vrms working isolation per channel)
  • Programmability – channel-by-channel configuration using NI-DAQ software
  • Measurement & Automation Explorer (MAX) – virtual channels
  • Calibration – internal software calibration as well as traditional external NIST-traceable calibration certificate
  • LabVIEW software routines for calibration
  • Connectivity – connect your signals using thermocouple jacks and module specific terminal blocks
  • TBX-1316 – ±1000 VDC (Cat. III) input range (200:1 attenuation)


SCXI-Based Test Platform

Measurement Channel Type Signal Conditioning SCXI Modules
Voltage Analog Input Isolation, Attenuation 1125 (TBX-1316)
Current Analog Input Scaling, Attenuation 1104
Pressure Analog Input Scaling 1102C
Humidity Analog Input Scaling 1102C
Flow Rate Analog Input Scaling 1126 or PXI-6602
Temperature Analog Input Scaling, Amplification 1102B
Emergency Shut Off Digital Output Switching 1161
Nitrogen Purge Digital Output Switching 1161
Pressure Valves Analog Output Amplification 1124
Heaters and Fans GPIB or Digital Output Switching (w/dig out) PXI-GPIB or PXI-6602
Load GPIB PXI-GPIB


FieldPoint Distributed Data Acquisition

National Instruments FieldPoint distributed I/O products are also frequently used for fuel cell testing. FieldPoint include capabilities that improve the reliability and maintainability of distributed systems. Local intelligence is very valuable for a fuel cell test platform because it provides onboard diagnostics and easy maintenance to maximize system uptime. Furthermore, distributed systems with embedded processing and control functionality can help you implement embedded, stable distributed control systems.

  • Modularity – innovative architecture that modularizes the communications, I/O functions, and signal termination
  • Isolation – protects your system from transient signals; may also be required when the sensor is on a different ground plane than the measurement sensor
  • FP-AI110 - 250 Vrms working voltage channel-to-ground isolation
  • Connectivity – screw terminals for easy signal connection


FieldPoint-Based Test Platform

Measurement Channel Type Signal Conditioning FieldPoint Modules
Voltage Analog Input Isolation, Attenuation FP-AI-100
Current Analog Input Scaling, Attenuation FP-AI-111
Pressure Analog Input Scaling FP-AI-110 or 100
Humidity Analog Input Scaling FP-AI-110 or 100
Flow Rate Analog Input Scaling FP-CTR-500
Temperature Analog Input Scaling, Amplification FP- TC-120
FP-TC-K
Emergency Shut Off Digital Output Switching FP-DO-401
FP-DO-DC
Nitrogen Purge Digital Output Switching FP-PWM-520
Pressure Valves Analog Output Amplification FP-AO-200
FP-AO-Vx
FP-A0-Cxxx
Heaters and Fans GPIB or Digital Output Switching (w/dig out) FP-PWM-520
Load GPIB PXI-GPIB

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6. National Instruments Software


The LabVIEW Datalogging and Supervisory Control Module meet the needs of fuel cell researchers and testers with its built-in high-channel-count data acquisition, data logging, interface, and security capabilities. Fuel cell test systems may require hundreds of data acquisition sensors and controls for temperature, humidity, atmospheric pressure, oxygen, and other parameters. A typical test may include increasing a parameter such as temperature to a certain point, holding it steady, and graphing the resulting voltage and current across the fuel cell. The LabVIEW Datalogging and Supervisory Control module includes an extensive historical database in which all measurement and test data acquired can be saved for later review and correlation with DIAdem, National Instruments software for technical data management. With DIAdem, the user can load in test data and perform interactive offline analysis and report generation.

Uptime, reliability and safety are critical factors for fuel cell test stands used in validation, life-cycle and production test systems. In these critical applications, manufacturers are using real-time solutions such as LabVIEW Real-Time to deliver a robust system. Vendors are reporting exceptional reliability (less than 1 second of downtime a year). LabVIEW’s ease of use, flexibility, code reuseability and ability to scale from benchtop measurements to real-time control makes it an ideal software development environment for fuel cell applications in every phase of their product life cycle. In addition, when manufacturers move fuel cells to mass production they can scale LabVIEW and National Instruments products into their production floor for manufacturing test. Fuel cell manufacturers can take full advantage stability and scalability of LabVIEW.

All National Instruments software offerings provide easy integration with any of the hardware products previously mentioned. Test software runs on a PXI (CompactPCI) computer that seamlessly interacts with any of the systems listed above. For more information about PXI see ni.com/pxi

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7. Summary


The key to all computer-based measurement systems offered by National Instruments is scalability, flexibility, and ease of use. These systems, which consist of NI hardware and software, can be used from design and validation testing in the initial stages of R&D all the way to final production.

Although the overall goals of R&D, 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 specifications. In actual use, monitoring is essential to the life and operation of a stack. Fortunately, the different stages of fuel cell implementation all have similar needs – a well-designed test system.

Software that is easily understood and flexible for expanding your test platform makes adding measurements to your platform effortless. Having a unified, standard test platform throughout the R&D and production processes can save both time and money. A test platform based on PC technology with the open architecture of the PXI/Compact PCI platform provides a good test foundation by blending mainstream PC technologies and rugged reliability while delivering a high degree of modularity. With millions of dollars being invested each year and interest in fuel cell development being advanced 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.

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