Deployment issues with portable fuel cells

The Challenge
Direct Methanol Fuel Cells (DMFC) and other consumer size fuel cell devices are making headlines these days. With the struggle for efficiency and regulation of liquid fuel in power supplies, the allure of long operating life and a rapid simple “recharge” procedure out weighs the difficulties in bringing this technology to the mainstream. However, there are still issues with managing these tiny power plants. Some of these concerns include safety, power regulation, fuel capacity and system monitoring.

In looking at the basic design for Direct Methanol Fuel Cells (DMFC), the challenges become more obvious. Storage of flammable fuel is an immediate issue on aircraft as well as other transportation systems. However, similar issues have been addressed in the past. Lithium Ion batteries were considered extremely dangerous and now are common place on aircraft. Technology can solve most of the safety issues and this article will address both the safety issues and power conversion of fuel cells.

The Solution: Regulation and Technology
Safety of any system where catastrophic failure can injure or kill a user (or those around them) is always a primary concern. Energy storage device manufacturers have had to deal with this issue for years. In the case of fuel cells, these devices are energy generators, but still require a reservoir of fuel which has potential for fire or explosion.

There are several ways to address the issue. One way is to regulate the use of fuel cells and limit the amount of fuel in each device. Additionally, providing mechanical safety mechanisms can make tampering extremely difficult. Furthermore monitoring of a fuel cell can add an additional layer of safety by shutting down the system during unsafe conditions. This is not unlike Lithium Ion batteries. These have mechanical safety systems inside the cells as well as monitoring systems that are engaged during charging or load. The added benefit of monitoring a battery or DMFC system’s performance is that the remaining energy can be calculated.

Figure 1 shows a sample block diagram along with several sensors used for monitoring a fuel cell system’s health. These include a pressure sensor on the methanol fuel source, temperature sensors on various sections of the system, a methanol sensor to ensure the proper concentration, and load sensors on the power supply. The pressure sensor is typically a strain gauge configured in a bridge topology. This requires a precision amplifier or instrumentation amplifier to provide the correct gain for the analog to digital converter. The amplifier requires low drift over temperature changes due to the range of operation of the system. In the example system, a National Semiconductor LMP2011 was chosen due to the extremely low drift and wide operating temperature of the device.

Figure 1: TypicalDMFC with Monitoring and Safety Features
To simplify monitoring thermal performance of the system, digital temperature sensors are recommended. These devices use a two wire system and can be optionally bussed together to reduce wiring. In the example, several LM92 digital temperature sensors were used which have a worse case accuracy of ± 0.33º C. Accurate temperature measurements of the cell, as well as the power regulators ensure safety and maximum performance.

A microcontroller is used to provide the brains for monitoring the operation and safety of the system. However, many modern microcontrollers have built in analog to digital converters (ADCs) that may have limitations in fuel cell applications. When the fuel cell system is in stand-by mode and the cell stack is not producing power, it may be best to use external ADCs and power them down to conserve energy. In this example, ADC121S101 external ADCs were used. These converters are serial 12 bit, single channel devices. Single channel devices provide the most flexibility in power management, but multi-channel devices are available as well from many manufacturers.

Output Power Regulation
Fuel cells exhibit a problem with cell voltage versus load current due to the electrochemical nature of the design. This variation under varying load (such as in a notebook computer) can cause the output voltage of a stack to vary significantly. This behavior is called Fuel Cell Polarization and is illustrated in Figure 2. There are three regions of cell polarization which are the “activation region”, the “resistive region” and the “diffusion region”. The activation loss is due to the basic energy required for cell operation. As the load increases it enters the resistive loss region. This is the normal operating area of the cell and the loss is due to ohm’s law of V=i*R as current increases. The diffusion loss is the point where the cell can no longer generate enough power and the cell voltage begins to sag. This behavior is unsuitable for directly powering most electronic systems, therefore voltage regulation is a necessity.

If the output voltage required is always lower than the minimum stack voltage, then a simple Buck converter can be used. Mainly, the issue with a buck converter topology is that the efficiency drops as the load is reduced. This is due to losses in the switching controller, resistive losses in diodes and FETs, as well as hysteretic losses in the inductor. To improve the efficiency dynamic range, National Semiconductor and other suppliers have created power converters that alter their operation frequency depending on the load. These devices are called Hysteretic converters (such as the LM27212 controller) and employ a constant on period followed by a variable off period. In this manner, as the load decreases, the off period increases and lowers the internal power consumption of the converter.

In cases where the output voltage may lie between the minimum and maximum voltages provided by the stack, a Buck-Boost, Septic, or Flyback topology can be used. These topologies share characteristics of both the buck and the boost topologies in that they can control the output voltage even when the input level drops below the output. For higher power applications (50-1000 watts), typically a forward, push-pull or current fed push-pull topology is used. These are much more complex power converters and require some expertise to design correctly. The semiconductor industry has simplified some of the design requirements by integrating much of the control loop functions into devices, such as the LM5041 buck fed, push-pull, current mode converter and others.

Conclusions
With the use of both regulations on mechanical safety along with the proper monitoring and control electronics, DMFC systems can be made extremely safe as was done in the past with the Lithium Ion batteries so common today. Simple electronics along with sensors can determine the safe operating condition of the cell and either allow operation or shut down the system to prevent failure or hazards.

Also, power converter technology has reached new levels of efficiency and power delivery at reasonable costs. Many different devices supporting various topologies are available for simplifying the design of these converters. By tightly coupling the control and safety functions with the power management functions, a user friendly, and extremely reliable and safe fuel cell system can be implemented.

Click here for Illustrations:

Figure 1

Figure 2