Fuel cells offer many advantages over batteries, but the design challenges faced when implementing fuel cells for portable application are substantial. High temperatures, tempermental catalysts, and fuel sources that can be difficult to handle pose problems for any kind of fuel cell system. However, the design solutions available when constrained by the needs of a portable hand-held device quickly dwindle. This article describes one particular solution developed by myself and Yildiz Bayazitoglu at Rice University. Designers often utilize hydrogen over other types of fuel for fuel cells because they offer advantages in terms of reliability (less catalyst fouling), and emissions (only water vapor is emitted), but carrying sufficient hydrogen can be problematic even for something as massive as an automobile. Several engineers have proposed solutions ranging from storing hydrogen within solid metal oxides or carbon nanotube matricies. But the easiest and safest hydrogen carrier by many measures is methanol, especially when mixed with water at a 1:1 molar ratio (water is actually useful as it provides additional hydrogen together with oxygen to combine with methanol's carbon atom).
Unlike a battery, a fuel cell just needs additional fuel added. For small systems this could be accomplished via a cartridge or pressurized reservoir. Nearly instantaneous recharging is a key benefit of these systems. Commercial versions of fuel cells including refillable fuel reservoirs have been developed for laptop computers. But creating an even smaller fuel cell for a hand held device such as a small tablet computer or cell or satellite phone poses unique challenges. The reforming reaction needs high temperatures, as high as 200 degrees Celsius. Insulation, which takes up space, can protect a user's hands and improve efficiency, but so much must be used as to make the device actually comfortable, and not just safe enough to prevent skin burns.
In response to these issues, we developed a design for a very small scale methanol to hydrogen microreformer. You can read more about this design in this patent document or this journal article.
Our miniaturized design relies on fluid channels routed in a spiral shape only a quarter of a millimeter in width. We created a computer program that optimized the shape and size of the channel design according to a few specific criteria including overall pressure drop, overall thermal efficiency, and complete reaction of the methanol and water into hydrogen. The objectives included creating a device that could supply a hydrogen fuel cell with sufficient fuel to power a 1 Watt handheld device, while keeping the temperature of that device comfortable to hold, quick to power up, and energy efficient.
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To meet these objectives, the design needed to rely on a very narrow catalyst-lined channel etched into a silicon substrate. Channels of this size begin to experience new physics not experienced by typical plumbing projects. The size of the molecules compared to the size of the channel can result in the flow being more similar to beads in a channel instead of a continuous fluid in a pipe. These effects combine with the complication of phase change and chemical conversion taking place during the transit from the microreformer's entrance to its exit. The final design only experiences some slight deviations from Newtonian fluid mechanics, but the relative benefits of much smaller channels had to be fully explored.
The microreformer requires heat to achieve the chemical breakdown of water and methanol into hydrogen and carbon dioxide, in addition to converting the liquid water-methanol mixture into a gas. The key is to ensure that this energy is significantly less than the energy potential of the hydrogen gas exiting the device. If you divide the total potential power provided by the volume of hydrogen gas created (assuming a typical, not perfect hydrogen fuel cell) minus the energy spent on the reforming operation and the energy lost as heat through the insulation by the total potential power you will calculate the overall efficiency of the device. Simulations of our optimized design indicate a theoretical efficiency of 70% a little less than many rechargeable batteries in the 80%-90% range. But, further improvements may be possible.
It should be noted that the consumer market is thoroughly trained in the use of and reasonable expectations for rechargeable batteries. Prospective buyers and designers are familiar with the benefits and limitations of the use and maintenance of batteries, and the very different drawbacks of portable fuel cells could be a significant impediment to widespread implementation. A more likely set of users would likely be military personnel, adventurers, and those involved in pipeline or transmission line inspections. Other groups of people are too integrated into the modern and increasingly pervasive electrical grid for fuel cells to offer many advantages. Those having to operate critically important electronic devices far away from typical civilization, however, could benefit from the unique characteristics of methanol fuel cell power systems.
