Figure 1. Portable Power Generation System
PNNL researchers are developing a compact, chemical process system for converting liquid hydrocarbons to hydrogen to power portable or miniature fuel cells. This system, or fuel processor, is unique in that the size and weight are dramatically reduced compared to conventional approaches, thus truly enabling compact and lightweight power generation.
Fuel cells show great promise as alternatives to batteries or power generators. The Proton Exchange Membrane (PEM) fuel cell has advantages because of its low operating temperature, high power density, and advanced stage of technical development. However, the fuel used by the PEM fuel cell is hydrogen, which is not easily transported or stored. For these reasons, and in order to take advantage of the existing fuel infrastructure, the PEM fuel cell needs to be integrated with a fuel processor that can convert liquid hydrocarbons into hydrogen. The fuel cell can then convert the hydrogen into electricity.
Using Battelle technology, a complete miniature fuel cell/fuel processor system would weigh about one kilogram, much of the weight resulting from fuel storage. Fueled by liquid hydrocarbons such as butane, it would continuously provide 5 watts of base load electric power with 10 watts of peak power for one week. This system would include two modules that run the fuel processor continuously to provide the base load and use a compact lithium power battery for load leveling and to meet peak electric power demands. The use of modular power generation and a battery unit enables flexibility, reliability, and extended duration.
The system miniaturization technology that makes a compact fuel processor possible is based on the enhanced heat and mass transfer exhibited when fluids flow in and around microstructures. These microstructures may consist of machined microchannels up to 500 microns wide or other special structures engineered to enhance chemical reactions or separations. Using many microstructures in parallel, chemical systems can be deployed with radical reductions in size and weight compared to conventional systems.
The process unit operations required for the fuel processor are embodied in parallel "sheets" that are machined with many parallel micro-scale features. Combinations of reactor, heat exchange, and control sheets are stacked together to form an integrated system that performs needed operations such as steam reforming and/or partial oxidation, water-gas shift reaction, carbon monoxide removal, heat exchange, and sulfur sorption.
The first component of the fuel processor, the vaporizer, has been demonstrated at the full scale required for a 25-kW fuel cell, using methanol as the liquid hydrocarbon fuel. A device with dimensions of 7 x 10 x 2.5cm vaporized methanol at a rate of 208 mL/min. Heat was provided by catalytic combustion of a dilute hydrogen stream that would be supplied as the exhaust from the fuel cell anode. The versatility of sheet architecture allows this vaporizer to be easily downsized for the 5-watt module. The same miniaturization techniques are being tested at the bench scale for additional system components: steam reforming, partial oxidation, water-gas shift, and preferential oxidation reactors.
The technology is protected by United States Patent 5,611,214 ("Microcomponent Sheet Architecture," issued March 18, 1997). Foreign patent applications based on this case and additional United States patent applications are pending.
|