Direct Methanol Fuel Cell (DMFC)

Portable fuel cells have gained attention and the most promising development is the direct methanol fuel cell. This small unit is inexpensive to manufacture, convenient to use and does not require pressurized hydrogen gas. The DMFC has good electrochemical performance and refilling is done by squirting in liquid or replacing the cartridge. This enables continued operation without downtime.

Manufactures admit that a direct battery replacement by the fuel cell is years away. To bridge the gap, the micro fuel cell serves as a charger to provide continuous operation for the onboard battery. Furthermore, methanol is toxic and flammable, and there are limitations to how much fuel passengers can carry on an aircraft. In 2008 the Department of Transportation issued a ruling to permit passengers and crew to carry an approved fuel cell with an installed methanol cartridge and up to two additional spare cartridges of 200 ml (6.76 fl oz). This provision does not yet extend to bottled hydrogen.

Figure 2 shows a micro fuel cell by Toshiba and Figure 3 demonstrates refueling with methanol that is 99.5 percent pure.

Figure 2: Micro fuel cell. This prototype micro fuel cell is capable of providing 300mW of continuous power. Courtesy of Toshiba	Figure 3: Toshiba fuel cell with refueling cartridge.The fuel in a 10ml tank is 99.5 percent pure methanol. Courtesy of Toshiba

Improvements are being made, and Toshiba unveiled prototype fuel cells for laptops and other applications generating 20 to 100 watts. The units are compact and the specific energy is comparable with that of a NiCd battery. Meanwhile, Panasonic claims to have doubled the power output with a similar size, specifying a calendar life of 5,000 hours if the fuel cell is used intermittently for 8 hours per day. The low longevity of these fuel cells has been an issue to be reckoned with.

Attempts are being made with small fuel cells running on stored hydrogen. Increased efficiency and smaller size are the advantages of pure hydrogen over methanol. These miniature systems have no pumps and fans and are totally silent. A 21cc cartridge is said to provide the equivalent energy of about 10 AA alkaline batteries with a runtime between refueling of 20 hours. This lends itself to portable computing, wireless communications and flashlights for the bicycle lone rider.

Military and recreational users are also experimenting with the miniature fuel cell. Figure 4 illustrates a portable fuel cell made by SFC Smart Fuel Cell. The EFOY fuel cell comes in different capacities that ranges from 600 to 2,160 watt-hours per day.

Figure 4: Portable fuel cell for consumer market. The fuel cell converts hydrogen and oxygen to electricity and clean water is the only by-product. Fuel cells can be used indoors as an electricity generator. Courtesy of SFC Smart Fuel Cell AG (2010)

Table 5 describes the applications and summarizes the advantages and limitations of common fuel cells. The table also includes the Molten Carbonate (MCFC) and Phosphoric Acid (PAFC), classic fuel cell systems that have been around for a while and have unique advantages.

Table 5: Advantages and limitations of various fuel cell systems.
Fuel cell developments have been gradual; the specific power is low and a direct battery replacement may never be feasible.

Developments

Limitations involve slow start-up times, low power output, sluggish response on power demand, poor loading capabilities, narrow power bandwidth, short service life and high cost. Similar to batteries, the performance of all fuel cells degrades with age, and the stack gradually loses efficiency. Such performance losses are much less apparent with the ICE.

Fuel cells below 1kW are normally non-pressurized and only use a fan to aid in oxygen supply; fuel cells above 1kW are pressurized and include a compressor that lowers efficiency and the system can get rather noisy. The relatively high internal resistance of fuel cells poses a further challenge. Each cell of a stack produces about 1 volt in open circuit; a heavy load causes a notable voltage drop. Similar to the battery, the power bandwidth decreases with age. Individual cells in the stack are also known to cause failures and contaminants are large contributors. Figure 6 illustrates the voltage and power bandwidth as a function of load.

Figure 6: Power band of a portable fuel cell High internal resistance causes the cell voltage to drop rapidly with load. The power band is limited to between 300 and 800mA. Courtesy of Cadex

Fuel cells operate best at a 30 percent load factor; higher loads reduce efficiency. This and poor throttle response place the fuel cell into a support mode or a charger to keep batteries charged. A stand-alone power source, as the developers had hoped, has not materialized.