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.  

Paradox of the fuel cell

The fuel cell enjoyed the height of popularity in the 1990s, when scientists and stock promoters envisioned a world running on a clean and inexhaustible resource — hydrogen. They predicted that cars would run on fuel cells, and that household electricity would also be generated by fuel cells. The stock prices skyrocketed but marginal performance, high manufacturing costs and limited service life moderated the hydrogen dream. It was said that the fuel cell would transform the world as the microprocessor did in the 1970s. A clean and inexhaustible source of energy would become available that would solve the environmental concerns of burning fossil fuel. From 1999 through 2001, more than 2,000 organizations got actively involved in fuel cell development, and four of the largest public fuel cell companies in North America raised over a billion US dollars in public stock offerings. What went wrong? Hydrogen is not a source of energy per se but a medium to transport and store energy similar to electricity that charges a battery. To envision “burning an endless supply of hydrogen,” the fuel must first be produced, because hydrogen cannot be pumped from the earth as is possible with oil. While fossil fuel lends itself well to producing hydrogen, taking this valuable fuel to unleash hydrogen makes little sense when it costs as much or more for extraction as burning it directly. The only benefit is reduced greenhouse gases. Just as the attempt to fly airplanes on steam failed in the mid-1800s, it is conceivable that the fuel cell will never be the powerhouse scientists had hoped for. But there is renewed interest in the automotive field in Japan. Fuel cells are replacing battery banks and diesel generators in office buildings as they can be installed in tight storage places with minimal maintenance and without the need for exhaust. Fuel cells allow continuous and pollution-free operation of forklifts in warehouses, whereas 40M fuel cells generate clean electricity in remote locations. The ultimate dream is propelling vehicles with the clean fuel cell. Fuel cells may one day taxi airplanes with electric wheel motors, lowering pollution and preserving fuel by up to 4 percent by not using the jet engines. Water produced from the fuel cell while charging the batteries could serve as on-board drinking water; generative braking could further assist in the charging. The ultimate dream is propelling vehicles with the clean fuel cell.