Fuel Cells

Although not new in concept, fuel cells have received considerable attention lately for their potential to be clean and reliable devices for generating electricity. Fuel cells can not yet compete economically with more traditional energy technologies, but rapid technical advances are being made. Fuel cells will certainly be widely used in future decades.

What Is a Fuel Cell?

A fuel cell is an electrochemical device used to create electricity. Much like the batteries that are found under the hoods of automobiles and in CD players and flashlights, it converts chemical energy to electrical energy. But unlike a typical battery which holds a limited fuel supply in a sealed container, a fuel cell requires an ongoing supply of fuel to create a continuous flow of electricity. The fuels, hydrogen and oxygen, are fed to the two terminals of the fuel cell and a chemical reaction occurs which produces electricity along with heat and water. Fuel cells have great potential for a variety of applications including transportation as well as stationary and portable power uses.

The Parts of a Fuel Cell

A typical fuel cell system for stationary power applications has three main parts: a fuel cell stack, a fuel processor, and the power electronics. The fuel cell stack is the heart of a fuel cell system. This is where the separate fuel streams of hydrogen and oxygen physically come together, react, and create electricity. This part of the fuel cell system is called the stack because layers of individual fuel cell modules are actually stacked next to or on top of each other in order to increase the power capacity of the system.

The fuel processor is the part of a commercial fuel cell system that produces a supply of hydrogen fuel. The other fuel, oxygen, is readily available in the air, and it is relatively simple to separate from nitrogen and other trace gases in the air. Hydrogen, although in great abundance on earth, is not so easily isolated. Currently, the most economical and practical source for hydrogen gas is fossil fuels, such as natural gas, gasoline, methane, and propane. These fossil fuels are all composed of carbon and hydrogen atoms. The fuel processor frees the hydrogen from the carbon to produce a pure hydrogen gas for the fuel cell stack. In the future, it may become cost competitive to get “renewable” hydrogen from water via electrolysis powered by renewable electricity generating sources such as the sun and wind.

Power electronics comprise the third main component of a fuel cell system. The electrochemical reaction that takes place in the fuel cell stack yields only direct current (DC) electricity. Because most residential and commercial applications use alternating current (AC), the power electronics convert the DC electricity to the more useable AC electricity.

Environmental Benefits

Even though fossil fuels are consumed in the electrochemical reaction inside of a fuel cell, fuel cells do not do not produce the same unhealthy air pollution emissions that are generated by burning gasoline in cars or burning coal and other fossil fuels in power plants. With fuel cells, there is no combustion, so fewer gases are released into the environment. For example, almost no sulfur oxides (SO x) or nitrogen oxides (NO x) are emitted, and emissions do not include any particulate matter. The greenhouse gas carbon dioxide (CO 2) is a byproduct, but because noxious air emissions are so low from fuel cell systems, certifications and permits are rarely needed to install commercial fuel cell systems. Also, the high electrical efficiency of fuel cells provides much more electricity per unit of carbon released than conventional generators of similar size.

If, in the long run, fuel cells could use hydrogen produced by electrolysis powered by electricity generated from renewable sources, the environmental benefits would be even greater. In that case, the outputs of fuel cells would be electricity, heat, and water vapor (produced when the hydrogen and oxygen combine in the fuel cell).

Security, Reliability, and Other Benefits

Fuel cells have a distinct advantage over other clean generators such as wind turbines and photovoltaics in that they can produce continuous power as long as they are supplied with a constant supply of hydrogen. The ability to produce continuous power makes fuel cells well suited for supporting critical loads for security applications. The power output of fuel cells is also high quality in that it is “clean” computer-grade power free from voltage disturbances such as sags, spikes, or transients.

Another advantage of fuel cells is that the power house of the system, the fuel cell stack, does not contain any moving parts, which typically lead to mechanical breakdowns in traditional generators. In theory, and as the technology matures, fuel cells may become more reliable than conventional engines. Also, in addition to low noxious air emissions, fuel cells can produce significant amounts of power (electrical and thermal) with much less noise than standard generators. These factors are viewed favorably when siting fuel cell systems in populated areas and even inside facilities.

Disadvantages of Fuel Cells

Fuel cells are not the perfect solution to the world’s energy needs. There are several obstacles that need to be overcome before widespread use of fuel cells occurs:

• Too expensive. The biggest hurdle for fuel cells is cost. Although some fuel cell systems are in use today, very few are currently cost effective. For stationary fuel cells, typical capital costs for installed systems exceed $5,000 per kilowatt – well above the target capital cost of $1,000-$1,500 used by most energy generation developers.
• Early technology risks. Fuel cells are still in a relatively early stage of development and even the few commercially available models have limited fleet operating experience. This emerging technology requires risk-taking early adopters as end users in order to ultimately expose more consumers to the benefits of fuel cells.
• Reliability and durability . In order to become widely accepted as a clean distributed generator, fuel cells must prove their adaptability for a variety of applications. Also, certain fuel cell system components—like the cell stack which can require a costly replacement every one to five years depending on the model—must be developed to have a longer lifespan or be easily and cheaply replaced.

Fuel Cell Applications

All fuel cells operate on the principle of combining oxygen and hydrogen to generate electricity, heat, and water, but they have many potential shapes and sizes suited for diverse applications. The three main applications of fuel cells are: transportation, portable uses, and stationary installations.

Many vehicle manufacturers, including DaimlerChrysler, Ford, General Motors, Honda, and Toyota , are actively researching and developing transportation fuel cells for future use in cars, trucks, and buses. These fuel cells will need to be relatively small, light, and durable. The manufacturers will also need to overcome the real but solvable challenges associated with storing hydrogen gas onboard vehicles.

Because the electrochemical reaction in a fuel cell is much more efficient than the burning process in a conventional internal-combustion engine, fuel-cell vehicles are much more efficient than conventionally powered vehicles. They also have advantages over battery-electric vehicles because they do not need to be slowly recharged, but can instead be refueled quickly at a filling station much like conventional cars.

Portable fuel cells are small, contained fuel cells designed for a variety of purposes. These units could provide back-up electrical generation for military, temporary, or special needs applications. As technology advances and fuel cells are able to be constructed ever smaller, the potential applications for portable fuel cells are limitless. Laptop computers, cellular phones, video recorders, and hearing aids could be powered by portable fuel cells.

Stationary fuel cells are the largest, most powerful fuel cells. They are designed for installation in permanent settings such as hospitals, banks, airports, military bases, universities, and homes. The market advantage of stationary fuel cell systems is that they could provide a clean source of on-site power to a variety of end users. These fuel cell systems have the potential to provide end users with these added values:

• Assured power in applications requiring high availability of power to maintain the operation of mission critical equipment even when the electric grid fails.
• Combined heat and power (CHP) applications where the thermal output of the fuel cell can be used to meet the heating or cooling requirements of the building being served.
• Industrial applications which can capture waste gases to serve as a source of hydrogen for fuel cell systems. For example, waste water treatment plants create hydrogen-rich, anaerobic digester gas.

Conclusions

Fuel cell technology has existed for over a hundred years and the research and development community is continually making advances to realize the potential of this exciting technology. The fuel cell industry is on the verge of commercializing this technology for portable applications with stationary and transportation applications in early field demonstration stages. With fuel cells’ inherent capability to produce clean, efficient, continuous power, they will certainly begin to be more widely used in the near future.