Fuel Cell Technologies
Fuel cells use a combination of hydrogen and oxygen to produce electricity and heat through a process called electrolysis. The hydrogen fuel can be created in a number of ways, and a variety of technologies are being explored by today’s fuel cell developers for different applications.
Technology overview
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 stationary and portable power uses and transportation.
Parts of a fuel cell system
While fuel cell systems can vary in a number of ways, they generally have three main components: the fuel cell stack, the fuel processor, and power electronics. The fuel cell system may also have a heat recovery system for converting excess heat into usable steam, hot water, or electricity.
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. While hydrogen is in great abundance on earth, it is not easily isolated for use as a fuel. Currently, the most economical and practical source for hydrogen gas is fossil fuels, such as natural gas, gasoline, methane, and propane, though 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. 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.
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 stacked next to or on top of each other in order to increase the power capacity of the system.
The type of electrolyte used in a fuel cell stack determines the necessary conditions for chemical reactions within the cell, required catalysts, and operating temperature range. Different characteristics are desirable for different types of fuel cell applications; for instance, a fuel cell system operating at very high temperatures or one with large size/weight requirements is not suitable for use in a small portable application.
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 directly usable AC electricity.
A heat recovery system may be used to increase the fuel cell system’s overall efficiency. It is most often used in systems operating at high temperatures, such as solid oxide and molten carbonate systems. These systems are often most useful for activities that require both electricity and heat, such as industrial processes.
To learn more, you can visit the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE), which devotes part of its website to fuel cells at www.eere.energy.gov/hydrogenandfuelcells. The site includes an animated introduction to how fuel cells work, and detailed information on fuel cell types.
Fuel cell types and applications
The three main applications of fuel cells are stationary installations, portable uses, and transportation,.
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 that can capture waste gases to serve as a source of hydrogen for fuel cell systems. For example, wastewater treatment plants create hydrogen-rich, anaerobic digester gas.
Polymer electrolyte membrane fuel cells (PEM), phosphoric acid fuel cells (PAFC), molten carbonate (MCFC), and solid oxide (SOFC) systems may all be used for stationary power. Alkaline fuel cells (AFC) are appropriate for military applications.
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 other niche applications. As technology advances and even smaller fuel cells are constructed, the potential applications for portable fuel cells are limitless. Laptop computers, cellular phones, video recorders, and hearing aids could eventually be powered by portable fuel cells.
PEM fuel cells are well-suited for portable applications. AFCs were the first type of fuel cells used for providing electricity and water aboard U.S. spacecraft, but are not yet cost-effective for mainstream commercial markets.
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. 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.
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. Currently, PEMs are the focus of transportation research, along with PAFC.
Emerging fuel cell technologies
Direct methanol fuel cells (DMFC) are a relatively new type of fuel cell in which pure methanol is mixed with steam to release hydrogen inside the fuel cell, rather than having a separate reformer to release the hydrogen from methanol, ethanol, and hydrocarbon fuels. Methanol is also easier to transport and store than pure hydrogen. DMFC technology is several years behind most other fuel cell technologies, but may be a promising option for future fuel cell applications like transportation.
Regenerative fuel cells are another new type of fuel cell, and have the advantage of using renewable energy to generate power. A solar-powered process called electrolysis is used to separate water into hydrogen and oxygen, which are fed into the fuel cell. The byproducts of the reaction are electricity, water, and heat. The water is fed back into the electrolyser (where electrolysis takes place), and the process repeats, forming a closed, continuous loop of electricity production. NASA and other groups are currently researching this technology.
Fuel Cell benefits and limitations
It is important to remember that, while fuel cells present a number of potential environmental and efficiency advantages, there are also economic barriers and environmental drawbacks that must be overcome before large-scale fuel cell technology adoption can occur. In the next section, we discuss Fuel Cell Benefits and Barriers.