Fuel cells have been receiving a great deal of press coverage over the last couple of years, often touted as the technology that will some day lead the world away from the environmental impacts surrounding fossil fuels and combustion technologies. As this media fanfare suggests, fuel cells do not use combustion to extract the energy in fuels, as do combustion turbines (CT) and reciprocating engines (IC). Instead, fuel cells use a chemical process to create electric power, much like conventional battery technologies. Unlike batteries, however, fuel cells do not have to be recharged. They can operate continuously as long as fuel is supplied.

The required fuel is hydrogen and the basic chemical reaction for all fuel cells uses hydrogen and oxygen to create electric power, heat energy, and water vapor. Thus, fuel cells that are powered by hydrogen do not emit any of the greenhouse gases or harmful pollutants that are byproducts of combustion technologies. They are also highly efficient, with certain designs reaching electric power generation efficiencies as high as 60 %, which is equivalent to a combined cycle, natural gas-fired combustion turbine.

The following characteristics make fuel cells very attractive for niche applications:

• Negligible emissions
• Very high electric generation efficiencies at full and partial load
• Few moving parts and low maintenance
• High quality power output with quick response to load fluctuations
• Excellent heat recovery characteristics for combined heat and power (CHP)
• Quiet operation

There are technical and economic issues that will inhibit the widespread utilization of fuel cells in the near term. Currently, the few systems that are commercially available have capital costs around $3,000 per kW, which is about three times the cost of the most expensive reciprocating engines, and ten times the cost of the least expensive engines. In addition, hydrogen fuel is not available in the natural environment and must be generated using fuel processing or renewable energy technologies. There are economic, environmental, and remaining technical issues surrounding the various methods of hydrogen production.

Discussions regarding fuel cells are often accompanied with talk about transitioning from a fossil fuel-based to hydrogen-based economy. Before that can happen, a hydrogen infrastructure needs to be created. With hundreds of existing oil refineries, thousands of fossil fuel-fired power plants, and extensive natural gas pipeline delivery systems in the U.S., Western Europe, and Japan alone, it will take years and substantial investments to complete the transition. The good news is that reformer technologies to convert natural gas and liquid fuels have shown great promise. These issues, along with the operating principles and potential benefits of fuel cells, are discussed below.

Technology

The operating principle of a fuel cell was discovered by Sir William Grove in 1839, but the technology remained confined to laboratory research projects for over a hundred years. In the late 1950's, a division of United Technologies, now know as UTC Fuel Cells, developed an alkaline fuel cell for NASA's Apollo space program (see photograph provided below). Fuel cell development activity increased in the mid 1970's as a result of the global energy crisis and the growing concern about dependence on foreign oil supplies.

In the 1980's and early 1990's, Dr. Geoffery Ballard and the company he founded, Ballard Power Systems, were instrumental in accelerating the development of fuel cell technology. Investors took notice when Ballard entered a collaborative agreement with automobile manufacturer Daimler-Benz (later to become Daimler-Chrysler) in 1993 to pursue development of fuel cells for automotive applications. Today, there are hundreds of companies from Asia to Europe involved in the development of fuel cell and hydrogen fuel-related technologies.

Getting back to Sir William Grove's early experiments, he identified that an electric current could be created through an external circuit when hydrogen and oxygen gas was exposed to a platinum plate sitting in an electrolyte solution (an electrolyte is a conductive medium in which an electric current is accompanied by a flow of electrically charged atoms, referred to as ions). There are several types of fuel cell technologies being pursued today, each of which has its own electrochemical process. The basic reaction involves two hydrogen molecules (2H2) and one oxygen molecule (O2) creating two water molecules (2H2O) as follows:

2H2 + O2 -> 2H2O

The diagram below illustrates the electrochemical process carried out in one of Ballard Power System's modern proton exchange membrane (PEM) fuel cells. In Ballard's system, hydrogen molecules separate into protons and electrons after contacting the catalyst. The free electrons create a usable electric current as they travel through an external circuit from the anode (the negatively charged electrode) to the cathode (the positively charged electrode). The protons travel through the membrane and then meet up with oxygen from air and the electrons which travel to the cathode via the external circuit. Emissions include clean air and water vapor.

Diagram provided courtesy of Ballard Power Systems

Regardless of the specific technology there are two half step reactions occurring, one at the anode and one at the cathode. In Grove's design, the reaction at the anode involves the ionization, or disassociation of protons (4H+) from electrons (4e-), of two hydrogen molecules as follows:

2H2 -> 4H+ + 4e-

The reaction at the cathode involves a reaction between an oxygen molecule (O2), the positively charged hydrogen protons (4H+) that travel through the electrolyte, and the electrons (4e-) which travel through an external circuit. The reaction at the cathode releases thermal energy which can be harnessed for CHP applications. This half-step reaction is as follows:

O2 + 4H+ + 4e- -> 2H2O

One electrochemical reaction produces a small electric current in the form of four electrons. Modern fuel cells are designed to maximize the number of reactions that are occurring. One method is to maximize the surface area of the membrane catalyst, while simultaneously reducing material costs. The use of platinum, the reactant material of choice for PEM fuel cell catalysts, is one example. Other technologies involve temperature and improving hydrogen concentration from fuel reformers. To achieve a particular output, individual PEM cells are strung together into a fuel cell stack. The image below illustrates one of Ballard's fuel cell stacks. The number of cells in a stack determines voltage and the effective surface area of the cells determines current.

Diagram provided courtesy of Ballard Power Systems

Unlike CT and IC engines, electric power generation through chemical reactions is not constrained by the Carnot Law, which specifies the maximum theoretical efficiency that a heat engine can reach. Fuel cells have high efficiencies compared to combustion technologies, particularly in lower output ranges. However, it is important to remember that there needs to be a source of hydrogen to operate fuel cells and it is not found naturally in the environment like oil and coal. If hydrogen is being processed from a fossil fuel such as natural gas, there will be energy losses associated with the fuel processing.

Fuel cell designs vary by the chemicals used as their electrolyte and are classified as such. The five electrolyte types described below differ in their operating characteristics, temperatures, power densities, and their most suitable end uses. All but the alkaline fuel cell have acid-based electrolytes. Because aqueous acids are either volatile or unstable, all the newer fuel cells are based on polymer, molten, or solid-acid electrolytes.

1. Alkaline Fuel Cells (AFC)-the first fuel cells used for terrestrial applications, powering an Allis-Chalmers tractor in 1959. Unlike acid electrolyte fuel cells, alkaline fuel cells react to form a solid carbonate if exposed to carbon dioxide. Since alkaline fuel cells have lower power densities and require on-board reforming of hydrogen, which produces carbon dioxide, this type was abandoned for transportation applications. They are used, however, in the Space Shuttle to provide both power and drinking water for the astronauts. Engineers learned in the 1980's how to make carbonate-insensitive cathodes in order to lessen the susceptibility to carbon dioxide, and several companies in the past five years have rediscovered the alkaline fuel cell for terrestrial applications. A key advantage is that AFCs need far less platinum catalyst than acid-based fuel cells. An estimate of the platinum needed for large-volume production of acid-based fuel cells predicted skyrocketing prices for platinum. Unlike the acid electrolyte fuel cells, AFCs can be made from inexpensive materials. Traditionally, they use a solution of potassium hydroxide (lye) in water as their electrolyte. Alkaline fuel cells, which operate at temperatures of about 200°C (392F), are among the most efficient (70 %), but their power densities are not as high as those for acid electrolyte fuel cells.

2. Phosphoric Acid Fuel Cells (PAFC)-the only type of fuel cell that has been sold in significant quantities to date. Now considered a first generation technology, PAFCs had early success because they produced enough waste heat from the fuel cell stack to steam reform methane into the hydrogen-rich gas required as a fuel. PAFCs are currently in limited use for power generation at a number of hospitals, hotels, schools, and similar commercial installations. They are relatively cheap, well developed, about 40 % efficient, and have an expected lifetime of about 40,000 hours. They are produced in 200kW modules and have been used to build power plants as large as 11MW. Of the more than 200 PAFC installations in the U.S., all but two are operated in CHP mode. The heat is used for space heating or hot water, but is not sufficient for other cogeneration applications. The Japanese are particularly advanced in PAFC research and design, with power plants from a few kilowatts to a few megawatts already in operation.

3. Proton Exchange Membrane (PEM)-these fuel cells have a special polymer electrolyte membrane which looks like an ordinary sheet of plastic. This membrane separates the anode and cathode sides, which contain platinum-coated carbon electrodes. Because of the limitations imposed by the membrane's thermal properties, PEMs operate at about 80°C (200°F), much lower than the other designs. The fuel cell's low-temperature heat is not adequate for traditional cogeneration, but allows for a quick startup time, which is advantageous for transportation and backup-power applications. Numerous companies are manufacturing or developing PEMs, and several small PEM systems are commercially available now.

4. Molten Carbonate Fuel Cells (MCFC)-this type is so named because their electrolyte is a molten alkali carbonate mixture, retained in a matrix. The carbonate electrolyte is solid at room temperature, but liquid at the operating temperatures of 650° to 800°C (1,200 - 1,472F). This high operating temperature means a long startup time. The high temperatures achieved allow MCFCs to operate without a separate reformer. MCFCs are inherently more efficient than a fuel cell requiring external fuel processing. In MCFCs, the cathode must be supplied with carbon dioxide, which reacts with the oxygen and electrons to form carbonate ions, which carry the ionic current through the electrolyte. The generated heat can be used for internal reformation of methane, a bottoming cycle, and for fuel processing and cogeneration. Because MCFCs require large amounts of ancillary equipment, which would render a small operation uneconomic, they have been developed only for large-scale power generation.

5. Solid Oxide Fuel Cells (SOFC)-the newest type of fuel cell and still undergoing field trials. The electrolyte is solid zirconium dioxide, which is operated at a temperature of around 1,000°C (1,800F). SOFCs have very good power densities and are 50-60 % efficient. Like MCFCs, SOFCs have long startup times as it takes a long time for the electrolyte to heat up. Like MCFCs, the high-grade waste heat can be used for internal reforming and many other applications, including steam that a steam turbine can use to generate extra electricity in a combined cycle (a bottoming cycle). Even after this process, the heat produced can be used in cogeneration applications. Because of the SOFC's internal reforming, a number of different fuels can be used, including pure hydrogen, and methane, and the nature of the emissions from this fuel cell will vary correspondingly with the fuel mix.

While a fuel cell itself gives off only CO2 and water; fuel reformers do emit low levels of pollutants-typically two orders of magnitude less than conventional combustion-based processes (e.g., NOx emissions below 0.02 pounds per MWh). The fuel cell's electrodes that comprise the "stack" degrade over time, reducing the unit's efficiency. Fuel cells are designed to permit the "stack" to be removed. It is estimated that "stack" replacement is required every four to six years when the fuel cell is operated under continuous conditions. Fuel cells, like photovoltaics, produce direct current (DC) power and require additional electronics to produce 60 Hz alternating current (AC)

The are practical and technical challenges remaining for fuel cell's including high capital costs, fuel reforming systems, hydrogen storage and delivery, and dealing with sensitivities to fuel impurities and water management. There have been many achievements with fuel cell cost reductions, but will remain a niche technology until costs can be further reduced. Fuel reforming has many remaining technical issues. Fuel cell manufacturers are finding developing hydrogen reforming systems is required. Certain impurities in reformed hydrogen can "poison" fuel cell catalysts, reducing output voltage and efficiency.

Strengths/Limitations

As was stated in the Introduction, negligible emissions, very high electric generation efficiencies at full and partial load, few moving parts and low maintenance, high quality power output with quick response to load fluctuations, excellent heat recovery characteristics for combined heat and power (CHP), and quiet operation are the characteristics which make fuel cells very attractive for distributed energy applications. The electric load characteristics of businesses, universities, hospitals, schools, offices and many other applications are a good fit for the technology. Siting concerns are reduced due to overall lower environmental impact.

High capital costs, the limited number of commercially available products, remaining technical barriers for certain designs, and the lack of a support and hydrogen infrastructure will likely keep fuel cells a niche player is distributed generation for the next several years. In higher quality back-up power applications, the disparity in cost between fuel cells and alternate technologies decreases. The telecommunications industry, in particular, has expressed interest in fuel cell power.

Related Links

These links provide you supplemental information on fuel cells. The vendor links will provide direct access to the Websites for manufacturers and packagers of fuel cells to obtain specifications. The general distributed energy links enables you to search other leading DE Websites for fuel cell analysis.