"Stacking Up," Feature Article, June 2003

FEATURE FOCUS: Advanced Energy Systems

stacking up

Efficiency on a small scale and a means of curbing emissions make fuel cells an investment for the future.

by Michael R. von Spakovsky

A fuel cell has made it into Yale. This summer, the university will fire up a new 250-watt fuel cell system, which will meet about a quarter of the electricity needs of the school's Environmental Sciences Building.

The installation is interesting on a number of levels. It's an experiment in distributed power generation at one of the world's leading scholarly institutions. The system will feed a headquarters for the study of our world's environment, and it's in Connecticut, the state that has begun to bill itself as "the fuel cell capital of the world."

The Hyundai Santa Fe, billed as a zero-emission sport utility vehicle, carries an electric motor, controls, and water systems under the hood. A 75 kW hydrogen fuel cell, from UTC, resides under the back seat.

The Connecticut Clean Energy Fund put up the $1.25 million to purchase the fuel cell power plant from a local company, FuelCell Energy Inc., in Danbury. Subhash Chandra, the chief technology officer and managing director of the fund, pointed out that Connecticut is also home to United Technologies Corp.'s fuel cell unit in South Windsor and to Proton Energy Systems Inc. in Wallingford, as well as to a number of smaller companies that are involved directly or indirectly in the field.

According to Chandra, the fund he heads has spent almost $29 million on fuel cell projects over the past couple of years. It has put money into the Connecticut Global Fuel Cell Center at the University of Connecticut and directly into fuel cell companies as equity investments, he said.

LOOKING TO THE FUTURE

The motive for funding fuel cells goes beyond boosterism for local industry, though. As pressures mount on available resources and the environment, fuel cell systems can play a major role in the future of stationary and mobile power generation.

They make highly efficient power plants. Especially on a small scale, they reach much higher average efficiencies than other technologies do. Higher efficiency not only means more electricity per pound of fuel, but also fewer emissions of all kinds per kilowatt-hour. In other words, the only product of a fuel cell's electrochemical reactions is water, while the chemical reactions for fuel reforming, if present in the system, occur at temperatures low enough to make emissions of nitrogen oxides almost nonexistent. The elimination or significant reduction of other emissions (e.g., sulfur dioxides and carbon dioxide) is also a feature of these systems. In addition, the heat generated in high-temperature fuel cells can be captured and used to generate steam in a combined cycle.

Yale's fuel cell system, the DFA 300, is listed by the manufacturer as having a low heating value peak electrical efficiency of 47 percent, which is very high indeed for a 250-watt generating plant. FuelCell Energy claims that NO_x emissions are less than 0.3 part per million by volume. Both carbon monoxide and volatile organic compounds are less than 10 parts per million.

Yale's installation will use the heat energy to power environmental control systems in a part of the building that houses delicate specimens and artifacts.

Fuel cell systems are expensive now, but as with any technology, prices are expected to fall as sales increase and manufacturing improves. A spokesman for FuelCell Energy said that the company's eventual goal is to price an installed fuel cell system at between $1,000 and $1,500 per kilowatt. That may not compete with the cost of a natural gas plant on a large scale, but it can provide an economical and environmentally friendly source of local power generation.

So, the question arises: How do fuel cell systems stack up (no pun intended) against competing technologies? The simple answer: quite well. One reason is that during the last three decades, there has been a significant surge in both research and development.

By no means a new energy conversion process, interest in fuel cells ceased in the late 19th century and did not re-emerge until the work of Francis Bacon and his co-workers in 1932, gaining much impetus during the 1960s from the American space program. What is remarkable is the comparatively high efficiencies and relatively low emissions that these devices already exhibit in an early stage of development, both for transportation and for portable and stationary power production. Advantages exhibited by these types of systems are: high energy conversion efficiencies at full and partial load (the latter provides a significant advantage for fuel cells over other more conventional systems); performance roughly independent of system size and load factor; lower emissions (but not zero emissions); no moving parts in the stacks, resulting in lower maintenance costs; high power density (i.e., units are compact); modularity (a feature conducive to meeting different power needs); low operating temperatures; and relatively low pressures.

FIVE WAYS TO GO

There are five basic types of fuel cells that have seen, or are seeing, significant development. A sixth, the alkaline fuel cell, is no longer considered a viable candidate for most applications of interest.

One of the most promising types of fuel cells is the proton exchange membrane, or PEM, fuel cell. It is often considered as a potential replacement for the internal combustion engine in transportation applications. The efficiency of a PEM fuel cell stack operating on hydrogen and pressurized air at typical current conditions would be approximately 50 percent. The PEM fuel cell also provides very high power density. Automotive fuel cell systems based on PEM technology have demonstrated a power density as high as 1.35 kilowatts per liter, which is comparable to that of an internal combustion engine.

This power is produced while the cell operates at a relatively low temperature of 60° to 80°C. The low operating temperature lets the fuel cell warm up quickly.

TUMBLING COSTS

Unfortunately, the low operating temperature leads to very slow chemical kinetics. Precious metal catalysts, typically platinum, must be used at the electrodes to facilitate the reactions. As recently as 10 years ago, the cost of the catalyst alone was as high as $184 per kilowatt of electricity, making the PEM fuel cell too expensive for most applications. In recent years, advances in the design of the electrodes and the application of the catalyst have led to catalyst costs approaching a design goal of $3.50 per kilowatt of electricity.

Nonetheless, further advances in technology and manufacturing are needed to reduce the cost of other cell components, particularly the collector plates, which are typically machined from graphite. Assuming the application of the mass manufacturing techniques that would be associated with a large-scale deployment of PEM fuel cell technology, such as in automobiles, the U.S. Department of Energy has established a goal of $35 per kilowatt for the fuel cell stack. Even at 10 times this price, PEM technology would be attractive in a wide range of stationary and portable power applications.

Bill Cardin, an engineer at Proton Energy Systems, runs checks on parts of a system that will contain both a fuel cell and an electrolyzer.

The direct methanol, or DM, fuel cell typically uses a polymer membrane as the electrolyte. The fuel is methanol, dissolved in liquid water and supplied to the anode. Since it is a liquid, methanol is easy to transport, and since it is used directly in the stack, there is no need for a fuel processor to separate hydrogen, a requirement of most other systems. However, because the reaction rate for methanol on currently available catalysts is slow, direct methanol fuel cells have relatively low efficiencies and power densities. Furthermore, since methanol is soluble in the polymer membrane, it can cross over to the cathode where it reacts without producing electrical power, thus further reducing efficiency.

Direct methanol fuel cells can be competitive with batteries in terms of storage density. At present, the most promising applications for DM fuel cells appear to be as replacements for batteries in small portable power applications, where the simplicity of the system and the portability of the liquid methanol fuel outweigh the relatively low efficiency.

PHOSPHORIC ACID SYSTEMS

The phosphoric acid, or PA, fuel cell was the first fuel cell to be commercially available. PA systems operate with efficiencies that are comparable to proton exchange membrane fuel cells, but power densities are lower. The operating temperature of the PA fuel cell is approximately 200°C. This temperature is high enough to facilitate the recovery of heat produced within the stack for water and space heating in buildings. However, the operating temperature is not high enough to overcome the need for precious metal catalysts.

Furthermore, despite a number of years of intensive development, the costs of these systems have not come down sufficiently. Phosphoric acid fuel cell systems, each of which includes a natural gas fuel processor, are commercially available at an installed cost of approximately $5,000 to $5,600 per kilowatt.

Nonetheless, phosphoric acid systems have been practically demonstrated through a number of projects. One of the largest demonstration projects is the U.S. Department of Defense Fuel Cell Demonstration Program that has placed 30 systems in a variety of applications. They include boiler plants, hospitals, dormitories, and office buildings at military installations, including the U.S. Military Academy at West Point, Edwards Air Force Base in California, and the New London, Conn., submarine base. The DOD program is detailed on a Web site, http://www.dodfuelcell.com.

Five 200 kW fuel cell power plants, made by UTC, supply the electrical power to the main postal sorting facility in Anchorage, Alaska. The station processes more than a million pieces of mail a day.

While this program and others have demonstrated the technical feasibility of applying PA fuel cell systems, the widespread economic feasibility of these systems will depend on reducing costs by at least a factor of two.

The type of system that is being installed at Yale, the molten carbonate fuel cell, is typically designed for mid-size to large stationary power applications or for shipboard use. The molten carbonate, or MC, fuel cell operates at a very high temperature, approximately 650°C. At this temperature, precious metal catalysts are not required for the fuel cell reaction. In addition, the heat available from the stack can be used to produce steam and hot water.

Furthermore, at this temperature, fuel gases other than hydrogen can be used by reforming the fuel within the cell stack in a process called "internal reforming." This greatly simplifies the "balance of plant" equipment required to operate the fuel cell. Heavier hydrocarbons may still require external processing.

Development efforts for molten carbonate fuel cells are focused on reducing costs and increasing the life of cell components in the harsh, high-temperature environment within the stack. Systems based on this technology are expected to be available within five years at costs ranging from $2,000 to $3,000 per kilowatt of electricity.

Target markets for the technology include small
distributed generation systems for utilities as well as building cogeneration systems at sizes of 0.1 to 2.0 MW of electricity.

The solid oxide, or SO, fuel cell operates at the highest temperatures of all fuel cell systems, 800° to 1,000°C. These high temperatures simplify system configuration by permitting internal reforming. They also facilitate the development of cogeneration systems as well as hybrid power systems that use fuel cells as topping cycles for gas, steam, or combined turbine cycles.

REFINEMENTS NEEDED

Developers of solid oxide fuel cells are seeking ways to reduce manufacturing cost, improve system integration, and lower the operating temperature to a range of 550° to 750°C. The lower operating temperature would still provide the advantages of internal reforming without the material problems associated with very high-temperature operation.

Systems based on SO fuel cells are being considered for a variety of purposes, ranging from small applications such as residential power systems and vehicle auxiliary power units, where the simplified fuel processing requirements are attractive, to large utility-scale applications.

Many adherents believe that fuel cell systems promise to provide benefits in a variety of applications. Systems based on PEM and direct methanol technology promise to make power more portable and convenient, and PEM technology also promises to provide a more efficient, cleaner technology for the automotive industry. PEM, phosphoric acid, molten carbonate, and solid oxide fuel cells are likely to be applied in cogeneration applications that use the exhaust heat.

With combined-cycle and cogeneration thermodynamic (exergy) efficiencies potentially above 70 and even 80 percent, these applications promise to reduce energy use and environmental impact. Many research and development organizations, manufacturers, and regulatory agencies are working to ensure that fuel cell systems fulfill their promise in each of these areas.

SIDEBAR: TESTING FUEL CELLS AT CHINA LAKE

Michael R. von Spakovsky, an ASME Fellow, is a professor of mechanical engineering and director of the Energy Management Institute at Virginia Polytechnic Institute and State University in Blacksburg.