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What is a Fuel Cell?

   Fact Sheet
      A Publication of the Fuel
      Cell Commercialization
      Group, Washington, D.C.


Fuel cells, although known in concept for more than 150 years, now are poised to make significant contributions to stationary power generation. This Fuel Cell Commercialization Group (FCCG) fact sheet provides a brief history of fuel cell development, explains the working concept and basic electrochemistry, describes how fuel cells are combined into stacks and modules to achieve megawatt-scale power production, and outlines some of the unique considerations that go into making up a complete fuel cell power plant.

Finally Coming of Age
Sir William Grove is widely attributed to be the “Father of the Fuel Cell.” Grove’s insights were gained from his experiments in 1839 on the electrolysis of water. Grove reasoned that it should be possible to reverse the process, reacting hydrogen with oxygen to generate electricity. The term “fuel cell” was coined in 1889 by Ludwig Mond and Charles Langer, who attempted to build the first practical device using air and industrial coal gas.

Attempts in the early 20th Century to build fuel cells that could convert coal or carbon directly to electricity continued to fail because of a lack of understanding of materials and electrode kinetics. Meanwhile, the internal combustion engine was developed, whose process was well understood. Petroleum was discovered and rapidly exploited; electric vehicles and other electrochemical approaches to energy production were quickly supplanted.

The first successful fuel cell devices resulted from inventions in 1932 by engineer Francis Bacon. He improved on the expensive platinum catalysts employed by Mond and Langer with a hydrogen-oxygen cell using a less corrosive alkaline electrolyte and inexpensive nickel electrodes. However, the technical challenges were daunting and it was not until 1959, a quarter of a century later, that Bacon and his coworkers were able to demonstrate a practical five-kilowatt system capable of powering a welding machine. In October of that same year, Harry Karl Ihrig of Allis-Chalmers Manufacturing Company demonstrated his famous 20-horsepower fuel cell-powered tractor.

In the late 1950s, a then little-known federal agency called the National Aeronautics and Space Administration (NASA) began to search in earnest for a compact electricity generator to provide onboard power for an upcoming series of manned space missions. After discarding nuclear reactors as too risky, batteries as too heavy and short-lived, and solar power as too cumbersome, NASA turned to fuel cells.

NASA eventually funded more than 200 research contracts into all aspects of fuel cell technology. Today, after reliably supplying electricity (and water) to the Apollo and Space Shuttle missions, fuel cells have proven their role in space.

These successes led to predictions in the 1960s that fuel cells would be the panacea to the world’s energy problems. The same qualities that make fuel cells ideal for space exploration — small size, high efficiency, low emissions, minimal water use or net water production — appeal as well to stationary power producers. Yet bringing the technology down to earth has proven to be a tricky proposition.

Nearly 30 years and US$1 billion in research have been devoted to address the barriers to the use of fuel cells for stationary applications. An alkaline electrolyte, such as used for space applications, requires very pure hydrogen, creating problems with the use of common fuels such as natural gas or coal. Common fuels also shorten the lifetimes of electrochemical components in cells akin to the NASA design.

Fortunately, during the ensuing decades, a number of manufacturers, the Electric Power Research Institute, the American Gas Association, the Gas Research Institute, committed groups of electric and gas utilities, and various federal agencies have supported numerous demonstration initiatives and ongoing research and development into stationary applications. In parallel, efforts in Europe and Japan also received increased support and now constitute significant governmentally backed initiatives.

The technology with the earliest promise for central station generation, phosphoric acid fuel cells, now is being offered commercially. An 11-megawatt unit was demonstrated in Tokyo, Japan, and more than one hundred 200-kilowatt units have been installed worldwide. More advanced designs, such as carbonate fuel cells and solid oxide fuel cells, are the focus of major electric utility efforts to bring the technology to the market.

Full-sized (commercial) cells and full-height stacks have been successfully demonstrated for the carbonate fuel cell design. Commercialization efforts in support of FuelCell Energy’s carbonate fuel cell design are well underway, with significant buyer interest being shown.

It has taken more than 150 years to develop the basic science and to realize the necessary materials improvement for fuel cells to become a commercial reality. The fuel cell is finally coming of age.


The Fuel Cell Concept
Fuel cells often are described as continuously operating batteries or an electrochemical engine. Like batteries, fuel cells produce power without combustion or rotating machinery. Fuel cells make electricity by combining hydrogen ions, drawn from a hydrogen-containing fuel, with oxygen atoms. Batteries provide the fuel and oxidizer internally, which is why they must be recharged periodically. Fuel cells, on the other hand, utilize a supply of these key ingredients from outside the system and produce power continuously, as long as the fuel supply is maintained.

The fuel cell uses these ingredients to create chemical reactions that produce either hydrogen- or oxygen-bearing ions at one of the cell’s two electrodes. These ions then pass through an electrolyte (which conducts electricity), such as phosphoric acid or carbonate, and react with oxygen atoms. The result is an electric current at both electrodes, plus waste heat and water vapor as exhaust products. This current is proportional to the size (area) of the electrodes. The voltage is limited electrochemically to about 1.23 volts per electrode pair, or cell. These cells then can be “stacked” until the desired power level is reached.

The challenge in fuel cell development for practical applications has been to improve the economics through the use of low-cost components with acceptable life and performance. Pure hydrogen and oxygen reactants have been replaced with common fossil fuels and air. Low-cost electrodes and electrolytes have been developed. Engineering, materials improvements, and manufacturing processes are now being developed to produce fuel cells with sufficiently high power, acceptable lifetimes, and affordable costs.

As each of these challenges is met, the promise of a factory-fabricated power generator scalable to virtually any size range with highly automated operation is being realized.

 

FUEL CELL TYPE

 

Polymer
Electrolyte
Membrane

Phosphoric
Acid

Carbonate

Solid
Oxide

Electrolyte

Ion Exchange
Membrane

Phosphoric
Acid

Alkali Carbonates
Mixture

Yttria Stabilized
Zirconia

Operating Temp., °C

80

200

650

1,000

Charge Carrier

H+

H+

CO3=

O=

Electrolyte State

Solid

Immobilized
Liquid

Immobilized
Liquid

Solid

Cell Hardware

Carbon- or
Metal-Based

Graphite-
Based

Stainless
Steel

Ceramic

Catalyst

Platinum

Platinum

Nickel

Perovskites

Cogeneration Heat

None

Low Quality

High

High

Fuel Cell Efficiency, %LHV

<40

40-45

50-60

50-60

Fuel Cell System Approaches
A major distinguishing characteristic of different fuel cells is in the electrolyte used. For stationary power generation, the three major fuel cells are phosphoric acid, carbonate, and solid oxide. While other differences exist among these fuel cells, it is the type of electrolyte used that gives them their name.

The phosphoric acid approach is the most mature of the technologies. Platinum is required as a catalyst for the electrodes. Converting (or “reforming”) of the natural gas used as fuel to a hydrogen-rich gas the system needs occurs outside the fuel cell stacks. Due to system complexity, capital costs are higher and efficiencies are lower than those projected for the two alternatives.

Compared to the phosphoric acid type, the carbonate fuel cell operates at higher temperatures, can operate at or slightly above ambient pressure, and uses less expensive, nickel-based electrodes. Reforming can occur inside the fuel cell stacks. FCE’s technology, which is being supported by the FCCG, uses internal reforming and is called the Direct Fuel Cell. DFCs are inherently more efficient compared to external reforming fuel cell systems and can generate power at 50 to 60 percent efficiency in a single cycle. This far surpasses generation technologies such as gas turbines, internal combustion engines, and steam turbines.

The solid oxide fuel cell approach is the least mature of the three. It uses a coated zirconia ceramic as the electrolyte. The electrochemical conversion process occurs at very high temperatures, supporting internal reforming. The cells themselves may be either flat plates or tubular. There are basic manufacturing challenges with all-ceramic construction, as yet unsolved, in mass producing the cells. Solid oxide fuel cells promise to operate at moderately high efficiencies with a high-grade waste heat product.

A polymer electrolyte membrane fuel cell has been proposed for submegawatt stationary power plant applications. This fuel cell operates at 175°F, uses platinum catalyst, and is susceptible to poisoning by carbon monoxide and other impurities. While this type of cell may be attractive for vehicular applications, especially when hydrogen is available as fuel, its deployment for stationary power will require significant effort.

The remaining discussion in this Fact Sheet focuses on FCE’s carbonate fuel cell design.


Basic Fuel Cell Electrochemistry
The operating principles for a carbonate fuel cell are simple in concept. The reactants — fuel and air (the oxidant) — are fed to the cell’s electrodes. Ions are transported through the electrolyte sandwiched between the electrodes, creating a current equal to the externally connected load. The basic reactions are:

Overall:
Natural Gas + Air ==> Steam + Carbon Dioxide + Electricity + Heat

Reforming Reactions:
Natural Gas + Steam (water + heat) ==> Hydrogen + Carbon Monoxide

Anode Reaction:
Hydrogen + Carbon Monoxide + Carbonate Ion ==> Steam + Carbon Dioxide + Heat + Electrons (electricity)

Cathode Reaction:
Carbon Dioxide + Oxygen + Electrons ==> Carbonate Ion

While natural gas is the primary fuel, with appropriate cleanup any hydrogen-rich gas — including gas from landfills, digesters, coal mines, or liquid fuels — can be supplied to the fuel cell. Note that electricity, heat, water vapor, and carbon dioxide are the products of these basic reactions.

The Cell Itself
The construction of an individual fuel cell resembles a sandwich. In the DFC, fuel and oxidant are fed through separate manifolds to the anode and cathode compartments of the cells, divided by a bipolar separator plate. The anode is bathed with fuel, the cathode with oxidant (air and carbon dioxide). These electrodes consist mainly of porous, sintered nickel (anode) or nickel oxide (cathode). Layered between the electrodes is the carbonate electrolyte contained in a porous (ceramic) matrix.

The separator plates, electrodes, and electrolyte layers are known as “repeating” components. The separator plates can be cut inexpensively and bent to size. The electrodes and electrolyte layers are produced on continuous tape-casting machines from relatively inexpensive materials and cut to length. Cell production lends itself well to automated factory fabrication.

Individual cells generate a relatively small voltage, on the order of 0.7 to 1.0 Volt each (after accounting for resistance losses).

Scaling Up The Technology
The current produced by an individual fuel cell is approximately a linear function of cell surface area. For FCE’s commercial cells, the two-foot by four-foot cell area is a trade-off between acceptable current (amperage) levels and manufacturing and transportation constraints.


A 100-Kilowatt Stack


To develop higher voltages, cells are “stacked” and connected in series. As the diagram indicates, stack design considerations include manifolds for uniform gas distribution to each cell and to maintain cell compression and mechanical integrity at the stack’s high operating temperatures. For FCE’s commercial megawatt-class power plants, individual stacks contain about 340 cells. Between each 10-cell grouping is a special catalyst-containing cell to improve internal reforming. Each stack has interconnections for fuel, air, and electricity.

Several stacks then are combined into a truck-transportable “module,” fabricated at the factory with all relevant connections, for shipping and installation at the site. The desired output from the power plant is obtained by combining a number of modules at the site. FCE’s megawatt-class power plants will contain one or two modules, each containing four stacks.

DFC power plants can achieve high electrical efficiencies at small sizes without the need for a bottoming cycle. The waste heat produced is well-suited to cogeneration or process heat applications.


The Internal Reforming Process
FCE’s design is significant because the fuel is “reformed” to hydrogen-rich gas internally in the stack, hence the name Direct Fuel Cell (DFC); this eliminates the fuel processing unit required by phosphoric acid fuel cells and other designs. Three significant advantages result from internal reforming: (1) costly separate equipment to process the fuel is eliminated, leading to lower overall capital costs; (2) equipment count is lower, leading to simpler operation and higher reliability; and (3) efficiency of the system is increased.

Power Plant Considerations
The diagram indicates FCE’s “simplified” power plant design.

(1) Sulfur and other impurities are removed from the natural gas (CH4) in a cleanup bed.

(2) Fuel and steam are fed to the cell’s anode section. The fuel is internally reformed and electrochemically oxidized by carbonate ions formed at the cathode by the reaction of oxygen and carbon dioxide.

(3) The anode exhaust stream is mixed with air and fed to the cathode. This is the source of the oxygen and carbon dioxide in (4). The cathode exhaust, resembling flue gas, is cooled with the extracted heat used to preheat and vaporize the water. Thermal energy at ~800°F is available for cogeneration (5).

(6) DC power produced by the fuel cells is conditioned by a high-efficiency inverter to meet AC electrical grid requirements.

Power output is controlled by varying fuel and oxidant feeds to the fuel cells. The inverter controls real and reactive power output.

The stacks have a projected commercial life of 40,000 hours. The stacks degrade gradually over their projected life and must be replaced periodically. The stack replacement decision is in reality an economic one, trading off performance loss versus fuel costs, within the thermal management limits of the balance-of-plant (BOP). The degree of stack or module flanging or valving dictates how much of the plant needs to be taken off-line when a stack is replaced.

Power plant heat rate and output are functions of the current density at which the cells operate. As cells age, heat rate can be maintained by allowing the cells to operate at a lower current density (thus lowering plant output), or rated output can be maintained by operating the cells at a higher current density (with an increase in heat rate). The FCE design assumes two shutdowns for planned and unplanned maintenance.

FCE’s commercialization program consists of a series of progressively linked steps to scale-up the power plant. Particular emphasis is being placed on BOP design, integration, operability, and performance.

A Winning Combination
The fuel cell’s mature capital costs will be competitive economically with other technologies, especially where strict environmental compliance is required. Due to short lead times (manufacture to installation), investments are incurred only when the capacity is needed. Because of the plant’s high efficiencies and reliability, particularly when operating at partial load, operating costs are competitive. Siting and operating flexibilities and benefits unique to fuel cells can lead to additional, site-specific dollar savings.

With their minimal environmental impact, competitive costs, and unsurpassed operational benefits and flexibility, fuel cells are truly a winning combination.

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For Further Details
For information on the FCCG commercialization program, contact:

Program Coordinator
Fuel Cell Commercialization Group
1800 M Street, N.W., Suite 300
Washington, DC 20036-5802, U.S.A.
Phone: +1.202.296.3471
Fax: +1.202.223.5537
eMail: fccg@ttcorp.com

For more information about H2 Solutions, Inc., contact or click here:

H2 Solutions, Inc.
+01-831-635-0509 Phone
+01-831-635-0300 Fax

 
 
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