DOE FUEL CELL HANDBOOK

DOE FUEL CELL HANDBOOK

Fuel cells are one of the cleanest and most efficient technologies for generating electricity. Since there is no combustion, there are none of the pollutants commonly produced by boilers and furnaces. For systems designed to consume hydrogen directly, the only products are electricity, water and heat. Fuel cells are an important technology for a potentially wide variety of applications including on-site electric power for households and commercial buildings; supplemental or auxiliary power to support car, truck and aircraft systems; power for personal, mass and commercial transportation; and the modular addition by utilities of new power generation closely tailored to meet growth in power consumption. These applications will be in a large number of industries worldwide.

This Handbook provides a foundation in fuel cells for persons wanting a better understanding of the technology, its benefits, and the systems issues that influence its application. Trends in technology are discussed, including next-generation concepts that promise ultra-high efficiency and low cost, while providing exceptionally clean power plant systems. Polymer electrolyte, alkaline, phosphoric acid, molten carbonate, and solid oxide fuel cell technology descriptions have been updated from the previous edition. Manufacturers are focusing on reducing fuel cell life cycle costs. In this edition, over 5,000 fuel cell patent abstracts, and their claims have been included. In addition, the handbook features a new fuel cell power conditioning section, and overviews on the hydrogen industry and rare earth minerals market.

The Fuel Cell handbook consists of eleven sections. The following is a brief description of the information presented in each section of the handbook.

Original CONTENTS OF THE BOOK in BLUE with EXPLANATIONS and FIGURES Inserted.

Section 1 - TECHNOLOGY OVERVIEW

- Explanation- This section provides an overview of fuel cell technology. First it discusses the basic workings of fuel cells and basic fuel cell system components. Then, an overview of the main fuel cell types, their characteristics, and their development status is provided. Finally, this chapter reviews potential fuel cell applications.

Figure 1-3 shows a simple rendition of a fuel cell power plant. Beginning with fuel processing, a conventional fuel (natural gas, other gaseous hydrocarbons, methanol, naphtha, or coal) is cleaned, then converted into a gas containing hydrogen. Energy conversion occurs when dc electricity is generated by means of individual fuel cells combined in stacks or bundles. A varying number of cells or stacks can be matched to a particular power application. Finally, power conditioning converts the electric power from dc into regulated dc or ac for consumer use. Section 8.1 describes
the processes of a fuel cell power plant system.

1.6 Characteristics

The interest in terrestrial applications of fuel cells is driven primarily by their potential for high efficiency and very low environmental impact (virtually no acid gas or solid emissions). Efficiencies of present fuel cell plants are in the range of 30 to 55 percent based on the lower heating value (LHV) of the fuel. Hybrid fuel cell/reheat gas turbine cycles that offer efficiencies greater than 70 percent LHV, using demonstrated cell performance, have been proposed. Figure 1-4 illustrates demonstrated low emissions of installed PAFC units compared to the Los Angeles Basin (South Coast Air Quality Management District) requirements, the strictest requirements in the U. S. Measured emissions from the PAFC unit are < 1 ppm of NOX, 4 ppm of CO, and <1 ppm of reactive organic gases (non-methane) (5). In addition, fuel cells operate at a constant temperature, and the heat from the electrochemical reaction is available for cogeneration applications. Table summarizes the impact of the major constituents within fuel gases on the various fuel cells. The reader is referred to Sections 3 through 7 for detail on trace contaminants.

1.1	INTRODUCTION..	1-1
1.2	UNIT CELLS..	1-2
	1.2.1 Basic Structure..	1-2
	1.2.2 Critical Functions of Cell Components..	1-3
1.3	FUEL CELL STACKING..	1-4
	1.3.1 Planar-Bipolar Stacking..	1-4
	1.3.2 Stacks with Tubular Cells..	1-5
1.4	FUEL CELL SYSTEMS..	1-5
1.5	FUEL CELL TYPES..	1-7
	1.5.1 Polymer Electrolyte Fuel Cell (PEFC)..	1-9
	1.5.2 Alkaline Fuel Cell (AFC)..	1-10
	1.5.3 Phosphoric Acid Fuel Cell (PAFC)..	1-10
	1.5.4 Molten Carbonate Fuel Cell (MCFC)..	1-11
	1.5.5 Solid Oxide Fuel Cell (SOFC)..	1-12
1.6	CHARACTERISTICS..	1-12
1.7	ADVANTAGES/DISADVANTAGES..	1-14
1.8	APPLICATIONS, DEMONSTRATIONS, AND STATUS..	1-15
	1.8.1 Stationary Electric Power..	1-15
	1.8.2 Distributed Generation..	1-20
	1.8.3 Vehicle Motive Power..	1-22
	1.8.4 Space and Other Closed Environment Power..	1-23
	1.8.5 Auxiliary Power Systems..	1-23
	1.8.6 Derivative Applications..	1-32
1.9	REFERENCES..	1-32

Section 2 - FUEL CELL PERFORMANCE

- Explanation- The purpose of this section is to describe the chemical and thermodynamic relations governing fuel cells and how operating conditions affect their performance. Understanding the impacts of variables such as temperature, pressure, and gas constituents on performance allows fuel cell developers to optimize their design of the modular units and it allows process engineers to maximize the performance of systems applications.

A wide variety of fuel cell models has been developed. While fundamentally the constitutive equations such as those described in this chapter underlie all models, their level of detail, level of aggregation, and numerical implementation method vary widely. A useful categorization of fuel cell models is made by level of aggregation, as shown in Figure 2-9.

As implied in the figure, the outputs of the more detailed fundamental models can be used in lower-order models. This flow of information is, in fact, a critical application for high fidelity models. Recently, much work has been done in the development of algorithms to integrate or embed high-fidelity models into system analysis simulation tools.

NETL's 3-D Analysis

The National Energy Technology Laboratory (NETL) developed a 3-dimensional computational fluid dynamics (CFD) model to allow stack developers to reduce time-consuming build-and-test efforts. As opposed to systems models, 3-dimensional CFD models can address critical issues such as temperature profiles and fuel utilization; important considerations in fuel cell development.

CFD analysis computes local fluid velocity, pressure, and temperature throughout the region of interest for problems with complex geometries and boundary conditions. By coupling the CFDpredicted fluid flow behavior with the electrochemistry and accompanying thermodynamics, detailed predictions are possible. Improved knowledge of temperature and flow conditions at all points in the fuel cell lead to improved design and performance of the unit.

2.1	THE ROLE OF GIBBS FREE ENERGY AND NERNST POTENTIAL..	2-1
2.2	IDEAL PERFORMANCE..	2-4
2.3	CELL ENERGY BALANCE..	2-7
2.4	CELL EFFICIENCY..	2-7
2.5	ACTUAL PERFORMANCE..	2-10
2.6	FUEL CELL PERFORMANCE VARIABLES..	2-18
2.7	MATHEMATICAL MODELS..	2-24
	2.7.1 Value-in-Use Models..	2-26
	2.7.2 Application Models..	2-27
	2.7.3 Thermodynamic System Models..	2-27
	2.7.4 3-D Cell / Stack Models..	2-29
	2.7.5 1-D Cell Models..	2-31
	2.7.6 Electrode Models..	2-32
2.8	REFERENCES..	2-33

Section 3 - POLYMER ELECTROLYTE FUEL CELLS

- Explanation- This section includes an in depth discussion of Polymer electrolyte membrane fuel cells (PEFC), which are able to efficiently generate high power densities, thereby making the technology potentially attractive for certain mobile and portable applications.

3.1 Cell Components

Typical cell components within a PEFC stack include:

· the ion exchange membrane

· an electrically conductive porous backing layer

· an electro-catalyst (the electrodes) at the interface between the backing
layer and the membrane

·       cell interconnects and flowplates that deliver the fuel and oxidant to
       reactive sites via flow channels and electrically connect the cells
       (Figure 3-1).

PEFC stacks are almost universally of the planar bipolar type. Typically, the electrodes are cast as thin films that are either transferred to the membrane or applied directly to the membrane. Alternatively, the catalyst-electrode layer may be deposited onto the backing layer, then bonded to the membrane.

3.1	CELL COMPONENTS..	3-1
	3.1.1 State-of-the-Art Components..	3-2
	3.1.2 Component Development..	3-11
3.2	PERFORMANCE..	3-14
3.3	PEFC SYSTEMS..	3-16
	3.3.1 Direct Hydrogen PEFC Systems..	3-16
	3.3.2 Reformer-Based PEFC Systems ..	3-17
	3.3.3 Direct Methanol Fuel Cell Systems..	3-19
3.4	PEFC APPLICATIONS..	3-21
	3.4.1 Transportation Applications..	3-21
	3.4.2 Stationary applications..	3-22
3.5	REFERENCES..	3-22

Section 4 - ALKALINE FUEL CELL

- Explanation- This section discusses the Alkaline Fuel Cell (AFC), which was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle. Desirable attributes of the AFC include excellent performance compared to other candidate fuel cells due to its active O2 electrode kinetics and flexibility to use a wide range of electro-catalysts. The AFC continues to be used: it now provides on-board power for the Space Shuttle Orbiter with cells manufactured by UTC Fuel Cells.

Figures 4-1 and 4-2 depict the operating configuration of the _H2/O2alkaline fuel cell (8) and a H2/air cell (9). In both, the half-cell reactions are:

H₂ + 2OH→ 2H2O + 2e- (Anode)

½O2 + H2O + 2e- → 2OH^¯ (Cathode)


4.1	CELL COMPONENTS..	4-5
	4.1.1 State-of-the-Art Components..	4-5
	4.1.2 Development Components..	4-6
4.2	PERFORMANCE...	4-7
	4.2.1 Effect of Pressure..	4-8
	4.2.2 Effect of Temperature..	4-9
	4.2.3 Effect of Impurities..	4-11
	4.2.4 Effects of Current Density..	4-12
	4.2.5 Effects of Cell Life..	4-14
4.3	SUMMARY OF EQUATIONS FOR AFC..	4-14
4.4	REFERENCES..	4-16

Section 5 - PHOSPHORIC ACID FUEL CELL

- Explanation- This section discusses the Phosphoric Acid Fuel Cell (PAFC), which was the first fuel cell technology to be commercialized. The number of units built exceeds any other fuel cell technology, with over 85 MW of demonstrators that have been tested, are being tested, or are being fabricated worldwide.

Figure 5-1 depicts the operating configuration of the phosphoric acid cell. The electrochemical reactions occurring in PAFCs are H2→2H⁺+2e- at the anode, and ½ O₂ + 2H⁺ + 2e⁻ → H2O at the cathode. The overall cell reaction is ½ O 2 + H₂ —> H₂O

The electrochemical reactions occur on highly dispersed electro-catalyst particles supported on carbon black. Platinum (Pt) or Pt alloys are used as the catalyst at both electrodes.

5.	PHOSPHORIC ACID FUEL CELL		5-1
	5.1	CELL COMPONENTS..	5-2
		5.1.1 State-of-the-Art Components..	5-2
		5.1.2 Development Components..	5-6
	5.2	PERFORMANCE..	5-11
		5.2.1 Effect of Pressure..	5-12
		5.2.2 Effect of Temperature..	5-13
		5.2.3 Effect of Reactant Gas Composition and Utilization..	5-14
		5.2.4 Effect of Impurities..	5-16
		5.2.5 Effects of Current Density..	5-19
		5.2.6 Effects of Cell Life..	5-20
	5.3	SUMMARY OF EQUATIONS FOR PAFC..	5-21
	5.4	REFERENCES..	5-22

Section 6 - MOLTEN CARBONATE FUEL CELL

- Explanation- This section discusses Molten Carbonate Fuel Cells, which are being developed for natural gas and coal-based power plants for industrial, electrical utility, and military applications.

There are two alternate approaches to internal reforming molten carbonate cells: indirect internal reforming (IIR) and direct internal reforming (DIR). In the first approach, the reformer section is separate, but adjacent to the fuel cell anode. This cell takes advantage of the close-coupled thermal benefit where the exothermic heat of the cell reaction can be used for the endothermic reforming reaction. Another advantage is that the reformer and the cell environments do not have a direct physical effect on each other. A disadvantage is that the conversion of methane to hydrogen is not promoted as well as in the direct approach. In the DIR cell, hydrogen consumption reduces its partial pressure, thus driving the methane reforming reaction, Equation (6-34), to the right. Figure 6-12 depicts one developer's approach where IIR and DIR have been combined.


6.1	CELL COMPONENTS..	6-4
	6.1.1 State-of-the-Art Componments..	6-4
	6.1.2 Development Components..	6-9
6.2	PERFORMANCE..	6-13
	6.2.1 Effect of Pressure..	6-15
	6.2.2 Effect of Temperature..	6-19
	6.2.3 Effect of Reactant Gas Composition and Utilization..	6-21
	6.2.4 Effect of Impurities..	6-25
	6.2.5 Effects of Current Density..	6-30
	6.2.6 Effects of Cell Life..	6-30
	6.2.7 Internal Reforming..	6-30
6.3	SUMMARY OF EQUATIONS FOR MCFC..	6-34
6.4	REFERENCES..	6-38

Section 7 - SOLID OXIDE FUEL CELLS

- Explanation- This section discusses Solid Oxide Fuel Cells (SOFCs), which have an electrolyte that is a solid, non-porous metal oxide, usually Y2O3-stablilized ZrO2. The cell operates at 600-1000 o_C where ionic conduction by oxygen ions takes place.

Sensitivity to sulfur and other contaminants. Strong reversible poisoning of the anode occurs at feed concentrations ranging from about 1 ppm H2S when operating at 1000 °C down to less than 50 ppb when operating at 750 °C (Figure 7-2a (16, 17)). These concentrations require desulfurization of the anode feed, even if it is produced from low-sulfur fuels such as natural gas or ultra-low sulfur diesel or gasoline (Figure 7-2b). No data is available publicly on the impact of other species (water or hydrocarbons) or different sulfur species on sulfur tolerance, or on the effect after long periods of time (e.g. 40,000 hours or more). Another strong anode poison reported is HCl. Poisoning by these species is reversible after exposure at low concentrations, but irreversible after exposure at concentrations above about 200 ppm.

Lower operating temperatures would allow the use of ferritic steels, that could reduce the materials cost, and ferritic steels are typically easier to process with low-cost processing techniques. The corrosion resistance of steel depends on the formation of stable oxide layers on the surface (Figure 7-5). After extensive testing of commercial compositions, it was concluded that none possessed the corrosion resistance required, especially to withstand the thermal cycling requirements while still providing adequate contact resistance. Efforts were undertaken to develop more suitable compositions, which led to the development of several special alloys. Many developers now use the Krupp formulation Crofer22 APU.

Tubular SOFC

Although the Siemens Westinghouse design of tubular SOFC is by far the best-known and most developed, two other types of tubular SOFCs, shown in Figure 7-9 illustrate ways in which the cells are interconnected. Numerous other designs have been proposed, but are no longer pursued.

Figure 7-22 shows a sample of recently-pursued planar SOFC approaches. The anode-supported technology with metal interconnects will be described in some detail below. Mitsubishi tested a 15 kW system with its all-ceramic MOLB design for almost 10,000 hours with degradation rates below 0.5 percent per 1,000 hrs, but without thermal cycles, and with power densities ranging from 190 to 220 mW/cm² (under practical operating conditions). Because the interconnect is flat and relatively thin (the flow-passage is embedded in the MEA), less of the expensive LaCrO3 is required than if the flow-passages were in the interconnect. Nevertheless, cost reduction is still one of the main priorities for this stack technology. Thermal cycling is also thought to be a challenge with the system, which is targeted to small-scale distributed stationary power generation applications.

Stack Performance

A number of planar cell stack designs have been developed based on planar anode-supported SOFC with metal interconnects. Typically, cells for full-scale stacks are about 10 to 20 cm mostly square or rectangular (though some are round). Stacks with between 30 and 80 cells are the state-of-the-art. Figure 7-26 shows examples of state-of-the art planar anode-supported SOFC stacks and selected performance data (68,78, 79). The stacks shown are the result of three to seven generations of full-scale stack designs by each of the developers. The capacities of these stacks (2 to 12 kW operated on reformate and at 0.7 V cell voltage) is sufficient for certain small-scale stationary and mobile (APU) applications.

7.1 CELL COMPONENTS..

7.1.1 Electrolyte Materials..

7.1.2 Anode Materials..

7.1.3 Cathode Materials ..

7.1.4 Interconnect Materials..

7.1.5 Seal Materials..

7.2 CELL AND STACK DESIGNS..

7.2.1 Tubular SOFC..

7.2.1.1 Performance..

7.2.2 Planar SOFC..

7.2.2.1 Single Cell Performance..

7.2.2.2 Stack Performance..

7.2.3 Stack Scale-Up..

7.3 SYSTEM CONSIDERATIONS..

7.4 REFERENCES..

7-2

7-3

7-5

7-6

7-9

7-13

7-20

7-31

7-35

7-39

7-41

7-45

Section 8 - FUEL CELL SYSTEMS

- Explanation- This section discusses fuel cell power systems, which consist of a fuel processor, fuel cell power section, power conditioner, and potentially a cogeneration or bottoming cycle to utilize the rejected heat.

Figure 8-6 shows a block diagram of a representative fuel cell power plant. Natural gas flows to a fuel processor, where the methane is reformed to hydrogen-rich gas. The hydrogen gas reacts in the power producing section, which consists of a fuel cell. The DC power generated by the fuel cell must be converted to AC power; one of the power conditioning approaches identified above would be selected, based on the specific application.

8.2.9 System Issues: Power Conversion Cost and Size

400V DC is required to produce 120V/240V AC. If a fuel cell can produce 400V DC, then only an inverter stage is required, resulting in lowest cost for power conditioning. Present day commercially available fuel cells produce low voltage (12V to 100V). Therefore, either a line frequency transformer to increase the AC voltage or a DC-DC converter to boost the DC voltage is required, adding to cost, weight, and volume. Figure 8-24 shows a representative cost per kW of the power conditioning unit, as the voltage and current values are varied for a certain power level. It is clear from this figure that extremes of voltage at low power and high current at high power levels does not result in an optimum design. In general, higher voltage levels are required at higher power outputs to minimize the cost of power conditioning hardware. The other issue is power density and size of power conditioning unit. Using higher switching frequency for power conversion should result in smaller size. However, the switching losses are higher and a design compromise becomes necessary. Employing power semiconductor devices with lower losses combined with active cooling methods should yield an optimum size. Power integrated circuits can also be considered for further size reduction and become viable, if the fuel cell systems are produced in high volume.

8.1 SYSTEM PROCESSES..

8.1.1 Fuel Processing..

8.2 POWER CONDITIONING..

8.2.1 Introduction to Fuel Cell Power Conditioning Systems..

8.2.2 Fuel Cell Power Conversion for Supplying a Dedicated Load [2,3,4]..

8.2.3 Fuel Cell Power Conversion for Supplying Backup Power to a Load

Connected to a Local Utility..

8.2.4 Fuel Cell Power Conversion for Supplying a Load Operating in Parallel

With the Local Utility (Utility Interactive)..

8.2.5 Fuel Cell Power Conversion for Connecting Directly to the Local Utility..

8.2.6 Power Conditioners for Automotive Fuel Cells..

8.2.7 Power Conversion Architecture for a Fuel Cell Turbine Hybrid Interfaced

With a Local Utility..

8.2.8 Fuel Cell Ripple Current..

8.2.9 System Issues: Power Conversion Cost and Size..

8.2.10 REFERENCES (Sections 8.1 and 8.2)..

8.3 SYSTEM OPTIMIZATION..

8.3.1 Pressure..

8.3.2 Temperature..

8.3.3 Utilization..

8.3.4 Heat Recovery..

8.3.5 Miscellaneous..

8.3.6 Concluding Remarks on System Optimization..

8.4 FUEL CELL SYSTEM DESIGNS..

8.4.1 Natural Gas Fueled PEFC System..

8.4.2 Natural Gas Fueled PAFC System..

8.4.3 Natural Gas Fueled Internally Reformed MCFC System..

8.4.4 Natural Gas Fueled Pressurized SOFC System..

8.4.5 Natural Gas Fueled Multi-Stage Solid State Power Plant System..

8.4.6 Coal Fueled SOFC System..

8.4.7 Power Generation by Combined Fuel Cell and Gas Turbine System..

8.4.8 Heat and Fuel Recovery Cycles..

8-2

8-27

8-28

8-29

8-34

8-37

8-39

8-41

8-43

8-44

8-45

8-46

8-48

8-49

8-50

8-51

8-52

8-53

8-56

8-58

8-62

8-66

8-70

8.5.2 MCFC Network..

8-86

8.5.3 Recycle Scheme..

8-86

8.5.4 Reactant Conditioning Between Stacks in Series..

8-86

8.5.5 Higher Total Reactant Utilization..

8-87

8.5.6 Disadvantages of MCFC Networks..

8-88

8.5.7 Comparison of Performance..

8-88

8.5.8 Conclusions..

8-89

8.6

HYBRIDS..

8-89

8.6.1 Technology..

8-89

8.6.2 Projects..

8-92

8.6.3 World’s First Hybrid Project..

8-93

8.6.4 Hybrid Electric Vehicles (HEV)..

8-93

8.7

FUEL CELL AUXILIARY POWER SYSTEMS..

8-96

8.7.1 System Performance Requirements..

8-97

8.7.2 Technology Status..

8-98

8.7.3 System Configuration and Technology Issues..

8-99

8.7.4 System Cost Considerations..

8-102

8.7.5 SOFC System Cost Structure..

8-103

8.7.6 Outlook and Conclusions..

8-104

8.8

REFERENCES..

8-104

Section 9 - SAMPLE CALCULATIONS

- Explanation- This section presents sample problems to aid the reader in understanding the calculations behind a fuel cell power system. The sample calculations are arranged topically with unit operations in Section 9. 1, system issues in Section 9.2, supporting calculations in Section 9.3, and cost calculations in Section 9.4. A list of conversion factors common to fuel cell systems analysis is presented in Section 9.5, and a sample automotive design calculation is presented in Section 9.6.

9.	SAMPLE CALCULATIONS..		9-1
	9.1	UNIT OPERATIONS..	9-1
		9.1.1 Fuel Cell Calculations..	9-1
		9.1.2 Fuel Processing Calculations..	9-13
		9.1.3 Power Conditioners..	9-16
		9.1.4 Others..	9-16
	9.2	SYSTEM ISSUES..	9-16
		9.2.1 Efficiency Calculations..	9-17
		9.2.2 Thermodynamic Considerations..	9-19
	9.3	SUPPORTING CALCULATIONS..	9-22
	9.4	COST CALCULATIONS..	9-25
		9.4.1 Cost of Electricity..	9-25
		9.4.2 Capital Cost Development..	9-26
	9.5	COMMON CONVERSION FACTORS..	9-27
	9.6	AUTOMOTIVE DESIGN CALCULATIONS..	9-28
	9.7	REFERENCES..	9-29

Section 10 - APPENDIX


10.1	EQUILIBRIUM CONSTANTS..	10-1
10.2	CONTAMINANTS FROM COAL GASIFICATION..	10-2
10.3	SELECTED MAJOR FUEL CELL REFERENCES, 1993 TO PRESENT..	10-4
10.4	LIST OF SYMBOLS..	10-10
10.5 10.6 10.7 10.8 10.9 10.10	FUEL CELL RELATED CODES AND STANDARDS.. 10.5.5 Codes and Standards for the Installation of Fuel Cells.. 10.5.6 Codes and Standards for Fuel Cell Vehicles.. 10.5.7 Application Permits.. 10.5.8 References.. FUEL CELL FIELD SITE DATA... 10.6.1 Worldwide Sites.. 10.6.2 DoD Field Sites.. 10. 6.3 IFC Field Units.. 10.6.4 FuelCell Energy.. 10.6.5 Siemens Westinghouse.. HYDROGEN.. 10.7.1 Introduction.. 10.7.2 Hydrogen Production.. 10.7.3 DOE’s Hydrogen Research.. 10.7.4 Hydrogen Storage.. 10.7.5 Barriers.. HE OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY WORK IN FUEL CELLS.. RARE EARTH MINERALS.. 10.9.1 Introduction.. 10.9.2 Outlook.. REFERENCES..	10-14 10-19 10-19 10-19 10-21 10-21 10-21 10-24 10-24 10-24 10-24 10-31 10-31 10-32 10-34 10-35 10-36 10-36 10-38 10-38 10-40 10-41