Current Technology of Fuel Cells Systems
Content
97354
CURRENT TECHNOLOGY OF FUEL CELL SYSTEMS
Ali T-Raissi Arundhati Banerjee Kenneth G. Sheinkopf Florida Solar Energy Center 1679 Clearlake Road Cocoa, Florida 32922 407-638-1000 FAX 407-638-1010
ABSTRACT A great deal of research is taking place on fuel cells, which use hydrogen and oxygen as the fuel. One of the reasons for this interest is that fuel cells offer the best criteria for meeting the requirements of zem emission vehicles, and thus are expected to be the prime users of hydrogen in the near future. There are presently six different types of fuel cell technologies available - phospho:ic acid fuel cells, proton exchange membrane fuel cells, alkaline fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and direct methanol-air fuel cells. This paper looks at each of these six types and gives a brief overview of the technologies and their present state of development. The suitability of the various types toward use in the transportation sector is also studied. Because the selection of a fuel storage method is highly dependent on basic quirements of operational characteristics, the status of the fuel cell technologies is discussed with respect to their basic operating principle, acceptable contamination level, economics, and suitability toward transportation. To be used in the transportation sector, fuel cells must meet the demands of rapid startup, fast pickup, high power density, greater fuel efficiency, easy and safe handling, high lifetime and low cost. Unfortunately, none of the six types can satisfy all of these demands at this time, but each has its own advantages and benefits. This paper categorizes each type as to their strengths and weaknesses in meet:.ngthese needs.
to batteries, fuel cells are made of two electrodes with a conductive electrolyte sandwiched in between but unlike a battery, a fuel cell does not require recharging. It produces electrical energy as long as fuel is supplied to it’. The fuel for the fuel cell can be hydrogen or any other hydrogen-containing compound which on reprocessing can produce hydrogen. At the hydrogen electrode (anode), hydrogen ions(protons) and electrons are formed. Protons migrate through the electrolyte to the oxygen electrode (cathode) while electrons move through an extemal circuit to a load and then retum to the cathode. At the oxygen electrode, oxygen, hydrogen and electrons combine to form water. Thus, by forcing the electrons tfi take an extemal path, low temperature direct energy conversion is achieved*. Thermodynamically, a fuel cell converts the Gibb’s free energy change (*G) of a electrochemical reaction to electrical energy according to the following equation: aG = nFBEr where E, = reversible potential of the cell, n = no. of electrons arid F = Faraday’s constant. Considering the most commonly referred reaction of the fuel cell -- the reaction between hydrogen arid oxygen to produce water -- (H2 + 1/2 O,=H,O), *Go is 56.32 kcdmole, n is 2 and therefore E, turns out to be 1.23 volt (under
INTRODUCTION This paper looks at the six major types of fuel cells and gives their basic operating principles, acceptable contamination levels, technological status, and suitability in the transport sector. Fuel cells are e1ec:trochemical devices which directly convert chemical energy to electrical energy without combustion. Similar
‘David L., Handbook cfBatteries and Fuel Cells, McGraw Hill 1984. ’Swain, D.H. and Appleby, A.J., “Fuel Cells and Other Long Range Technology Options for Electric Vehicles: Knowledge Gaps and Development Priorities”, in The Urban Electric Vehicle, proceedings of an International Conference, Stockholm, Sweden, May 25-27, 1992, pp. 457-468, OECD Document.
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standard conditions (i,e.,T=25”C,P, = Poz = 1 atm, liquid Hz03). It is this electrical energy which can be effectively used to power the motor of an electric vehicle. The operation of a fuel cell involves startup, fuel and air delivery control as a function of the load. The products are heat and water which have to be removed from the system. In this regard, the fuel cell can be referred to as an electrochemical engine for electridhybrid vehicles. In practice, the efficiencies of fuel cells range from 40% t3 65% based on the lower heating value of hydrogen. Under operational conditions, the voltage output falls due to polarization effects. To make a useful voltage for practical purposes, multiple cells are connected in electrical series and are referred to as a fuel cell stack. The fuel cell stack and its necessary auxiliaries are collectively called fuel cell systems. The major accessories include thermal management system, fuel supply and storage subsystems. There are presently six different types of fuel cell technologies: Phosphoric acid fuel cell (PAFC) 1) 2) Proton exchange membrane fuel cell (PEMFC) 3) Alkaline fuel cells (AFC) Solid Oxide Fuel cell (SOFC) 4) Molten carbonate fuel cell (MOFC) 5) Direct melhanol-air fuel cell (DMAFC) 6) The following classification is based on the type of electrolyte used in the fuel cells. Because the selection of a fuel storage method is highly dependent on basic requirements of operational characteristics, the status of fuel cell technologies is discussed with respect to their basic operating principle, acceptable contamination level, technological status and economics and suitability towards transportation sectors.
The system is extremely sensitive to CO and H,S which are commonly present in the reformed fuels. A major challenge for using reformed fuel, therefore, lies in the removal of CO to a level of <200 - 300 ppm. CO tolerance is better at temperatures >180°C. Removal of sulfur is still essential.
TechnoloPical Status The PAFC system is the most advanced fuel cell system for terrestrial applications. Its major use is in on-site integrated energy systems to provide electrical power in apartments, shopping centers, etc. These fuel cells are commercially available in the range from 24V, 250 watt portable units to 200kW on-site generators. PAFC systems of 0.5 - 1.OMW are being developed for use in stationary power plants of 1-11 MW capacity. Using natural gas as the fuel, a 200kW system is $2875/kW (1992 $). The power density of PAFC is 200 mW/cm2and the projected capital costs are $1000-$1500/kW5. (The projected power density for 36 kW brassboard PAFC fuel cell stack has been reported to be 0.12 kW/kg and 0.16 kW/L‘.) TransportationSector PAFCs are currently being considered for use in heavy duty vehicles. Their major problem for use in vehicular application is their slow start-up (since the cell has to be heated to over 200°C), high costs and excessive weight. Since PAFCs work best at a constant output, their application will be better in hybrid systems where a battery or other device meets the high-power demands of acceleration. PAFCs stand their best chance of success in heavy duty vehicles or locomotives. The 200kW unit can be used for long-range bus or truck applications while the larger megawatt capacity units are planned to be used as the power plant of a longrange locomotive unit.
PHOSPHORIC ACID FUEL CELLS (PAFCs) Basic Operating Principle The basic components of a phosphoric acid fuel cell (PAFC) are the electrodes consisting of finely dispersed Pt catalyst on carbon paper, S i c matrix holding the phosphoric acid and a bipolar graphite plate with flow channels for fuel and oxidant. The operating temperature ranges between 160-220°C and it can use either hydrogen or hydrogen produced from hydrocarbons(natura1 gas) or alcohols as the anodic reactant. In the case of hydrogen produced from a reformer with air as the anodic reactant, a temperature of 200°C and a pressure of 8 atm are required for better performance. PAFCs are advantageous from a thermal management point of view. The rejection of waste heat and product water is very efficient in this system and the waste heat at 200°C can be used efficiently for the endothermic steamreforming reaction4.
PROTON EXCHANGE MEMBRANE FUEL CELLS (PEMFC)/SP(E)FC Basic Operating Principle PEM fuel cells, also called solid polymer electrolyte fuel cells -SP(E)FC -- use a proton (hydrogen ion) conducting membrane which stays sandwiched between two platinum-catalyzed porous electrodes728. Initially, these membranes were based on polystyrene, but at present a Teflon-based product “Nafion” is
5
Snnivasan, S. et.al, “Overview of Fuel Cell Technology,” in Fuel Cell .. edited by Leo J.M.J. Blomen and Michel N. Mugenva, Plenum Press, N.Y. 1993.
System,
Acceptable Contamination Levels
%hi, C.V., Glemm, D.R. and Abens, S.G., “Developmentof a fuel cell power source for bus,” proceedings of the 2ShIntersociety Energy Conversion Engineering Conference, vol3, edited by PIA. Nelson, W.W. Schertz, and R.H. Till, American Institute of Chemical Engineers, N.Y. 1990, pp. 308-313.
’U.S. Department of Energy, Proton-Exchange Membrance Fuel Cell
Program, DE93000009, Nov. 1992.
3Appleby, A.J. and Foulkes, F.R., Fuel Cell Handbook, Van Nostrand
Reinhold, N.Y., 1989.
4U.S. Department of Energy, “Phosphoric Acid Fuel Cells,’’ DE93000003,Nov. 92.
8Gottesfeld,S . “Polymer Electrolyte Fuel Cells,” submitted to 11” International Seminar on Primary and Secondary Battery Technology and Applications, Feb. 28-March 3, 1994, Deerfield Beach, Florida (private communication).
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used. This offers high stability, high oxygen solubility and high mechanical strength. The cell operating temperature is quite low (80-90°C) and opepating pressures can be in the range of 1-8 atmosphere. The fuel cell requires humidified hydrogen and oxygen for its operation. The pressures, in general, are maintained equal on either side of the membrane. Operation at high pressure is necessary to attain high power densities, particularly when air is chosen as the anodic reactant.
and heat is difficult at these low temperatures. In space shuttles, closed loop hydrogen circulation as well as dielectric liquid circulation is used for heat management. Some of the terrestrial fuel cells are process air-cooled. Acceptable Contamination Level Alkaline fuel cell can operate only with pure H and pure 0,. , Even a small level (<250 ppm) of CO, is sufficien: to carbonate the electrolyte and can spoil the electrode. Several processes for cleaning of the electrode after contamination are availabl!e (Physical adsorption-Selexol process, Fluor solvent process, pressure swing adsorption) but each is expensive and none are totally effective Technolosical Status AFCs are extremely reliable arid offer very high power output in a relatively small package. Apart from space applications, this type of fuel cell has promise as a standby power source. The power density ranges from 100-200 mW/cm2, and the projected cost varies for different applications. For transportation applications, the goal is to reduce the capital cost to $50-$100/kW; for standby power, capital costs of $500/kW are reported to be acceptable. For defense and space applications, the allowance wais $5000- 15,000/kWI. Transportation Sector AFCs offer high power density and cold-start capabilities, but their application in the transportation sector is clearly dependent on the availability of on-board pure hydrogen storage and supply. The sensitivity towards CO, and high cost are the major constraints.
Acceptable Contamination Level The major contaminant of the PEMFC system is carbon monoxide. Even a trace amount of CO drastically reduces the performance levely~'oIt is best to use a fuel which is virtually free of CO for PEMFC. On the other hand, it is tolerant toward CO, contamination. Technological Stat@ PEMFCs have high power density (350 mW/cmz) and are now commercially available in power ranges (IOOW-5OOW) with the corresponding capital cost ranging from approximately $5,676$13,000. The lOkW to 200 kW power plants have been developed and demonstrated successfully but are waiting for large scale productionY.los". projected capital cost is approximately $200The $300/kW, given a 10-20 fold reduction in the membrane and catalyst cost and also considering large scale productions. Therefore, the major challenge ahead is to find a low-cost catalyst, low-cost membrane and an efficient water management option within the cell assembly. Transportation Seem Due to their high power density, rapid startup and variable power output, PEMFCs are suitable for use in the transportation sector. They are considered the best choice as far as present day available fuel cell technologiw are concerned. Their high power density and small size makes them primary candidates for light-duty vehicles, though they are also used for heavy-duty vehicles. PEMFCs are also being developed for space and underwater transportation applications. ALKALINE FUEL CELLS (AFCS) Basic Operating Principle Alkaline fuel cella represents the oldest and most widely-used fuel cell systems in the U.S. space program and have gone onboard most of the manned space missions. AFCs use potassium hydroxide as the electrolyte and hydroxyl ions are the conducting species. Because of the alkaline electrolyte, no noble metal catalyst is required '. AFCs operate at 613-80"C which is relatively low compared to
other fuel cells. Operating pressure is normally atmospheric pressure. From a system point of view, removal of product water
SOLID OXIDE FUEL CELLS (SOFCS)
Basic Operatins Principle Solid oxide fuel cells (SOFCs) are solid-state power systems and at present use ytria-stabilized zirconia as the electrolyte". The operating temperature is high, typically 1,OOO"C. SOFCs can be used as cogenerators to supply both electricity and high quality waste heat. In this cell, a conversion efficiency of more than 50470 can easily be attained. Acceptable Contamination Level Because of high temperature, the SOFCs can handle impurities in the incoming fuel better. SOFCs can operate equally well on dry or humidified hydrogen or carbon monoxide fuel or on mixture:;. The main poisoning factor for SOFC is H,S. Though the sulfur tolerance level is approximately two orders of magnitude greater than other fuel cells, the level is below 80 ppm. Technolosical Status This technology is still in the developmental stage. SOFCs are -. very attractive in electrical utility and industrial applications. The high operating temperature allows them to use H, and CO from natural gas steam reformers and coal gasification plants, a major
'Final Report, United 'Technology, Hamilton Standard For LANL W X 53.D6272-1.
"'Analytic Power Corporation, Boston, USA, private communication.
"Ballard Power Systems Inc., Canada, Technical Marketing Dept., private communication.
"Goldstein, R. "Solid Oxide Fuel Cell Development" EPRI Journal, Oct./Nov. 1992.
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advantage as far as fuel selection is c ~ n c e r n e d ’ ~ ,The present ’~. day estimated capital cost is $1500/kW but is expected to be reduced with improvements in technology. Transportation Sector Although the high power density and potential for intemal fuel processing of SOFC is very attractive from the transportation point of view, the high operating temperature and long warm-up time rules out their use in vehicular applications. MOLTEN CARBONATE FUEL CELLS (MCFCS)
DIRECT METHANOL-AIR FUEL CELLS (DMAFCS) DMAFCs are the least developed among all the fuel cell technologies. Though methanol itself has simpler storage requirements that hydrogen lacks and is simpler to make and transport, its electrochemical activity is one-third that of hydrogen. Also, the conversion takes place at lower temperature (200°C) compared to other hydrocarbons and therefore the contaminant problem is higher than other fuel cells. The technology is underdeveloped for present consideration. SUMMARY
Basic Operatinq Principle In the molten carbonate fuel cell, a molten alkali carbonate mixture, retained in a porous lithium aluminate matrix, is used as the electrolyte. The conducting species is carbonate ions’j. The operating temperature is in the range of 600 - 800”C, high enough to produce suitable qualities of waste heat. This waste heat can be used for fuel processing and cogeneration, a bottoming cycle, or intemal reforming of methane ’. MCFCs normally have 75% fuel (hydrogen) utilization. The advanced form of MCFC referred to as internal reforming molten carbonate fuel cell (IRMCFC) may consume lower hydrocarbons (CH,) directlyi6. It is intrinsically efficient since the heat produced at the anode is used for reformation of hydrocarbons. Normally their efficiencies are 50% or higher. Acceptable Contamination Level MCFCs do not suffer from CO poisoning and, in fact, can use CO in the anode gas as the fuel. They are extremely sensitive (1 ppm) to the presence of sulfur in the reformed fuel or oxidant gas stream. The presence of HCl, HF, HBr, etc. causes corrosion, while trace metals can spoil the electrodes. The presence of particulates of coayfine ash in the reformed fuel can clog the gas passages. Technoloqical Status MCFC technologies are still under development. Their best application is in power generation and cogeneration. The projected capital cost is approximately $lOOO/kW. Transportation Sector MCFCs have not been considered for transportation application as yet.
A summary of the above discussion, operational characteristics and technological status of the six types of fuel cells is given in Table 1. For their acceptance into the transportation sector, fuel cells must meet the demands of rapid startup (preferably cold), fast pickup, high power density (light weight, compact), greater fuel efficiency, easy and safe handling, high lifetime and low cost (comparable to gasoline). Unfortunately, none of the above fuel cell technologies can satisfy all of these demands today. However, among the present day available technologies are:
. .
The PEMFC meets the demands of rapid startup, acceleration, high power density. But until its cost is lowered by 10-20 times, it will not be economically acceptable. PEMFC will be best for light-duty vehicle applications. The PAFC is also suitable for vehicular application, but its application is limited to buses and trucks because of its size and weight. Also, battery support is needed for acceleration since it can only produce a constant output. Finally, the AFC stands a better chance for transportation use as far as weight and startup are concerned. Its use is limited because of its inability to tolerate CO, contamination in the fuel and oxidant.
.
l3Nguyen, Q. Minh, “Ceramic Fuel Cells,” J. Am. Ceram. Soc., vol. 76, no. 3, 1993 pp. 563-588. Dollar, W.J. and Parker, W.J., “Fuel Cell Technology - Into The OS," Enerw World, vol. 199, June 1992. pp. 11-14. ”Terda, S. and Horiuchi, N., “Evaluation and Target Values for Materials Used for Advanced MCFC Stacks,” Conversion Technologies - Electrochemical Conversion in Proceedings of Ths 27Ih Intersociety Energy Conversion Engineering Conference, JECEC 92, San Diego, CA, vol. 3 1992, pp. 3.275-3.279. I6Masauki, Miyazaki et. Al., “Development of an Indirect Intemal Reforming Molten Carbonate Fuel Cell Stack”, ibid, pp. 3.287-3.292
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TABLE 1. OPERATIONAL CHARACTERISTICS AND TECHNOLOGICAL STATUS OF VARIOUS FUEL CELLS 'J'
Type of Fuel Cell
Operating, Temp("C:m
Choice of Fuel
Oxidant
Impurity
Power Density (mW/cmz) (present), projected
Projected Rated Power Level
Fuel Efficieucy
(kW)
Lifetime Projected (h)
Capital Costs Projected ($kW)
PAFC
160-200
hydrogen natural gas mcthanol
oxygen, air
CO, HIS
200,250
100-5000
40
>4Q,ooo
1000
PEMFC
80-90
hydrogen, methanol
oxygen, air
CO, H2S
350, >600
1-1000
45
>40,000
>200
SOFC
800-1000
hydrogen, natural gas, coal
oxygen, air
HS
240, 300
100-100,000
>50
r40,000
1500
AFC
60-90
pure hydrogen
pure oxygen
co2
100-200, >300
10-100
40
>10,000
>200
MCFC
660
hydrogen natural gas, coal
oxygen, air
S,H,S, HC1, HI,
100, >200
1000-100,000
50-75
>40,000
1000
DMAFC
not reported
methanol
___
not reported
40, >I00
1-100
30
>10,000
>zoo
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CURRENT TECHNOLOGY OF FUEL CELL SYSTEMS
Ali T-Raissi Arundhati Banerjee Kenneth G. Sheinkopf Florida Solar Energy Center 1679 Clearlake Road Cocoa, Florida 32922 407-638-1000 FAX 407-638-1010
ABSTRACT A great deal of research is taking place on fuel cells, which use hydrogen and oxygen as the fuel. One of the reasons for this interest is that fuel cells offer the best criteria for meeting the requirements of zem emission vehicles, and thus are expected to be the prime users of hydrogen in the near future. There are presently six different types of fuel cell technologies available - phospho:ic acid fuel cells, proton exchange membrane fuel cells, alkaline fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and direct methanol-air fuel cells. This paper looks at each of these six types and gives a brief overview of the technologies and their present state of development. The suitability of the various types toward use in the transportation sector is also studied. Because the selection of a fuel storage method is highly dependent on basic quirements of operational characteristics, the status of the fuel cell technologies is discussed with respect to their basic operating principle, acceptable contamination level, economics, and suitability toward transportation. To be used in the transportation sector, fuel cells must meet the demands of rapid startup, fast pickup, high power density, greater fuel efficiency, easy and safe handling, high lifetime and low cost. Unfortunately, none of the six types can satisfy all of these demands at this time, but each has its own advantages and benefits. This paper categorizes each type as to their strengths and weaknesses in meet:.ngthese needs.
to batteries, fuel cells are made of two electrodes with a conductive electrolyte sandwiched in between but unlike a battery, a fuel cell does not require recharging. It produces electrical energy as long as fuel is supplied to it’. The fuel for the fuel cell can be hydrogen or any other hydrogen-containing compound which on reprocessing can produce hydrogen. At the hydrogen electrode (anode), hydrogen ions(protons) and electrons are formed. Protons migrate through the electrolyte to the oxygen electrode (cathode) while electrons move through an extemal circuit to a load and then retum to the cathode. At the oxygen electrode, oxygen, hydrogen and electrons combine to form water. Thus, by forcing the electrons tfi take an extemal path, low temperature direct energy conversion is achieved*. Thermodynamically, a fuel cell converts the Gibb’s free energy change (*G) of a electrochemical reaction to electrical energy according to the following equation: aG = nFBEr where E, = reversible potential of the cell, n = no. of electrons arid F = Faraday’s constant. Considering the most commonly referred reaction of the fuel cell -- the reaction between hydrogen arid oxygen to produce water -- (H2 + 1/2 O,=H,O), *Go is 56.32 kcdmole, n is 2 and therefore E, turns out to be 1.23 volt (under
INTRODUCTION This paper looks at the six major types of fuel cells and gives their basic operating principles, acceptable contamination levels, technological status, and suitability in the transport sector. Fuel cells are e1ec:trochemical devices which directly convert chemical energy to electrical energy without combustion. Similar
‘David L., Handbook cfBatteries and Fuel Cells, McGraw Hill 1984. ’Swain, D.H. and Appleby, A.J., “Fuel Cells and Other Long Range Technology Options for Electric Vehicles: Knowledge Gaps and Development Priorities”, in The Urban Electric Vehicle, proceedings of an International Conference, Stockholm, Sweden, May 25-27, 1992, pp. 457-468, OECD Document.
1953
Authorized licensed use limited to: UNIV OF ENGINEERING AND TECHNOLOGY LAHORE. Downloaded on February 17, 2009 at 23:59 from IEEE Xplore. Restrictions apply.
standard conditions (i,e.,T=25”C,P, = Poz = 1 atm, liquid Hz03). It is this electrical energy which can be effectively used to power the motor of an electric vehicle. The operation of a fuel cell involves startup, fuel and air delivery control as a function of the load. The products are heat and water which have to be removed from the system. In this regard, the fuel cell can be referred to as an electrochemical engine for electridhybrid vehicles. In practice, the efficiencies of fuel cells range from 40% t3 65% based on the lower heating value of hydrogen. Under operational conditions, the voltage output falls due to polarization effects. To make a useful voltage for practical purposes, multiple cells are connected in electrical series and are referred to as a fuel cell stack. The fuel cell stack and its necessary auxiliaries are collectively called fuel cell systems. The major accessories include thermal management system, fuel supply and storage subsystems. There are presently six different types of fuel cell technologies: Phosphoric acid fuel cell (PAFC) 1) 2) Proton exchange membrane fuel cell (PEMFC) 3) Alkaline fuel cells (AFC) Solid Oxide Fuel cell (SOFC) 4) Molten carbonate fuel cell (MOFC) 5) Direct melhanol-air fuel cell (DMAFC) 6) The following classification is based on the type of electrolyte used in the fuel cells. Because the selection of a fuel storage method is highly dependent on basic requirements of operational characteristics, the status of fuel cell technologies is discussed with respect to their basic operating principle, acceptable contamination level, technological status and economics and suitability towards transportation sectors.
The system is extremely sensitive to CO and H,S which are commonly present in the reformed fuels. A major challenge for using reformed fuel, therefore, lies in the removal of CO to a level of <200 - 300 ppm. CO tolerance is better at temperatures >180°C. Removal of sulfur is still essential.
TechnoloPical Status The PAFC system is the most advanced fuel cell system for terrestrial applications. Its major use is in on-site integrated energy systems to provide electrical power in apartments, shopping centers, etc. These fuel cells are commercially available in the range from 24V, 250 watt portable units to 200kW on-site generators. PAFC systems of 0.5 - 1.OMW are being developed for use in stationary power plants of 1-11 MW capacity. Using natural gas as the fuel, a 200kW system is $2875/kW (1992 $). The power density of PAFC is 200 mW/cm2and the projected capital costs are $1000-$1500/kW5. (The projected power density for 36 kW brassboard PAFC fuel cell stack has been reported to be 0.12 kW/kg and 0.16 kW/L‘.) TransportationSector PAFCs are currently being considered for use in heavy duty vehicles. Their major problem for use in vehicular application is their slow start-up (since the cell has to be heated to over 200°C), high costs and excessive weight. Since PAFCs work best at a constant output, their application will be better in hybrid systems where a battery or other device meets the high-power demands of acceleration. PAFCs stand their best chance of success in heavy duty vehicles or locomotives. The 200kW unit can be used for long-range bus or truck applications while the larger megawatt capacity units are planned to be used as the power plant of a longrange locomotive unit.
PHOSPHORIC ACID FUEL CELLS (PAFCs) Basic Operating Principle The basic components of a phosphoric acid fuel cell (PAFC) are the electrodes consisting of finely dispersed Pt catalyst on carbon paper, S i c matrix holding the phosphoric acid and a bipolar graphite plate with flow channels for fuel and oxidant. The operating temperature ranges between 160-220°C and it can use either hydrogen or hydrogen produced from hydrocarbons(natura1 gas) or alcohols as the anodic reactant. In the case of hydrogen produced from a reformer with air as the anodic reactant, a temperature of 200°C and a pressure of 8 atm are required for better performance. PAFCs are advantageous from a thermal management point of view. The rejection of waste heat and product water is very efficient in this system and the waste heat at 200°C can be used efficiently for the endothermic steamreforming reaction4.
PROTON EXCHANGE MEMBRANE FUEL CELLS (PEMFC)/SP(E)FC Basic Operating Principle PEM fuel cells, also called solid polymer electrolyte fuel cells -SP(E)FC -- use a proton (hydrogen ion) conducting membrane which stays sandwiched between two platinum-catalyzed porous electrodes728. Initially, these membranes were based on polystyrene, but at present a Teflon-based product “Nafion” is
5
Snnivasan, S. et.al, “Overview of Fuel Cell Technology,” in Fuel Cell .. edited by Leo J.M.J. Blomen and Michel N. Mugenva, Plenum Press, N.Y. 1993.
System,
Acceptable Contamination Levels
%hi, C.V., Glemm, D.R. and Abens, S.G., “Developmentof a fuel cell power source for bus,” proceedings of the 2ShIntersociety Energy Conversion Engineering Conference, vol3, edited by PIA. Nelson, W.W. Schertz, and R.H. Till, American Institute of Chemical Engineers, N.Y. 1990, pp. 308-313.
’U.S. Department of Energy, Proton-Exchange Membrance Fuel Cell
Program, DE93000009, Nov. 1992.
3Appleby, A.J. and Foulkes, F.R., Fuel Cell Handbook, Van Nostrand
Reinhold, N.Y., 1989.
4U.S. Department of Energy, “Phosphoric Acid Fuel Cells,’’ DE93000003,Nov. 92.
8Gottesfeld,S . “Polymer Electrolyte Fuel Cells,” submitted to 11” International Seminar on Primary and Secondary Battery Technology and Applications, Feb. 28-March 3, 1994, Deerfield Beach, Florida (private communication).
1954
used. This offers high stability, high oxygen solubility and high mechanical strength. The cell operating temperature is quite low (80-90°C) and opepating pressures can be in the range of 1-8 atmosphere. The fuel cell requires humidified hydrogen and oxygen for its operation. The pressures, in general, are maintained equal on either side of the membrane. Operation at high pressure is necessary to attain high power densities, particularly when air is chosen as the anodic reactant.
and heat is difficult at these low temperatures. In space shuttles, closed loop hydrogen circulation as well as dielectric liquid circulation is used for heat management. Some of the terrestrial fuel cells are process air-cooled. Acceptable Contamination Level Alkaline fuel cell can operate only with pure H and pure 0,. , Even a small level (<250 ppm) of CO, is sufficien: to carbonate the electrolyte and can spoil the electrode. Several processes for cleaning of the electrode after contamination are availabl!e (Physical adsorption-Selexol process, Fluor solvent process, pressure swing adsorption) but each is expensive and none are totally effective Technolosical Status AFCs are extremely reliable arid offer very high power output in a relatively small package. Apart from space applications, this type of fuel cell has promise as a standby power source. The power density ranges from 100-200 mW/cm2, and the projected cost varies for different applications. For transportation applications, the goal is to reduce the capital cost to $50-$100/kW; for standby power, capital costs of $500/kW are reported to be acceptable. For defense and space applications, the allowance wais $5000- 15,000/kWI. Transportation Sector AFCs offer high power density and cold-start capabilities, but their application in the transportation sector is clearly dependent on the availability of on-board pure hydrogen storage and supply. The sensitivity towards CO, and high cost are the major constraints.
Acceptable Contamination Level The major contaminant of the PEMFC system is carbon monoxide. Even a trace amount of CO drastically reduces the performance levely~'oIt is best to use a fuel which is virtually free of CO for PEMFC. On the other hand, it is tolerant toward CO, contamination. Technological Stat@ PEMFCs have high power density (350 mW/cmz) and are now commercially available in power ranges (IOOW-5OOW) with the corresponding capital cost ranging from approximately $5,676$13,000. The lOkW to 200 kW power plants have been developed and demonstrated successfully but are waiting for large scale productionY.los". projected capital cost is approximately $200The $300/kW, given a 10-20 fold reduction in the membrane and catalyst cost and also considering large scale productions. Therefore, the major challenge ahead is to find a low-cost catalyst, low-cost membrane and an efficient water management option within the cell assembly. Transportation Seem Due to their high power density, rapid startup and variable power output, PEMFCs are suitable for use in the transportation sector. They are considered the best choice as far as present day available fuel cell technologiw are concerned. Their high power density and small size makes them primary candidates for light-duty vehicles, though they are also used for heavy-duty vehicles. PEMFCs are also being developed for space and underwater transportation applications. ALKALINE FUEL CELLS (AFCS) Basic Operating Principle Alkaline fuel cella represents the oldest and most widely-used fuel cell systems in the U.S. space program and have gone onboard most of the manned space missions. AFCs use potassium hydroxide as the electrolyte and hydroxyl ions are the conducting species. Because of the alkaline electrolyte, no noble metal catalyst is required '. AFCs operate at 613-80"C which is relatively low compared to
other fuel cells. Operating pressure is normally atmospheric pressure. From a system point of view, removal of product water
SOLID OXIDE FUEL CELLS (SOFCS)
Basic Operatins Principle Solid oxide fuel cells (SOFCs) are solid-state power systems and at present use ytria-stabilized zirconia as the electrolyte". The operating temperature is high, typically 1,OOO"C. SOFCs can be used as cogenerators to supply both electricity and high quality waste heat. In this cell, a conversion efficiency of more than 50470 can easily be attained. Acceptable Contamination Level Because of high temperature, the SOFCs can handle impurities in the incoming fuel better. SOFCs can operate equally well on dry or humidified hydrogen or carbon monoxide fuel or on mixture:;. The main poisoning factor for SOFC is H,S. Though the sulfur tolerance level is approximately two orders of magnitude greater than other fuel cells, the level is below 80 ppm. Technolosical Status This technology is still in the developmental stage. SOFCs are -. very attractive in electrical utility and industrial applications. The high operating temperature allows them to use H, and CO from natural gas steam reformers and coal gasification plants, a major
'Final Report, United 'Technology, Hamilton Standard For LANL W X 53.D6272-1.
"'Analytic Power Corporation, Boston, USA, private communication.
"Ballard Power Systems Inc., Canada, Technical Marketing Dept., private communication.
"Goldstein, R. "Solid Oxide Fuel Cell Development" EPRI Journal, Oct./Nov. 1992.
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advantage as far as fuel selection is c ~ n c e r n e d ’ ~ ,The present ’~. day estimated capital cost is $1500/kW but is expected to be reduced with improvements in technology. Transportation Sector Although the high power density and potential for intemal fuel processing of SOFC is very attractive from the transportation point of view, the high operating temperature and long warm-up time rules out their use in vehicular applications. MOLTEN CARBONATE FUEL CELLS (MCFCS)
DIRECT METHANOL-AIR FUEL CELLS (DMAFCS) DMAFCs are the least developed among all the fuel cell technologies. Though methanol itself has simpler storage requirements that hydrogen lacks and is simpler to make and transport, its electrochemical activity is one-third that of hydrogen. Also, the conversion takes place at lower temperature (200°C) compared to other hydrocarbons and therefore the contaminant problem is higher than other fuel cells. The technology is underdeveloped for present consideration. SUMMARY
Basic Operatinq Principle In the molten carbonate fuel cell, a molten alkali carbonate mixture, retained in a porous lithium aluminate matrix, is used as the electrolyte. The conducting species is carbonate ions’j. The operating temperature is in the range of 600 - 800”C, high enough to produce suitable qualities of waste heat. This waste heat can be used for fuel processing and cogeneration, a bottoming cycle, or intemal reforming of methane ’. MCFCs normally have 75% fuel (hydrogen) utilization. The advanced form of MCFC referred to as internal reforming molten carbonate fuel cell (IRMCFC) may consume lower hydrocarbons (CH,) directlyi6. It is intrinsically efficient since the heat produced at the anode is used for reformation of hydrocarbons. Normally their efficiencies are 50% or higher. Acceptable Contamination Level MCFCs do not suffer from CO poisoning and, in fact, can use CO in the anode gas as the fuel. They are extremely sensitive (1 ppm) to the presence of sulfur in the reformed fuel or oxidant gas stream. The presence of HCl, HF, HBr, etc. causes corrosion, while trace metals can spoil the electrodes. The presence of particulates of coayfine ash in the reformed fuel can clog the gas passages. Technoloqical Status MCFC technologies are still under development. Their best application is in power generation and cogeneration. The projected capital cost is approximately $lOOO/kW. Transportation Sector MCFCs have not been considered for transportation application as yet.
A summary of the above discussion, operational characteristics and technological status of the six types of fuel cells is given in Table 1. For their acceptance into the transportation sector, fuel cells must meet the demands of rapid startup (preferably cold), fast pickup, high power density (light weight, compact), greater fuel efficiency, easy and safe handling, high lifetime and low cost (comparable to gasoline). Unfortunately, none of the above fuel cell technologies can satisfy all of these demands today. However, among the present day available technologies are:
. .
The PEMFC meets the demands of rapid startup, acceleration, high power density. But until its cost is lowered by 10-20 times, it will not be economically acceptable. PEMFC will be best for light-duty vehicle applications. The PAFC is also suitable for vehicular application, but its application is limited to buses and trucks because of its size and weight. Also, battery support is needed for acceleration since it can only produce a constant output. Finally, the AFC stands a better chance for transportation use as far as weight and startup are concerned. Its use is limited because of its inability to tolerate CO, contamination in the fuel and oxidant.
.
l3Nguyen, Q. Minh, “Ceramic Fuel Cells,” J. Am. Ceram. Soc., vol. 76, no. 3, 1993 pp. 563-588. Dollar, W.J. and Parker, W.J., “Fuel Cell Technology - Into The OS," Enerw World, vol. 199, June 1992. pp. 11-14. ”Terda, S. and Horiuchi, N., “Evaluation and Target Values for Materials Used for Advanced MCFC Stacks,” Conversion Technologies - Electrochemical Conversion in Proceedings of Ths 27Ih Intersociety Energy Conversion Engineering Conference, JECEC 92, San Diego, CA, vol. 3 1992, pp. 3.275-3.279. I6Masauki, Miyazaki et. Al., “Development of an Indirect Intemal Reforming Molten Carbonate Fuel Cell Stack”, ibid, pp. 3.287-3.292
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TABLE 1. OPERATIONAL CHARACTERISTICS AND TECHNOLOGICAL STATUS OF VARIOUS FUEL CELLS 'J'
Type of Fuel Cell
Operating, Temp("C:m
Choice of Fuel
Oxidant
Impurity
Power Density (mW/cmz) (present), projected
Projected Rated Power Level
Fuel Efficieucy
(kW)
Lifetime Projected (h)
Capital Costs Projected ($kW)
PAFC
160-200
hydrogen natural gas mcthanol
oxygen, air
CO, HIS
200,250
100-5000
40
>4Q,ooo
1000
PEMFC
80-90
hydrogen, methanol
oxygen, air
CO, H2S
350, >600
1-1000
45
>40,000
>200
SOFC
800-1000
hydrogen, natural gas, coal
oxygen, air
HS
240, 300
100-100,000
>50
r40,000
1500
AFC
60-90
pure hydrogen
pure oxygen
co2
100-200, >300
10-100
40
>10,000
>200
MCFC
660
hydrogen natural gas, coal
oxygen, air
S,H,S, HC1, HI,
100, >200
1000-100,000
50-75
>40,000
1000
DMAFC
not reported
methanol
___
not reported
40, >I00
1-100
30
>10,000
>zoo
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