The Development of Fuel Cell
Technology for Electric Power
Generation: From NASA’s
Manned Space Program to
the
BHydrogen Economy[
High-efficiency fuel cells may be suitable for hydrogen-fueled electric power plants and
transportation engines, but further development of this technology is needed.
By John H. Scott
ABSTRACT
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This paper discusses the place of fuel cells among the various energy conversion technologies relevant to the hydrogen economy and the fossil fuel economy. Also summa-rized are the fundamental principles of fuel cell thermody-namics and of fuel cell power plant engineering. Further discussed are the differing requirements placed on power generation systems by spacecraft and terrestrial applications and how those requirements have directed investment in the development of fuel cell technology over the past half century.KEYWORDS
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Batteries; energy conversion; energy storage; fuel cellsI .
I N T R O D U C T I O N
This paper first reviews where fuel cells fit among the various technologies for converting chemical energy to electrical energy and how fuel cell technology is important to the concept of the Bhydrogen economy.[ The basic thermodynamics and electrochemistry of fuel cells are then explained, and the practical engineering challenges and limitations inherent in various types of fuel cell power plants for both stationary and transpor-tation power generation applications are reviewed.
Finally, the rationale by which the unique requirements placed on electric power systems by the manned space program led NASA to fund the development of the first practical fuel cell power plants and to continue their development today in a certain direction is discussed. With this is contrasted how the requirements placed on commercial power generation plants by the recentBgreen power[ and Bhydrogen economy[ initiatives have accel-erated investment in fuel cell technology but driven its development in a direction quite different from that of NASA’s manned space program.
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E N E R G Y C O N V E R S I O N A N D
H Y D R O G E N E C O N O M Y
The world economy’s energy needs are met primarily by the conversion of the energy stored in the chemical bonds of substances found on Earth to a form of energy that is economically useful (e.g., electricity or mechanical motion). This energy conversion fundamentally cannot take place without release of reaction products into the atmosphere that may potentially have a deleterious effect on the environment. Key to an understanding of how chemical energy conversion technology can affect the impact of the world energy economy on the environment is an understanding of the basic principles of such energy conversion. There are fundamentally two means of converting chemical bond energy into an economically useful form of energy: heat engine conversion and direct conversion (See Fig. 1).
Manuscript received May 11, 2005; revised October 28, 2005. This work was supported by the National Aeronautics and Space Administration.
The author is with the Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058 USA (e-mail: [email protected]). Digital Object Identifier: 10.1109/JPROC.2006.883702
A. Heat Engine Conversion
Heat engines convert chemical energy to electrical or mechanical energy with the intermediate steps of releasing chemical bond energy as heat and then using that heat to either drive a mechanical system (that may in turn drive an electric generator) or to directly drive an electric current. The laws of thermodynamics limit the efficiency of heat engine conversion to that defined by the Carnot cycle, in which the idealized maximum efficiency is determined by the difference between the temperature at which heat is added to the system (theBtopping[ temperature) and the temperature at which waste heat is rejected from the
system (theBbottoming[ temperature). It has rarely proven practical to build engines with isothermal processes at the topping and bottoming temperatures, thus the actual efficiencies obtainable with practical heat engines are much lower than that of an ideal Carnot cycle operating between the same topping and bottoming temperatures.
The most common heat engines in the world economy are based on fluid cycles. In such devices, a working fluid is mechanically compressed, heated, expanded through a device that drives a generator, and then cooled to its original state. The Carnot cycle was originally developed as a model of this idealized fluid cycle heat engine. Fluid cycle heat engines currently in common use include: 1) the Diesel/Otto cycle (on which the automotive internal combustion engine operates); 2) the Brayton cycle (on which jet engines and gas turbines operate); 3) the Rankine cycle (on which reciprocating steam engines and steam turbines operate); and 4) the Stirling cycle (currently used in small high-efficiency refrigerators).
Other energy conversion systems for which efficiency can be modeled using Carnot theory include thermo-electrics and photovoltaics. Thermoelectric conversion is a relatively low efficiency system that operates, with no moving parts, on the Seebeck effect (as do thermocouples). It is often applied in conjunction with nuclear heat sources in spacecraft on missions to deep space. Photovoltaic con-version, also a relatively low efficiency system, is the basis for solar electricity generation. Though solar cells are not
Fig. 1.Chemical-to-electric energy conversion. Heat engine conversion and direct conversion have fundamentally different ideal limits on thermodynamic efficiencyðthÞ.
strictly heat engines, as they convert radiated electromag-netic energy rather than heat into electricity, their effi-ciency is sometimes modeled using Carnot theory. B. Direct Conversion
Direct conversion systems convert chemical energy directly to electrical energy without the release of heat as an intermediate step. Such conversion systems do not follow Carnot theory. The efficiency of a direct conver-sion system is thus not dependent upon a large difference between a topping and bottoming temperature. The effi-ciency of a direct conversion system is limited rather by the Gibbs Free Energy change ðGÞ associated with the electrochemical reactions taking place. In all direct con-version systems, the electrochemistry of the active ma-terials sets up a galvanic couple between two electrodes (the anode and the cathode) that are separated by an electrolyte. A fuel is oxidized at the anode and an oxidant is reduced at the cathode, releasing ions to flow through the electrolyte between them. This flow of ions sets up an electric potential difference between the electrodes, which can be used to drive a useful electric current. As detailed in Section III, the Gibbs Free Energy release in the oxidation and reduction reactions drives this electric potential and, thus, drives the efficiency of a direct conversion system.
Direct conversion systems can be categorized into batteries and fuel cells (Fig. 1). Batteries are the most common and widely used direct conversion system. In a battery, the electrodes are themselves consumed by the oxidation/reduction reactions underway. The electrodes are consumed irreversibly in what is known as a primary battery, but, in rechargeable (or Bsecondary[) batteries, the electrode materials are regenerated during recharging. Thus, the total energy available from a primary battery, or from a secondary battery between recharges, is limited by the amount of electrode material contained in the device. Myriad combinations of electrode materials (fuels and oxidants) can be reacted in a battery.
By contrast, the fuel cell is a direct conversion device in which neither of the electrodes is consumed. The elect-rodes act as screens to which the reacting chemicals can be continuously fed and which provide sites where catalyzed reactions can take place between the fuel, oxidant, and ions being transported through the electrolyte. Thus, the total energy available from a fuel cell system is, like in most heat engines, limited by the amount of reactant storage available. Also, as detailed in Section III, the fuel and oxidant combinations that can be reacted in a fuel cell are limited primarily to that of hydrogen and oxygen.
For completeness, it should be noted that another direct conversion device that can be characterized as either a fuel cell or a battery has an anode that is itself oxidized and a cathode that is a screen to bring an oxidant into contact with the electrolyte. As the anode is usually a metal and the oxidant is often the oxygen from the atmosphere,
such devices are known as Bmetal-air[ batteries or Bcartridge[ fuel cells. Such a device might take the form of a cartridge that is periodically replaced, with the energy available limited by the amount of metal fuel in the cartridge, thus resembling a primary battery.
C. Energy Conversion and Energy Economies The vast bulk of the electric grid and transportation energy consumed in the world economy is produced via conversion of the energy contained in hydrocarbon chemical bonds. This is done almost entirely by oxidizing hydrocarbon fuels to release heat, which is converted to electricity (or to useful mechanical energy) by means of some type of heat engine. As these hydrocarbon fuels consist of variants of coal, oil, and natural gas, the energy economy extant is known as the Bfossil fuel economy[ or theBhydrocarbon economy[ and is thought by many envi-ronmental scientists to be the source of mankind’s most destructive impact on the Earth. Oxidation of such fuels inevitably produces carbon dioxide, which, barring the application of sequestration technology long thought uneconomical, is released into the atmosphere. The accu-mulation of such Bgreenhouse[ gases in the atmosphere, which has been accelerating since the start of the Industrial Revolution in the 18th century, is thought by many researchers to be causing an inexorable increase in the temperature of the atmosphere with potentially disas-trous environmental consequences. Also, the inefficiencies of any practical method of oxidizing hydrocarbons result in the release of elemental carbon (e.g., Bsoot[) into the atmosphere. In yet another impact, when the atmosphere is used to supply oxygen to reactions at the high topping temperatures at which heat engine conversion must be conducted (e.g., the flame temperature of burning natural gas: > 1500C) in order to be reasonably efficient, oxides of nitrogen ðNOXÞ, the predominant chemical precursor to air pollution, are produced as well.
The concept of theBhydrogen economy[ is born from the idea that release of energy via the oxidation of hydrogen, which produces only water at the point of conversion, can provide for the world economy’s energy needs without the environmental impacts associated with the Bfossil fuel economy.[ The key limitation on this concept stems from the fact that hydrogen cannot be mined nor is it efficiently separable from the atmosphere. As detailed in Section IV, production of hydrogen requires either the electrolysis of water, which could be powered by electricity sources (e.g., hydropower, solar, wind, nuclear) not themselves dependent upon fossil fuels but is a rather inefficient use of that electricity, or the reforming of fossil fuels, which would release carbon dioxide just as would oxidation of those fuels for heat engine conversion (albeit at a lower rate due to the higher oxidation efficiencies obtainable from most reforming processes). Another consideration is that, while no elemental carbon would be released, oxidation of hydrogen in heat engine
conversion would have to take place at topping tempera-tures high enough to produce oxides of nitrogen just as in fossil fuel engines, though often at a lower rate. Key to overcoming this limitation is the fact that, as detailed in Section III, hydrogen can be oxidized at a low temperature (as low as 80C) via direct conversion in a fuel cell. An energy conversion system based on hydrogen and fuel cells would have the advantage not of only eliminating the emissions of key air pollutants at the point of conversion (a major consideration in areas such as the Los Angeles basin) but also of decreasing the emission of greenhouse gases to the extent hydrogen can be produced from electrolysis via non-fossil fuel electricity generation or from high efficiency reforming processes. Thus, the development of economical fuel cell technology is core to the realization of the environmental advantages of a Bhydrogen economy.[
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F U E L C E L L E L E C T R O C H E M I S T R Y
In the basic fuel cell (Fig. 2), the electrodes provide a means of contact between reactants that are continuously supplied to the electrolyte. The fuel (hydrogen) is supplied to a catalyst-coated anode, and the oxidant (oxygen, either pure or atmospheric) is supplied to a catalyst-coated cathode. (It should be noted that other fuels, notably carbon monoxide, methane, and methanol, can theoreti-cally be oxidized in some fuel cells, but the hydrogen oxidation reaction generally predominates. As discussed in Section II, this is the primary reason for the importance of fuel cells in theBhydrogen economy[). The catalyst at the electrodes enables relatively low temperature oxidation of hydrogen and reduction of oxygen which releases ions to flow through the electrolyte and produce water at one of
the electrodes; which one depends on the chemistry of the electrolyte. The operating temperature range of a fuel cell is determined by the range in which the catalysts are effective and in which the electrolyte is mechanically durable and conductive of ions. The passage of these ions sets up an electric potential difference between the electrodes which can drive a current through a load, thus completing the circuit.
As with any direct conversion device, the amount of electric energy produced by a fuel cell is, in idealized terms, a function of the Gibbs free energy released by the reaction taking place
G ¼ H TS: (1)
This relationship can be used to define the theoretical upper limit of a fuel cell’s efficiency, and algebra with thermodynamic relations reveals that this theoretical maximum efficiency is directly proportional to the electric potential setup by the catalyzed reaction
th G nF E=H ¼ 1 ðTS=HÞ: (2)
For a hydrogen/oxygen fuel cell with reactants at standard conditions (of 1 atm and 25 C), the ideal potential is 1.23 V. Further calculus with the Gibbs free energy equation and other fundamental relations of thermodynamics reveals that this ideal potential actually drops with increasing reactant temperature (3) and rises with increasing reactant pressure (4)
ð@Eo=@TÞP¼ S=nF (3) ð@Eo=@PÞT ¼ V=nF : (4) Integrating the Gibbs free energy equation with boundary conditions relevant to a hydrogen/oxygen fuel cell results in the fuel cell Nernst equation (named for a 19th century electrochemist)
E ¼ Eoþ ðRT=2F Þln½PH2=PH2O
þ ðRT=2F Þln ðPO2Þ1=2
h i
: (5)
The Nernst equation yields the theoretical limit of cell voltage and thus the highest efficiency that laws of thermodynamics will allow for a fuel cell with a given reactant pressure and temperature. References [1] and [2] provide more rigorous derivations of these equations from thermodynamic principles.
Of course, the thermodynamically ideal efficiency cannot be achieved in a practical fuel cell, as the cell voltage achievable is reduced by the losses affecting the
Fig. 2.Individual fuel cell. Each fuel cell creates electric potential across its electrodes that is a function of pressure and temperature of reactants fed and of losses inherent in the electrode/electrolyte combination selected. Individual cells are wired in series and parallel to provide the voltage and current required of the power plant output.
performance of any direct conversion device. These losses increase (and the cell voltage thus decreases) as current density (current flowing per unit cell area) increases. Dif-ferent loss mechanisms predominate in difDif-ferent regimes of current density (Fig. 3). At lower current densities, cell voltage is decreased by the rate limiting effects of oxidation/reduction kinetics, which are known as Bactivation[ losses. As current density increases, ohmic resistance to the flow of ions through the electrolyte begins to drive down cell voltage due to what are known as Bohmic[ losses. Then, at the highest current densities, fluid flow resistance will lead to starvation at reaction sites, thus forcing cell voltage down even more rapidly due to Bconcentration[ losses. It is worth noting that these losses all tend to become less pronounced, and achievable cell voltage thus increases, as operating temperature increases. Most of the engineering research in fuel cell technologies, particularly in the realm of nanoengineering, is focused on finding ways to mitigate the effect of these losses and produce fuel cells that approach idealized efficiency limits over a useful operating life. Nanotechnology is yielding dividends in the fabrication of durable electrodes that enable very high contact area per unit and thus offer decreased effective current density, as well as in the devel-opment of electrode and electrolyte materials with low ohmic resistance.
The combination of thermodynamic limitations and the effects of losses results in a general principle for fuel cells: Cell voltage, and thus efficiency, decreases with increasing current density but generally tends to increase with operating temperature and pressure. One implication of this principle is that designers of automotive fuel cells, for whom volume and weight are just as or more important than fuel effi-ciency, will tend to design compact fuel cells that operate at high current density, while designers of stationary power plants may choose larger, heavier, but lower current density fuel cells that offer higher fuel efficiency. Another implication of this principle is that fuel cell efficiency increases as power demand falls off from the maximum
power output (i.e., from the design current density) of the cell. This is in contrast to heat engines involving rotating or reciprocating machinery, in which case efficiency markedly decreases as power demand falls off from the level associated with the optimal design speed of the engine. (This is one reason why automobile internal combustion power trains have multispeed transmissions.) However, as the compressors and pumps generally used to feed reactants to the fuel cell (see Section IV) themselves fall off in efficiency if demand significantly varies from their design speed, the net efficiency of a complete fuel cell power plant often remains nearly constant as power demand increases up to the design-rated value.
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F U E L C E L L P O W E R P L A N T
E N G I N E E R I N G
The basic components of a complete fuel cell electric power plant are identified in Fig. 4. Individual fuel cells are connected in series and parallel to yield what is known as a fuel cellBstack[ that produces the current and voltage required. The other components of the power plant, known as theBbalance of plant,[ function to feed reactants to the stack, to remove water, waste heat, and other reac-tion products from the stack, and to condireac-tion the electric power output. All of these components have inefficiencies resulting in more waste heat and require net energy input that acts as a parasitic loss on the net fuel cell power output. As the output of a fuel cell is dc electricity, it must be passed through an inverter to yield the ac electricity used in most power grids. Inefficiencies in this inverter also produce waste heat that must be removed. Oxygen is fed to the stack from either the atmosphere or a reservoir
Fig. 4.Fuel cell power plant components. Complete fuel cell power plant system includes equipment for hydrogen production, reactant and water management, and electric power conditioning.
Fig. 3.Polarization losses. Degradation of cell voltage from thermodynamic ideal worsens through different regimes as current density (in Amperes per centimeter2) increases.
of pure oxygen with either high storage pressure or a compressor (with its own parasitic power requirements) driving the flow.
The more challenging engineering problem is the effi-cient provision of hydrogen. Hydrogen cannot be mined nor is it readily available from the atmosphere. It must be provided from one of two processes: 1) electrolysis or 2) reforming. Depending on the application, the compo-nents for these processes may or may not be integrated in the plant with the fuel cell stack.
A. Electrolysis
An electrolyzer is basically a fuel cell driven in reverse, with water (liquid or vapor) fed to catalyst-coated electrodes and dc electricity driving ions through an elect-rolyte to produce pure hydrogen and oxygen. An electro-lyzer and a fuel cell can be combined into what is known as a regenerative fuel cell system, which in certain applica-tions can offer an energy storage system with weight advantages over batteries. It is through electrolysis that nonfossil-fuel electricity sources, such as hydro, solar, and nuclear, can be used to produce hydrogen as a trans-portation fuel with no emission of carbon into the atmo-sphere. However, electrolysis can itself be a relatively inefficient (roughly 50%–70%, give or take) use of elec-tricity, and (when compared to natural gas) hydrogen is relatively expensive to compress and pipe over long dis-tances. Thus, electrolysis units are rarely found useful as part of a fuel cell-fed electric utility grid.
B. Reforming
The more economically practical and energy efficient means of hydrogen provision is from hydrocarbon fuels via a process known asBreforming.[ This is the process by which the aerospace industry currently makes hydrogen for rocket propellant and by which German industry made hydrogen to buoy Zeppelins during the First World War. In a fuel cell power plant system, this process takes place in a unit known as the Bfuel processor[ or Bre-former.[ A reformer cracks hydrogen from a hydrocarbon fuel and produces a gas known as Breformate,[ rich in hydrogen but also containing carbon dioxide ðCO2Þ and carbon monoxide (CO). Processes to accomplish this include Bsteam reforming[ (6), an endothermic process in which the vaporized hydrocarbon and steam react in the presence of a nickel catalyst, andBpartial oxidation[ (7), an exothermic process in which the hydrocarbon is oxidized without combustion in the presence of a platinum catalyst
e.g., for methane: CH4þ H20 , CO þ 3H2 (6) e.g., for methane: CH4þ
1
2O2, CO þ 2H2: (7) These two processes are often combined into a process known as Bautothermal reforming,[ which operates at
temperatures ranging from 600 C to 900 C and is endothermic, thus requiring some burning of the hydro-carbon fuel to provide the net energy input.
The reformate produced at this step in these pro-cesses is rich in CO. While CO2 is generally inert in a fuel cell, CO is a poison to most fuel cell catalysts. Dif-ferent catalyst/electrolyte combinations have varying degrees of tolerance to it. CO cleanup can be accom-plished in a fuel processor by cooling the reformate to encourage theBwater gas shift[ reaction (8), in which CO and water exothermically combine to produce CO2 and hydrogen, and then warming it backup to encourage Bpreferential oxidation[ (9) of CO into CO2on a platinum catalyst bed
CO þ H2O , CO2þ H2 (8) CO þ1
2O2, CO2: (9)
Also, virtually no fuel cell catalyst can tolerate sulfur in the fuel stream. Thus, the fuel processor must remove the sulfur content common to fossil fuels. This is normally done by reacting the vaporized hydrocarbon with hydrogen on a zinc catalyst bed near the input of the fuel processor. Note that the need to desulfurize crude oil via the same catalyzed reaction has resulted in an industrial demand for hydrogen that has, in turn, motivated the construction of hydrogen pipeline networks to service clusters of refiner-ies and petrochemical plants.
There are two paramount engineering limitations relevant to reformer design. First, as it is a relatively high-temperature energy-consuming unit, there can be significant warm up delays on startup (on the order of minutes at least). Also, for the same reasons, sudden step changes in demand for reformate can force incomplete reactions in the unit, leading to deposition of elemental carbon. This phenomenon, known as Bcoking,[ can clog the catalyst beds and significantly degrade the fuel pro-cessor’s performance. Second, the size, number of reaction stages, and parasitic energy needs of the fuel processor are directly related to the purity of hydrogen required in the reformate and to the heaviness and cleanliness of the hydrocarbon fuel. Thus, a fuel processor that is only required to convert a pure, light hydrocarbon to a rela-tively carbon monoxide-rich (orBdirty[) reformate can be small, low temperature, and integrated into the fuel cell stack; whereas, a reformer which must convert heavy, dirty marine fuel oil into relatively clean reformate must run very hot and is often ten times the size of the fuel cell stack it feeds. It is worth noting here that some hydrocarbons, most notably methanol, can be directly oxidized in certain types of fuel cells without the reforming step, albeit at relatively low efficiency (often less than 20% at current densities of interest). Along with theBcartridge[ fuel cells
and metal-air batteries mentioned in Section II-B, such Bdirect methanol fuel cells[ (DMFC) are being developed in competition with lithium-ion batteries for powering small electronic devices.
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S T A T E - O F - T H E - A R T
Fig. 5 summarizes how the capabilities and limitations discussed above play out in the performance of selected, practical, state-of-the-art fuel cell power plants. Fuel cells are characterized primarily by the electrolyte used. There are other types of fuel cells beyond those described in Fig. 5 (notably the molten carbonate and phosphoric acid chemistries which are marketed to specialized commercial applications), but these three can be used to characterize the range of capabilities and limitations. Note that the thermodynamic efficiency ranges identified in Fig. 5 represent an assessment of the state-of-the-art extant and allow for all of the losses between the chemical energy in the refined hydrocarbon fuel and output to an ac power bus. The Bwell to wheels[ efficiency often quoted in reference to automobile fuel cell applications includes losses associated with refining fossil fuels and with the vehicle power train and is thus usually 10 to 20 percen-tage points lower than the figures quoted here.
A. Alkaline Fuel Cells
The alkaline fuel cell was the first practical fuel cell power plant. It was developed by NASA for the Apollo Program and is used as the Space Shuttle’s energy source today. In the alkaline fuel cell, ions flow between the
electrodes through an aqueous solution of potassium hydroxide suspended in an asbestos matrix. The catalyst is generally either relatively inexpensive nickel or the expensive noble metal platinum (selection dependant upon the importance of efficiency and durability in the fuel cell application). It is important to note that the chemical nature of the electrolyte limits alkaline fuel cells to reactants of reagent purity. CO, and even CO2 (which is considered inert in most fuel cells), in the fuel or oxidant stream will react with the electrolyte and permanently degrade its ionic conductivity. Thus, even the trace amounts of CO2 in the atmosphere must be filtered out of air supplied to the cathode. Also, the relatively low operating temperature, which is attractive in many applications and allows the plant to respond rapidly to step changes in power demand, can allow the production of liquid water at the anode. If not properly managed by the balance of the plant, this two-phase regime can result in either dry-out or flooding of the electrolyte, either of which destroys the fuel cell. The primary limitation on the operating life of these plants is the fact that the electrolyte is highly corrosive and will eventually leach out and degrade the stack materials. NASA recently completed a program to double the operating life of the Space Shuttle’s alkaline fuel cells. This was accomplished primarily by redesigning the seals on the separator plates to impose a longer leak path for the electrolyte. Finally, note that the relatively low thermodynamic efficiency quoted in Fig. 5 is due to the fact that an alkaline cell can only operate for significant periods with reactants of reagent purity, nor-mally available only from electrolysis fed by a power grid. Thus, alkaline fuel cells have proven tobe of very limited use in commercial applications.
aEfficiency figure assumes hydrogen and oxygen of reagent purity produced from natural gas-fired, combined cycle electricity generation (th¼ 50%) feeding a large, stationary
electrolyzer (th¼ 70%).
Fig. 5.Representative fuel cell power plant performance. Selection of electrode and electrolyte chemistry drives capabilities and limitations of practical fuel cell power plant.
B. Proton Exchange Membrane Fuel Cells
The proton exchange membrane (PEM) fuel cell is currently being commercialized and is the focus of a great deal of engineering study, especially for applications in the automobile industry. In PEM fuel cells, the electrolyte is a sheet of engineered polymer plastic that conducts ions when hydrated and warmed to the proper temperature. The electrodes, with platinum catalyst, are generally deposited on either side of the membrane and formed into a membrane electrode assembly (MEA). Separator plates with seals and reactant flow channels machined on each side separate the individual MEAs when assembled into the stack. The capability of PEM fuel cell technology that renders it attractive to a broad range of commercial applications, particularly automobiles, is the ability of the electrolyte to work withBclean reformate.[ CO2 is effec-tively inert in the cell and low concentrations (maybe ten parts per million) of CO can be tolerated. Higher concentrations of CO can poison (albeit reversibly) the electrode catalyst, so considerable shifting and preferential oxidation of the reformate is required in the fuel processor. While the low temperature PEM fuel cell can respond very rapidly to step changes in load, a high-temperature reformer feeding it cannot without significant impacts to its life and efficiency. Thus, most developers of automobile applications assume that clean reformate will be produced at a constant rate and stored at a fueling station, from which the vehicle, which needs rapid power plant re-sponse, can draw and store hydrogen. The BHydrogen Highway[ project under development in California is based on this concept.
There are two other notable limitations associated with PEM technology. First, at PEM operating temperatures, liquid water is produced at the cathode. The humidity of the reactant gases must thus be carefully managed in order to avoid dry-out or flooding of the cell, neither of which destroys the cell but either of which significantly impacts efficiency and shortens operating life. Cost-effective mass production of PEM stacks with the tightly toleranced flow channels necessary to manage water is proving to be a significant challenge, particularly for engineers developing automobile applications. Second, it is important to note that the end-to-end fuel efficiency of a hydrocarbon-fueled PEM system is generally no better than that of a state-of-the-art diesel generating plant. Thus, while broad appli-cation of PEM technology in the economy will result in a notable reduction in the emission of oxides of nitrogen (the component of air pollution produced by the flame temperatures of internal combustion engines) and of carbon particulates (e.g., Bsoot[), emission of carbon-based Bgreenhouse gases[ would only be reduced to the extent that renewable energy (hydro, solar, nuclear) is used to produce hydrogen by electrolysis, itself a relatively energy-inefficient process. A solution to both of these problems that is currently under significant study is known asBhigh temperature[ PEM. Under this concept, advanced
chemistry and nanotechnology techniques are used to create polymer membranes that can both conduct ions and remain durable at temperatures above 100 C, thus offering both the reduced polarization losses (and, therefore, increased efficiency) usually found in higher temperature devices and the elimination of the problem of managing liquid water in the fuel cell stack.
C. Solid Oxide Fuel Cells
The solid oxide fuel cell is the focus of much interest oriented toward applications in auxiliary power and distributed power generation. In a solid oxide fuel cell, the electrolyte is a metal oxide (often zirconia and yttria) ceramic that generally requires temperatures above 800C to become conductive of ions. The electrodes are porous cerments impregnated with non-noble metal catalysts that also require high temperatures to become active. It is important to note that such catalysts (commonly nickel in a yttrium oxide/zirconium oxide cerment at the anode and lanthanum strontium manganite at the cathode) are much less expensive than the platinum used in lower temper-ature fuel cells. Individual electrolyte/electrode cells are separated by metal or ceramic interconnects.
The catalysts involved are not poisoned by CO and even allow the direct oxidation of carbon monoxide at the anode, which means that the cell can run on Bdirty[ re-formate. This eliminates the need for the CO cleanup stages in a fuel processor. However, oxidation of hydrogen in the reformate favors the water gas shift reaction in the fuel cell itself, so the hydrogen oxidation reaction still predominates. Also, as the operating temperatures of the fuel cells are near that of autothermal reforming, the fuel processor can often be easily packaged with the stack, improving the plant’s power output per unit mass (or Bspecific power[). This capability, along with fuel effi-ciencies notably higher than those available from other fuel cells and practical heat engines, makes the solid oxide fuel cell an attractive focus of industrial research and development.
Challenges facing developers of solid oxide fuel cells are centered on the relatively slow startup and response to step changes in demand inherent in a high temperature device and on the associated limitations in cycle life. The interconnects, electrodes, and electrolyte are all materials with different heat capacities and thermal expansion coefficients. Thus, the repeated large temperature swings associated with startup and load following will eventually result in cracking of the cell assemblies. One solution to this problem involves arranging the cells in a concentric tubular configuration, rather than the planer configuration found in most fuel cells. However, such configurations significantly detract from the volumetric efficiencies (kilowatts per unit volume) that help make fuel cells attractive in the first place. In an attempt to solve these problems, notable advanced research in this field is focused on the development of Bthin film[ solid oxide
fuel cells. Nanoengineering techniques are being used to develop ceramic electrolytes that conduct ions at lower temperatures (possibly 500 C) with ohmic polarization losses still low enough to yield attractive cell voltage and thus attractive fuel efficiency.
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S Y S T E M S E N G I N E E R I N G
With these results established, the means by which the differing requirements of commercial and spacecraft power systems have led fuel cell technology to develop as it has can be understood.
For commercial and most military power systems, there are three paramount figures of merit on which a design is optimized: 1) life cycle cost (dollars per kilo-watt) for both manufacture and fuel; 2) specific power (kilowatts per kilogram), which primarily expresses the cost of transporting the plant; and 3) emissionsVthe de-gree to which the plant offers reduced emissions of air pollutants, greenhouse gasses, and (particularly impor-tant for the U.S. military’s BSilent Watch[ concept) noise. All of these considerations must be traded within the constraint of safe operability in the field by relatively untrained personnel.
Tradeoffs among these figures of merit lead systems engineers in commercial industry to select different types of fuel cells for different applications.
A power plant with tolerance for clean reformate and relatively long life under quick startup and highly varying power demand would likely yield the lowest life cycle cost for an automotive application, even at the sacrifice of a large improvement in fuel efficiency over the internal com-bustion engines currently in use. In contrast, engineers developing systems for high-efficiency low-emission distrib-uted power generation or auxiliary power would choose a fuel cell system with the highest possible efficiency and, as such applications tend to be used to provide a steady power output over relatively long periods, might be willing to sacrifice quick startup and load following ability.
For spacecraft power systems, design priorities are simpler. With launch costs to low earth orbit hovering at $10 000 per pound, nothing but specific energy (kilowatt hours per kilogram) really matters. Engineers are thus led to the highest efficiency fuel cell possible that can provide sufficient operating life under the quick-start and highly varying demand conditions in a spacecraft. However, the requirement to maximize specific energy must be met within a constraint of maximum mission reliability. As stochastic risk assessment models rarely prove useful with the extremely small historical operating hours on many spacecraft components, manned spacecraft designers generally meet a requirement that all systems be two-fault tolerant to catastrophic failure. This generally means that the power system must have at least three complete, independent strings (from generation to load interface).
This is why, despite the critical need to minimize weight, the Space Shuttle has three fuel cell power plants when it could (with all but the most critical avionics and heaters powered down) limp home on one.
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T E C H N O L O G Y D E V E L O P M E N T
R O A D M A P S
Primarily because of their production expense (stemming from both the cost of catalyst and the precision machining required) fuel cells did not compete well with other energy conversion methods from the time of their invention in the late 19th century through the first half of the 20th century. The first energy conversion problems for which fuel cells were the optimal solution came about in the 1960s from NASA’s manned space program. When examining the specific energy capabilities of various energy conversion systems, fuel cells looked to offer the minimum mass solution for the power levels and mission duration requirements of manned spacecraft from Gemini onward. NASA has since followed a well-defined roadmap in its development of fuel cell technology.
NASA’s assessment of the requirements for and constraints on spacecraft power systems not only pointed to fuel cells as the mass optimal solution for the Gemini and Apollo spacecraft and, later, for the Space Shuttle, but also identified certain requirements that are unique for spacecraft fuel cells. Given the need to minimize total mass, spacecraft fuel cell cathodes are fed by pure oxygen, rather than by mixtures diluted with inerts like nitrogen. In the quantities needed for manned spacecraft missions, such pure reactants, both hydrogen and oxygen, can be made available for relatively insignificant cost. Pure reactants do increase the achievable efficiency of fuel cells, but, in the case of pure oxygen, also present a more corrosive and hazardous environment for the power plant. One other key requirement unique to spacecraft fuel cell power plants is that, as they are not just one of many generators feeding a large grid, they must be able to both endure and support the power bus voltage under repeated large swings in power demand. The Space Shuttle’s fuel cells are required to return to required bus voltage within less than a quarter of a second after a 12-to-1 load swing. A third requirement unique to spacecraft fuel cells is the need to operate over a wide range (0 to 4 g) of gravi-tation. This makes the problem of two-phase flow man-agement in low temperature fuel cells even more challenging than in terrestrial applications.
For the Space Shuttle, the availability of pure reactants made alkaline fuel cell technology the most fuel efficient and, therefore, the highest specific energy power system option. These plants have operated with extremely high reliability for the life of the Shuttle program. Their relatively short operating life (2500 h) influenced NASA to modify the design during the last few years to improve its corrosion resistance and double the operating life.
However, with the retirement of the Space Shuttle imminent as of this writing, this upgrade is unlikely to be implemented in the fleet. Alkaline technology would still likely yield the highest specific energy and therefore optimal solution for future spacecraft where pure reactants are available and extremely long operating life is not necessary.
The continued emphasis on maximizing specific energy in spacecraft power and propulsion systems has led to a trend in thinking for the design of future manned spacecraft wherein the same reactants are used for both rocket propulsion and power generation, thereby allowing common storage tanks. This trend, along with a desire for even longer operating life and higher reliability for future missions, is pointing NASA’s roadmap along a con-sistent path.
For future manned spacecraft designs on near-term horizons, much research has been focused on PEM fuel cell technology. This technology offers the potential for longer operating life than does alkaline, provided that the problem of gravity-independent water management in the cell stack can be solved. As PEM cells produce water on the cathode, solution of this water management problem without the introduction of the hazards associated with rotating machinery in a pure oxygen environment has proven challenging. Another capability offered to space-craft designers by PEM technology is its ability to work with reformate. One concept under study includes the use of ethanol or methane as the fuel and pure oxygen as the oxidant. Such a combination can be burned through a rocket engine with reasonable efficiency and pure ethanol or methane could be reformed readily to supply the fuel cell. One limitation on this come sfrom the fact that, while the PEM fuel cell can easily survive and respond to load swings, the reformer cannot. A solution to this problem would likely involve the addition to the system of either reformate storage tanks or energy storage capaci-tors, the additional weight of which would decrease the total specific energy of the integrated system. More advanced concepts for vehicles for space exploration take the concept of common reactant storage a step further, using water, hydrogen, and oxygen as the only fluids. Concepts under study include use of PEM-based regen-erative fuel cells as energy storage devices in conjunction with water-based propulsion. Advanced concepts for planetary surface exploration vehicles consider the use of fuel cells of various chemistries to generate power from hydrocarbon fuels that are themselves generated from planetary resources. For example, one concept under study involves the production via the Sabatier reaction of methane and water from CO2 in the Martian atmo-sphere and hydrogen from electrolysis. The methane would be intended primarily for use as a rocket fuel but, as it is more easily stored than hydrogen, could also be used to supply a reformer and fuel cell that might power a planetary surface rover. NASA sees fuel cells as just one of
a suite of energy conversion and storage technologies that could be used to support space exploration. Fuel cell technology will likely play a key role in NASA’s new initiative for exploration of the Moon and beyond.
While the interest from NASA’s space program provided the early investment in practical fuel cell technology, theBgreen power[ and Bhydrogen economy[ initiatives beginning in the 1990s have stimulated governmental and industrial investment in fuel cell technology to levels that are orders of magnitude beyond that which the space program envisions. Many government agencies, including the aeronautics side of NASA, and a broad spectrum of industry are engaged in this effort. While significant advances in the state of the art are being generated by this investment, the requirements on commercial power systems are driving the technology in a direction quite different than that being pursued in the manned space program. Industry’s need to extract hydrogen from heavy, dirty, commercial-grade fossil fuels has led much of this investment to focus on technologies associated with reforming and with fuel cells stacks that can work efficiently with reformate. Considering the figure of merit tradeoffs discussed in Section VI, the automotive industry is thus following roadmaps that guide investment in the development of PEM technology as a replacement for the internal combustion engine. While PEM technology’s Bwell-to-wheels[ fuel efficiency (and thus total cycle CO2 emission rate) is inferior to that of some other fuel cell chemistries and only somewhat better than the potential efficiency of the battery hybrid engines currently in production, it still vastly outperforms the internal combustion engine in the reduction of NOx and particulate emissions at the point of use, which is the primary reason for the automotive industry’s interest in fuel cells. In contrast, technology roadmaps laid out for the commercialization of fuel cell power plants for use as small commercial or residential generators or as auxiliary power plants for aircraft and large trucks are focusing their efforts on solid oxide technology. Solid oxide fuel cells offer maximum efficiency (and, therefore lower emissions of CO2) in an application where relatively slow startup and power demand response is acceptable. Interagency and cross-industry roadmaps have been laid out around both of these technologies to guide what may develop into a major transformation of the U.S. energy economy.
V I I I .
C O N C L U S I O N
Fuel cell technology was long a solution in search of a problem. The manned space program brought it from an interesting laboratory experiment to a viable power generation option. With the tremendous commercial investment now being applied, fuel cell technology is fast becoming a practical tool in the effort to develop a Bhydrogen economy[ and lighten mankind’s footprint on the planet.h
A c k n o w l e d g m e n t
The author wishes to thank the IEEE for the invitation to create this publication and Ms. K. Bradley,
Mr. R. Byron, and Mr. K. Araghi of NASA’s Lyndon B. Johnson Space Center for their technical review and support of the preparation of the manuscript.
R E F E R E N C E S
[1] Fuel Cell Handbook, 7th ed., U.S. Dep. Energy, National Energy Technol. Lab., Morgantown, WV, 2004, pp. 2.1–2.24.
[2] A. Culp, Principles of Energy Conversion, 3rd. New York: McGraw-Hill, 1979, pp. 394–400.
[3] Fuel Cell Handbook, 7th ed., U.S. Dep. Energy, National Energy Technol. Lab., Morgantown, WV, 2004, pp. 1.17–1.20.
A B O U T T H E A U T H O R
John H. Scott was born in Memphis, TN. He received the B.Sc. degree in mechanical engineer-ing, in 1982, from Rice University, Houston, TX. He received the M.Sc. degree in mechanical engineer-ing and MBA degree in operations management, both from the University of California, Los Angeles (UCLA), in 1984 and 1986, respectively.
He joined TRW Space and Technology’s Applied Technology Division, Redondo Beach, CA, in 1986, and then joined NASA’s Lyndon B. Johnson Space
Center, Houston, TX, in 1988. At the Johnson Space Center, he has held increasingly responsible engineering and project management positions in support of the Space Shuttle and International Space Station Programs and, as of this writing, serves as Chief of the Johnson Space Center’s Energy Conversion Branch. His current technical interests include the development of fuel cell, battery, and other power conversion technology in support of NASA’s manned space program.
Mr. Scott is a member of the American Society of Mechanical Engineers.
