Fuel Cells: The Future of Energy Conversion Technology, Lab Reports of Health sciences

Download Fuel Cells: The Future of Energy Conversion Technology and more Lab Reports Health sciences in PDF only on Docsity! The authors acknowledge the following reviewers: Los Alamos National Laboratory Shimshon Gottesfeld Charles F. Keller Steffen MØller-Holst Antonio Redondo Office of Advanced Automotive Technologies, U.S. Department of Energy JoAnn Milliken LA-UR-99-3231 Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the US Department of Energy under contract W-7405- ENG-36. All company names, logos, and products mentioned herein are trademarks of their respective companies. Reference to any specific company or product is not be construed as an endorsement of said company or product by the Regents of the University of California, the United States Government, the US Department of Energy, nor any of their employees. The Los Alamos National Laboratory strongly supports academic freedom and a researcher’s right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. 3 “The mission of our global fuel cell project center is nothing less than to make us the leader in commercially viable fuel cell powered vehicles.” Harry J. Pearce, Vice Chairman, Board of Directors, General Motors. May 1998 In March 1998, Chicago became the first city in the world to put pollution-free, hydrogen fuel cell powered buses in their public transit system. (Courtesy: Ballard Power Systems) The FutureCar Challenge, sponsored by the U.S. Department of Energy, presents a unique assignment to students from North America’s top engineering schools: convert a conventional midsize sedan into a super efficient vehicle without sacrificing performance, utility, and safety. In 1999, the Virginia Tech team entered the competition with a fuel cell vehicle. The first fuel cell powered bicycle to compete in the American Tour-de-Sol. (Courtesy: H-Power) The automobile, it is fair to say, changed the industrial andsocial fabric of the United States and most countries around the globe. Henry Ford epitomized “Yankee ingenuity” and the Model T helped create the open road, new horizons, abundant and inexpensive gasoline...and tailpipe exhaust. More people are driving more cars today than ever before — more than 200 million vehicles are on the road in the U.S. alone. But the car has contributed to our air and water pollution and forced us to rely on imported oil from the Middle East, helping to create a significant trade imbalance. Today many people think fuel cell technology will play a pivotal role in a new technological renaissance — just as the internal combustion engine vehicle revolutionized life at the beginning of the 20th century. Such innovation would have a global environmental and economic impact. “In today’s world, solving environmental problems is an investment, not an expense.” William Clay Ford, Jr. Chairman and CEO, Ford Motor Company, September 1998 Fuel cells are not just laboratory curiosities. While there is much work that needs to be done to optimize the fuel cell system (remember, the gasoline internal combustion engine is nearly 120 years old and still being improved), hydrogen fuel cell vehicles are on the road — now. Commuters living in Chicago and Vancouver ride on fuel cell buses. You can take a ride around London in a fuel cell taxi and even compete in the American Tour de Sol on a fuel cell bicycle. Every major automobile manufacturer in the world is developing fuel cell vehicles. To understand why fuel cells have received such attention, we need to compare them to existing energy conversion technologies. Where the Action in Fuel Cells is Today Allied Signal Volvo Ballard DaimlerChrysler Detroit Edison DuPont Shell Ford General Motors Honda Mazda Georgetown University Case Western Reserve University Los Alamos National Laboratory Motorola Penn State University Princeton University Rolls-Royce Argonne National Laboratory Sanyo DAIS Siemens British Gas Plug Power University of Michigan Texas A&M University ARCO Epyx International Fuel Cells H-Power Energy Partners Hydrogen Burner W.L. Gore A.D. Little Institute of Gas Technology Vairex Electrochem Giner Jet Propulsion Laboratory Toyota University of California Exxon Westinghouse Renault 3M Nissan BMW PSA Peugeot Citroën Texaco University of Florida Tokyo Electric Power (This is just a partial l ist) Carnot Cycle vs. Fuel Cells T he theoretical thermodynamic derivation of Carnot Cycle showsthat even under ideal conditions, a heat engine cannot convert all the heat energy supplied to it into mechanical energy; some of the heat energy is rejected. In an internal combustion engine, the engine accepts heat from a source at a high temperature (T1), converts part of the energy into mechanical work and rejects the remainder to a heat sink at a low temperature (T2). The greater the temperature difference between source and sink, the greater the efficiency, Maximum Efficiency = (T1 – T2) / T1 where the temperatures T1 and T2 are given in degrees Kelvin. Because fuel cells convert chemical energy directly to electrical energy, this process does not involve conversion of heat to mechanical energy. Therefore, fuel cell efficiencies can exceed the Carnot limit even when operating at relatively low temperatures, for example, 80°C. The Very Basics • A fuel cell is an electrochemical energy conversion device. It is two to three times more efficient than an internal combustion engine in converting fuel to power. • A fuel cell produces electricity, water, and heat using fuel and oxygen in the air. • Water is the only emission when hydrogen is the fuel. As hydrogen flows into the fuel cell on the anode side, a platinum catalyst facili- tates the separation of the hydrogen gas into electrons and protons (hydrogen ions). The hydrogen ions pass through the membrane (the center of the fuel cell) and, again with the help of a platinum catalyst, combine with oxygen and electrons on the cathode side, producing water. The electrons, which cannot pass through the membrane, flow from the anode to the cathode through an external circuit containing a motor or other electric load, which con- sumes the power generated by the cell. The voltage from one single cell is about 0.7 volts – just about enough for a light bulb – much less a car. When the cells are stacked in series, the operating voltage increases to 0.7 volts multiplied by the number of cells stacked. Hydrogen Catalyst Cathode (+) Water Oxygen Anode (-) Electrons Protons Membrane/ Electrolyte 5 W hat internal combustionengines, batteries, and fuel cells have in common is their purpose: all are devices that convert energy from one form to another. As a starting point, let’s consider the internal combustion engine — used to power virtually all of the cars driven on U.S. highways today. These engines run on noisy, high temperature explosions resulting from the release of chemical energy by burning fuel with oxygen from the air. Internal combustion en- gines, as well as conventional utility power plants, change chemical energy of fuel to thermal energy to generate mechanical and, in the case of a power plant, electrical energy. Fuel cells and batteries are electro- chemical devices, and by their very nature have a more efficient conver- sion process: chemical energy is converted directly to electrical energy. Internal combustion engines are less efficient because they include the conversion of thermal to me- chanical energy, which is limited by the Carnot Cycle. If cars were powered by electricity generated from direct hydrogen fuel cells, there would be no combustion involved. In an automotive fuel cell, hydrogen and oxygen undergo a relatively cool, electrochemical reaction that directly produces electrical energy. This electricity would be used by motors, including one or more connected to axles used to power the wheels of the vehicle. The direct hydrogen fuel cell vehicle will have no emissions even during idling — this is especially important during city rush hours. There are How do Fuel Cells Compare to Internal Combustion Engines and Batteries? some similarities to an internal combustion engine, however. There is still a need for a fuel tank and oxygen is still supplied from the air. Many people incorrectly assume that all electric vehicles (EVs) are pow- ered by batteries. Actually, an EV is a vehicle with an electric drive train powered by either an on-board battery or fuel cell. Batteries and fuel cells are similar in that they both convert chemical energy into electricity very efficiently and they both require minimal maintenance because neither has any moving parts. However, unlike a fuel cell, the reactants in a battery are stored internally and, when used up, the battery must be either recharged or replaced. In a battery-powered EV, rechargeable batteries are used. With a fuel cell powered EV, the fuel is stored externally in the vehicle’s fuel tank and air is obtained from the atmosphere. As long as the vehicle’s tank contains fuel, the fuel cell will produce energy in the form of electricity and heat. The choice of electrochemical device, battery or fuel cell, depends upon use. For larger scale applications, fuel cells have several advantages over batter- ies including smaller size, lighter weight, quick refueling, and longer range. The polymer electrolyte membrane (PEM) fuel cell is one in a family of five distinct types of fuel cells. The PEM fuel cell, under consideration by vehicle manufacturers around the world as an alternative to the internal combustion engine, will be used to illustrate the science and technology of fuel cells. The P2000, from Ford Motor Company, is a zero-emission vehicle that utilizes a direct hydrogen polymer electrolyte fuel cell. (Courtesy of Ford Motor Co.) Hydrogen Fuel Cell Car Chemical Energy (gaseous flow) Electrical Energy Fuel Cells HydrogenTank Mechanical Energy Traction Inverter Module Turbo- compressor Hydrogen is supplied to the Fuel Cells Air is supplied to the fuel cells by turbocompressor The traction inverter module converts the electricity for use by the electric motor/ transaxles The electric motor/ transaxle converts the electric energy into the mechanical energy which turns the wheels Oxygen from the air and hydrogen combine in the fuel cells to generate electricity that is sent to the traction inverter module Electric Motor/ Transaxle The Electrodes All electrochemical reactions consist of two separate reactions: an oxidation half-reaction occurring at the anode and a reduction half-reaction occurring at the cathode. The anode and the cathode are separated from each other by the electrolyte, the membrane. In the oxidation half-reaction, gaseous hydrogen produces hydrogen ions, which travel through the ionically con- ducting membrane to the cathode, and electrons which travel through an external circuit to the cathode. In the reduction half-reaction, oxygen, supplied from air flowing past the cathode, combines with these hydrogen ions and electrons to form water and excess heat. These two half-reactions would normally occur very slowly at the low operating temperature, typically 80˚C, of the polymer electrolyte membrane fuel cell. Thus, catalysts are used on both the anode and cathode to increase the rates of each half-reaction. The catalyst that works the best on each electrode is platinum, a very expensive material. The final products of the overall cell reaction are electric power, water, and excess heat. Cooling is required, in fact, to maintain the temperature of a fuel cell stack at about 80˚C. At this temperature, the product water produced at the cathode is both liquid and vapor. This product water is carried out of the fuel cell by the air flow. Electrochemistry of Fuel Cells Oxidation half reaction 2H2 ➔ 4H + + 4e- Reduction half reaction O2 + 4H + + 4e- ➔ 2H2O Cell reaction 2H2 + O2 ➔2H2O T he physical and electrochemical processes that occur at eachelectrode are quite complex. At the anode, hydrogen gas (H2) must diffuse through tortuous pathways until a platinum (Pt) particle is encountered. The Pt catalyzes the dissociation of the H2 molecule into two hydrogen atoms (H) bonded to two neighboring Pt atoms. Only then can each H atom release an electron to form a hydrogen ion (H+). Current flows in the circuit as these H+ ions are conducted through the membrane to the cathode while the electrons pass from the anode to the outer circuit and then to the cathode. The reaction of one oxygen (O2) molecule at the cathode is a 4 electron reduction process (see above equation) which occurs in a multi-step sequence. Expensive Pt based catalysts seem to be the only catalysts capable of generating high rates of O2 reduction at the relatively low temperatures (~ 80˚C) at which polymer electrolyte membrane fuel cells operate. There is still uncertainty regarding the mechanism of this complex process. The performance of the polymer electrolyte membrane fuel cells is limited primarily by the slow rate of the O2 reduction half reaction which is more than 100 times slower than the H2 oxidation half reaction. Definitions: Catalyst: A substance that participates in a reaction, increasing its rate, but is not consumed in the reaction. Current: The flow of electric charge through a circuit. Electrode: An electronic conductor through which electrons are exchanged with the chemical reactants in an electrochemical cell. Electron: An elementary particle having a negative charge. 1 nanometer: = 10-9 m = 10-7 cm = 10-6 mm = 10-3 µm = 1 nm Oxidation half reaction: A process in which a chemical species changes to another species with a more positive charge due to the release of one or more electrons. It can occur only when combined with a reduction half reaction. Reduction half reaction: A process in which a chemical species changes to another species with a less positive charge due to the addition of one or more electrons. It can occur only when combined with an oxidation half reaction. 9 Polymer electrolyte membrane with porous electrodes that are composed of platinum particles uniformly supported on carbon particles. Why a Fuel Cell Goes “Platinum” T he half reactions occurring at each electrode can only occur at ahigh rate at the surface of the Pt catalyst. Platinum is unique because it is sufficiently reactive in bonding H and O intermediates as required to facilitate the electrode processes, and also capable of effectively releasing the intermediate to form the final product. For example, the anode process requires Pt sites to bond H atoms when the H2 molecule reacts, and these Pt sites next release the H atoms, as H+ + e- H2 + 2Pt ➔ 2 Pt-H 2 Pt – H ➔ 2 Pt + 2 H+ + 2e- This requires optimized bonding to H atoms — not too weak and not too strong — and this is the unique feature of a good catalyst. Real- izing that the best catalyst for the polymer electrolyte membrane fuel cell is expensive, lowering Pt catalyst levels is an on-going effort. One of the best ways to accomplish this is to construct the catalyst layer with the highest possible surface area. Each electrode consists of porous carbon (C) to which very small Pt particles are bonded. The electrode is somewhat porous so that the gases can diffuse through each electrode to reach the catalyst. Both Pt and C conduct electrons well, so electrons are able to move freely through the electrode. The small size of the Pt particles, about 2 nanometers in diameter, results in an enormously large total surface area of Pt that is accessible to gas molecules. The total surface presented by this huge number of small particles is very large even when the total mass of Pt used is small. This large Pt surface area allows the electrode reactions to proceed at many Pt surface sites simultaneously. This high dispersion of the catalyst is one key to generating significant electron flow, i.e. current, in a fuel cell. Water and Fuel Cell Performance “W ater management” is key to effective operation of apolymer electrolyte membrane fuel cell. Although water is a product of the fuel cell reaction, and is carried out of the cell during its operation, it is interesting that both the fuel and air entering the fuel cell must still be humidified. This additional water keeps the polymer electrolyte membrane hydrated. The humidity of the gases must be carefully controlled. Too little water prevents the membrane from conducting the H+ ions well and the cell current drops. If the air flow past the cathode is too slow, the air can’t carry all the water produced at the cathode out of the fuel cell, and the cathode “floods.” Cell performance is hurt because not enough oxygen is able to penetrate the excess liquid water to reach the cathode catalyst sites. Future Opportunities • Impurities often present in the H2 fuel feed stream bind to the Pt catalyst surface in the anode, preventing H2 oxidation by blocking Pt catalyst sites. Alternative catalysts which can oxidize H2 while remaining unaffected by impurities are needed to improve cell performance. • The rate of the oxygen reduction process at the air electrode is quite low even at the best Pt catalysts developed to date, resulting in significant performance loss. Alternative catalysts that promote a high rate of oxygen reduction are needed to further enhance fuel cell performance. • Future alternative catalysts must be less expensive than Pt to lower the cost of the cell. Polymer Electrolyte Membrane Pathway(s) allowing conduction of electrons Pathway(s) allowing conduction of hydrogen ions Pathway(s) allowing access of gas to catalyst surface Carbon Platinum The Membrane/Electrode Assembly T he combination of anode/membrane/cathode isreferred to as the membrane/electrode assembly. The evolution of membrane/electrode assemblies in polymer electrolyte membrane fuel cells has passed through several generations. The original membrane/ electrode assemblies were constructed in the 1960s for the Gemini space program and used 4 milligrams of platinum per square centimeter of membrane area (4 mg/cm2). Current technology varies with the manu- facturer, but total platinum loading has decreased from the original 4 mg/cm2 to about 0.5 mg/cm2. Laboratory research now uses platinum loadings of 0.15mg/cm2 . This corresponds to an improvement in fuel cell perfor- mance since the Gemini program, as measured by amperes of current produced, from about 0.5 amperes per milligram of platinum to 15 amperes per milligram of platinum. The thickness of the membrane in a membrane/elec- trode assembly can vary with the type of membrane. The thickness of the catalyst layers depends upon how much platinum is used in each electrode. For catalyst layers containing about 0.15 mg Pt/cm2, the thickness of the catalyst layer is close to 10 microns, less than half the thickness of a sheet of paper. It is amazing that this membrane/electrode assembly, with a total thickness of about 200 microns or 0.2 millimeters, can generate more than half an ampere of current for every square centimeter of membrane/electrode assem- bly at a voltage between the cathode and anode of 0.7 volts, but only when encased in well engineered components — backing layers, flow fields, and current collectors. Membrane/electrode assembly Future Opportunities Optimization of membrane/electrode assembly (MEA) construction is on-going. Fundamental research into the catalyst layer/ membrane interface is needed to further understand the processes involved in current generation. New MEA designs which will increase fuel cell performance are needed. As always, the science and technology of MEAs are interconnected; whether improved understanding will lead to better MEA design or a different design will lead to improved understanding remains to be seen. Making a Membrane/Electrode Assembly M embrane/electrode assembly construction varies greatly,but the following procedure is one of several used at Los Alamos National Laboratory where fuel cell research is actively pursued. The catalyst material is first prepared in liquid “ink” form by thoroughly mixing together appropriate amounts of catalyst (a powder of Pt dispersed on carbon) and a solution of the membrane material dissolved in alcohols. Once the ink is prepared, it is applied to the surface of the solid membrane in a number of different ways. The simplest method involves painting the catalyst “ink” directly onto a dry, solid piece of membrane. The wet catalyst layer and the membrane are heated until the catalyst layer is dry. The membrane is then turned over and the procedure is repeated on the other side. Catalyst layers are now on both sides of the membrane. The dry membrane/electrode assembly is next rehydrated by immersing in lightly boiling dilute acid solution to also ensure that the membrane is in the H+ form needed for fuel cell opera- tion. The final step is a thorough rinsing in distilled water. The membrane/electrode assembly is now ready for insertion into the fuel cell hardware. 0.2 mm Anode Cathode Polymer electrolyte membrane 13 Efficiency, Power and Energy of Polymer Electrolyte Membrane Fuel Cell Energy conversion of a fuel cell can be summarized in the following equation: Chemical energy of fuel = Electric energy + Heat energy A single, ideal H2/air fuel cell should provide 1.16 volts at zero current (“open circuit” conditions), 80°C and 1 atm gas pressure. A good measure of energy conversion efficiency for a fuel cell is the ratio of the actual cell voltage to the theo- retical maximum voltage for the H2/air reaction. Thus a fuel cell operating at 0.7 V is generating about 60% of the maximum useful energy available from the fuel in the form of electric power. If the same fuel cell is operated at 0.9 V, about 77.5% of the maximum useful energy is being delivered as electricity. The remaining energy (40% or 22.5%) will appear as heat. The characteristic performance curve for a fuel cell represents the DC voltage delivered at the cell terminals as a function of the current density, total current divided by area of membrane, being drawn from the fuel cell by the load in the external circuit The power (P), expressed in units of watts, delivered by a cell is the product of the current (I) drawn and the terminal voltage (V) at that cur- rent (P = IV). Power is also the rate at which energy (E) is made available (P = E/t) or conversely, energy, expressed in units of watt-hours, is the power available over a time period (t) (E = Pt). As the mass and volume of a fuel cell system are so important, additional terms are also used. Specific power is the ratio of the power produced by a cell to the mass of the cell; power density is the ratio of the power produced by a cell to the volume of the cell. High specific power and power density are important for transportation applications, to minimize the weight and volume of the fuel cell as well as to minimize cost. Derivation of Ideal Fuel Cell Voltage Prediction of the maximum available voltage from a fuel cell process involves evaluation of energy differences between the initial state of reac- tants in the process (H2 +1/2 O2) and the final state (H2O). Such evaluation relies on thermodynamic functions of state in a chemical process, primarily the Gibbs free energy. The maximum cell voltage ( E) for the hydrogen/air fuel cell reaction ( H2 + 1/2 O2 ➔ H2O) at a specific tem- perature and pressure is calculated [ E = - G/nF], where G is the Gibbs free energy change for the reaction, n is the number of moles of electrons involved in the reaction per mole of H2, and F is Faraday’s constant, 96, 487 coulombs (joules/volt), the charge transferred per mole of electrons. At a constant pressure of 1 atmosphere, the Gibbs free energy change in the fuel cell process (per mole of H2) is calculated from the reaction temperature (T), and from changes in the reaction enthalpy ( H) and entropy ( S) G = H - T S = - 285,800 J – (298 K)(-163.2 J/K) = - 237,200 J For the hydrogen/air fuel cell at 1 atmosphere pressure and 25˚C (298 K), the cell voltage is 1.23 V. E = - G/nF = - (-237,200 J/2 x 96,487 J/V) = 1.23 V As temperature rises from room temperature to that of an operating fuel cell (80˚C or 353 K), the values of H and S change only slightly, but T changes by 55˚. Thus the absolute value of G decreases. For a good estimation, assuming no change in the values of H and S G = - 285,800 J/mol – (353 K)(-163.2 J/mol K) = - 228,200 J/mol Thus, the maximum cell voltage decreases as well (for the standard case of 1 atm), from 1.23 V at 25˚C to 1.18 V at 80˚C E = - (-228,200 J/2 x 96,487 J/V) = 1.18 V An additional correction for air, instead of pure oxygen, and using humidified air and hydrogen, instead of dry gases, further reduces the maximum voltage obtainable from the hydrogen/air fuel cell to 1.16 V at 80˚C and 1 atmosphere pressure. 0 Current Density (mA/cm2) Ce ll V olt ag e ( mV ) 1200 800 600 200 0 200 600 800 1000 1400 1600 400 400 1200 1000 l ll Graph of voltage vs. current density of a hydrogen/air polymer electrolyte membrane fuel cell. Rate of Heat Generation in an Operating Fuel Cell Assume a 100 cm 2 fuel cell is operating, under typical conditions of 1 atmosphere pressure and 80˚C, at 0.7 V and generating 0.6 A/cm2 of current, for a total current of 60 A. The excess heat generated by this cell can be estimated as follows: Power due to heat = Total power generated – Electrical power Pheat = Ptotal – Pelectrical = (Videal x Icell) – (Vcell x Icell) = (Videal – Vcell) x Icell = (1.16 V – 0.7 V) x 60 A = 0.46 V x 60 coulombs/sec. x 60 seconds/min. = 1650 J/min This cell is generating about 1.7 kJ of excess heat every minute it operates, while generating about 2.5 kJ of electric energy per minute. S ince fuel cells operate at less than 100% efficiency,the voltage output of one cell is less than 1.16 volt. As most applications require much higher voltages than this, (for example, effective commercial electric motors typically operate at 200 – 300 volts), the required voltage is obtained by connecting individual fuel cells in series to form a fuel cell “stack.” If fuel cells were simply lined-up next to each other, the anode and cathode current collectors would be side by side. To decrease the overall volume and weight of the stack, instead of two current collectors, only one plate is used with a flow field cut into each side of the plate. This type of plate, called a “bipolar plate,” separates one cell from the next, with this single plate serving to carry hydrogen gas on one side and air on the other. It is important that the bipolar plate is made of gas-impermeable material. Otherwise the two gases would intermix, leading to direct oxidation of fuel. Without separation of the gases, electrons pass directly from the hydrogen to the oxygen and these electrons are essentially “wasted” as they cannot be routed through an external circuit to do useful electrical work. The bipolar plate must also be electronically conductive because the electrons produced at the anode on one side of the bipolar plate are conducted through the plate where they enter the cathode on the other side of the bipolar plate. Two end-plates, one at each end of the complete stack of cells, are connected via the external circuit. In the near term, different manufacturers will provide a variety of sizes of fuel cell stacks for diverse applications. The area of a single fuel cell can vary from a few square centimeters to a thousand square centimeters. A stack can consist of a few cells to a hundred or more cells connected in series using bipolar plates. For applications that require large amounts of power, many stacks can be used in series or parallel combinations. A 3 cell fuel cell stack with two bipolar plates and two end plates. The Polymer Electrolyte Membrane Fuel Cell Stack Polymer electrolyte membrane fuel cell stack. (Courtesy: Energy Partners) End-plateEnd-plate Bipolar plates Hydrogen flow fields Air flow fields e- e- 15 T here are several other types of polymer electrolytemembrane fuel cells for transportation applications, although none have reached the same stage of develop- ment and simplicity as the hydrogen/air. Reformate/Air Fuel Cell In addition to the direct hydrogen fuel cell, research is currently underway to develop a fuel cell system that can operate on various types of hydrocarbon fuels — including gasoline, and alternative fuels such as metha- nol, natural gas, and ethanol. Initially, this fuel-flexible fuel strategy will enable reformate/air fuel cell systems to use the exisiting fuels infrastructure. A hydrogen/air polymer electrolyte membrane fuel cell would be fueled from an onboard reformer that can convert these fuels into hydrogen-rich gas mixtures. Processing hydrocar- bon fuels to generate hydrogen is a technical challenge and a relatively demanding operation. Hydrocarbon fuels require processing temperatures of 700˚C - 1000˚C. Sulfur, found in all carbon-based fuels, and carbon monoxide generated in the fuel processor, must be removed to avoid poisoning the fuel cell catalyst. Although the reformate/air fuel cell lacks the zero emission characteristic of the direct hydrogen fuel cell, it has the potential of lowering emissions signifi- cantly vs. the gasoline internal combustion engine. The near-term introduction of reformate/air fuel cells is expected to increase market acceptance of fuel cell technology and help pave the way for the widespread use of direct hydrogen systems in the future. Other Types of Polymer Electrolyte Membrane Fuel Cell Systems Computer model of 50kW fuel cell stack with reformer. (Courtesy: International Fuel Cells) Processing Hydrocarbon Fuels into Hydrogen As long as hydrogen is difficult to store on a vehicle, on-boardfuel processors will be needed to convert a hydrocarbon fuel, such as methanol or gasoline, to a H2 rich gas for use in the fuel cell stack. Currently, steam reforming of methanol to H2 is the conventional technology, although partial oxidation of gasoline to H2 is attractive because of the gasoline infrastructure already in place in most countries. Both types of fuel processors are complex systems. The steam reforming of methanol involves the reaction of steam and pre-vaporized methanol at 200˚C (gasoline requires tempera- tures over 800˚C) to produce a mixture of H2, carbon dioxide (CO2), carbon monoxide (CO), and excess steam. This mixture passes through another reactor, called a shift reactor, which uses catalysts and water to convert nearly all of the CO to CO2 as well as additional H2. There can be a third stage in which air is in- jected into the mixture in a third type of reactor, the preferential oxidation reactor. Oxygen in the air reacts with the remaining CO over a Pt-containing catalyst to convert CO to CO2. The final gas mixture contains about 70% H2 , 24% CO2, 6% nitrogen (N2) and traces of CO. With the partial oxidation reformer system, liquid fuel is first vaporized into a gas. The gas is then ignited in a partial oxidation reactor which limits the amount of air so that primarily H2 , CO and CO2 are produced from the combustion. This mixture is passed through a shift reactor to convert the CO to CO2 and then through a preferential oxidation reactor to convert any remaining CO to CO2. Conventional partial oxidation takes place at ~1000˚C and catalytic partial oxidation takes place at ~700˚C. The final reformate composition is about 42% N2, 38% H2, 18% CO2 , less than 2% CH4 and traces of CO. Renewable regenerative fuel cell utilizing the energy source of the sun to produce power (Courtesy: Aerovironment) In the near term, fuel availability could be an important reason for operating fuel cell vehicles on gasoline. Today, oil refiners in the U.S. are spending over $10 billion to comply with reformulated gasoline regulations to help lower tailpipe emissions. Fuel cell vehicles operating on alternative fuels will require new and expensive fueling infrastructures. However, alternative fuels will provide superior environmental performance. In the long term, incremental investments in a new domestic fuel infrastructure will be necessary for the 21st century. RFG : Reformulated Gasoline M 100 : Methanol E 100 : Ethanol H2 : Hydrogen CNG : Compressed Natural Gas Regenerative Fuel Cell A regenerative fuel cell, currently being developed for utility applications, uses hydrogen and oxygen or air to produce electricity, water, and waste heat as a conventional fuel cell does. However, the regenerative fuel cell also performs the reverse of the fuel cell reaction, using electricity and water to form hydrogen and oxygen. In the reverse mode of the regenerative fuel cell, known as electrolysis, electricity is applied to the electrodes of the cell to force the dissociation of water into its components. The “closed” system of a regenerative fuel cell could have a significant advantage because it could enable the operation of a fuel cell power system without requiring a new hydrogen infrastructure. There are two concerns to be addressed in the development of regenerative fuel cells. The first is the extra cost that would be incurred in making the fuel cell reversible. Unmanned solar plane powered by a renewable regenerative fuel cell (Courtesy: Aerovironment) The second drawback to the operation of the regenerative fuel cell is the use of grid electricity to produce the hydrogen. In the United States, most electricity comes from burning fossil fuels. The fossil fuel ➔ electricity ➔ hydrogen energy route generates signifi- cantly more greenhouse gases than simply burning gasoline in an internal combustion engine. Although the concept of a regenerative fuel cell is attrac- tive, until renewable electricity, e.g. electricity from solar or wind sources, is readily available, this technology will not reduce greenhouse gas emissions. 19 Characteristics of Potential Fuel Cell Fuels Production RFG M 100 E 100 H2 CNG Storage Cost est./ gal. eq Safety Distribution Infrastructure Environmental Attributes Large existing production operation Uses imported feedstock No energy security or trade balance benefits Conventional storage tanks Low flashpoint Narrow flammability limits Potentially carcinogenic when inhaled Existing infrastructure and distribution system Reduction in greenhouse gases Much lower reactive hydrocarbon and sulfur oxide emissions than gasoline Abundant domestic/imported natural gas feedstock Can be manufactured renewably from domestic biomass - not currently being done Requires special storage because fuel can be corrosive to rubber, plastic and some metals Toxic and can be absorbed through the skin No visible flame Adequate training required to operate safely Infrastructure needs to be expanded High greenhouse gas emissions when manufactured from coal Zero emissions when made renewably Made from domestic renewable resources: corn, wood, rice, straw, waste, switchgrass. Many technologies still experimental Production from feedstocks are energy intensive Requires special storage because fuel can be corrosive to rubber, plastic and some metals Wide flammability limit Adequate training required to operate safely Less toxic than methanol and gasoline Nearly no infrastructure currently available Food/fuel competition at high productions levels Zero carbon dioxide emissions as a fuel Significant emissions in production Domestic manufacturing: Steam reforming of coal, natural gas or methane Renewable solar Compressed gas cylinders Cryogenic fuel tanks Metal hydrides Carbon nanofibers Currently storage systems are heavy and bulky Low flammability limit Disperses quickly when released Nearly invisible flame Odorless and colorless Non-toxic Adequate training required to operate safely Needs new infrastructure High emissions when manufactured from electrolysis Lower emissions from natural gas Zero emissions when manufactured renewably $.05-.15 more than gasoline $1.10- $1.15 $.79- $1.91 Abundant domestic/imported feedstock Can be made from coal CNG needs to be compressed during refueling and requires special nozzles to avoid evaporative emissions Stored in compressed gas cylinders Low flashpoint Non-carcinogenic Dissipates into the air in open areas High thermal efficiency Adequate training required to operate safely Limited infrastructure Non-renewable Possible increase in nitrogen oxide emissions $.85 $.90 U.S. Congress, Office of Technology Assessment, “Replacing Gasoline-Alternative Fuels for Light-Duty Vehicles” OTA-E-364, September, 1990. Union of Concerned Scientists, Summary of Alternative Fuels, 1991. U.S. Department of Energy, Taking an Alternative Route, 1994. National Alternative Fuel and Clean Cities Hotline: http://www.afdc.doe.gov Jason Mark. “Environmental and Infrastructure Trade-Offs of Fuel Choices for Fuel Cell Vehicles.” Future Transportation Technology Conference, San Diego, CA. August 6-8, 1997. SAE Technical Paper 972693. A laptop computer using a fuel cell power source can operate for up to 20 hours on a single charge of fuel. (Courtesy: Ballard Power Systems) References: Natural Gas Fuel Cells, Federal Technology Alert, Federal Energy Management Program. U.S. Department of Energy, November 1995. J.L. Preston, J.C. Trocciola, and R.J. Spiegel. “Utilization and Control of Landfill Methane by Fuel Cells” presented at U.S. EPA Greenhouse Gas Emissions & Mitigation Research Symposium, June 27-29, 1995, Washington, D.C. G.D. Rambach and J.D. Snyder. An Examination of the Criteria Necessary for Successful Worldwide Deployment of Isolated, Renewable Hydrogen Stationary Power Systems. XII World Hydrogen Energy Conference, Buenos Aires, June, 1998. Keith Kozloff, “Power to Choose.” Frontiers of Sustainability. World Resources Institute, Island Press, 1997. Resources: Federal Energy Technology Center http://www.fetc.doe.gov/ World Fuel Cell Council http://members.aol.com/fuelcells/ Michael Corbett. Opportunities in Advanced Fuel Cell Technologies: Volume One, Stationary Power Generation 1998 – 2009, Kline & Co., Inc. Fairfield, N.J. August, 1998. Definitions: Quad: A unit of heat energy, equal to 1015 British thermal units. The world's first prototype polymer electrolyte membrane fuel cell (on the right) used to provide all residential power needs for a home in Latham, New York. This 7kW unit is attached to a power conditioner/storage unit that stores excess electricity. (Courtesy: Plug Power) 23 Applications electric utility portable power transportation military space electric utility transportation electric utility electric utility Advantages • Solid electrolyte reduces corrosion & management problems • Low temperature • Quick start-up • Cathode reaction faster in alkaline electrolyte — so high performance • Up to 85 % efficiency in co-generation of electricity and heat • Impure H2 as fuel • High temperature advantages* • High temperature advantages* • Solid electrolyte advantages (see PEM) Disadvantages • Low temperature requires expensive catalysts • High sensitivity to fuel impurities • Expensive removal of CO2 from fuel and air streams required • Pt catalyst • Low current and power • Large size/weight • High temperature enhances corrosion and breakdown of cell components • High temperature enhances breakdown of cell components *High temperature advantages include higher efficiency, and the flexibility to use more types of fuels and inexpensive catalysts as the reactions involving breaking of carbon to carbon bonds in larger hydrocarbon fuels occur much faster as the temperature is increased. C=Carbon H=Hydrogen Trends in energy use: Hydrogen-to-Carbon ratio increases as we become less dependent on carbon-based fuels. (Courtesy: “Wired” 10/97) H ydrogen is the most attractive fuel for fuel cells — having excellentelectrochemical reactivity, providing adequate levels of power density in a hydrogen /air system for automobile propulsion, as well as having zero emissions characteristics. Historically, the trend in energy use indicates a slow transition from fuels with high carbon content, beginning with wood, to fuels with more hydro- gen. Fossil fuels release varying quantities of carbon dioxide into the atmosphere — coal having the highest carbon content, then petroleum, and finally natural gas — the lowest carbon dioxide emitter per thermal unit. Hydrogen obviously releases no carbon dioxide emissions when burned. Hydrogen (H2) is the most abundant element in the universe, although practically all of it is found in combination with other elements, for ex- ample, water (H2O), or fossil fuels such as natural gas (CH4). Therefore, hydrogen must be manufactured from either fossil fuels or water before it can be used as a fuel. Today, approximately 95% of all hydrogen is produced by “steam reforming” of natural gas, the most energy-efficient, large-scale method of production. Carbon dioxide (CO2) is a by-product of this reaction. CH4 + 2H2O ➔ 4H2 + CO2 Hydrogen can also be produced by gasification of carbon containing materi- als such as coal — although this method also produces large amounts of carbon dioxide as a by-product. Electrolysis of water generates hydrogen and oxygen. H2O ➔ H2 + 1/2O2 The electricity required to electrolyze the water could be generated from either fossil fuel combustion or from renewable sources such as hydro- power, solar energy or wind energy. In the longer term, hydrogen generation could be based on photobiological or photochemical methods. While there is an existing manufacturing, distribution, and storage infrastruc- ture of hydrogen, it is limited. An expanded system would be required if hydrogen fuel were to be used for automotive and utility applications. In 1809, an amateur inventor submitted a patent for this hydrogen car. Hydrogen As a FuelWood Coal Oil NaturalGas Hydrogen C H 25 While a single hydrogen production/distribution/ storage system may not be appropriate for the diverse applications of fuel cells, it is certainly possible that a combination of technologies could be employed to meet future needs. All of the system components are cur- rently available — but cost effective delivery and dispensing of hydrogen fuel is essential. If hydrogen were to become available and affordable, this would reduce the complexity and cost of fuel cell vehicles — enhancing the success of the technology. “Hydrogen Economy” is an energy system based upon hydrogen for energy storage, distribution, and utiliza- tion. The term, coined at General Motors in 1970, caught the imagination of the popular press. During the oil crisis in the early 70’s, the price of crude oil sharply increased, concern over stability of petroleum reserves and the potential lack of a secure energy source grew, and government and industry together developed plans and implementation strategies for the introduction of hydrogen into a world energy system. However, the lessening of tensions in the Middle East led to a lowering of crude oil prices and the resumption of business as usual. Petroleum has continued to be the fuel of choice for the transportation sector worldwide. Hydrogen fuel has the reputation of being unsafe. However, all fuels are inherently dangerous — how much thought do you give to the potential dangers of gasoline when you drive your car? Proper engineering, education, and common sense reduce the risk in any potentially explosive situation. A hydrogen vehicle and supporting infrastructure can be engineered to be as safe as existing gasoline systems. Dealing with the perception and reality of safety will be critical to the successful wide introduction of hydrogen into our energy economy. References: Peter Hoffmann. “The Forever Fuel: The Story of Hydrogen. Boulder: Westview Press,” 1981. James Cannon. “Harnessing Hydrogen: The Key to Sustainable Transportation.” New York: Inform, 1995. ”Strategic Planning for the Hydrogen Economy: The Hydrogen Commercialization Plan.” National Hydrogen Association. November, 1996. U.S. Department of Energy. Hydrogen Program Plan, FY 1993 - FY 1997. Richard G. Van Treuren. “Odorless, Colorless, Blameless.” Air & Space, May, 1997. National Hydrogen Association http://www.ttcorp.com/nha/ advocate/ad22zepp.htm Resources: HyWeb http://www.HyWeb.de Hydrogen InfoNet http://www.eren.doe.gov/ hydrogen/infonet.html The New Sunshine Program http://www.aist.go.jp/nss/ text/wenet.htm C.E. Thomas. Hydrogen Vehicle Safety Report. National Technical Information Service. U.S. Department of Commerce. Springfield, Virginia. July, 1997. “Shell has established a Hydrogen Economy team dedicated to investigate opportunities in hydrogen manufacturing and new fuel cell technologies...” Chris Fay, Chief Executive, Shell UK 1930 ’40 ’50 ’60 ’70 ’80 ’90 2000 ’10 ’20 ’30 ’40 ’50 An nu al oil pr od uc tio n ( bil lio ns of ba rre ls) 0 5 10 15 20 25 30 World World projected World outside Persian Gulf World outside projected 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 1950 ’60 ’70 ’80 ’90 2000 Bi llio ns of ba rre ls/ ye ar Lower 48 Projected Year Year N o one can predict what willhappen to world wide oil prices or global oil demand. The world’s production of oil reached a record level of 65 million barrels a day in 1997, and global demand is rising more than 2% a year. Americans spend roughly $100,000 per minute to purchase foreign oil, and the U.S. transportation sector uses over 10% of the world’s oil. Consumption of oil by passenger vehicles, which include automobiles and light duty trucks, exceeds all of the United States’ domestic production. Re- serves of fossil fuels are large but finite, and there is growing evidence to suggest that world production of crude oil will peak early in the 21st century. The Energy Information Agency forecasts that worldwide demand for oil will increase 60% by 2020. By 2010, Middle East OPEC states (Organization of Petroleum Exporting Countries), considered to U.S. oil production in the lower 48 states (upper right) peaked in 1970 as predicted by a bell shaped curve. World oil production is expected to follow suit. (Courtesy: Science, vol. 281, Aug. 21,1998, p.1128; C. Campbell & J. Laher rè re) Oil Reserves, Transportation, and Fuel Cells be unpredictable and often unstable, will have over 50% of the world oil business, and the switch from growth to decline in oil production could cause economic and political tension. As excess oil production capacity begins to decline over the coming decades, oil prices can be expected to rise, and the transporta- tion sector is likely to be most heavily affected by these fluctua- tions. World wide, transportation relies almost totally on oil, and there are few viable short-term fuel options. Every gallon of gasoline manufactured, distributed, and then consumed in a vehicle releases roughly 25 pounds of carbon dioxide. About 25% of all human-generated greenhouse gases come from trans- portation — more than half of that from light-duty vehicles. Unlike air pollutants (carbon monoxide, nitrogen oxides, hydrocarbons, and particulates — soot, smoke, etc.), greenhouse gas emissions (primarily carbon dioxide, methane, nitrous oxide, water vapor, etc.) from vehicles cannot be easily or inexpen- sively reduced by using add-on control devices such as a catalytic converter. In addition, unlike air pollutants, greenhouse gas emissions are not regulated by the Environ- mental Protection Agency. The relationship between gasoline consumption and carbon dioxide emissions is fixed. Today, increasing fuel economy, reducing vehicle miles traveled, and switching to lower or non-carbon fuels will begin to decrease carbon dioxide emissions. The introduction of fuel cells into the transportation sector will increase fuel efficiency, decrease foreign oil dependency, and become an important strategy/technology to mitigate climate change. As fuel cell vehicles begin to operate on fuels from natural gas or gasoline, green- house gas emissions will be reduced by 50%. In the future, the combina- tion of high efficiency fuel cells and fuels from renewable energy sources would nearly eliminate greenhouse gas emissions. The early transition to lower carbon-based fuels will begin to create cleaner air and a stronger national energy security. Since 1985, energy use is up • 40% in Latin America • 40% In Africa • 50% in Asia 29 T here is a growing scientific consensus that increasinglevels of greenhouse gas emissions are changing the earth’s climate. The natural greenhouse gases include carbon dioxide (CO2), water vapor (H2O), nitrous oxide (N2O), methane (CH4) and ozone (O3), and are essential if the Earth is to support life. With the exception of water vapor, carbon dioxide is the most plentiful. Since the beginning of the Industrial Revolution in 1765, burning fossil fuels and the increased energy needs of a growing world population have added man-made, or anthropogenic, greenhouse gas emissions into the environment. Carbon dioxide constitutes a tiny fraction of the earth’s atmosphere — about one molecule in three thousand — but is the single largest waste product of modern industrial society. The concentration of carbon dioxide in the atmosphere has risen from about 280 parts per million by volume to the current level of over 360 parts per million by volume and anthropogenically caused atmospheric concentration of methane has doubled. In the past 100 years, levels of nitrous oxide have risen about 15%. Increasing concentrations of greenhouse gases trap more terrestrial radiation in the lower atmosphere (troposphere), artificially enhancing the natural greenhouse effect. The average temperature of the Earth has warmed about 1°C since the mid-19th century when measurements began, and fragmentary records suggest the Earth is warmer than it has been in nearly 2,000 years. “The balance of evidence suggests that there is a discernible human influence on global climate.” United Nations Intergovernmental Panel on Climate Change, 1995 Under the most optimistic scenarios proposed by the United Nations Intergovernmental Panel on Climate Change, carbon dioxide is expected to rise to approxi- mately 600 parts per million by volume during the next century — more than double the level held for 10,000 years since the end of the last ice age. Climate Change, Greenhouse Gases, and Fuel Cells: What is the Link? The CO2 level has increased sharply since the beginning of the Industrial Era and is already outside the bounds of natural variability seen in the climate record of the last 160,000 years. Continuation of current levels of emissions are predicted to raise concentrations to over 600 ppm by 2100. (Courtesy: Office of Science and Technology Po l i cy) Based on these scenarios, the Intergovernmental Panel on Climate Change has concluded that the increase in greenhouse gases may be expected to cause a rise in the global average temperature of between 1°C and 3.5°C in the 21st century. In 1997, global carbon emissions amounted to more than six billion tons — more than a ton for every human being on the planet. 1998 was the warmest year on record, and no one is absolutely certain what these temperature increases will do — changes in precipitation, extreme weather, and sea level rise are all possible. The climate modeling and resulting scientific conclusions are not universally accepted because climate codes have difficulties simulating such events. The picture is far from clear, but it appears that climate is driven by a variety of forcing mechanisms — and anthropogenic forcing must be placed within the total context that includes the long-term variations of the earth’s orbit, solar variability, and the natural cycles of nature. However, as all of these data are taken into account, evidence is increasing that the climate model predictions cannot be too far wrong and that we are warming the Earth. Compelling societal implications place even more significance on prudent policy directions. References: Jason Mark. “Zeroing out Pollution: The Promise of Fuel Cells.” Union of Concerned Scientists, 1996. Daniel Sperling . “A New Agenda,” ACCESS, Number 11, University of California Transportation Center, Fall 1997. Reinhold Wurster. “PEM Fuel Cells in Stationary and Mobile Appli- cations.” Electric and Lighting Industry Biennial, Buenos Aries, September, 1997. W. Wayt Gibbs. “Transportation’s Perennial Problems.” Scientific American. Oct. 1977. Toward a Sustainable Future: “Addressing the Long-Term Effects of Motor Vehicle Transportation on Climate and Ecology.” Transportation Research Board: National Research Council, National Academy Press, 1997. James J. MacKenzie. “Driving the Road to Sustainable Ground Transportation.” World Resources Institute. “Frontiers of Sustainability: Environmentally Sound Agriculture, Forestry, Transportation, and Power Production.” Island Press, 1997. “Cars and Trucks and Global Warming.” Union of Concerned Scientists, N.D. M. Kerr. “The Next Oil Crisis Looms Large — and Perhaps Close.” Science. August 21, 1998. Resources: President’s Council on Sustainable Development http:// www.whitehouse.gov/PCSD/ Rocky Mountain Institute http:// www.rmi.org California Air Resources Board http://www.arb.ca.gov/ homepage.htm Instrumental Temperature Record from 1860 – 1999 indicates a global warming over the past century, with many peaks and val- leys suggesting the natural year-to-year variability of climate. (Courtesy: Hadley Centre for Climate P red i c t i on & Research) The Science of Global Climate Change The regulating factor for global climate change depends on a fundamental principle, theFirst Law of Thermodynamics, also known as the Law of Conservation of Energy. Mathematically this can be represented as follows: dQ = dU – dW where dQ = heat added to the system, dU = change in the internal energy of the system, and dW = work extracted. Energy cannot be gained or lost in a stable system; it can only change forms. Such a system is said to follow an “Energy Balance Model.” To maintain stability, the Earth-ocean-atmosphere system absorbs energy from the Sun, radiates it in the form of infrared (heat) energy, and transports it in the form of both latent heat and sensible heat flux. Several natural events (volcanic eruptions, forest fires, fluctuating intensity of solar radiation, varying cloud cover, and others) and human activities (fuel combustion, aerosol production, and industrial and land use practices that release or remove heat-trapping greenhouse gases, and others) can affect the balance between absorption and emission of radiation. 1840 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 -0.10 -0.20 -0.30 1860 1880 1900 1920 1940 1960 1980 2000 Global surface temperatures, 1860 - 1999 Ch an ge in te mp er at ur e ( de gr ee s C ) T he U.S. Department of Energy through its Office of TransportationTechnologies is pursuing critical technological advances that can help to create new and improved national transportation systems. The Office of Transportation Technologies supports research that is often too financially risky for private industry to develop on its own. Partnerships are developed with industry, working with national laboratories, as a way to strengthen resources. The mission of the Office of Transportation Technologies is to reduce U.S. dependence on petroleum. Within the Office of Transportation Technologies, the Office of Advanced Automotive Technologies focuses its efforts on developing cleaner and more energy-efficient technologies for automobiles of the future. The Transporta- tion Fuel Cell Program is just one of many exciting research activities. The Partnership for a New Generation of Vehicles (PNGV) Program is a partnership between 11 government agencies and the United States Council for Automotive Research, a cooperative research effort among DaimlerChrysler Corporation, Ford Motor Company, and General Motors Corporation, to develop commercially-viable vehicle technology that, over the long-term, can preserve personal mobility, reduce the impact of cars and light trucks on the environment and reduce U.S. dependency on foreign oil. The Alternative Fuels Research and Development Program has been developing alternative fuels technologies in partnership with industry for more than 20 years. The CARAT Program (Cooperative Automotive Research for Advanced Technology) supports universities and small businesses to accelerate the development and production of innovative technologies that address barriers to producing ultra-efficient vehicles including the design and development of advanced, energy-efficient automotive components and systems. The Graduate Automotive Technology Education Program (GATE) is a multidisciplinary automotive engineering program for graduate students that focuses on technologies critical to the development and production of future automobiles. Benefits of Office of Transportation Technologies Program • Reducing dependence upon foreign oil • Increasing energy savings • Improving air quality by reducing destructive air pollution and greenhouse gases To learn more about the Office of Transportation Technologies: www.ott.doe.gov