Is the venerable internal-combustion engine on the way out? It's entirely possible, and it' s long overdue. But replace it with what? Is there such a thing as a truly "green" car? (Consider, for example, the GM model shown in Fig. 7.6) Fuel cells are widely believed to hold the key to pollution-free motor vehicles and longer-running electric devices. Fuel cells can provide energy for devices as small as a laptop computer, or as large as utility power stations, and just about anything in between, inducing every type of motor vehicle. Just what is this sweeping development that holds promise of repowering our world?
Fuel cells use the chemical energy in hydrogen to generate electricity. That's the short answer. A tad more technical response informs us that a fuel cell is an electrochemical energy conversion device that converts hydrogen gas and oxygen from air into water and heat, and in the process, produces electricity. So elegantly simple. Is it too good to be true? If hydrogen were readily available, and inexhaustible, as it may well be, the generation of electricity would no longer require fossil fuels, turbines, or generators, and we'd live in a cleaner, quieter world. Hydrogen could revolutionize the world. The idea borders on fantasy—or does it?
Hydrogen filler neck
Fuel cell stack
Hydrogen filler neck
Tank control unit
Liquid hydrogen storage tank
Liquid hydrogen storage tank
Tank control unit
Figure 7.6. The GM HydroGen3 automobile appears to be the prototype of all hydrogen fuel-cell-powered vehicles. (Figure courtesy of the General Motors Corporation.)
During 1874/75, Jules Verne, the eminent and prolific French novelist, wrote his three-volume masterpiece The Mysterious Island, based on the true story of Alexander Selkirk ' s survival (remember Robinson Crusoe?) on an unin-habital island for 5 years. The Mysterious Island is an enthralling tale of five men and a dog, who escape from Richmond, Virginia, during the Civil War, in a hot-air balloon, landing on a faraway and uncharted island, where they learn to survive. One day, as winter was approaching and the men were sitting near a glowing fireplace after dinner, smoking their pipes, one of them asks Cyrus Smith, an engineer, what might happen if one day the earth's supply of coal will be exhausted, with no more machines, no more railways, no more steamships, no more factories, no more of anything that the progress of modern life requires. "What would take its place?" asked Pencroff. "Do you have any idea, Mr. Cyrus?" "What will people burn in place of coal?"
" Water, " answered Cyrus Smith. " Water broken down into its component elements. And broken down by electricity. Yes, I believe that water will one day be used as fuel, that the hydrogen and oxygen of which it is constituted will be used, simultaneously or in isolation, to furnish an inexhaustible source of heat and light, more powerful then coal can ever be" . Imagine. Water, the coal of the future. Here we are 132 years after Jules Verne predicted a future in which all our needs would be supplied by an inexhaustible source of hydrogen. The world is ready for a hydrogen economy to interdict the disloca tions of global warming, to strengthen national energy security, and to improve energy efficiency. Just what is a hydrogen economy, and what do we know about hydrogen and its availability?
The hydrogen economy is a world in which hydrogen, a component of many organic chemicals, and water, is readily available to everyone, everywhere, and the United States is no longer dependent on a single source of fuel, as hydrogen is produced in abundance domestically, cleanly, and cost- effectively, from a range of sources such as biomass, nuclear power, and water. In this economy, hydrogen-powered fuel cells will be as common as gasoline was in the twentieth century—powering our homes, cars, trains, planes, ships, offices, and factories. Developing countries have hydrogen-based systems to fuel their energy needs.
With fuel cells as the means whereby hydrogen will be used to produce electricity, the question for us is: How do fuel cells work, and how do they differ from batteries, if they do? We know that batteries are power storage devices, with all their chemicals are inside, and are converted to electricity when switched on. This means that eventually the battery will "go dead," and must be recharged or replaced. Recharging is limited, as eventually they are no longer rechargeable. A fuel cell combines its hydrogen fuel, and oxygen from air to produce electricity steadily for as long as fuel is provided, which means that it can never "go dead" and needs no recharging. As long as hydrogen and oxygen flow into the cell, electricity will always flow out. The fuel cell provides DC (direct current), voltage that can be used to power motors, lights, or any appliance, large or small. Most fuel cells systems consist of four basic components:
• Current converter
• Fuel processor
• Heat recovery system
Additional components control humidity, temperature, gas pressure, and wastewater. Our concern is with the basic four. The fuel cell stack generates electric current from chemical reactions within the cell. A typical fuel cell stack can consist of hundreds of cells. The amount of power produced depends on fuel cell type, cell size, temperature at which it operates, and the pressure at which the gases are supplied to the cell.
The fuel processor converts fuel into a form usable by the cell. If hydrogen is fed into the system, a processor may be needed only to filter out impurities in the gas. Current inverters and conditioners adapt the electrical current from the cell to suit the electricial needs of the application: a simple electric motor, an automobile engine, or a complex utility grid. Since fuel cells produce direct current, and our homes and businesses use alternating current, the direct current must be converted to alternating current. Both AC and DC must be conditioned, which means controlling current flow in terms of amperes, voltage, and frequency to meet the needs of the end user.
As fuel cells generate heat as a byproduct, a heat recovery system may be used to produce steam or hot water that can be converted to electricity via a traditional turbine/generator system. Because heat is captured, the overall efficiency of the system increases. But this depends on the type of fuel cell system. Of primary importance is the fact that fuel cells are significantly more energy - efficient than are combustion- based power generation processes. A conventional combustion-based power plant typically generates electricity at efficiencies of 30-35%; fuel cell plants can generate electricity at efficiencies of up to 60%. When used to generate electricity and heat (cogeneration), they can attain efficiencies of 85%. Internal-combustion engines in today ' s automobiles convert less than 30% of the energy in gasoline. Vehicles using electric motors powered by hydrogen fuel cells attain efficiencies of 40-60% .
Before describing the various types of fuel cell systems, we can take a deeper look at the system. The heart or core of the fuel cell is its stack, which is made of many thin, flat cells layered together. While the term fuel cell often refers to the entire stack, in fact, it refers only to the individual cells. A single cell produces a small amount of electricity, but many cells stacked together provide sufficient power to drive a car. Increasing the number of cells in a stack increases the voltage; increasing the area of the cells increases the current.
When hydrogen is fed to a fuel cell, it encounters the first catalyst - coated electrode, the anode, where hydrogen molecules release electrons and protons. The protons migrate through the electrolyte membrane to the second catalyst-coated electrode, the cathode, where they react with oxygen to form water. The electrons can' t pass through the electrolyte membrane to the cathode. Unable to do so, they travel around it. This traveling around of electrons produces the electric current.
The electrodes (anode and cathode), catalyst, and polymer electrolyte membrane together constitute the membrane electrode assembly of a polymer electrolyte membrane (PEM)—t he fuel cell type of current concern. Each membrane assembly consists of two electrodes, anode and cathode, and an exceedingly thin layer of catalyst.
The anode, the negative side of the fuel cell, conducts the electrons freed from the hydrogen molecules so that they can be used in an external circuit. Channels etched into the anode disperse the hydrogen gas equally over the surface of the catalyst. The cathode, the positive side of the fuel cell, contains channels that distribute oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.
The polymer electrolyte membrane (PEM), a specially treated material that looks like saran wrap, conducts only positively charged ions, and blocks the electrons. This is the key to fuel cell technology; permitting only the necessary ions to pass between the anode and cathode. All electrochemical reactions in a fuel cell consist of two separate reactions: an oxidation half-reaction at the anode, and a reduction half-reaction at the cathode:
Anode side: 2H2 ^ 4H + +4e-Cathode side: O2 + 4H + +4e-^ 2H2O Net reaction: 2H2 + O2 ^ 2H2O
Normally these two reactions would occur slowly with the low operating temperatures of the PEM cell. To speed things up, to facilitate the reactions, each electrode is coated on one side with a catalyst layer that increases the velocity of the reaction of hydrogen with oxygen. This catalyst is most often made of platinum powder, which is tremendously expensive—one of the most pressing current problems.
When an H- molecule contacts the platinum catalyst, it splits into two H+ ions and two electrons (e-), which are conducted through the anode, making their way through the external circuit—powering a motor, then returning to the cathode side of the cell. At the same time, on the cathode side, oxygen gas (O2) is being forced through the catalyst, where it forms two oxygen atoms. Each has a strong negative charge that attracts the two H+ ions through the membrane, where they combine with the oxygen atom and two of the electrons from the external circuit to form a water molecule . This reaction in a single fuel cell produces about 0.7 V. To raise this meager voltage to levels required to power an engine, many cells are combined to form a fuel cell stack, as shown in Figures 7.7a-7.7d. These are the essentials. Fuel cells are simple; no moving parts, yet theoretically capable of powering the world, cleanly and efficiently and limitlessly. But not for some time. A number of problems must be overcome. Fuel cells are far more costly than fossil fuels, but the rising price of oil is narrowing the gap. The durability of fuel cells has yet to be established; they must show themselves to be at least as durable as current engines, and be able to operate efficiently in the dead of winter, and heat of summer. The size and weight of current fuel cells are still excessive for automobiles of all sizes. Today fuel cell stacks are extremely large and heavy in order to produce the required current to power cars and light-rail tracks. Their size and weight include the stack (with its membranes, bipolar plates, catalysts, and gas diffusion media) but not the ancillary components; namely, the heat exchanger, humidifiers, sensors, and compressor/expander. Pressure to reduce both size and weight comes from rising oil prices and global warming concerns. There isn' t a car manufacturer that hasn' t invested heavily in fuel cells. Within the coming decade the problem should be solved as there are a number of different types of fuel cells being developed. In fact, recently, chemists at the University of North Carolina developed a new, much improved polymer membrane, based on perfluoropolyester that doesn't dissolve when the water content rises, and which can operate efficiently at 120°C, well above the standard 80°C. This new polymer conducts protons 3 times better than does the standard Nafion (DuPont) polymer membrane, which will markedly improve the performance of automotive fuel cells .
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