The Hydrogen Air Fuel Cell

The fuel cell was discussed in Chapter 6. The automobile fuel cell will likely make a profound impact on the private automobile; a repeat of the discussion of how fuel cells function is warranted. A fuel cell is a special type of battery. In commonplace batteries, the chemicals and hardware used to produce the electric current are placed within the battery package. As the battery is used, the reaction products remain within the confines of the package. Because of the fixed mass of reactive chemicals, a battery has a fixed total power output. When it is depleted, it must be replaced or recharged. Recharging is the slow process of driving the chemical reaction in reverse with an external source of electric power.

In the fuel cell, only the hardware is placed within the package. The chemicals used to generate the electric current are stored outside the package and the reaction products are vented to the atmosphere. Fuel cells cannot become discharged. As long as fuel is available, a fuel cell can continue to provide power. It never needs replacing or recharging to continue operation. Refer back to Figure 6.7 for a schematic diagram of a fuel cell and Figure 6.8 and 6.9 for a picture of the Ballard automotive fuel cell.

The difference in the service provided by a battery system and a fuel system is well illustrated by comparing a battery-powered lantern to a gasoline-fueled lantern. When a battery powered lantern stops producing light it can be regenerated by replacing or recharging the batteries. The gasoline-fueled lantern continues to produce light as long as there is fuel. At the same starting weight, the gasoline lantern will provide light far longer than the battery lantern. The fuel provides 20 to 100 times more energy per unit weight than a battery. It takes dozens of kilograms of single use batteries to provide lighting time equal to one-kilogram charge of gasoline. Changing batteries or refueling can be accomplished in a minute or two. Recharging a battery can take hours.

Like a battery, the fuel cell has a group of metal plates (electrodes) separated slightly by an ionic electric conductor termed an electrolyte. An electrolyte is a material that conducts an electric current by the movement of ions. Metals conduct electric current by the movement of electrons. The metal electrodes are arranged in pairs so one can be subjected to hydrogen pressure and the other to air (or oxygen) pressure. The appropriate electrodes are treated with a catalyst that activates the hydrogen and the oxygen. At the hydrogen electrode, hydrogen gives up an electron and forms a hydrogen ion. The hydrogen ion enters the electrolyte. At the air electrode, an oxygen atom accepts two electrons and reacts with a water molecule to produce two hydroxyl ions. The hydroxyl ions enter the electrolyte. In the electrolyte, the hydrogen ion and the hydroxyl ions react to produce water. The electrons given up by the hydrogen, and accepted by the oxygen, constitute an electric current that can flow outside the fuel cell and drive electrical devices.211

Many types of electrolytes have been used in fuel cells. Water solutions of acids, such as phosphoric, sulfuric, and trifluoroacetic acids (acidic electrolytes), and bases such as sodium hydroxide or potassium hydroxide (alkaline electrolytes), can be incorporated into efficient cells. Cells using water solutions as electrolytes have complex problems of water management and electrolyte retention under conditions of severe physical motion. These will probably not be suitable for automobile service. For stationary applications described in Chapter 6 the water based electrolytes may offer advantages.

Alkaline electrolyte fuel cells are relatively inexpensive and show high efficiency. They are preferred when pure oxygen is supplied. Unfortunately, alkaline electrolytes react with the slightly acidic carbon dioxide of the atmosphere. This reaction changes the chemical nature of the electrolyte. After a few hours of operation with air as the source of oxygen, the alkaline fuel cell no longer functions efficiently.

The high temperature fuel cells (300 degrees Celsius and higher), discussed in chapter 6, use electrolytes made of solid ceramics, such as aluminum oxide, or molten salts, such as alkaline carbonates. These fuel cells also resist carbon dioxide. The high temperature of operation is difficult to maintain in the relatively small size required for automobiles. Thick insulation would be required to protect the other systems of the automobile from the heat. This thick insulation would be bulky and heavy, creating space and weight challenges in automobile design. Energy would be wasted maintaining the operating temperature during non-use periods. In an accident, breaking the container would allow the red-hot materials to start fires and burn passengers. These characteristics probably make high temperature fuel cells unsuitable for automotive use. They remain under evaluation for power plant applications. The automotive fuel cell will use ambient air as its source of oxygen and will need an acidic electrolyte.

211 Raia, Ernest, "Fuel Cells", High Technology, December 1984

Fuel cells built for the space program using an acidic solid polymer electrolyte. Because the electrolyte is a solid, it can be used in any position including zero gravity. Automotive air-hydrogen fuel cells will likely use solid polymer electrolytes. Figure 6.7 showed a highly simplified schematic diagram of a single cell of a solid polymer fuel cell. Figure 6.8 shows multi-celled solid polymer fuel cell developed by Ballard Ltd with solid polymer technology.

Figure 6.7 showed a low-pressure air pump driving a continuous flow of filtered air over the catalytic oxygen electrode. An excess of air is required to sweep the inert nitrogen away from the electrode. This flow of air also removes waste heat and water produced by the reaction. Electrons, water and oxygen react at the oxygen electrode to produce hydroxyl ions. The negatively charged hydroxyl ions pass into the solid polymer electrolyte. Pure hydrogen is piped directly to the catalytic hydrogen electrode. At the hydrogen electrode, electrons and hydrogen ions are produced and pass into the solid polymer. The electrons produced by the hydrogen electrode pass out of the fuel cell to be used as electric current. After performing work outside the cell, they return to the oxygen electrode. Within the solid polymer electrolyte, the negative hydroxyl ions and positive hydrogen ions react to produce water. Water is the waste product of the reaction. A small amount of water must be retained within the electrolyte to give proper conductivity. The water, not retained in the electrolyte, is vented from the fuel cell as water vapor in the low-pressure air stream.

Unlike a common chemical battery, the metal plates and electrolytes in the fuel cell are unchanged by the chemical reactions producing the electric current. Hydrogen and air enter the fuel cell and water and an electric current are produced. These characteristics provide the fuel cell with three great advantages compared to other batteries.

First, the fuel cell cannot become discharged as can a battery. As long as hydrogen and air are available, the fuel cell can produce a current. Second, the fuel cell case does not have to be designed with space to hold all the input chemicals or the reaction products. The hydrogen is stored externally, the oxygen is obtained from the air and the product water is vented back to the air as vapor. Third, there is no requirement for a long recharging time. Recharging requires only refilling the hydrogen tank as shown in Figure 7.6.

The electrochemical generation of power in the fuel cell is not limited by the same factors limiting combustion engines. Under carefully controlled conditions of low power output, a fuel cell can approach 100% efficiency in conversion of the potential energy of hydrogen into electrical energy. In the future, when fuel cells have been optimized for use in automobiles, it is reasonable to expect high efficiency.

In common service, the internal combustion automobile engine operates far off optimum, and only delivers an efficiency of 10% to 20%. This means that 10% to 20% of the energy available in the gasoline actually gets used in propelling the automobile. The hybrid automobiles introduced by Toyota 212 and Honda abundantly demonstrated this characteristic. These cars have about the same performance as competitive small automobiles but consume only half as much fuel. These automobiles operate on the battery for acceleration and cruising. The internal combustion engine's primary task is charging the batteries. The engine can be operated at the optimum efficient speed during the charging process. This raises the average engine efficiency to 30% to 35%. This nearly doubles the kilometers per liter of the automobile. This is an excellent improvement over the standard automobile. It is also nearing the theoretical maximum for an internal combustion engine.

212 Normile, Dennis, "Toyota's Hybrid Hits the Streets First", Science, Vol. 285, No. 5428, July 30,1999, Page 706

Figure 7.6 Stuart Energy Systems' Personal Automobile Hydrogen Re-fueler

This module can be kept in a garage to re-fuel an automobile using line current from current utility power sources to make Hydrogen

Figure 7.6 Stuart Energy Systems' Personal Automobile Hydrogen Re-fueler

This module can be kept in a garage to re-fuel an automobile using line current from current utility power sources to make Hydrogen

When the energy efficiencies of the electric motors and transmission in the fuel cell automobile are taken into account the total system efficiency will be about 50% to 70%. The fuel cell automobile will have about four times the energy efficiency of current automobiles and possibly twice that of the new generation of hybrid automobiles. These comparisons are shown in Table 7.2

Vehicle type

Weight

Kilometers Liter

Vehicle Efficiency

Fuel

Toyota Corolla - Standard

1130

13

~10%-20%

Gasoline

Toyota Prius - Hybrid

1250

21

~25%-35%

Gasoline

Honda Civic - Standard

1130

13

~10%-20%

Gasoline

Honda Insight - Hybrid

850

25

~25%-35%

Gasoline

Notional Fuel Cell Auto

1200

(Comp, gas)

~50%-70%

Hydrogen

Table 7.2 Efficiency of Hybrid and Fuel Cell Automobiles

The reason current automobiles are inefficient is that most of the time an automobile power plant is operating at a small fraction of its potential power output. For example, an auto with a 75-kilowatt (100 hp) engine only uses about 5 to 7 kilowatts for cruising at a constant speed of 100 kilometers per hour. The peak power potential of an engine is only used during acceleration. The peak efficiency of a piston engine occurs near the operational conditions of peak power. The efficiency of an internal combustion engine is greatly reduced from its peak value when it is operated at part load, as when cruising. As a result, under most operating conditions, an automobile engine is operating well off peak efficiency. To produce maximum power output the internal combustion engine must be operated at several thousand revolutions per minute. This results in the need for a complex transmission and clutch arrangement to allow the automobile to start into motion and to match the engine speed with the vehicle speed.

The fuel cell is most efficient at small loads. The efficiency decreases as the load increases and it is least efficient at full load. Because of this characteristic, the fuel cell operates efficiently under the most common driving conditions of partial load. The fuel cell drive system can deliver the same power to the wheels at zero revolutions per minute as at high speeds. Because of these relationships, the fuel cell is better suited as an automobile power source than is the internal combustion engine.

This improvement in efficiency offers a number of advantages to the air-hydrogen fuel cell powered vehicle. With a four times greater efficiency, only one fourth as much fuel (in terms of energy) need be carried on the vehicle to achieve the same range. This will result in a much smaller hydrogen tank, one more easily accommodated within the envelope of the automobile. Since the amount of hydrogen needed is so small, it is feasible to use simple, low cost, high-pressure gas storage for the hydrogen. This is the system used in the vehicle being refueled in Figure 7.6.

The cost of operating the vehicle will be strongly influenced by the small amount of hydrogen required for the hydrogen fuel cell. If hydrogen can be produced at the same relative cost per unit of energy as gasoline, then the fuel cost of operating the fuel cell vehicle will be only one fourth the cost of operating a similar vehicle fueled with gasoline. If the future cost of hydrogen is two or three times more per unit energy than gasoline, the cost of operating the fuel cell vehicle will still be lower. Only when the cost of the hydrogen is 4 times more than gasoline, on an energy basis, will the operating costs be the same.

The generic air-hydrogen fuel cell automobile is shown in Figure 7.7. It has virtually none of the fuel related disadvantages associated with today's automobile. The fuel cell produces little sound when operating. Consequently, the only noise from the fuel cell automobile will be the soft hum of the electric motor and the scuff of rubber tires on the road. The fuel cell emits only water vapor. No pollutants of any type are possible. The nitrogen oxides, the only pollutants from the internal combustion engine operating on hydrogen, are not produced by the fuel cell.

Each wheel has its own drive motor

Each wheel has its own drive motor

Figure 7.7 A Generic Fuel Cell Powered Automobile

Using Compressed Gas Hydrogen Storage

Hydrogen pipe line from storage tank

Gaseous hydrogen storage tanks Damage resistant package

Figure 7.7 A Generic Fuel Cell Powered Automobile

Using Compressed Gas Hydrogen Storage

At its current state of development, air-hydrogen fuel cells are somewhat heavier and more costly than internal combustion engines of the same power rating. As discussed in Chapter 6 there are ongoing efforts to reducing both the weight and the cost of fuel cells. Improvements in the air-hydrogen fuel cell will be a major goal in the use of hydrogen as a fuel. In the future, replacing the internal combustion engine with the air-hydrogen fuel cell will greatly improve the operational characteristics of the automobile. These improvements will take the form of vastly improved fuel energy efficiency, much smaller fuel load, no emission of any type of pollutants, potentially low maintenance and almost soundless operation. Figure 7.8 shows the type of station the future owner of a fuel cell automobile will encounter. It may be possible to re-fuel in your own garage using hydrogen from the domestic supply line. As shown in Figure 7.6, it will still be necessary to have refueling stations located at the side of the road for those who do not re-fuel at home and when necessary during a long trip.

Figure 7.9 show the fuel cell demonstration automobile developed by the Ford Motor Company. Ford's vehicles store hydrogen as high-pressure gas. Figure 7.10 shows a second Ford fuel cell automobile pictured next to the refueling station developed by Hydrogen Consultants of Denver Colorado. Figure 7.11 shows a fuel cell bus next to its refueling station. This bus is in service carrying passengers in Southern California.

Another small advantage to the fuel cell is the lack of need for crankcase oil. In 1991, 5.250 billion liters of crankcase motor oil were discarded in the United States. 213 A significant amount of this oil was used as fuel but about 25% was illegally dumped or placed in a landfill. All this contributed to environmental pollution. The Fuel cell automobile does not use any crank case oil so there will be none to throw away.

213 http://pasture.ecn.purdue.edu/~epados/waste/src/oiifact.htm

Figure 7.8 Refueling Station

Street side Refueling with Hydrogen Gas Picture supplied by Stuart Energy Systems

Figure 7.8 Refueling Station

Street side Refueling with Hydrogen Gas Picture supplied by Stuart Energy Systems

In the discussion of automobile engines, it was noted that small internal combustion engines would be difficult to adapt to hydrogen because small efficient liquid hydrogen tanks are not feasible. With the development of the technology for the automotive fuel cell, small engine systems can be replaced by fuel cell electric motor systems. The high fuel efficiency of the fuel cell should provide adequate operating time with hydrogen stored as a compressed gas.

Figure 7.9 Ford Fuel Cell Demonstration Automobile

Picture Supplied by Ford Motor Company

Figure 7.9 Ford Fuel Cell Demonstration Automobile

Picture Supplied by Ford Motor Company

One option for refueling small engines would be gas cylinder exchange. A small cylinder of hydrogen can be obtained from the local supply center and attached to the small fuel cell system. When empty the cylinder will be exchanged for a full cylinder. A second option would be the implementation of a pumping system that would allow the cylinder to be recharged at its use point from local pipeline supplied hydrogen. The personal fueler shown in Figure 7.6 could be used to fill small motors at home. The great thermodynamic efficiency of the fuel cell will allow the use of a convenient sized cylinder. These small systems will greatly benefit from the lack of noxious emission and operating noise.

The safety of hydrogen-fueled automobiles is a continuous source of discussion. Total safety for energetic fuel systems, whether hydrogen or hydrocarbon fueled, remains elusive. Gasoline causes poisoning, fires and produces carbon monoxide, but we have learned to handle it with an acceptable level of safety. Hydrogen systems will be designed to be safe under all normal operating conditions. The safety challenge is in evaluating the consequences of accidents. Hydrogen dangers are a bit different from those of gasoline. The details will depend on whether the hydrogen is stored as a cryogenic liquid or as high-pressure gas. In a crash fuel will be spilled. If Cryogenic storage is used, there is a danger from freezing if exposed to the cold liquid. With gasoline, the danger is direct poisoning from contact or breathing the fumes. Hydrogen is not toxic but it can smother if it prevents air from reaching a person. Hydrogen fires will flash and float away because of buoyancy. Gasoline fire persists and produces toxic carbon monoxide. With high-pressure gas, there is little danger from fire and none from freezing. If the tank is ruptured when fully charged it might act like a bomb. In total the danger from the use of hydrogen appear to be no greater, and possibly significantly less than those we are accustomed to with gasoline and natural gas. The preponderance of opinion of those experienced in hydrogen handling is that hydrogen systems can be made safe for every day use. For hydrogen, full understanding of system safety will be difficult to establish until multiple full-scale systems are available to test. For a detailed discussion of hydrogen safety see Chapter 11 of Peter Hoffmann Book, "Tomorrow's Energy," excerpted at the start of Chapter 5.

Figure 7.10 Ford Fuel Cell Automobile at a Refueling Station

Refueling station built by Air Products Inc. and Hydrogen Components Inc. Picture supplied by Hydrogen Components Inc. Denver Colorado

Figure 7.10 Ford Fuel Cell Automobile at a Refueling Station

Refueling station built by Air Products Inc. and Hydrogen Components Inc. Picture supplied by Hydrogen Components Inc. Denver Colorado m m

Figure 7.11 Hydrogen-air fuel cell Bus Owned by Sun Line Transit

Coachella Valley, California Picture supplied by Hydrogen Components Inc. Denver Colorado

Figure 7.11 Hydrogen-air fuel cell Bus Owned by Sun Line Transit

Coachella Valley, California Picture supplied by Hydrogen Components Inc. Denver Colorado

Some members of the oil industry have discussed equipping an automobile with a reactor that produces hydrogen from a fossil fuel. The notion is to capture the high efficiency of the fuel cell and still allow the use of fossil fuels. 214 The Shell study results displayed in Figure 7.5 shows that if a full cycle analysis is performed the inefficiency of the over all system is such that it actually produces more green house gas per kilometer than a hybrid automobile. The automobile with this system would not only have a fuel cell and an electric drive motor but would also have some type of mini oil refinery. This mini refinery would convert a fossil fuel to hydrogen and carbon dioxide. The two gasses would be separated and the hydrogen would be used to run the fuel cell. The carbon dioxide would be vented to the atmosphere. The only justification for this system is that it would allow the exploitation of the high thermodynamic efficiency of the fuel cell. Oil companies like this concept because it will allow them to continue to provide a historic fuel for transportation.

If this type of system can be developed to provide a low cost vehicle, it could be useful during the period of transition from fossil fuels to fusion hydrogen. It might potentially provide an efficient vehicle that could operate on either fuel, although tanks for 2 different fuels will present problems. This could allow the operator to take a long trip with the assurance that one of the two possible fuels would be available.

214 Malakoff, David, "U.S. Supercars: Around the Comer, or Running on Empty?", Science, Vol. 285, No. 5428, July 30, 1999, Page 680

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment