1.3 HYDROGEN STORAGE ON VEHICLES
A sufficient amount of hydrogen to provide satisfactory driving range must be stored onboard a hydrogen-powered vehicle. This is a significant challenge because, at normal temperature and pressure, a given volume of hydrogen is very light and contains very little energy. Hydrogen vehicle fuel storage systems on commercial vehicles are larger, heavier, and more expensive than diesel vehicle fuel storage systems. Given the limitations of onboard hydrogen storage, hydrogen-powered commercial vehicles may not provide comparable operating range to typical diesel-powered commercial vehicles.
There are five ways that the hydrogen can be stored on the vehicle:
• As a high-pressure gas,
• As a very low temperature liquid,
• Chemically bound or physically absorbed onto a material such as a solid “hydride,” As a component of a liquid hydrocarbon fuel (which is reformed), or
• As a component of water (H20) (which is hydrolyzed).
Currently the most common method of onboard hydrogen storage for vehicles powered by fuel cells and hydrogen ICEs is as a compressed gas. This is likely to continue to be true for the foreseeable future.
Wednesday, April 30, 2014
1.2.3 Hydrogen Injection Systems
A hydrogen injection system for a diesel engine produces small amounts of hydrogen and oxygen on demand by electrolyzing water carried onboard the vehicle. The electricity required is supplied by the engine’s alternator or 12/24-volt electrical system (see Section 1.5 for a description of electrolysis). The hydrogen and oxygen are injected into the engine’s air intake manifold, where they mix with the intake air. In theory, the combustion properties of the hydrogen result in more complete combustion of diesel fuel in the engine, reducing tailpipe emissions and improving fuel economy (CHEC, n.d.).
Limited laboratory testing of a hydrogen injection system installed on an older diesel truck engine operated at a series of constant speeds showed a 4 percent reduction in fuel use and a 7 percent reduction in particulate emissions with the system on (ETVC, 2005).
A hydrogen injection system for a diesel engine produces and uses significantly less hydrogen than a hydrogen fuel cell or hydrogen ICE, and does not require that compressed or liquid hydrogen be carried on the vehicle. The system is designed to produce hydrogen only when required, in response to driver throttle commands. When the system is shut-off, no hydrogen is present on the vehicle.
A hydrogen injection system for a diesel engine produces small amounts of hydrogen and oxygen on demand by electrolyzing water carried onboard the vehicle. The electricity required is supplied by the engine’s alternator or 12/24-volt electrical system (see Section 1.5 for a description of electrolysis). The hydrogen and oxygen are injected into the engine’s air intake manifold, where they mix with the intake air. In theory, the combustion properties of the hydrogen result in more complete combustion of diesel fuel in the engine, reducing tailpipe emissions and improving fuel economy (CHEC, n.d.).
Limited laboratory testing of a hydrogen injection system installed on an older diesel truck engine operated at a series of constant speeds showed a 4 percent reduction in fuel use and a 7 percent reduction in particulate emissions with the system on (ETVC, 2005).
A hydrogen injection system for a diesel engine produces and uses significantly less hydrogen than a hydrogen fuel cell or hydrogen ICE, and does not require that compressed or liquid hydrogen be carried on the vehicle. The system is designed to produce hydrogen only when required, in response to driver throttle commands. When the system is shut-off, no hydrogen is present on the vehicle.
1.2.2 Hydrogen Internal Combustion Engines
Theoretically any typical spark-ignited engine, like the gasoline engines used in most cars, can operate on a range of liquid or gaseous fuels, including hydrogen. However, due to differences in the chemical properties of the various fuels, the designs of engines optimized for each are quite different.
Because of the wide flammability range of hydrogen, an internal combustion engine (ICE) operating on hydrogen can operate with a much leaner air/fuel mixture than a typical gasoline engine, which improves efficiency. A hydrogen ICE developed by Ford Motor Company can operate with an air fuel ratio as high as 86:1, compared to 14.7:1 for typical gasoline engines (see Figure 7). This results in about a 25 percent improvement in efficiency (NEW-CARS, 2003).
Because hydrogen is a light gas, it displaces more volume in the combustion chamber than gasoline vapors, and super-charging is generally required to get equivalent power output as the same sized gasoline engine. Other design changes compared to typical gasoline engines may be required to reduce the possibility of pre-ignition, or knock, because of hydrogen’s low ignition energy. These may include the use of a disk-shaped combustion chamber to reduce turbulence in the cylinder, the use of more than one spark-plug, and the use of multiple exhaust valves (College of the Desert, 2001b).
Theoretically any typical spark-ignited engine, like the gasoline engines used in most cars, can operate on a range of liquid or gaseous fuels, including hydrogen. However, due to differences in the chemical properties of the various fuels, the designs of engines optimized for each are quite different.
Because of the wide flammability range of hydrogen, an internal combustion engine (ICE) operating on hydrogen can operate with a much leaner air/fuel mixture than a typical gasoline engine, which improves efficiency. A hydrogen ICE developed by Ford Motor Company can operate with an air fuel ratio as high as 86:1, compared to 14.7:1 for typical gasoline engines (see Figure 7). This results in about a 25 percent improvement in efficiency (NEW-CARS, 2003).
Because hydrogen is a light gas, it displaces more volume in the combustion chamber than gasoline vapors, and super-charging is generally required to get equivalent power output as the same sized gasoline engine. Other design changes compared to typical gasoline engines may be required to reduce the possibility of pre-ignition, or knock, because of hydrogen’s low ignition energy. These may include the use of a disk-shaped combustion chamber to reduce turbulence in the cylinder, the use of more than one spark-plug, and the use of multiple exhaust valves (College of the Desert, 2001b).
Besides the potential for better fuel economy because of improved efficiency, hydrogen ICEs offer other advantages over gasoline and diesel engines, including reduced exhaust emissions. Because there is no carbon in the fuel, a vehicle powered by a hydrogen ICE would have zero emissions of the greenhouse gas CO2. Tailpipe emissions of nitrogen oxides and volatile organic hydrocarbons would also be lower.
Typically the hydrogen fuel for a hydrogen ICE is carried on the vehicle as a high-pressure compressed gas (see Section 1.3 for a description of hydrogen storage systems).
In addition to Ford, at least two other companies have developed hydrogen ICEs for cars, either as prototypes or commercial products (CHHN, 2004). There are also fourteen buses currently operating in Berlin, Germany, and one in Thousand Palms, California, which are powered by heavy-duty hydrogen ICEs (Chandler and Eudy, 2006).
In addition to Ford, at least two other companies have developed hydrogen ICEs for cars, either as prototypes or commercial products (CHHN, 2004). There are also fourteen buses currently operating in Berlin, Germany, and one in Thousand Palms, California, which are powered by heavy-duty hydrogen ICEs (Chandler and Eudy, 2006).
1.2.1.1 Solid Oxide Fuel Cell APUs
Like PEM fuel cells, solid oxide fuel cells (SOFCs) are galvanic cells that directly produce electricity from hydrogen and oxygen through an electrochemical reaction. However, SOFCs are constructed of different materials and use a different chemical reaction from PEM fuel cells.
SOFCs operate at much higher temperatures than PEM fuel cells – between 1,100 °F and 1,800 °F.
When combined with a small fuel reformer they can also use diesel fuel or gasoline vapors as fuel, eliminating the need to carry hydrogen gas onboard.
In an SOFC, the electrolyte is not a plastic-like material as it is in a PEM cell; it is a ceramic material made of a solid metal oxide, usually zirconia oxide. This electrolyte does not need to be coated with an expensive platinum catalyst as in a PEM cell. As with a PEM fuel cell, the major by-products of the reactions inside the cell are electricity, water, and heat. See Figure 6. Also see Appendix A for a more detailed description of the construction of SOFCs and the chemical reactions that take place inside the cells.
Like PEM fuel cells, solid oxide fuel cells (SOFCs) are galvanic cells that directly produce electricity from hydrogen and oxygen through an electrochemical reaction. However, SOFCs are constructed of different materials and use a different chemical reaction from PEM fuel cells.
SOFCs operate at much higher temperatures than PEM fuel cells – between 1,100 °F and 1,800 °F.
When combined with a small fuel reformer they can also use diesel fuel or gasoline vapors as fuel, eliminating the need to carry hydrogen gas onboard.
In an SOFC, the electrolyte is not a plastic-like material as it is in a PEM cell; it is a ceramic material made of a solid metal oxide, usually zirconia oxide. This electrolyte does not need to be coated with an expensive platinum catalyst as in a PEM cell. As with a PEM fuel cell, the major by-products of the reactions inside the cell are electricity, water, and heat. See Figure 6. Also see Appendix A for a more detailed description of the construction of SOFCs and the chemical reactions that take place inside the cells.
Unlike a PEM cell, an SOFC does not need to be fueled with pure hydrogen gas. Because SOFCs operate at such high temperature and because oxygen ions are transferred through a solid oxide electrolyte material—not hydrogen ions—SOFCs support automatic “reforming” of gaseous hydrocarbon fuels like methane (natural gas) within the device. Reforming is the chemical process of separating the hydrogen from the carbon atoms in a hydrocarbon fuel (see Section 1.4). Diesel fuel and gasoline vapors can not be internally reformed by an SOFC, but can be used to fuel an SOFC if it is combined with a relatively simple fuel reformer/processor.
When using diesel fuel, the “reformate” produced by the fuel processor and introduced as the fuel at the anode of the SOFC will include hydrogen, nitrogen, carbon monoxide, and CO2. The exhaust from the SOFC will also include CO2 and nitrogen, as well as water and waste heat.
SOFCs operate at much higher temperatures than PEM fuel cells—between 1,100 °F and 1,800 °F—so the waste heat created during operation is also at a higher temperature and can, therefore, more easily be put to use, for example, to heat the interior of a vehicle as is typical of the waste heat from an ICE.
SOFCs operate at much higher temperatures than PEM fuel cells—between 1,100 °F and 1,800 °F—so the waste heat created during operation is also at a higher temperature and can, therefore, more easily be put to use, for example, to heat the interior of a vehicle as is typical of the waste heat from an ICE.
There are at least fifteen companies that have demonstrated prototype or commercial SOFC systems (HARC, 2004). Most of these systems are small, producing from 200 watts to 25 kilowatts of power. Several manufacturers are developing low-power systems specifically for use as an auxiliary power unit (APU) on commercial trucks (DELPHI, 2005)
Truck APUs are used to provide electrical power and sometimes heat to power truck accessories such as cabin lighting, air conditioning, and heating. Most often used with sleeper berth-equipped, truck-tractors they allow these loads, which are normally supplied by the truck’s main engine, to be supplied even when the main engine is off.
Truck APUs are used to provide electrical power and sometimes heat to power truck accessories such as cabin lighting, air conditioning, and heating. Most often used with sleeper berth-equipped, truck-tractors they allow these loads, which are normally supplied by the truck’s main engine, to be supplied even when the main engine is off.
Without an APU, many long-haul truckers end up idling their main engines for eight hours a day or more while resting in the sleeper-berth. This practice is wasteful and results in unnecessary harmful exhaust emissions. Testing by the U.S. Environmental Protection Agency has shown that a commercial truck’s main engine typically consumes about one gallon of fuel per hour while idling, while a properly sized ICE APU will burn only about one fifth as much (EPA, 2002). The use of an APU instead of main engine idling can therefore save a truck operator money and reduce pollution at the same time.
In comparison to an ICE APU, an SOFC APU could be more efficient, smaller and lighter, quieter, and produce fewer exhaust emissions (DELPHI, 2005). Because an SOFC with a fuel processor can be fueled directly with diesel fuel, there would be no need to carry compressed hydrogen on the vehicle (see Section 1.4).
1.2.1 Hydrogen Fuel Cell Engines
Fuel cells are often compared to both internal combustion engines (ICEs) and batteries and, in fact, they share some characteristics with each. All three types of devices are used to transform one type of energy into another. A diesel engine turns chemical energy contained in diesel fuel into heat energy through combustion with oxygen from the air, and then turns that heat energy into mechanical energy, turning the vehicle’s wheels through a transmission and drive shaft.
On the other hand, a battery is a galvanic cell; it uses reactions between chemicals stored inside to turn chemical energy directly into electricity, which can then be used to power a number of devices, including an electric motor to produce mechanical energy.
Like a diesel engine, a fuel cell requires fuel (hydrogen) and oxygen (air). However, like a battery, it is capable of directly producing electricity.
A fuel cell is also a galvanic cell; the hydrogen and oxygen do not combust inside the device. Instead, the hydrogen and oxygen react electrochemically and produce electricity. See Table 3 for a comparison of the major differences between fuel cells and ICEs and batteries.
As with a battery, the electricity produced by a fuel cell can be used to power any number of devices. In the case of a vehicle, it is most often used to power an electric motor to move the vehicle down the road. A fuel cell vehicle is, therefore, an electric vehicle, powered by an electric motor.
Fuel cells have been around for a long time and have been used in the United States space program since the 1960s (College of the Desert, 2001c). It has only been in the last few years, however, that they have been developed for use in conventional vehicles.
There are a number of different fuel cell technologies that employ different chemical reactions to combine hydrogen and oxygen to produce electricity. The most common technology used in vehicles is called a Proton Exchange Membrane (PEM) fuel cell. See Figure 1, which shows the layout and operation of a PEM fuel cell. Also see Appendix A for a more in-depth description of the construction of a PEM fuel cell and the chemical reactions that take place in the cell.
The maximum voltage that one PEM “cell” can produce is 1.2 VDC, but the actual voltage depends on how much current is being drawn from the cell. The cell can put out the greatest amount of power at between 0.5 and 0.6 volts, so that is where they are generally designed to operate. To create a device powerful enough to power a large vehicle, up to 1,200 cells are connected in series, to produce a peak power of 100 kW or more at between 300 and 600 VDC (nominal). Physically the individual PEM cells are usually stacked together, separated by a cooling plate between each set of cells. These cooling plates circulate a mixture of water and ethylene glycol to remove excess heat created during operation of the cells. These cooling plates are part of a cooling system that is similar in both design and function to the cooling systems used with diesel engines. A collection of individual fuel cells used to create a practical device is usually referred to as a fuel cell “stack” (see Figure 2).
The fuel cell stack must be supported by a number of auxiliary systems that together make up the “fuel cell engine.” In addition to a cooling system, the fuel cell engine needs a fuel system, an air system, and a water management system. See Figure 3 for a generic schematic of a PEM fuel cell engine. In a PEM fuel cell, engine hydrogen and air are saturated with water and fed into the fuel cell stack. Inside each PEM cell, the hydrogen and air react with each other across a thin plastic-like film, called a proton exchange membrane, but they never mix. Electricity is produced by each cell, and water and a small amount of heat are the only by-products. Excess water not needed to humidify the gases is exhausted, with air, out the tailpipe.
PEM fuel cells generally operate at relatively low temperatures (140 to 180 ºF) and pressures (from 0 to 15 psig). The exact layout and details of a fuel cell engine and its subsystems will depend on the specifics of the design and its specified operating parameters. Packaging and layout of the fuel cell engine in the vehicle can also vary significantly. See Figure 4 for a photo of a fuel cell engine and electric drive motor that was installed in a transit bus.
PEM fuel cell engines fueled by hydrogen produce virtually none of the volatile organic hydrocarbon or nitrogen oxide tailpipe emissions that come from combustion of fuel in gasoline and diesel engines, and which together produce ground-level ozone, or “smog” in the atmosphere in the presence of sunlight. They also produce virtually none of the harmful particulate emissions produced by diesel engines and zero carbon dioxide emissions. Carbon dioxide is a major by-product of fuel combustion in diesel and gasoline engines. As a so-called “greenhouse gas,” carbon dioxide is a contributor to global warming.
In addition to reduced exhaust emissions, the potential benefits of using hydrogen fuel cells to power commercial vehicles include lower total energy use due to improved efficiency of the fuel cell compared to an internal combustion engine. The actual “wells-to-wheels” efficiency of a fuel cell vehicle will depend on how the system is designed, as well as how the hydrogen fuel is produced. Many fuel cell vehicles are designed with a hybrid propulsion system that incorporates a large battery to supplement the fuel cell. The battery provides power during acceleration, allowing the fuel cell to be smaller. It is also used to capture energy that is normally wasted in braking, which can later be re-used, increasing net efficiency, especially in stop-and-go city driving. See Figure 5 for a comparison of “wells-to-wheels” fuel use (liters per mile)2 for vehicles with different types of power sources. This figure is illustrative only and does not include all potential combinations of fuel and propulsion technology.
One liter per mile is equivalent to 0.26 gal/mile.
Usually pure hydrogen is used to fuel a PEM fuel cell engine. While a mixture of hydrogen and carbon dioxide can be used, other “contaminants” must be kept to a minimum in the fuel supplied to the cells, especially carbon monoxide (CO) and sulfur. Both CO and sulfur can reduce the activity of the platinum catalysts used in the PEM cells, reducing the amount of power that the cells can produce (EG&G, 2004).
Fuel cells are often compared to both internal combustion engines (ICEs) and batteries and, in fact, they share some characteristics with each. All three types of devices are used to transform one type of energy into another. A diesel engine turns chemical energy contained in diesel fuel into heat energy through combustion with oxygen from the air, and then turns that heat energy into mechanical energy, turning the vehicle’s wheels through a transmission and drive shaft.
On the other hand, a battery is a galvanic cell; it uses reactions between chemicals stored inside to turn chemical energy directly into electricity, which can then be used to power a number of devices, including an electric motor to produce mechanical energy.
Like a diesel engine, a fuel cell requires fuel (hydrogen) and oxygen (air). However, like a battery, it is capable of directly producing electricity.
A fuel cell is also a galvanic cell; the hydrogen and oxygen do not combust inside the device. Instead, the hydrogen and oxygen react electrochemically and produce electricity. See Table 3 for a comparison of the major differences between fuel cells and ICEs and batteries.
As with a battery, the electricity produced by a fuel cell can be used to power any number of devices. In the case of a vehicle, it is most often used to power an electric motor to move the vehicle down the road. A fuel cell vehicle is, therefore, an electric vehicle, powered by an electric motor.
Fuel cells have been around for a long time and have been used in the United States space program since the 1960s (College of the Desert, 2001c). It has only been in the last few years, however, that they have been developed for use in conventional vehicles.
There are a number of different fuel cell technologies that employ different chemical reactions to combine hydrogen and oxygen to produce electricity. The most common technology used in vehicles is called a Proton Exchange Membrane (PEM) fuel cell. See Figure 1, which shows the layout and operation of a PEM fuel cell. Also see Appendix A for a more in-depth description of the construction of a PEM fuel cell and the chemical reactions that take place in the cell.
The maximum voltage that one PEM “cell” can produce is 1.2 VDC, but the actual voltage depends on how much current is being drawn from the cell. The cell can put out the greatest amount of power at between 0.5 and 0.6 volts, so that is where they are generally designed to operate. To create a device powerful enough to power a large vehicle, up to 1,200 cells are connected in series, to produce a peak power of 100 kW or more at between 300 and 600 VDC (nominal). Physically the individual PEM cells are usually stacked together, separated by a cooling plate between each set of cells. These cooling plates circulate a mixture of water and ethylene glycol to remove excess heat created during operation of the cells. These cooling plates are part of a cooling system that is similar in both design and function to the cooling systems used with diesel engines. A collection of individual fuel cells used to create a practical device is usually referred to as a fuel cell “stack” (see Figure 2).
The fuel cell stack must be supported by a number of auxiliary systems that together make up the “fuel cell engine.” In addition to a cooling system, the fuel cell engine needs a fuel system, an air system, and a water management system. See Figure 3 for a generic schematic of a PEM fuel cell engine. In a PEM fuel cell, engine hydrogen and air are saturated with water and fed into the fuel cell stack. Inside each PEM cell, the hydrogen and air react with each other across a thin plastic-like film, called a proton exchange membrane, but they never mix. Electricity is produced by each cell, and water and a small amount of heat are the only by-products. Excess water not needed to humidify the gases is exhausted, with air, out the tailpipe.
PEM fuel cells generally operate at relatively low temperatures (140 to 180 ºF) and pressures (from 0 to 15 psig). The exact layout and details of a fuel cell engine and its subsystems will depend on the specifics of the design and its specified operating parameters. Packaging and layout of the fuel cell engine in the vehicle can also vary significantly. See Figure 4 for a photo of a fuel cell engine and electric drive motor that was installed in a transit bus.
PEM fuel cell engines fueled by hydrogen produce virtually none of the volatile organic hydrocarbon or nitrogen oxide tailpipe emissions that come from combustion of fuel in gasoline and diesel engines, and which together produce ground-level ozone, or “smog” in the atmosphere in the presence of sunlight. They also produce virtually none of the harmful particulate emissions produced by diesel engines and zero carbon dioxide emissions. Carbon dioxide is a major by-product of fuel combustion in diesel and gasoline engines. As a so-called “greenhouse gas,” carbon dioxide is a contributor to global warming.
In addition to reduced exhaust emissions, the potential benefits of using hydrogen fuel cells to power commercial vehicles include lower total energy use due to improved efficiency of the fuel cell compared to an internal combustion engine. The actual “wells-to-wheels” efficiency of a fuel cell vehicle will depend on how the system is designed, as well as how the hydrogen fuel is produced. Many fuel cell vehicles are designed with a hybrid propulsion system that incorporates a large battery to supplement the fuel cell. The battery provides power during acceleration, allowing the fuel cell to be smaller. It is also used to capture energy that is normally wasted in braking, which can later be re-used, increasing net efficiency, especially in stop-and-go city driving. See Figure 5 for a comparison of “wells-to-wheels” fuel use (liters per mile)2 for vehicles with different types of power sources. This figure is illustrative only and does not include all potential combinations of fuel and propulsion technology.
One liter per mile is equivalent to 0.26 gal/mile.
Usually pure hydrogen is used to fuel a PEM fuel cell engine. While a mixture of hydrogen and carbon dioxide can be used, other “contaminants” must be kept to a minimum in the fuel supplied to the cells, especially carbon monoxide (CO) and sulfur. Both CO and sulfur can reduce the activity of the platinum catalysts used in the PEM cells, reducing the amount of power that the cells can produce (EG&G, 2004).
Monday, April 28, 2014
1.2 HYDROGEN USE AS A MOTOR FUEL
There are several ways that hydrogen can be used as a motor fuel. It can be used to directly replace gasoline or diesel fuel in specially designed internal combustion engines (ICEs), or it can be used to supplement these typical fuels in existing engines. In either of these cases, the vehicle drive system will be identical to those used on most gasoline-powered or diesel-powered vehicles. The engine will drive the vehicle’s wheels through a transmission, drive shaft, and front or rear axle.
Hydrogen can also be used as the fuel source for a “fuel cell engine,” in which case the vehicle’s drive system will be very different. A fuel cell directly creates electricity, which can be used to power an electric motor to drive the vehicle’s wheels. A fuel cell vehicle is, therefore, an electric vehicle, but one that creates its own electricity and does not need to be plugged in to recharge batteries. A small fuel cell can also be used to create electricity to directly power the auxiliary systems on a commercial truck (for example heating, air conditioning, and lighting in a sleeper berth), which are typically powered by the truck’s main engine. Using such a fuel cell auxiliary power unit (APU) would allow the driver to shut off the truck’s main diesel engine while resting, saving fuel and reducing pollution.
Regardless of whether the hydrogen will be used in a fuel cell main engine, a fuel cell APU, or an internal combustion engine, there are different ways that it can be stored on the vehicle. As described below, these different storage technologies can introduce significantly different potential hazards, including very high pressure (gaseous hydrogen storage), very low temperature (liquid hydrogen storage), or high temperature (liquid fuel reforming) (see Table 2).
Currently both fuel cells and hydrogen ICEs are in the early stages of commercialization. All of the major auto companies have fielded concept, prototype, or demonstration fuel cell sedans and sport utility vehicles in the last several years, with at least fifteen different models introduced since 2000 (Barnitt and Eudy, 2005; USFCC, 2006). Most of these vehicles have been operated by the companies themselves or have been fielded to government agencies and fleet customers as part of technology development or demonstration programs. The California Fuel Cell Partnership reports that its members have placed 134 light-duty fuel cell vehicles in service in California since 2000 (CAFCP, n.d.). In addition, there are currently nine fuel cell transit buses in service in the United States and Canada, and over 20 in Europe and Asia (Chandler and Eudy, 2006).
It is expected that commercial fuel cells will be introduced into government and transit bus fleets between 2010 and 2020, with sales to commercial vehicle fleets and the public sometime between 2020 and 2030 (DOE, 2002). It is also expected that the first use of hydrogen fuel in the commercial truck sector will be to power fuel cell APUs rather than to power fuel cell or hydrogen ICE main propulsion engines. At least one company has announced plans to introduce commercial fuel cell APUs as early as 2011 (Delphi, 2005).
Most current prototype fuel cell vehicles carry their hydrogen fuel as a compressed gas, and it is expected that this will continue to be the case for the earliest commercial vehicles. It may be desirable to store liquid hydrogen onboard a commercial vehicle because it has a higher energy density and would increase the range between fill-ups. However, onboard liquid hydrogen storage is more costly, and it is more likely that liquid hydrogen will be stored at fueling stations to supply gaseous hydrogen to vehicles. Other storage technologies, such as metal and chemical hydrides, are much further from commercial readiness (DOE, n.d.). Several fuel cell buses have been demonstrated that “reform,” or extract hydrogen from, liquid methanol onboard (Georgetown University, 2003), and there are fuel cell APU systems under development that will derive their hydrogen from onboard reforming of diesel fuel or gasoline (Delphi, 2005). In addition, there are several commercial “hydrogen injection” systems available for retrofit on diesel engines (CHEC, n.d.). These systems produce small amounts of hydrogen by electrolysis of water carried on the vehicle, which is injected into the diesel engine along with the diesel fuel.
The remainder of this chapter provides a brief overview of the types of systems that might be found on a vehicle to store or use hydrogen fuel.
There are several ways that hydrogen can be used as a motor fuel. It can be used to directly replace gasoline or diesel fuel in specially designed internal combustion engines (ICEs), or it can be used to supplement these typical fuels in existing engines. In either of these cases, the vehicle drive system will be identical to those used on most gasoline-powered or diesel-powered vehicles. The engine will drive the vehicle’s wheels through a transmission, drive shaft, and front or rear axle.
Hydrogen can also be used as the fuel source for a “fuel cell engine,” in which case the vehicle’s drive system will be very different. A fuel cell directly creates electricity, which can be used to power an electric motor to drive the vehicle’s wheels. A fuel cell vehicle is, therefore, an electric vehicle, but one that creates its own electricity and does not need to be plugged in to recharge batteries. A small fuel cell can also be used to create electricity to directly power the auxiliary systems on a commercial truck (for example heating, air conditioning, and lighting in a sleeper berth), which are typically powered by the truck’s main engine. Using such a fuel cell auxiliary power unit (APU) would allow the driver to shut off the truck’s main diesel engine while resting, saving fuel and reducing pollution.
Regardless of whether the hydrogen will be used in a fuel cell main engine, a fuel cell APU, or an internal combustion engine, there are different ways that it can be stored on the vehicle. As described below, these different storage technologies can introduce significantly different potential hazards, including very high pressure (gaseous hydrogen storage), very low temperature (liquid hydrogen storage), or high temperature (liquid fuel reforming) (see Table 2).
Currently both fuel cells and hydrogen ICEs are in the early stages of commercialization. All of the major auto companies have fielded concept, prototype, or demonstration fuel cell sedans and sport utility vehicles in the last several years, with at least fifteen different models introduced since 2000 (Barnitt and Eudy, 2005; USFCC, 2006). Most of these vehicles have been operated by the companies themselves or have been fielded to government agencies and fleet customers as part of technology development or demonstration programs. The California Fuel Cell Partnership reports that its members have placed 134 light-duty fuel cell vehicles in service in California since 2000 (CAFCP, n.d.). In addition, there are currently nine fuel cell transit buses in service in the United States and Canada, and over 20 in Europe and Asia (Chandler and Eudy, 2006).
It is expected that commercial fuel cells will be introduced into government and transit bus fleets between 2010 and 2020, with sales to commercial vehicle fleets and the public sometime between 2020 and 2030 (DOE, 2002). It is also expected that the first use of hydrogen fuel in the commercial truck sector will be to power fuel cell APUs rather than to power fuel cell or hydrogen ICE main propulsion engines. At least one company has announced plans to introduce commercial fuel cell APUs as early as 2011 (Delphi, 2005).
Most current prototype fuel cell vehicles carry their hydrogen fuel as a compressed gas, and it is expected that this will continue to be the case for the earliest commercial vehicles. It may be desirable to store liquid hydrogen onboard a commercial vehicle because it has a higher energy density and would increase the range between fill-ups. However, onboard liquid hydrogen storage is more costly, and it is more likely that liquid hydrogen will be stored at fueling stations to supply gaseous hydrogen to vehicles. Other storage technologies, such as metal and chemical hydrides, are much further from commercial readiness (DOE, n.d.). Several fuel cell buses have been demonstrated that “reform,” or extract hydrogen from, liquid methanol onboard (Georgetown University, 2003), and there are fuel cell APU systems under development that will derive their hydrogen from onboard reforming of diesel fuel or gasoline (Delphi, 2005). In addition, there are several commercial “hydrogen injection” systems available for retrofit on diesel engines (CHEC, n.d.). These systems produce small amounts of hydrogen by electrolysis of water carried on the vehicle, which is injected into the diesel engine along with the diesel fuel.
The remainder of this chapter provides a brief overview of the types of systems that might be found on a vehicle to store or use hydrogen fuel.
1. INTRODUCTION
1.1 BACKGROUND
Today, virtually all commercial trucks are powered by diesel fuel, while private cars are fueled by gasoline. While these petroleum-based fossil fuels have served society well for many years, their supply is limited, and their use creates pollution that contributes to poor air quality in many areas. Supported by our National Energy Policy, a new generation of technologies is currently being developed that allow the use of hydrogen as a fuel to power cars and trucks (see Table 1). In the future, hydrogen may be used in one of three ways to power vehicles:
•To produce electricity in a fuel cell,
•As a replacement for gasoline or diesel fuel in an internal combustion engine, or
• As a supplement to gasoline or diesel fuel used in an internal combustion engine.
Table 1. Why Hydrogen?
1.To reduce harmful pollution from vehicle exhaust
2.To reduce carbon dioxide (CO2) emissions, which contribute to global warming
3. To reduce our growing dependence on foreign oil
This document was developed by the Federal Motor Carrier Safety Administration as a reference for commercial vehicle fleet owners and operators who use hydrogen fuel in their vehicles, and it primarily focuses on safety. All motor fuels, including diesel fuel, gasoline, and natural gas pose risks of fire and explosion if handled improperly. Hydrogen is no different.
While there are risks, hydrogen can be as safe, or safer, than diesel and other fuels when vehicles and fuel stations are designed and operated properly.
While safe, hydrogen is different from other motor fuels, it has significantly different physical and chemical properties that affect how it must be safely stored and handled. Therefore, this document was designed to provide basic information about hydrogen properties and characteristics, as well as an overview of the vehicle systems than might use hydrogen fuel. It also provides basic guidelines for how vehicles, as well as fuel stations and maintenance facilities, should be designed and operated if hydrogen will be used. This information is provided so that fleet owners and operators will know what to look for—and what to do and not do—when using hydrogen fuel for their vehicles.
1.1 BACKGROUND
Today, virtually all commercial trucks are powered by diesel fuel, while private cars are fueled by gasoline. While these petroleum-based fossil fuels have served society well for many years, their supply is limited, and their use creates pollution that contributes to poor air quality in many areas. Supported by our National Energy Policy, a new generation of technologies is currently being developed that allow the use of hydrogen as a fuel to power cars and trucks (see Table 1). In the future, hydrogen may be used in one of three ways to power vehicles:
•To produce electricity in a fuel cell,
•As a replacement for gasoline or diesel fuel in an internal combustion engine, or
• As a supplement to gasoline or diesel fuel used in an internal combustion engine.
Table 1. Why Hydrogen?
1.To reduce harmful pollution from vehicle exhaust
2.To reduce carbon dioxide (CO2) emissions, which contribute to global warming
3. To reduce our growing dependence on foreign oil
This document was developed by the Federal Motor Carrier Safety Administration as a reference for commercial vehicle fleet owners and operators who use hydrogen fuel in their vehicles, and it primarily focuses on safety. All motor fuels, including diesel fuel, gasoline, and natural gas pose risks of fire and explosion if handled improperly. Hydrogen is no different.
While there are risks, hydrogen can be as safe, or safer, than diesel and other fuels when vehicles and fuel stations are designed and operated properly.
While safe, hydrogen is different from other motor fuels, it has significantly different physical and chemical properties that affect how it must be safely stored and handled. Therefore, this document was designed to provide basic information about hydrogen properties and characteristics, as well as an overview of the vehicle systems than might use hydrogen fuel. It also provides basic guidelines for how vehicles, as well as fuel stations and maintenance facilities, should be designed and operated if hydrogen will be used. This information is provided so that fleet owners and operators will know what to look for—and what to do and not do—when using hydrogen fuel for their vehicles.
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