2.1 GASEOUS HYDROGEN
Hydrogen gas is colorless, odorless, tasteless, and noncorrosive, and it is nontoxic to humans. It has the second widest flammability range in air of any gas, but leaking hydrogen rises and diffuses to a nonflammable mixture quickly. Hydrogen ignites very easily and burns hot, but tends to burn out quickly. A hydrogen flame burns very cleanly, producing virtually no soot, which means that it is also virtually invisible.
Friday, May 2, 2014
2. PROPERTIES OF HYDROGEN
Hydrogen is the most abundant element in our universe. In addition to being a component of all living things hydrogen and oxygen together make up water, which covers 70 percent of the earth. In its pure form, a hydrogen molecule is composed of two hydrogen atoms (H2) and is a gas at normal temperatures and pressures. It is the lightest gas (even lighter than helium) with only 7 percent of the density of air. If you get it cold enough (–423 °F), gaseous hydrogen will liquefy, and it can be transported and stored in this form.
There is virtually no “free” hydrogen on earth; all of it is combined with other elements (mostly oxygen or carbon) in other substances. Every molecule of water contains two hydrogen atoms and one oxygen atom. Hydrocarbon fuels such as coal, gasoline, diesel, and natural gas also contain hydrogen. In the case of gasoline and diesel fuel, there are approximately two hydrogen atoms for every carbon atom, while natural gas contains four hydrogen atoms for every carbon atom.
In order to directly use hydrogen as a fuel (whether in a fuel cell or in an internal combustion engine), it must be separated from these other elements. The hydrogen fuel used in vehicles is either derived from water (by electrolysis) or from a gaseous or liquid hydrocarbon fuel (by reforming). After being separated it must be stored—first at the fuel station and then on the vehicle. Some fuel stations include liquid hydrogen storage, but on the vehicle, hydrogen is usually stored as a gas at high pressure. It is also possible to store a liquid fuel (gasoline, diesel, and methanol) onboard a vehicle and then use an onboard reformer to separate the hydrogen just before it is used in the fuel cell engine. While this requires additional equipment on the vehicle, it removes the need for high-pressure gas storage.
This chapter provides an overview of the properties of both gaseous and liquid hydrogen that are necessary to understand how hydrogen differs from more familiar motor fuels, such as gasoline and diesel fuel, and what is required to handle and use it safely. 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. All fuels require particular design and handling practices based on their properties, and all present certain hazards when mishandled. Understanding the properties of hydrogen is necessary to understanding what is required to use it safely.
Building on the discussion of hydrogen properties, this chapter also provides an overview of the general principles that govern safe design and use of hydrogen fuel. These principles inform the design and operating guidelines presented in chapters 3 through 5.
Hydrogen is the most abundant element in our universe. In addition to being a component of all living things hydrogen and oxygen together make up water, which covers 70 percent of the earth. In its pure form, a hydrogen molecule is composed of two hydrogen atoms (H2) and is a gas at normal temperatures and pressures. It is the lightest gas (even lighter than helium) with only 7 percent of the density of air. If you get it cold enough (–423 °F), gaseous hydrogen will liquefy, and it can be transported and stored in this form.
There is virtually no “free” hydrogen on earth; all of it is combined with other elements (mostly oxygen or carbon) in other substances. Every molecule of water contains two hydrogen atoms and one oxygen atom. Hydrocarbon fuels such as coal, gasoline, diesel, and natural gas also contain hydrogen. In the case of gasoline and diesel fuel, there are approximately two hydrogen atoms for every carbon atom, while natural gas contains four hydrogen atoms for every carbon atom.
In order to directly use hydrogen as a fuel (whether in a fuel cell or in an internal combustion engine), it must be separated from these other elements. The hydrogen fuel used in vehicles is either derived from water (by electrolysis) or from a gaseous or liquid hydrocarbon fuel (by reforming). After being separated it must be stored—first at the fuel station and then on the vehicle. Some fuel stations include liquid hydrogen storage, but on the vehicle, hydrogen is usually stored as a gas at high pressure. It is also possible to store a liquid fuel (gasoline, diesel, and methanol) onboard a vehicle and then use an onboard reformer to separate the hydrogen just before it is used in the fuel cell engine. While this requires additional equipment on the vehicle, it removes the need for high-pressure gas storage.
This chapter provides an overview of the properties of both gaseous and liquid hydrogen that are necessary to understand how hydrogen differs from more familiar motor fuels, such as gasoline and diesel fuel, and what is required to handle and use it safely. 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. All fuels require particular design and handling practices based on their properties, and all present certain hazards when mishandled. Understanding the properties of hydrogen is necessary to understanding what is required to use it safely.
Building on the discussion of hydrogen properties, this chapter also provides an overview of the general principles that govern safe design and use of hydrogen fuel. These principles inform the design and operating guidelines presented in chapters 3 through 5.
1.5 ELECTROLYSIS OF WATER
The most abundant source of hydrogen on earth is water—every molecule of water contains one oxygen atom and two hydrogen atoms. It is relatively simple to separate the hydrogen in water from the oxygen using electricity to run an electrolyzer. An electrolyzer is a galvanic cell composed of an anode and a cathode submerged in a water-based electrolyte.
In many ways, the operation of an electrolyzer is the opposite of operating a hydrogen fuel cell. In a fuel cell, hydrogen and oxygen are supplied to the anode and the cathode, and they combine to form water while creating an electrical current that can be put to use (see Section 1.2.1 and Appendix A). In an electrolyzer, an electrical current is applied between the anode and the cathode, which causes the water in the electrolyte to break down, releasing oxygen gas at the anode and hydrogen gas at the cathode (see Figure 12).
Water and an onboard electrolyzer cannot be used to power a fuel cell or hydrogen ICE vehicle because of the large amount of electricity required to operate the electrolyzer. An electrolyzer can be used at a centralized fueling station to produce hydrogen, which is then compressed for on-site storage and delivery to vehicles. For a centralized electrolyzer, the electrical energy could be supplied from the electrical grid or from a dedicated renewable source, such as a wind turbine or solar cell array.
The most abundant source of hydrogen on earth is water—every molecule of water contains one oxygen atom and two hydrogen atoms. It is relatively simple to separate the hydrogen in water from the oxygen using electricity to run an electrolyzer. An electrolyzer is a galvanic cell composed of an anode and a cathode submerged in a water-based electrolyte.
In many ways, the operation of an electrolyzer is the opposite of operating a hydrogen fuel cell. In a fuel cell, hydrogen and oxygen are supplied to the anode and the cathode, and they combine to form water while creating an electrical current that can be put to use (see Section 1.2.1 and Appendix A). In an electrolyzer, an electrical current is applied between the anode and the cathode, which causes the water in the electrolyte to break down, releasing oxygen gas at the anode and hydrogen gas at the cathode (see Figure 12).
Water and an onboard electrolyzer cannot be used to power a fuel cell or hydrogen ICE vehicle because of the large amount of electricity required to operate the electrolyzer. An electrolyzer can be used at a centralized fueling station to produce hydrogen, which is then compressed for on-site storage and delivery to vehicles. For a centralized electrolyzer, the electrical energy could be supplied from the electrical grid or from a dedicated renewable source, such as a wind turbine or solar cell array.
1.4 REFORMING OF LIQUID FUELS
All liquid hydrocarbon fuels (gasoline, diesel fuel, kerosene, and methanol), as well as natural gas, contain significant hydrogen, which is chemically bound to carbon. Both diesel fuel and gasoline contain about two hydrogen atoms for every carbon atom, while natural gas contains four.
“Reforming” of a hydrocarbon fuel is a chemical process that converts the natural gas or liquid fuel into a hydrogen-rich gas. The product of this process is called “reformate,” and when used to fuel a PEM fuel cell, it is typically composed of a mixture of hydrogen gas, carbon dioxide, nitrogen, and water vapor. Reformate used to fuel an SOFC can also contain carbon monoxide. Depending on the fuel being reformed and the process used, the reformate could be anywhere from 40 to 75 percent hydrogen by volume (College of the Desert, 2001a).
There are a number of processes that can be used to reform different fuels. Fuel reforming often requires several different steps, each of which involves flowing the fuel or partially processed reformate across a catalyst bed in a closed vessel, or “reactor.” These reactors are generally constructed like heat exchangers. with the working fluid flowing through one set of channels coated with some kind of a catalyst, and another fluid (thermal oil or water-ethylene glycol)
flowing through another set of channels to either add or take away heat. Each process step may also require the addition of air or water to the inlet flow stream. The catalyst coating promotes chemical reactions in the vessel, which usually occur at relatively high temperatures and pressures. The necessary process heat may be produced by combusting some of the liquid fuel and/or depleted reformate after it leaves the fuel cell stack, in a burner. As a whole, the fuel reformer unit is close to being a “solid state” device, with very few moving parts.
Natural gas and alcohol fuels, like methanol, are easier to reform than gasoline or diesel fuel and also yield a reformate with higher hydrogen content. Both gasoline and diesel fuel are a mixture of different hydrocarbons, including aromatics and olefins that tend to form polymer gums and carbon during reforming, which can block the reformer catalyst sites (College of the Desert 2001a). Reforming of gasoline and diesel fuel, especially for use in a PEM fuel cell, usually requires additional processing steps.
Hydrogen is often produced at a centralized hydrogen fueling station by reforming natural gas on site. If so, additional processing steps are used to remove the carbon dioxide and other impurities from the reformate to produce very pure hydrogen gas. This hydrogen is then compressed for on-site storage and delivery to vehicles.
Different reformer designs are possible, but most will likely be packaged into a “hot box” that incorporates all of the process steps, including the process heater or burner, into a relatively compact unit housed in a single enclosure (see Figure 11). The plumbing inside this box may be very complicated, with the different systems feeding each other. The device will likely also have interconnections with the fuel cell stack outlet (for depleted reformate), the fuel cell water recovery system, the fuel cell cooling system, and the liquid fuel storage system.
The reformate leaving the fuel reformer is generally at approximately the same temperature and pressure at which the fuel cell stacks operate.
For SOFC APUs, the fuel reformer and SOFC stacks may be packaged into a single unit in a common enclosure, with only external fuel line, process air intake, exhaust outlet, and electrical connections to other vehicle systems.
Onboard reformers can also be used with fuel cell vehicles so that compressed or liquid hydrogen does not need to be carried on the vehicle. For example, Georgetown University has fielded a fuel cell transit bus operated on methanol fuel that is reformed onboard. The methanol fuel processor on this bus uses low temperature steam reformation and selective oxidation to make the hydrogen-rich reformate, which is fed to a PEM fuel cell.
In steam reformation, the methanol must first be vaporized and mixed with steam. The steam/methanol mixture then passes across a heated catalyst bed in the steam reformer, which converts the methanol and water to hydrogen gas, carbon dioxide, and carbon monoxide. Because PEM fuel cells cannot tolerate carbon monoxide, the reformate must go through a second catalytic process called selective oxidation, which converts the carbon monoxide into carbon dioxide. The final reformate is approximately two-thirds hydrogen, with the balance CO2, water, nitrogen, and less than 20 parts per million CO. The required heat for the process is provided by oxidizing the depleted reformate in a catalytic burner after it exhausts from the fuel cell stacks. See Figure 11 for a picture of this methanol fuel processor.
At least two companies are also working on a fuel reformer/processor to reform diesel fuel to power an SOFC APU. Unlike a PEM fuel cell, an SOFC can tolerate CO, so this fuel processor is based on catalytic partial oxidation and does not require the second, selective oxidation processing step.
Compared to onboard storage and use of compressed or liquid hydrogen in a PEM fuel cell engine or SOFC APU, onboard reforming of hydrocarbon fuels creates more tailpipe emissions. In particular, the vehicle will emit carbon dioxide, as well as small amounts of nitrogen oxides created during fuel reforming.
All liquid hydrocarbon fuels (gasoline, diesel fuel, kerosene, and methanol), as well as natural gas, contain significant hydrogen, which is chemically bound to carbon. Both diesel fuel and gasoline contain about two hydrogen atoms for every carbon atom, while natural gas contains four.
“Reforming” of a hydrocarbon fuel is a chemical process that converts the natural gas or liquid fuel into a hydrogen-rich gas. The product of this process is called “reformate,” and when used to fuel a PEM fuel cell, it is typically composed of a mixture of hydrogen gas, carbon dioxide, nitrogen, and water vapor. Reformate used to fuel an SOFC can also contain carbon monoxide. Depending on the fuel being reformed and the process used, the reformate could be anywhere from 40 to 75 percent hydrogen by volume (College of the Desert, 2001a).
There are a number of processes that can be used to reform different fuels. Fuel reforming often requires several different steps, each of which involves flowing the fuel or partially processed reformate across a catalyst bed in a closed vessel, or “reactor.” These reactors are generally constructed like heat exchangers. with the working fluid flowing through one set of channels coated with some kind of a catalyst, and another fluid (thermal oil or water-ethylene glycol)
flowing through another set of channels to either add or take away heat. Each process step may also require the addition of air or water to the inlet flow stream. The catalyst coating promotes chemical reactions in the vessel, which usually occur at relatively high temperatures and pressures. The necessary process heat may be produced by combusting some of the liquid fuel and/or depleted reformate after it leaves the fuel cell stack, in a burner. As a whole, the fuel reformer unit is close to being a “solid state” device, with very few moving parts.
Natural gas and alcohol fuels, like methanol, are easier to reform than gasoline or diesel fuel and also yield a reformate with higher hydrogen content. Both gasoline and diesel fuel are a mixture of different hydrocarbons, including aromatics and olefins that tend to form polymer gums and carbon during reforming, which can block the reformer catalyst sites (College of the Desert 2001a). Reforming of gasoline and diesel fuel, especially for use in a PEM fuel cell, usually requires additional processing steps.
Hydrogen is often produced at a centralized hydrogen fueling station by reforming natural gas on site. If so, additional processing steps are used to remove the carbon dioxide and other impurities from the reformate to produce very pure hydrogen gas. This hydrogen is then compressed for on-site storage and delivery to vehicles.
Different reformer designs are possible, but most will likely be packaged into a “hot box” that incorporates all of the process steps, including the process heater or burner, into a relatively compact unit housed in a single enclosure (see Figure 11). The plumbing inside this box may be very complicated, with the different systems feeding each other. The device will likely also have interconnections with the fuel cell stack outlet (for depleted reformate), the fuel cell water recovery system, the fuel cell cooling system, and the liquid fuel storage system.
The reformate leaving the fuel reformer is generally at approximately the same temperature and pressure at which the fuel cell stacks operate.
For SOFC APUs, the fuel reformer and SOFC stacks may be packaged into a single unit in a common enclosure, with only external fuel line, process air intake, exhaust outlet, and electrical connections to other vehicle systems.
Onboard reformers can also be used with fuel cell vehicles so that compressed or liquid hydrogen does not need to be carried on the vehicle. For example, Georgetown University has fielded a fuel cell transit bus operated on methanol fuel that is reformed onboard. The methanol fuel processor on this bus uses low temperature steam reformation and selective oxidation to make the hydrogen-rich reformate, which is fed to a PEM fuel cell.
In steam reformation, the methanol must first be vaporized and mixed with steam. The steam/methanol mixture then passes across a heated catalyst bed in the steam reformer, which converts the methanol and water to hydrogen gas, carbon dioxide, and carbon monoxide. Because PEM fuel cells cannot tolerate carbon monoxide, the reformate must go through a second catalytic process called selective oxidation, which converts the carbon monoxide into carbon dioxide. The final reformate is approximately two-thirds hydrogen, with the balance CO2, water, nitrogen, and less than 20 parts per million CO. The required heat for the process is provided by oxidizing the depleted reformate in a catalytic burner after it exhausts from the fuel cell stacks. See Figure 11 for a picture of this methanol fuel processor.
At least two companies are also working on a fuel reformer/processor to reform diesel fuel to power an SOFC APU. Unlike a PEM fuel cell, an SOFC can tolerate CO, so this fuel processor is based on catalytic partial oxidation and does not require the second, selective oxidation processing step.
Compared to onboard storage and use of compressed or liquid hydrogen in a PEM fuel cell engine or SOFC APU, onboard reforming of hydrocarbon fuels creates more tailpipe emissions. In particular, the vehicle will emit carbon dioxide, as well as small amounts of nitrogen oxides created during fuel reforming.
1.3.3 Hydrogen Storage in Materials
There are a number of other ways to store hydrogen in solid or liquid materials, for release on demand. The two most studied approaches are adsorption of hydrogen into solid metal hydrides and “chemical” storage as part of a chemical hydride. Both of these approaches are inherently safer than storing hydrogen as a high-pressure gas or a cryogenic liquid, and the process of releasing the hydrogen from the storage medium is less complex than reforming of hydrocarbon fuels. At present, these systems are heavy and bulky and require further development to be practical.
Metal hydride storage systems are based on the fact that some metals can adsorb significant amounts of hydrogen under high pressure and moderate temperatures. The hydrogen is either adsorbed onto the surface of the metal or actually incorporated into the crystalline lattice of the solid metal. When heated to some higher temperature at low pressure, the hydrogen is released from the metal. In a vehicle hydrogen storage system, waste heat from the fuel cell or ICE engine would typically be used to release the hydrogen (DOE, n.d.). Such a system could potentially be “re-fueled” onboard the vehicle by connecting it to a high-pressure hydrogen source.
Chemical hydrides are compounds that include significant numbers of hydrogen atoms chemically bound to other types of atoms, for example, sodium borohydride, which is composed of one sodium atom, one boron atom, and four hydrogen atoms (NaBH4). In a hydrogen storage system based on a chemical hydride, the hydrogen is released on demand through a chemical reaction with either water or an alcohol. The solid hydride is made into a slurry with an inert liquid, and when hydrogen is required, water is added, releasing hydrogen (DOE, n.d.). Unlike metal hydrides, chemical hydrides cannot be regenerated on the vehicle; after releasing all of its hydrogen the spent slurry must be removed and regenerated off-site.
Current metal and chemical hydride fuel storage systems are heavy and bulky; they can only store and release 6 percent or less of their weight as hydrogen (DOE, n.d.) (i.e., only 6 percent of the total weight of the system is the hydrogen fuel; the rest of the weight is the container). These systems have even lower energy densities than compressed gaseous hydrogen storage systems. More work is required to develop truly practical storage systems for vehicles based on these technologies.
There are a number of other ways to store hydrogen in solid or liquid materials, for release on demand. The two most studied approaches are adsorption of hydrogen into solid metal hydrides and “chemical” storage as part of a chemical hydride. Both of these approaches are inherently safer than storing hydrogen as a high-pressure gas or a cryogenic liquid, and the process of releasing the hydrogen from the storage medium is less complex than reforming of hydrocarbon fuels. At present, these systems are heavy and bulky and require further development to be practical.
Metal hydride storage systems are based on the fact that some metals can adsorb significant amounts of hydrogen under high pressure and moderate temperatures. The hydrogen is either adsorbed onto the surface of the metal or actually incorporated into the crystalline lattice of the solid metal. When heated to some higher temperature at low pressure, the hydrogen is released from the metal. In a vehicle hydrogen storage system, waste heat from the fuel cell or ICE engine would typically be used to release the hydrogen (DOE, n.d.). Such a system could potentially be “re-fueled” onboard the vehicle by connecting it to a high-pressure hydrogen source.
Chemical hydrides are compounds that include significant numbers of hydrogen atoms chemically bound to other types of atoms, for example, sodium borohydride, which is composed of one sodium atom, one boron atom, and four hydrogen atoms (NaBH4). In a hydrogen storage system based on a chemical hydride, the hydrogen is released on demand through a chemical reaction with either water or an alcohol. The solid hydride is made into a slurry with an inert liquid, and when hydrogen is required, water is added, releasing hydrogen (DOE, n.d.). Unlike metal hydrides, chemical hydrides cannot be regenerated on the vehicle; after releasing all of its hydrogen the spent slurry must be removed and regenerated off-site.
Current metal and chemical hydride fuel storage systems are heavy and bulky; they can only store and release 6 percent or less of their weight as hydrogen (DOE, n.d.) (i.e., only 6 percent of the total weight of the system is the hydrogen fuel; the rest of the weight is the container). These systems have even lower energy densities than compressed gaseous hydrogen storage systems. More work is required to develop truly practical storage systems for vehicles based on these technologies.
1.3.2 Liquid Hydrogen Storage
Very few fuel cell or hydrogen ICE vehicles have been deployed with onboard liquid hydrogen storage. Liquid hydrogen storage systems are smaller and lighter than comparable compressed hydrogen storage systems, but are more complex and expensive and have other disadvantages. Bulk liquid hydrogen storage systems are more commonly used at centralized vehicle fueling stations.
The boiling point of hydrogen at atmospheric pressure is –423 °F; above that temperature hydrogen exists as a gas, and it will only liquefy if the temperature drops below the boiling point. Compressors and heat exchangers can be used to lower the temperature of hydrogen gas to produce liquid hydrogen, which must then be kept at this very low temperature or it will “boil off” again as a gas. To maintain its temperature, liquid hydrogen is stored in specialized, heavily insulated, containers called “dewars,” “cryotanks,” or “cryogenic vessels.”
A typical cryogenic container is made of metal and is double-walled. The inner tank is wrapped in multiple insulating layers and is enclosed by the second outer metal tank. Air is removed from the space between tank walls to create a vacuum. This design minimizes heat transfer by radiation, convection. or conduction.
Even the best cryotanks allow some heat through the tank walls. As the liquid hydrogen inside absorbs the heat, some of it evaporates, raising the tank pressure. Cryotanks are generally designed to operate near atmospheric pressure and are not designed to hold high pressures. Therefore, as tank pressure rises, some gaseous hydrogen must be vented to relieve the pressure.
All cryotanks are equipped with pressure relief safety valves for gas venting. In a pressure relief valve, a spring holds a plunger against the valve opening with a specific amount of pressure. When the pressure inside the tank rises above the spring pressure, the plunger moves back against the spring and the valve opens, releasing some gas. As gas vents, the pressure inside the tank falls. When the pressure falls enough, the spring pushes the plunger back against the valve opening, closing the valve. Pressure relief valves are different from PRD/TRDs (see Section 1.3.1) because they are designed to open and close numerous times during their life, and to vent only part of the tank contents each time they open.
The amount of venting from an on-vehicle liquid hydrogen storage system will depend on the design of the system, the ambient temperature, and how often the vehicle is used. Many of the cryogenic tanks currently in use for bulk storage and delivery can store liquid hydrogen for a week or more without any venting loss (Linde, n.d.). Nonetheless, vehicle storage facilities and maintenance operating plans need to account for the possibility of hydrogen venting, particularly from vehicles parked indoors for long term.
See Figure 9 for an illustration of a liquid hydrogen fuel system for a vehicle. In addition to the super-insulated cryotank, a typical on-vehicle liquid hydrogen storage system will include a filling port, a safety (pressure relief) valve, and a heat exchanger. The safety valve is connected to a line or plenum, which directs vented hydrogen gas through a diffuser out of the top of the vehicle. Inside the tank, there is a filling line, a gas extraction line, a liquid extraction line, one or more level probes, and an electric heater. The heater is used to raise the pressure inside the tank to force out hydrogen gas in response to fuel demand. Mounting hardware holds the tank securely to the vehicle.
The gas released from the liquid hydrogen storage tank is extremely cold. Before entering the fuel cell or hydrogen ICE fuel delivery system, the gas passes through the heat exchanger, which raises the temperature. Typically the heat exchanger is connected to the same cooling system used to control the fuel cell stack or ICE temperature. Once through the heat exchanger, the hydrogen is close to the operating temperature of the fuel cell stack or ICE.
In the past, some fueling couplings used with liquid hydrogen required heating and rinsing to separate the two parts and to disconnect them from the vehicle after fueling. Newer designs have improved the safety and speed of fueling operations through the use of a special coaxial “cold withdrawal coupling.” This allows the operator to immediately disconnect from the vehicle after refueling has stopped and to rapidly refuel multiple vehicles without waiting for the coupling to warm up in between (Linde, n.d.).
The fueling operation used with liquid hydrogen is similar to fueling with compressed hydrogen. The connection between the vehicle and the fuel station is manual. To fuel, the operator inserts the male part of the coupling from the fuel station into the female part of the coupling on the vehicle. When a positive connection is made, the operator turns a lever to lock the coupling and fuel starts to flow (see Figure 10).
There is a data connection in the fuel coupling connected to the vehicle’s control system. Using signals from the probes inside the storage tank, the vehicle signals the fuel station when the tank is full. After the liquid hydrogen has stopped flowing, the operator unlocks the coupling and removes it.
On-vehicle liquid hydrogen storage systems will be larger than the diesel fuel tanks on current trucks, but smaller than compressed hydrogen storage systems. Liquid hydrogen with the same amount of energy as 100 gallons of diesel fuel would take up four times as much space as the diesel fuel, but less than one third as much space as the same amount of gaseous hydrogen stored at 5,000 psi. The weight of the liquid hydrogen storage system would be about 50 percent greater than the weight of the diesel fuel system when full, but less than half the weight of the compressed hydrogen fuel system (College of the Desert, 2001a). As with compressed hydrogen storage, the weight of the containment vessel for liquid hydrogen accounts for the majority of the total weight of the system.
The size and weight advantage of liquid hydrogen storage compared to compressed hydrogen storage is balanced by higher cost and complexity of the storage system, the energy required to liquefy the hydrogen, and ongoing hydrogen venting. Given these disadvantages, to date very few fuel cell or hydrogen ICE vehicles have been deployed with onboard liquid hydrogen storage. Bulk liquid hydrogen storage at a centralized vehicle fueling station is much more common. Bulk liquid hydrogen storage tanks have similar construction to onboard vehicle storage tanks.
Very few fuel cell or hydrogen ICE vehicles have been deployed with onboard liquid hydrogen storage. Liquid hydrogen storage systems are smaller and lighter than comparable compressed hydrogen storage systems, but are more complex and expensive and have other disadvantages. Bulk liquid hydrogen storage systems are more commonly used at centralized vehicle fueling stations.
The boiling point of hydrogen at atmospheric pressure is –423 °F; above that temperature hydrogen exists as a gas, and it will only liquefy if the temperature drops below the boiling point. Compressors and heat exchangers can be used to lower the temperature of hydrogen gas to produce liquid hydrogen, which must then be kept at this very low temperature or it will “boil off” again as a gas. To maintain its temperature, liquid hydrogen is stored in specialized, heavily insulated, containers called “dewars,” “cryotanks,” or “cryogenic vessels.”
A typical cryogenic container is made of metal and is double-walled. The inner tank is wrapped in multiple insulating layers and is enclosed by the second outer metal tank. Air is removed from the space between tank walls to create a vacuum. This design minimizes heat transfer by radiation, convection. or conduction.
Even the best cryotanks allow some heat through the tank walls. As the liquid hydrogen inside absorbs the heat, some of it evaporates, raising the tank pressure. Cryotanks are generally designed to operate near atmospheric pressure and are not designed to hold high pressures. Therefore, as tank pressure rises, some gaseous hydrogen must be vented to relieve the pressure.
All cryotanks are equipped with pressure relief safety valves for gas venting. In a pressure relief valve, a spring holds a plunger against the valve opening with a specific amount of pressure. When the pressure inside the tank rises above the spring pressure, the plunger moves back against the spring and the valve opens, releasing some gas. As gas vents, the pressure inside the tank falls. When the pressure falls enough, the spring pushes the plunger back against the valve opening, closing the valve. Pressure relief valves are different from PRD/TRDs (see Section 1.3.1) because they are designed to open and close numerous times during their life, and to vent only part of the tank contents each time they open.
The amount of venting from an on-vehicle liquid hydrogen storage system will depend on the design of the system, the ambient temperature, and how often the vehicle is used. Many of the cryogenic tanks currently in use for bulk storage and delivery can store liquid hydrogen for a week or more without any venting loss (Linde, n.d.). Nonetheless, vehicle storage facilities and maintenance operating plans need to account for the possibility of hydrogen venting, particularly from vehicles parked indoors for long term.
See Figure 9 for an illustration of a liquid hydrogen fuel system for a vehicle. In addition to the super-insulated cryotank, a typical on-vehicle liquid hydrogen storage system will include a filling port, a safety (pressure relief) valve, and a heat exchanger. The safety valve is connected to a line or plenum, which directs vented hydrogen gas through a diffuser out of the top of the vehicle. Inside the tank, there is a filling line, a gas extraction line, a liquid extraction line, one or more level probes, and an electric heater. The heater is used to raise the pressure inside the tank to force out hydrogen gas in response to fuel demand. Mounting hardware holds the tank securely to the vehicle.
The gas released from the liquid hydrogen storage tank is extremely cold. Before entering the fuel cell or hydrogen ICE fuel delivery system, the gas passes through the heat exchanger, which raises the temperature. Typically the heat exchanger is connected to the same cooling system used to control the fuel cell stack or ICE temperature. Once through the heat exchanger, the hydrogen is close to the operating temperature of the fuel cell stack or ICE.
In the past, some fueling couplings used with liquid hydrogen required heating and rinsing to separate the two parts and to disconnect them from the vehicle after fueling. Newer designs have improved the safety and speed of fueling operations through the use of a special coaxial “cold withdrawal coupling.” This allows the operator to immediately disconnect from the vehicle after refueling has stopped and to rapidly refuel multiple vehicles without waiting for the coupling to warm up in between (Linde, n.d.).
The fueling operation used with liquid hydrogen is similar to fueling with compressed hydrogen. The connection between the vehicle and the fuel station is manual. To fuel, the operator inserts the male part of the coupling from the fuel station into the female part of the coupling on the vehicle. When a positive connection is made, the operator turns a lever to lock the coupling and fuel starts to flow (see Figure 10).
There is a data connection in the fuel coupling connected to the vehicle’s control system. Using signals from the probes inside the storage tank, the vehicle signals the fuel station when the tank is full. After the liquid hydrogen has stopped flowing, the operator unlocks the coupling and removes it.
On-vehicle liquid hydrogen storage systems will be larger than the diesel fuel tanks on current trucks, but smaller than compressed hydrogen storage systems. Liquid hydrogen with the same amount of energy as 100 gallons of diesel fuel would take up four times as much space as the diesel fuel, but less than one third as much space as the same amount of gaseous hydrogen stored at 5,000 psi. The weight of the liquid hydrogen storage system would be about 50 percent greater than the weight of the diesel fuel system when full, but less than half the weight of the compressed hydrogen fuel system (College of the Desert, 2001a). As with compressed hydrogen storage, the weight of the containment vessel for liquid hydrogen accounts for the majority of the total weight of the system.
The size and weight advantage of liquid hydrogen storage compared to compressed hydrogen storage is balanced by higher cost and complexity of the storage system, the energy required to liquefy the hydrogen, and ongoing hydrogen venting. Given these disadvantages, to date very few fuel cell or hydrogen ICE vehicles have been deployed with onboard liquid hydrogen storage. Bulk liquid hydrogen storage at a centralized vehicle fueling station is much more common. Bulk liquid hydrogen storage tanks have similar construction to onboard vehicle storage tanks.
1.3.1 Compressed Hydrogen Storage
When stored as a gas, hydrogen can be fed directly into a fuel cell or ICE without further processing. However, like all gases hydrogen is difficult to compress. In order to get enough fuel onto a vehicle to be able to go several hundred miles between fill-ups, but without taking up too much space, the hydrogen must be stored at very high pressure. Most current vehicle systems store hydrogen at a pressure of 5,000 pounds per square inch (psi). In the future, hydrogen storage pressures may be as high as 10,000 psi (DOE, n.d.).
Even at these pressures, a gaseous hydrogen storage system will be much larger and heavier than the diesel fuel tanks on current trucks. Hydrogen with the same amount of energy as 100 gallons of diesel fuel, if stored at 5,000 psi, would take up over twelve times as much space—over 170 cubic feet.
Because high-pressure storage tanks must be very strong to contain the pressure, the total weight of such a system when full would be over 2,500 pounds—almost four times more than the weight of a full 100 gallon diesel tank (College of the Desert, 2001a).
In a diesel tank, the weight of the fuel would be over 90 percent of the total, while in a gaseous hydrogen storage system, the opposite is true—the weight of the hydrogen fuel would only be 10 percent of the total, with the remaining 90 percent the weight of the tank.
High-pressure storage cylinders can be made of metal (steel or aluminum) or they can be made with a thin metal or plastic liner that holds the gas, covered with a composite overwrap that provides most of the strength. The designs for these cylinders are subjected to rigorous qualification tests to ensure that they can withstand the forces that they might be subjected to in service on a vehicle, including in a crash.
Hydrogen storage systems for commercial vehicles will likely be composed of multiple storage cylinders connected to a common manifold. See Figure 8, which shows an automotive hydrogen fuel storage system that includes two high-pressure storage cylinders. Systems composed of more than one storage cylinder will normally include a manual isolation valve for each cylinder that can be used during servicing, as well as one or more electrically activated valves that can be used to automatically isolate the fuel supply in the case of a leak or other system problem.
When stored as a gas, hydrogen can be fed directly into a fuel cell or ICE without further processing. However, like all gases hydrogen is difficult to compress. In order to get enough fuel onto a vehicle to be able to go several hundred miles between fill-ups, but without taking up too much space, the hydrogen must be stored at very high pressure. Most current vehicle systems store hydrogen at a pressure of 5,000 pounds per square inch (psi). In the future, hydrogen storage pressures may be as high as 10,000 psi (DOE, n.d.).
Even at these pressures, a gaseous hydrogen storage system will be much larger and heavier than the diesel fuel tanks on current trucks. Hydrogen with the same amount of energy as 100 gallons of diesel fuel, if stored at 5,000 psi, would take up over twelve times as much space—over 170 cubic feet.
Because high-pressure storage tanks must be very strong to contain the pressure, the total weight of such a system when full would be over 2,500 pounds—almost four times more than the weight of a full 100 gallon diesel tank (College of the Desert, 2001a).
In a diesel tank, the weight of the fuel would be over 90 percent of the total, while in a gaseous hydrogen storage system, the opposite is true—the weight of the hydrogen fuel would only be 10 percent of the total, with the remaining 90 percent the weight of the tank.
High-pressure storage cylinders can be made of metal (steel or aluminum) or they can be made with a thin metal or plastic liner that holds the gas, covered with a composite overwrap that provides most of the strength. The designs for these cylinders are subjected to rigorous qualification tests to ensure that they can withstand the forces that they might be subjected to in service on a vehicle, including in a crash.
Hydrogen storage systems for commercial vehicles will likely be composed of multiple storage cylinders connected to a common manifold. See Figure 8, which shows an automotive hydrogen fuel storage system that includes two high-pressure storage cylinders. Systems composed of more than one storage cylinder will normally include a manual isolation valve for each cylinder that can be used during servicing, as well as one or more electrically activated valves that can be used to automatically isolate the fuel supply in the case of a leak or other system problem.
All high-pressure hydrogen storage cylinders must also be equipped with a pressure relief device (PRD) and/or a thermal relief device (TRD) to protect against cylinder rupture if the pressure inside the cylinder gets too high. A PRD includes a metal disk designed to rupture at a set pressure, releasing the gas inside the cylinder (Air Products, 2004). The most likely reason for overpressure in a hydrogen fuel cylinder is a vehicle fire. If engulfed in flames, the pressure inside the tank will rise as the temperature rises. TRDs are, therefore, made with a plug of fusible metal that begins to melt and deform at a set temperature (Air Products, 2004). As the plug deforms, it can no longer hold the pressure inside the cylinder and gas escapes. Some devices combine both a rupture disk and a fusible plug. PRDs and TRDs are not pressure relief valves (see Section 1.3.2). Once the disc ruptures or the fusible plug melts, all of the gas in the cylinder escapes, and they cannot be reset; they must be replaced.
Typically, the outlets from all PRD/TRDs are run into a common manifold that exits the vehicle at or near the roof line to ensure that any escaping gas is directed upward away from vehicle occupants or pedestrians.
Fueling with compressed hydrogen is similar to fueling with other high-pressure gases, such as compressed natural gas (CNG). The on-vehicle fueling ports and fueling nozzles used are very similar to those used with CNG, though they are designed to operate at higher pressures.
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