3.2.1 Design
The exterior of the vehicle should be marked with diamond-shaped labels that say “Liquid Hydrogen” in white letters on a blue background (see Figure 16). For commercial vehicles, one label should be located on the rear of the power unit and one label should be located on each side of the power unit cab, below the DOT numbers. The hydrogen labels should be legible from fifty feet in day light.
All cryotanks used to hold liquid hydrogen must be permanently marked “hydrogen,” securely mounted to the vehicle, and protected from damage by road debris.
All liquid hydrogen cryotanks must have a safety pressure relief valve installed. The outlet(s) from the valve(s) should empty into a hydrogen diffuser, whose outlet is located at or above the top surface of the vehicle. The hydrogen diffuser should be designed to mix the exiting hydrogen gas with enough air that under normal operations, the resultant flow will have a hydrogen concentration less than 25 percent of the lower flammable limit.
Each liquid hydrogen cryotank should have a manual shutoff valve installed that will allow that cylinder to be isolated from the rest of the fuel system for maintenance.
Each liquid hydrogen cryotank should be equipped with a liquid level gauge that can be read from the vehicle cab and a pressure gauge that can be read locally on or near the tank.
While certification standards for on-vehicle liquid hydrogen tanks have not yet been finalized, at a minimum, tanks should be tested/certified in the same way that current liquefied natural gas (LNG) tanks are tested.
For each tank design, this includes a 10-foot and a 30-foot drop test of a full tank to ensure that the tank will not leak even if subjected to a severe crash, and a 20-minute flame test to ensure that the tank will not immediately vent even if impacted by a fire (SAE, 1997).
The fuel system should include one or more electrically activated valves that will isolate the hydrogen cryotank(s), individually or as a group, from the rest of the system when closed. These valve(s) should “fail safely” so that they will close if the control signal is lost due to a system fault.
All liquid and gaseous hydrogen fuel lines should be securely mounted to the vehicle and routed away from heat sources. To the extent possible, fuel line connections should be minimized since leaks are most likely at joints. Fuel lines should not be routed through the passenger compartment.
All components of the fuel system that will come into contact with liquid hydrogen, including cryotank(s), fill lines, valves, and sealing materials should be constructed of materials that have been tested to be compatible with the low temperatures of liquid hydrogen. All gaseous hydrogen fuel lines and valves (downstream of the cryotank heat exchanger) shall be constructed of materials that have been tested to be compatible with hydrogen and not subject to hydrogen embrittlement.
All components of the fuel system and engine system that will carry or contain liquid or gaseous hydrogen should be electrically grounded and bonded to the vehicle chassis to preclude the buildup of static electricity.
All components of the fuel system that will carry or contain liquid hydrogen must be well-insulated, labeled, and be located to prevent casual contact by vehicle operators or maintenance personnel. The outer insulating layer must be vapor sealed to prevent air infiltration. Any fuel line that may be isolated between two closed valves with residual liquid hydrogen still inside (i.e., a fill line) must contain a pressure relief valve to vent hydrogen that vaporizes as the line heats up. These valve(s) should vent to a common plenum, with the pressure relief valve(s) on the liquid hydrogen cryotank(s).
Any compartment into which hydrogen could leak (from a fuel line connection or valve or from the fuel cell stack) should be ventilated such that gaseous hydrogen cannot collect in concentrations greater than 25 percent of hydrogen’s lower flammable limit. Hydrogen carrying components should not be located such that hydrogen can leak into the passenger compartment under any circumstance. Because fuel cell stacks can develop internal leaks over time, they will likely be installed in their own enclosure, which will have both ventilation holes and a ventilation fan to force air through the enclosure to flush out any leaked hydrogen so that it can not collect.
One or more hydrogen sensors should be installed on the vehicle. The number and location of these sensors will depend on the hydrogen fuel and engine system design. These hydrogen sensor(s) should be connected to the vehicle control system to provide an alarm and automatic system shutdown if a hydrogen concentration greater than a preset threshold is detected. This threshold could be anywhere from 25 percent to 50 percent of the lower flammable limit for hydrogen (1–2 percent hydrogen concentration).
The fuel system may also have an excess flow valve installed that is designed to close off fuel flow and trigger an automatic system shutdown when flow in excess of a set threshold is detected. The threshold is set to be greater than the maximum flow that could be used by the fuel cell or hydrogen ICE at full power. Flows greater than this amount indicate that there is probably a leak in the system.
The vehicle may also have an inertial crash sensor installed that can automatically trigger a vehicle shutdown when a crash is detected. Some vehicles may include a switch to override automatic shutdown and allow the vehicle to continue to operate for a short time. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high-speed traffic or off of a railroad track.
The vehicle control system should be configured so that automatic system shutdown can be triggered by detection of leaked hydrogen, excess fuel flow, a vehicle crash, or other system fault. Automatic system shutdown should include closing valve(s) to isolate hydrogen in the hydrogen storage cylinders, disconnecting traction power, and de-energizing high voltage equipment. During system shutdown, hydrogen should be vented from all other fuel and engine system components.
The control system should include a single main on/off switch that allows the vehicle operator to shut down the fuel cell system, disconnect traction power, de-energize high voltage equipment, and shut off the hydrogen fuel supply (isolating all hydrogen in the liquid hydrogen cryotank(s)). This switch should be located in the passenger cab easily accessible to the operator, similar to a conventional ignition switch.
The vehicle control system should include an interlock to the vehicle fueling port such that fueling cannot begin unless the fuel cell system is shutdown and the vehicle traction system is de-energized so that the vehicle cannot move.
The on board liquid hydrogen filling receptacle must be electrically bonded to the vehicle chassis, and some method must be provided to electrically connect the vehicle chassis to the fuel station ground during fueling. This can be done through the fueling nozzle (preferred) or with a separate ground strap.
A dust cap permanently mounted to the vehicle should be provided for the onboard fuel filling port, to keep out dirt and debris when the vehicle is not being fueled.
The vehicle fuel system should include fittings and other provisions necessary to safely remove hydrogen fuel from the liquid hydrogen cryotank(s) and purge them with nitrogen or helium as required for maintenance.
After system shutdown, hydrogen will typically be vented from the vehicle’s low-pressure gaseous fuel system and fuel cell stack. The outlet for this venting hydrogen should be at or above the top surface of the vehicle. If, under normal operations, venting hydrogen will achieve concentrations greater than 25 percent of the lower flammable limit (1 percent hydrogen concentration), the hydrogen should vent through a hydrogen diffuser. The same hydrogen diffuser can be used for both this function and to diffuse hydrogen released through the fuel system pressure relief valve(s).
Wednesday, May 28, 2014
3.2 LIQUID HYDROGEN SYSTEMS
Liquid hydrogen storage on vehicles is much less common than high-pressure compressed hydrogen storage. If your vehicle does store liquid hydrogen onboard, many of the design and operating considerations listed in Section 3.1 will apply, but since liquid hydrogen is stored at very low temperatures, the additional considerations listed below will also apply.
Liquid hydrogen storage on vehicles is much less common than high-pressure compressed hydrogen storage. If your vehicle does store liquid hydrogen onboard, many of the design and operating considerations listed in Section 3.1 will apply, but since liquid hydrogen is stored at very low temperatures, the additional considerations listed below will also apply.
3.1.2 Operation and Maintenance
Anyone who will operate or maintain hydrogen-fueled vehicles should receive hydrogen safety training. At a minimum, this training should cover the characteristics of hydrogen, operation of onboard safety systems, hydrogen fueling operations, and actions to take in an emergency.
During maintenance, never substitute fuel system replacement parts that have not been specifically tested and certified for use with hydrogen (for example, lines, valves, and regulators designed for use with natural gas).
While they may look and function the same, they may be subject to hydrogen embrittlement. In addition, compressed natural gas fuel systems typically operate at lower pressures (maximum 3,600 psi) than hydrogen fuel systems.
Periodically check all connections in the hydrogen fuel system for leaks using procedures outlined in the manufacturer’s service manual. Tighten or repair all leaking joints, no matter how small the leak. Leak checks should also be conducted after repair or replacement of any fuel system lines or valves.
Never loosen any joint in the fuel system while the connected components are under pressure. Shut down the system and isolate and vent components as directed in the manufacturer’s service manual. Torque all joints to the levels specified in the service manual. Do not over tighten. Overtorquing can cause leaks.
Air must never be allowed to enter the hydrogen fuel system. If exposed to the atmosphere, some components, particularly hydrogen fuel cylinders, must be purged with nitrogen before being refilled with hydrogen. See the manufacturer’s service manual for specific purging procedures.
Periodically check the exterior surface of hydrogen fuel cylinders for nicks, dents, and cuts that could weaken the structure. See the manufacturer’s service manual for information on the allowable level of wear and damage before cylinders need to be replaced. The Federal Motor Vehicle Safety Standards applicable to natural gas fuel cylinders (FMVSS 304, 49 CFR 571.304) specify that a visual inspection by a “qualified container inspector”11 must be conducted “at least every 36 months or 36,000 miles or at the time of re-installation.” The inspection procedures for damage assessment are outlined in pamphlet C-612 from the
Compressed Gas Association. While standards specifically applicable to hydrogen cylinders have not yet been developed, at a minimum, the requirements applicable to natural gas fuel cylinders should be followed.
Local laws and regulations may require more frequent cylinder inspections, for example, in conjunction with annual registration safety inspections. The fuel system, including the high-pressure storage tanks, should also be visually inspected after any accident, and be retested or replaced as required.
Periodically check and calibrate hydrogen sensors in accordance with the schedule and procedures in the manufacturer’s service manual.
Periodically check operation of the fan in the hydrogen diffuser and any ventilation fans in accordance with
the schedule and procedures in the manufacturer’s service manual.
The fuel system will likely include a coalescing filter to remove any oil that might carry over into the hydrogen fuel from the fuel station compressor. Check and empty or replace this filter periodically in accordance with the schedule and procedures in the manufacturer’s service manual.
Do not ignore warning lights or alarms. Do not attempt to override automatic system shutdown unless absolutely necessary (e.g., to move vehicle off of railroad tracks).
Always make sure that the main switch is off before servicing the vehicle. Before working on the fuel cell system or gaseous hydrogen storage system, also disconnect the vehicle’s 12/24-VDC battery and close the manual fuel valves to isolate hydrogen in the storage cylinders.
Do not try to repair damaged fuel lines—replace them.
Do not walk on hydrogen fuel cylinders or expose them to damage from impact or abrasion. Do not allow strong chemicals, such as battery acid or metal cleaning solvents, to contact the hydrogen fuel cylinders.
Always electrically ground and bond the vehicle when fueling and defueling. Connect the ground strap or cable at the fuel station if one is provided.
Before fueling, check that the onboard fuel port is free of dirt and debris. Always replace the fuel port dust cover after fueling.
Do not smoke or use a cell phone when servicing or fueling the vehicle.
If the vehicle must be defueled for servicing of the hydrogen fuel system, the rate of fuel release must be carefully controlled. Follow the instructions in the manufacturer’s service manual. Unless the hydrogen storage tanks will be removed, always leave a small amount of pressure in the tanks so that the internal pressure is a few psi above atmospheric pressure. Any time tank pressure falls below atmospheric, it is possible for air to enter, and the tank must be purged with nitrogen before refilling with hydrogen.
Anyone who will operate or maintain hydrogen-fueled vehicles should receive hydrogen safety training. At a minimum, this training should cover the characteristics of hydrogen, operation of onboard safety systems, hydrogen fueling operations, and actions to take in an emergency.
During maintenance, never substitute fuel system replacement parts that have not been specifically tested and certified for use with hydrogen (for example, lines, valves, and regulators designed for use with natural gas).
While they may look and function the same, they may be subject to hydrogen embrittlement. In addition, compressed natural gas fuel systems typically operate at lower pressures (maximum 3,600 psi) than hydrogen fuel systems.
Periodically check all connections in the hydrogen fuel system for leaks using procedures outlined in the manufacturer’s service manual. Tighten or repair all leaking joints, no matter how small the leak. Leak checks should also be conducted after repair or replacement of any fuel system lines or valves.
Never loosen any joint in the fuel system while the connected components are under pressure. Shut down the system and isolate and vent components as directed in the manufacturer’s service manual. Torque all joints to the levels specified in the service manual. Do not over tighten. Overtorquing can cause leaks.
Air must never be allowed to enter the hydrogen fuel system. If exposed to the atmosphere, some components, particularly hydrogen fuel cylinders, must be purged with nitrogen before being refilled with hydrogen. See the manufacturer’s service manual for specific purging procedures.
Periodically check the exterior surface of hydrogen fuel cylinders for nicks, dents, and cuts that could weaken the structure. See the manufacturer’s service manual for information on the allowable level of wear and damage before cylinders need to be replaced. The Federal Motor Vehicle Safety Standards applicable to natural gas fuel cylinders (FMVSS 304, 49 CFR 571.304) specify that a visual inspection by a “qualified container inspector”11 must be conducted “at least every 36 months or 36,000 miles or at the time of re-installation.” The inspection procedures for damage assessment are outlined in pamphlet C-612 from the
Compressed Gas Association. While standards specifically applicable to hydrogen cylinders have not yet been developed, at a minimum, the requirements applicable to natural gas fuel cylinders should be followed.
Local laws and regulations may require more frequent cylinder inspections, for example, in conjunction with annual registration safety inspections. The fuel system, including the high-pressure storage tanks, should also be visually inspected after any accident, and be retested or replaced as required.
Periodically check and calibrate hydrogen sensors in accordance with the schedule and procedures in the manufacturer’s service manual.
Periodically check operation of the fan in the hydrogen diffuser and any ventilation fans in accordance with
the schedule and procedures in the manufacturer’s service manual.
The fuel system will likely include a coalescing filter to remove any oil that might carry over into the hydrogen fuel from the fuel station compressor. Check and empty or replace this filter periodically in accordance with the schedule and procedures in the manufacturer’s service manual.
Do not ignore warning lights or alarms. Do not attempt to override automatic system shutdown unless absolutely necessary (e.g., to move vehicle off of railroad tracks).
Always make sure that the main switch is off before servicing the vehicle. Before working on the fuel cell system or gaseous hydrogen storage system, also disconnect the vehicle’s 12/24-VDC battery and close the manual fuel valves to isolate hydrogen in the storage cylinders.
Do not try to repair damaged fuel lines—replace them.
Do not walk on hydrogen fuel cylinders or expose them to damage from impact or abrasion. Do not allow strong chemicals, such as battery acid or metal cleaning solvents, to contact the hydrogen fuel cylinders.
Always electrically ground and bond the vehicle when fueling and defueling. Connect the ground strap or cable at the fuel station if one is provided.
Before fueling, check that the onboard fuel port is free of dirt and debris. Always replace the fuel port dust cover after fueling.
Do not smoke or use a cell phone when servicing or fueling the vehicle.
If the vehicle must be defueled for servicing of the hydrogen fuel system, the rate of fuel release must be carefully controlled. Follow the instructions in the manufacturer’s service manual. Unless the hydrogen storage tanks will be removed, always leave a small amount of pressure in the tanks so that the internal pressure is a few psi above atmospheric pressure. Any time tank pressure falls below atmospheric, it is possible for air to enter, and the tank must be purged with nitrogen before refilling with hydrogen.
Sunday, May 18, 2014
3.1.1 Design
The exterior of the vehicle should be marked with diamond-shaped labels that say “Compressed Hydrogen” in white letters on a blue background (see Figure 16). For commercial vehicles, one label should be located on the rear of the power unit and one label should be located on each side of the power unit cab below the DOT numbers. The hydrogen labels should be legible from fifty feet in day light.
The hydrogen fuel system will likely include two pressure regulators, which reduce the gas pressure in stages from the fuel storage cylinders to a fuel cell or hydrogen ICE. The three stages are:
• The fuel storage system at up to 5,000 psi,
• The motive pressure circuit at up to approximately 175 psi, and
• The low pressure circuit at approximately 15 psi.
Each stage will include pressure relief devices, isolation valves, and other valves to regulate the flow of gas under all conditions. All components should be designed to withstand at least three times their expected maximum working pressure (NFPA, 2005).
Storage cylinders used to hold compressed hydrogen gas must be tested and certified by the manufacturer to withstand the normal forces expected during vehicle operation (such as working pressure, pressure shocks from fueling, and vibration) over a 15-year life, as well as certain extreme events such as would be encountered in a vehicle crash. These cylinders should be permanently marked “hydrogen,” securely mounted to the vehicle with the certification label visible, and protected from damage by road debris (i.e., if mounted between the frame rails, they should be protected by a cover).
All hydrogen storage cylinders must have a PRD or TRD installed. The outlets from each PRD/TRD should be connected to a common manifold that exits at or above the top surface of the vehicle, with outlet flow directed away from vehicle occupants and pedestrians.
Each hydrogen storage cylinder should have a manual shut-off valve installed that will allow that cylinder to be isolated from the rest of the fuel system for maintenance.
The fuel system should include one or more electrically activated valves that will isolate the hydrogen storage cylinders, individually or as a group, from the rest of the system when closed. These valve(s) should “fail safely” so that they will close if the control signal is lost due to a system fault.
All hydrogen fuel lines should be securely mounted to the vehicle and routed away from heat sources. To the extent possible, fuel line connections should be minimized since leaks are most likely at joints. Fuel lines should not be routed through the passenger compartment.
All components of the fuel system, including cylinders, lines, valves, and sealing materials should be constructed of materials that have been tested to be compatible with hydrogen and not subject to hydrogen embrittlement.
All components of the fuel system and engine system that will carry or contain hydrogen should be electrically grounded and bonded to the vehicle chassis to preclude the build up of static electricity.
Any compartment into which hydrogen could leak (from a fuel line connection or valve or from the fuel cell stack) should be ventilated such that hydrogen cannot collect in concentrations greater than 25 percent of hydrogen’s lower flammable limit. Hydrogen carrying components should not be located such that hydrogen can leak into the passenger compartment under any circumstance.
Because fuel cell stacks can develop internal leaks over time, they will likely be installed in their own enclosure, which will have both ventilation holes and a ventilation fan to force air through the enclosure to flush out any leaked hydrogen so that it can not collect.
One or more hydrogen sensors should be installed on the vehicle. The number and location of these sensors will depend on the hydrogen fuel and engine system design. These hydrogen sensor(s) should be connected to the vehicle control system to provide an alarm and automatic system shutdown if a hydrogen concentration greater than a preset threshold is detected. This threshold could be anywhere from 25 percent to 50 percent of the lower flammable limit for hydrogen (1–2 percent hydrogen concentration).
The fuel system may also have an excess flow valve installed that is designed to close off fuel flow and trigger an automatic system shutdown when flow in excess of a set threshold is detected. The threshold is set to be greater than the maximum flow that could be used by the fuel cell or hydrogen ICE at full power. Flows greater than this amount indicate that there is probably a leak in the system.
The vehicle may also have an inertial crash sensor installed, which can automatically trigger a vehicle shutdown when a crash is detected.
The vehicle control system should be configured so that automatic system shutdown can be triggered by detection of leaked hydrogen, excess fuel flow, a vehicle crash, or other system fault. Automatic system shutdown should include closing valve(s) to isolate hydrogen in the hydrogen storage cylinders, disconnecting traction power, and de-energizing high-voltage equipment. During system shutdown, hydrogen should be vented from all other fuel and engine system components. Some vehicles may include a switch to override automatic shutdown and allow the vehicle to continue to operate for a short time. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high-speed traffic or off of a railroad track.
The control system should also include a single main on/off switch that allows the vehicle operator to shut down the fuel cell system, disconnect traction power, de-energize high-voltage equipment, and shut off hydrogen fuel supply (isolating all hydrogen in the hydrogen storage cylinders). This switch should be located in the operator’s cab easily accessible to the operator, similar to a conventional ignition switch. Some vehicles may also have one or more secondary means of shutting down the system, for example, by opening a battery disconnect switch accessible from outside the vehicle.
The vehicle control system should include an interlock to the vehicle fueling port such that fueling cannot begin unless the fuel cell system is shutdown and the vehicle traction system is de-energized so that the vehicle cannot move.
The onboard fuel filling receptacle must be electrically bonded to the vehicle chassis, and some method must be provided to electrically connect the vehicle chassis to the fuel station ground during fueling. This can be done through the fueling nozzle (preferred) or with a separate ground strap.
A dust cap permanently mounted to the vehicle should be provided for the onboard fuel filling receptacle, to keep out dirt and debris when the vehicle is not being fueled.
The vehicle fuel system should include fittings and other provisions necessary to de-fuel safely from the hydrogen storage cylinders and purge the cylinders with nitrogen, as required for maintenance.
After system shutdown, hydrogen will typically be vented from the low-pressure sections of vehicle’s fuel system and fuel cell stack. The outlet for this venting hydrogen should be at or above the top surface of the vehicle. If, under normal operations, venting hydrogen will achieve concentrations greater than 25 percent of the lower flammable limit (1.0 percent hydrogen concentration), the hydrogen should vent through a hydrogen diffuser. The hydrogen diffuser should be designed to mix the exiting hydrogen gas with enough air that under normal operations the resultant flow will have a hydrogen concentration less than 25 percent of the lower flammable limit.
The exterior of the vehicle should be marked with diamond-shaped labels that say “Compressed Hydrogen” in white letters on a blue background (see Figure 16). For commercial vehicles, one label should be located on the rear of the power unit and one label should be located on each side of the power unit cab below the DOT numbers. The hydrogen labels should be legible from fifty feet in day light.
The hydrogen fuel system will likely include two pressure regulators, which reduce the gas pressure in stages from the fuel storage cylinders to a fuel cell or hydrogen ICE. The three stages are:
• The fuel storage system at up to 5,000 psi,
• The motive pressure circuit at up to approximately 175 psi, and
• The low pressure circuit at approximately 15 psi.
Each stage will include pressure relief devices, isolation valves, and other valves to regulate the flow of gas under all conditions. All components should be designed to withstand at least three times their expected maximum working pressure (NFPA, 2005).
Storage cylinders used to hold compressed hydrogen gas must be tested and certified by the manufacturer to withstand the normal forces expected during vehicle operation (such as working pressure, pressure shocks from fueling, and vibration) over a 15-year life, as well as certain extreme events such as would be encountered in a vehicle crash. These cylinders should be permanently marked “hydrogen,” securely mounted to the vehicle with the certification label visible, and protected from damage by road debris (i.e., if mounted between the frame rails, they should be protected by a cover).
All hydrogen storage cylinders must have a PRD or TRD installed. The outlets from each PRD/TRD should be connected to a common manifold that exits at or above the top surface of the vehicle, with outlet flow directed away from vehicle occupants and pedestrians.
Each hydrogen storage cylinder should have a manual shut-off valve installed that will allow that cylinder to be isolated from the rest of the fuel system for maintenance.
The fuel system should include one or more electrically activated valves that will isolate the hydrogen storage cylinders, individually or as a group, from the rest of the system when closed. These valve(s) should “fail safely” so that they will close if the control signal is lost due to a system fault.
All hydrogen fuel lines should be securely mounted to the vehicle and routed away from heat sources. To the extent possible, fuel line connections should be minimized since leaks are most likely at joints. Fuel lines should not be routed through the passenger compartment.
All components of the fuel system, including cylinders, lines, valves, and sealing materials should be constructed of materials that have been tested to be compatible with hydrogen and not subject to hydrogen embrittlement.
All components of the fuel system and engine system that will carry or contain hydrogen should be electrically grounded and bonded to the vehicle chassis to preclude the build up of static electricity.
Any compartment into which hydrogen could leak (from a fuel line connection or valve or from the fuel cell stack) should be ventilated such that hydrogen cannot collect in concentrations greater than 25 percent of hydrogen’s lower flammable limit. Hydrogen carrying components should not be located such that hydrogen can leak into the passenger compartment under any circumstance.
Because fuel cell stacks can develop internal leaks over time, they will likely be installed in their own enclosure, which will have both ventilation holes and a ventilation fan to force air through the enclosure to flush out any leaked hydrogen so that it can not collect.
One or more hydrogen sensors should be installed on the vehicle. The number and location of these sensors will depend on the hydrogen fuel and engine system design. These hydrogen sensor(s) should be connected to the vehicle control system to provide an alarm and automatic system shutdown if a hydrogen concentration greater than a preset threshold is detected. This threshold could be anywhere from 25 percent to 50 percent of the lower flammable limit for hydrogen (1–2 percent hydrogen concentration).
The fuel system may also have an excess flow valve installed that is designed to close off fuel flow and trigger an automatic system shutdown when flow in excess of a set threshold is detected. The threshold is set to be greater than the maximum flow that could be used by the fuel cell or hydrogen ICE at full power. Flows greater than this amount indicate that there is probably a leak in the system.
The vehicle may also have an inertial crash sensor installed, which can automatically trigger a vehicle shutdown when a crash is detected.
The vehicle control system should be configured so that automatic system shutdown can be triggered by detection of leaked hydrogen, excess fuel flow, a vehicle crash, or other system fault. Automatic system shutdown should include closing valve(s) to isolate hydrogen in the hydrogen storage cylinders, disconnecting traction power, and de-energizing high-voltage equipment. During system shutdown, hydrogen should be vented from all other fuel and engine system components. Some vehicles may include a switch to override automatic shutdown and allow the vehicle to continue to operate for a short time. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high-speed traffic or off of a railroad track.
The control system should also include a single main on/off switch that allows the vehicle operator to shut down the fuel cell system, disconnect traction power, de-energize high-voltage equipment, and shut off hydrogen fuel supply (isolating all hydrogen in the hydrogen storage cylinders). This switch should be located in the operator’s cab easily accessible to the operator, similar to a conventional ignition switch. Some vehicles may also have one or more secondary means of shutting down the system, for example, by opening a battery disconnect switch accessible from outside the vehicle.
The vehicle control system should include an interlock to the vehicle fueling port such that fueling cannot begin unless the fuel cell system is shutdown and the vehicle traction system is de-energized so that the vehicle cannot move.
The onboard fuel filling receptacle must be electrically bonded to the vehicle chassis, and some method must be provided to electrically connect the vehicle chassis to the fuel station ground during fueling. This can be done through the fueling nozzle (preferred) or with a separate ground strap.
A dust cap permanently mounted to the vehicle should be provided for the onboard fuel filling receptacle, to keep out dirt and debris when the vehicle is not being fueled.
The vehicle fuel system should include fittings and other provisions necessary to de-fuel safely from the hydrogen storage cylinders and purge the cylinders with nitrogen, as required for maintenance.
After system shutdown, hydrogen will typically be vented from the low-pressure sections of vehicle’s fuel system and fuel cell stack. The outlet for this venting hydrogen should be at or above the top surface of the vehicle. If, under normal operations, venting hydrogen will achieve concentrations greater than 25 percent of the lower flammable limit (1.0 percent hydrogen concentration), the hydrogen should vent through a hydrogen diffuser. The hydrogen diffuser should be designed to mix the exiting hydrogen gas with enough air that under normal operations the resultant flow will have a hydrogen concentration less than 25 percent of the lower flammable limit.
3. GUIDELINES FOR DESIGN AND OPERATION OF HYDROGEN SYSTEMS ON VEHICLES
Safe use of hydrogen as an alternative fuel requires that vehicles be designed and operated with the physical and chemical properties of hydrogen in mind. These properties are discussed in chapter 2. In most aspects, commercial vehicles powered by hydrogen will be quite similar to those powered by diesel fuel, but the propulsion system will be significantly different and some hydrogen-specific design elements are required. Likewise, operation of these vehicles will be similar to operation of diesel-fueled vehicles, with a few specific exceptions.
This chapter briefly discusses the most important hydrogen-specific aspects of vehicle design and operation. The issues discussed are of necessity generalized and are based on current codes and standards and best practices (see Appendix B). Each vehicle manufacturer will develop their own specific designs, which are likely to vary significantly in their details, while adhering to the same over-all design principles.
The information in this chapter is intended to familiarize commercial vehicle operators with the types of safety systems they are likely to see on hydrogen-fueled vehicles and the general operating principles they will need to use with them. Vehicle operators should look to the manuals provided by the manufacturer with each hydrogen-fueled vehicle for specific operating and maintenance instructions.
Safe use of hydrogen as an alternative fuel requires that vehicles be designed and operated with the physical and chemical properties of hydrogen in mind. These properties are discussed in chapter 2. In most aspects, commercial vehicles powered by hydrogen will be quite similar to those powered by diesel fuel, but the propulsion system will be significantly different and some hydrogen-specific design elements are required. Likewise, operation of these vehicles will be similar to operation of diesel-fueled vehicles, with a few specific exceptions.
This chapter briefly discusses the most important hydrogen-specific aspects of vehicle design and operation. The issues discussed are of necessity generalized and are based on current codes and standards and best practices (see Appendix B). Each vehicle manufacturer will develop their own specific designs, which are likely to vary significantly in their details, while adhering to the same over-all design principles.
The information in this chapter is intended to familiarize commercial vehicle operators with the types of safety systems they are likely to see on hydrogen-fueled vehicles and the general operating principles they will need to use with them. Vehicle operators should look to the manuals provided by the manufacturer with each hydrogen-fueled vehicle for specific operating and maintenance instructions.
2.4.2.3 Ventilating Enclosed Spaces
Hydrogen leaking into open air poses very little danger to anyone—it will quickly dissipate to nonflammable levels. Hydrogen that leaks into an enclosed space potentially presents a much greater hazard. When designing a hydrogen fueled vehicle, it is important to minimize all potential for hydrogen to leak into the passenger compartment, trunk, cargo space, wheel wells, and other enclosed spaces. This is done through careful placement of fuel tanks, lines, and connections. It may also be advisable to provide ventilation openings in locations that might not otherwise require them, specifically to vent any leaked hydrogen.
Another important consideration is placement of the outlet for any PRDs/TRDs. These outlets should be at the top surface of the vehicle and pointed away from the passenger or cargo compartment.
Buildings that will be used to house or maintain hydrogen-fueled vehicles should be designed so that there are no dead pockets at the ceiling where leaked hydrogen might collect and not be swept in by the building’s ventilation system.
Hydrogen leaking into open air poses very little danger to anyone—it will quickly dissipate to nonflammable levels. Hydrogen that leaks into an enclosed space potentially presents a much greater hazard. When designing a hydrogen fueled vehicle, it is important to minimize all potential for hydrogen to leak into the passenger compartment, trunk, cargo space, wheel wells, and other enclosed spaces. This is done through careful placement of fuel tanks, lines, and connections. It may also be advisable to provide ventilation openings in locations that might not otherwise require them, specifically to vent any leaked hydrogen.
Another important consideration is placement of the outlet for any PRDs/TRDs. These outlets should be at the top surface of the vehicle and pointed away from the passenger or cargo compartment.
Buildings that will be used to house or maintain hydrogen-fueled vehicles should be designed so that there are no dead pockets at the ceiling where leaked hydrogen might collect and not be swept in by the building’s ventilation system.
2.4.2.2 Removing Ignition Sources
Hydrogen is very easily ignited. A spark from static electricity, a vehicle tailpipe, electrical device, or even a hot surface can all ignite a mixture of air and leaked hydrogen within its flammable range.
It is important to minimize potential ignition sources in areas where hydrogen might leak and collect.
On a vehicle, static electricity is removed by proper grounding and bonding of electrical components. Fuel tanks, lines, and connections should be deliberately placed so that they avoid surfaces that might be hot or a source of ignition. In a building that will be used to house or maintain hydrogen fueled vehicles, any leaked hydrogen will quickly rise. The only area in which it is likely to collect in flammable concentrations is within a few feet of the ceiling. Electrical lines and equipment and heating equipment should not be located in this area near the ceiling. If electrical equipment must be located near the ceiling, it should be sealed so that it is “explosion proof” or intrinsically safe.
Hydrogen is very easily ignited. A spark from static electricity, a vehicle tailpipe, electrical device, or even a hot surface can all ignite a mixture of air and leaked hydrogen within its flammable range.
It is important to minimize potential ignition sources in areas where hydrogen might leak and collect.
On a vehicle, static electricity is removed by proper grounding and bonding of electrical components. Fuel tanks, lines, and connections should be deliberately placed so that they avoid surfaces that might be hot or a source of ignition. In a building that will be used to house or maintain hydrogen fueled vehicles, any leaked hydrogen will quickly rise. The only area in which it is likely to collect in flammable concentrations is within a few feet of the ceiling. Electrical lines and equipment and heating equipment should not be located in this area near the ceiling. If electrical equipment must be located near the ceiling, it should be sealed so that it is “explosion proof” or intrinsically safe.
2.4.2.1 Removing the Fuel Source—Avoiding and Detecting Leaks
Good design for a fuel system involves two major principles:
• Avoiding leaks of hydrogen fuel
• Detecting leaks of hydrogen fuel
The most likely locations for a hydrogen leak are at joints and connections in the high-pressure hydrogen fuel system. Hydrogen gas is the smallest of all molecules and can, therefore, move more easily through joints than other gases. However, most hydrogen leaks can be avoided by designing hydrogen systems using appropriate materials and minimizing connections.
Proper maintenance practices, in accordance with the manufacturer’s instructions, are also critical. This includes the use of the proper tools when making and breaking connections, tightening to the correct torque as specified by the manufacturer, and use of only approved replacement parts.
In a properly designed and maintained hydrogen fuel system, the most likely location for a hydrogen release will be through the PRD/TRD. If the PRD/TRD is properly oriented, a release will pose little danger to the vehicle, the operator, or the public. Hydrogen-fueled vehicles should enter only buildings designed to handle hydrogen.
All vehicles that use hydrogen fuel should also be equipped with one or more sensors to detect hydrogen leaks.
These sensors should be linked to the vehicle control system. If hydrogen levels approaching the lower limit of flammability are detected, the system will automatically shut down the vehicle and close valves to isolate the hydrogen within the high-pressure tank. In most cases, this will stop the source of the leak and remove any hazard. Some vehicles may include an “override” switch that will allow the vehicle to operate for a short time, even after a hydrogen leak has been detected. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high speed traffic or off of a railroad track.
Buildings used to store or maintain hydrogen-fueled vehicles should also generally be equipped with hydrogen sensors.
These sensors will be hooked into a building alarm system, as well as the building’s ventilation system. If hydrogen levels approaching the lower limit of flammability are detected, the system will sound an alarm to warn people in the area to evacuate and will increase the ventilation rate in the building to help remove any accumulated hydrogen. Some buildings designed for hydrogen vehicles may also be equipped with ultraviolet flame detectors to detect hydrogen fires. If so, they will also be hooked to the building alarm system.
Good design for a fuel system involves two major principles:
• Avoiding leaks of hydrogen fuel
• Detecting leaks of hydrogen fuel
The most likely locations for a hydrogen leak are at joints and connections in the high-pressure hydrogen fuel system. Hydrogen gas is the smallest of all molecules and can, therefore, move more easily through joints than other gases. However, most hydrogen leaks can be avoided by designing hydrogen systems using appropriate materials and minimizing connections.
Proper maintenance practices, in accordance with the manufacturer’s instructions, are also critical. This includes the use of the proper tools when making and breaking connections, tightening to the correct torque as specified by the manufacturer, and use of only approved replacement parts.
In a properly designed and maintained hydrogen fuel system, the most likely location for a hydrogen release will be through the PRD/TRD. If the PRD/TRD is properly oriented, a release will pose little danger to the vehicle, the operator, or the public. Hydrogen-fueled vehicles should enter only buildings designed to handle hydrogen.
All vehicles that use hydrogen fuel should also be equipped with one or more sensors to detect hydrogen leaks.
These sensors should be linked to the vehicle control system. If hydrogen levels approaching the lower limit of flammability are detected, the system will automatically shut down the vehicle and close valves to isolate the hydrogen within the high-pressure tank. In most cases, this will stop the source of the leak and remove any hazard. Some vehicles may include an “override” switch that will allow the vehicle to operate for a short time, even after a hydrogen leak has been detected. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high speed traffic or off of a railroad track.
Buildings used to store or maintain hydrogen-fueled vehicles should also generally be equipped with hydrogen sensors.
These sensors will be hooked into a building alarm system, as well as the building’s ventilation system. If hydrogen levels approaching the lower limit of flammability are detected, the system will sound an alarm to warn people in the area to evacuate and will increase the ventilation rate in the building to help remove any accumulated hydrogen. Some buildings designed for hydrogen vehicles may also be equipped with ultraviolet flame detectors to detect hydrogen fires. If so, they will also be hooked to the building alarm system.
Wednesday, May 14, 2014
2.4.2 Avoiding Fire and Explosion
As shown in Figure 17, a fire or explosion requires three things:
• Fuel (i.e., a hydrogen leak),
• An ignition source, and
• Oxygen (from the air).
Safe design for use of any gaseous fuel, including hydrogen, involves paying attention to all three legs of the “fire triangle.”
As shown in Figure 17, a fire or explosion requires three things:
• Fuel (i.e., a hydrogen leak),
• An ignition source, and
• Oxygen (from the air).
Safe design for use of any gaseous fuel, including hydrogen, involves paying attention to all three legs of the “fire triangle.”
2.4.1 Labeling
As with other alternative-fueled vehicles, all commercial vehicles that store hydrogen fuel onboard should carry a label that identifies the type of fuel used in order to alert emergency response personnel to the types of hazards they might face if the vehicle is involved in an accident. These labels should conform with SAE J2578, a fuel cell safety recommended practice developed by the Society for Automotive Engineers (SAE, 2002).
This practice recommends a diamond-shaped label with a blue background and white lettering (see Figure 16). It also specifically recommends that the words “Compressed Hydrogen” or “Liquid Hydrogen” be contained in the blue diamond, depending on the form in which the hydrogen is stored on the vehicle.
SAE J2578 does not specify the location of the hydrogen label on the vehicle. For hydrogen-fueled cars, the manufacturer usually affixes the label to the rear of the vehicle, as shown in Figure 16. For commercial tractor-trailer units, a label on the rear of the power unit would likely be obscured by the trailer. For that reason, all commercial vehicles should include the hydrogen label on the rear of the vehicle and on each side of the power unit cab, below the DOT numbers mandated by 49 CFR 390.21. As with these DOT numbers, the hydrogen labels should be legible from fifty feet in daylight.
Any commercial trailer unit that will store hydrogen (for example, to fuel a transportation refrigeration unit) should also have SAE J2578-compliant hydrogen labels affixed. These labels should be located on the rear of the trailer and on each side of the trailer, in the vicinity of where the hydrogen fuel tank is located.
As with other alternative-fueled vehicles, all commercial vehicles that store hydrogen fuel onboard should carry a label that identifies the type of fuel used in order to alert emergency response personnel to the types of hazards they might face if the vehicle is involved in an accident. These labels should conform with SAE J2578, a fuel cell safety recommended practice developed by the Society for Automotive Engineers (SAE, 2002).
This practice recommends a diamond-shaped label with a blue background and white lettering (see Figure 16). It also specifically recommends that the words “Compressed Hydrogen” or “Liquid Hydrogen” be contained in the blue diamond, depending on the form in which the hydrogen is stored on the vehicle.
SAE J2578 does not specify the location of the hydrogen label on the vehicle. For hydrogen-fueled cars, the manufacturer usually affixes the label to the rear of the vehicle, as shown in Figure 16. For commercial tractor-trailer units, a label on the rear of the power unit would likely be obscured by the trailer. For that reason, all commercial vehicles should include the hydrogen label on the rear of the vehicle and on each side of the power unit cab, below the DOT numbers mandated by 49 CFR 390.21. As with these DOT numbers, the hydrogen labels should be legible from fifty feet in daylight.
Any commercial trailer unit that will store hydrogen (for example, to fuel a transportation refrigeration unit) should also have SAE J2578-compliant hydrogen labels affixed. These labels should be located on the rear of the trailer and on each side of the trailer, in the vicinity of where the hydrogen fuel tank is located.
2.4 HYDROGEN SAFETY PRINCIPLES
The most important safety principle in any situation is education—making anyone who will come into contact with a vehicle aware of a potential hazard. For hydrogen and other alternative-fueled vehicles, this is done with appropriate labeling, to let users, emergency responders and the public know that hydrogen is present.
As with other motor fuels, fire and explosion are the most significant everyday hazards associated with hydrogen. Also as with other fuels, a hydrogen leak from a vehicle’s fuel or engine system, or from a fueling station, provides the starting point for all fire and explosion hazards.
Safe design for using hydrogen therefore requires attention to these safety principles:
• Properly label all vehicles that use hydrogen fuel. Avoid fire and explosion by: Avoiding leaks through proper design and maintenance, Providing leak detection systems to detect leaks and, if a leak is detected, shut off the fuel system as soon as possible,
• Removing ignition sources from areas where leaked hydrogen might be present, and
• Properly ventilating all enclosed spaces where leaked hydrogen might accumulate.
These are essentially the same principles that apply to the use of any gaseous fuel (e.g., natural gas), but their application may be slightly different based on the properties of hydrogen. Each of these principles, as applied to hydrogen, is discussed further below.
The most important safety principle in any situation is education—making anyone who will come into contact with a vehicle aware of a potential hazard. For hydrogen and other alternative-fueled vehicles, this is done with appropriate labeling, to let users, emergency responders and the public know that hydrogen is present.
As with other motor fuels, fire and explosion are the most significant everyday hazards associated with hydrogen. Also as with other fuels, a hydrogen leak from a vehicle’s fuel or engine system, or from a fueling station, provides the starting point for all fire and explosion hazards.
Safe design for using hydrogen therefore requires attention to these safety principles:
• Properly label all vehicles that use hydrogen fuel. Avoid fire and explosion by: Avoiding leaks through proper design and maintenance, Providing leak detection systems to detect leaks and, if a leak is detected, shut off the fuel system as soon as possible,
• Removing ignition sources from areas where leaked hydrogen might be present, and
• Properly ventilating all enclosed spaces where leaked hydrogen might accumulate.
These are essentially the same principles that apply to the use of any gaseous fuel (e.g., natural gas), but their application may be slightly different based on the properties of hydrogen. Each of these principles, as applied to hydrogen, is discussed further below.
Thursday, May 8, 2014
DAZO HHO PWM ( Pulse Width Modulators ) FOR FREEZING TEMPERATURES
The PWM has become the new standard for current control for Advanced DAZO HHO Kit enthusiasts. Rather than just straight current flowing into your generator, these PWM boards control current and the resulting heat that can sometimes be a problem, by pulsing off and on rapidly. Some pulse at a fixed frequency, with current adjustable. Others allow both current and frequency to be controlled like our Heavy Duty 30 amps.
DO I NEED A PWM? That is a common question. If your HHO kit is cheaply made of acrylic or PVC that melts at 180-190 degrees F, then YES! If you don't control the current it will melt around the terminals and leak. Ours are designed to withstand even constant currents of 50 amps+ without damage. They don't need one to regulate current for protection like cheaper models.
FREEZING TEMPS? YES! You should get one in Extreme Freezing Areas, because the electrolyte concentration can be increased with a good PWM preventing freezing to - 40 degrees F!
30 AMP PWM Assembled in a Project Box! They are built using the Heavy Duty 30 AMP PWM and come with a fan and dial controls for adjusting the current AND frequency (Actual product may vary from picture). Current boxes do NOT come with an additional on/off switch as shown in photo. They DO come with a single fan, dual mosfets and heat sinks and two controls for current (amperage) control and frequency. Size is approximately 3"W x 4-1/2"L x 2-1/2" H. Powered by 12 volts- can be set up for 24 volt by special order. each connection is split into 2 terminals for power in + and -, and power out to load + and - for a total of 8 terminals to divide up the current.
These are nice units and come with ventilation slots cut into the sides to allow air to be drawn up and over the cooling heat sinks. Holes are pre-drilled in the back for the wires and connectors to attach to the board inside.
Tuesday, May 6, 2014
2.3.2 Fires and Explosion
Hydrogen has higher energy content (per pound) than diesel fuel and is very reactive, which results in a very vigorous fire. However, the buoyancy of gaseous hydrogen means that it rises rapidly and diffuses quickly, resulting in a very vertical and localized flame. Hydrogen fires also tend to burn out fairly quickly. By contrast, diesel fuel and gasoline fires tend to burn longer and spread over a much larger area, as the liquid fuel puddle expands.
There is virtually no risk of a thermal “explosion” of hydrogen stored in a closed tank unless oxygen has been allowed to enter the tank during fueling operations, which is virtually impossible if the fueling system has been designed properly. Hydrogen gas that leaks into an enclosed space, whether on a vehicle or in a building, can create an explosive mixture, depending on the volume of hydrogen relative to the volume of air in the space.
Diesel fuel is not very volatile, and the risk that accumulated vapors will result in an explosion is small, even for large leaks. Gasoline, however, is very volatile and vapors from leaks can create the risk of an explosion if they collect in an enclosed space on the vehicle or in a building.
Hydrogen has higher energy content (per pound) than diesel fuel and is very reactive, which results in a very vigorous fire. However, the buoyancy of gaseous hydrogen means that it rises rapidly and diffuses quickly, resulting in a very vertical and localized flame. Hydrogen fires also tend to burn out fairly quickly. By contrast, diesel fuel and gasoline fires tend to burn longer and spread over a much larger area, as the liquid fuel puddle expands.
There is virtually no risk of a thermal “explosion” of hydrogen stored in a closed tank unless oxygen has been allowed to enter the tank during fueling operations, which is virtually impossible if the fueling system has been designed properly. Hydrogen gas that leaks into an enclosed space, whether on a vehicle or in a building, can create an explosive mixture, depending on the volume of hydrogen relative to the volume of air in the space.
Diesel fuel is not very volatile, and the risk that accumulated vapors will result in an explosion is small, even for large leaks. Gasoline, however, is very volatile and vapors from leaks can create the risk of an explosion if they collect in an enclosed space on the vehicle or in a building.
2.3.1 Leaks
In some ways a gaseous hydrogen fuel leak is less dangerous than a leak of diesel fuel or gasoline. Leaking diesel fuel and gasoline can puddle and spread over a large area, and the puddles will persist because they evaporate slowly (Amerada Hess, 2001).
Gaseous hydrogen leaks tend to be vertical with only a relatively narrow area/volume in which a flammable mixture exists;9 the hydrogen quickly dissipates in open air to nonhazardous levels.
If designed properly, the most likely location of a major hydrogen leak from a vehicle will be through the PRD, which should vent away from the occupied area of the vehicle. PRDs are designed to vent the entire contents of a hydrogen tank in only a few minutes, after which there is no lingering risk of hydrogen fire or explosion if the release was in the open air. Large hydrogen leaks inside buildings are more dangerous unless the facility has been designed to evacuate the leaked gas and to minimize ignition sources at ceiling level.
Leaking liquid hydrogen can pool and spread, but will quickly evaporate as it is heated by the surrounding air. As it evaporates, the cloud of gaseous hydrogen formed over the spill may move horizontally as it rises and dissipates.
While diesel fuel and gasoline leaks are easily visible and accompanied by a strong characteristic smell, gaseous hydrogen leaks are invisible and odorless (see Table 5). The only indication of a gaseous hydrogen leak may be a whistling noise similar to escape of other high-pressure gases. A liquid hydrogen leak may be accompanied by an area of fog surrounding the leaking hydrogen and/or the formation of frost on the tank or lines in the vicinity of the leak, because the super cold hydrogen cools the surrounding air and causes water vapor to condense.
In some ways a gaseous hydrogen fuel leak is less dangerous than a leak of diesel fuel or gasoline. Leaking diesel fuel and gasoline can puddle and spread over a large area, and the puddles will persist because they evaporate slowly (Amerada Hess, 2001).
Gaseous hydrogen leaks tend to be vertical with only a relatively narrow area/volume in which a flammable mixture exists;9 the hydrogen quickly dissipates in open air to nonhazardous levels.
If designed properly, the most likely location of a major hydrogen leak from a vehicle will be through the PRD, which should vent away from the occupied area of the vehicle. PRDs are designed to vent the entire contents of a hydrogen tank in only a few minutes, after which there is no lingering risk of hydrogen fire or explosion if the release was in the open air. Large hydrogen leaks inside buildings are more dangerous unless the facility has been designed to evacuate the leaked gas and to minimize ignition sources at ceiling level.
Leaking liquid hydrogen can pool and spread, but will quickly evaporate as it is heated by the surrounding air. As it evaporates, the cloud of gaseous hydrogen formed over the spill may move horizontally as it rises and dissipates.
While diesel fuel and gasoline leaks are easily visible and accompanied by a strong characteristic smell, gaseous hydrogen leaks are invisible and odorless (see Table 5). The only indication of a gaseous hydrogen leak may be a whistling noise similar to escape of other high-pressure gases. A liquid hydrogen leak may be accompanied by an area of fog surrounding the leaking hydrogen and/or the formation of frost on the tank or lines in the vicinity of the leak, because the super cold hydrogen cools the surrounding air and causes water vapor to condense.
2.3 COMPARISON OF HYDROGEN TO OTHER MOTOR FUELS
This section directly compares the properties of gaseous and liquid hydrogen to those of diesel fuel to contrast the safety issues of hydrogen to those of more common motor fuels. Two specific areas will be highlighted: the behavior of fuel leaks, and the characteristics of fire and explosions with these fuels.
This section directly compares the properties of gaseous and liquid hydrogen to those of diesel fuel to contrast the safety issues of hydrogen to those of more common motor fuels. Two specific areas will be highlighted: the behavior of fuel leaks, and the characteristics of fire and explosions with these fuels.
2.2.3 Liquid Hydrogen Leaks
Leaking liquid hydrogen may spread on the ground for a short distance, but the liquid will quickly evaporate, creating a cloud of gaseous hydrogen over the liquid pool. The distance of spread and the rate of evaporation will depend on the size of the leak and on ambient conditions. The hydrogen cloud over the liquid pool will be very cold and dense, but will rise and dissipate as it is warmed by surrounding air. Often the cold hydrogen will condense water vapor in the air, creating a visible fog in the area of the leak. Frost or ice may also form on the storage vessel or lines in the area of the leak.
The cloud of cold gaseous hydrogen may move horizontally as it warms and rises, and may extend beyond the area of visible fog. This hydrogen cloud may be cold enough to cause frostbite to exposed skin and should be avoided.
Leaking liquid hydrogen is so cold that it can liquefy the oxygen and nitrogen in surrounding air. If liquid oxygen drips onto combustible material (for example, asphalt), it will significantly increase the fire hazard. This is not of major concern for the volumes of liquid hydrogen likely to be carried on a typical commercial vehicle. However, liquid hydrogen storage tanks at fuel stations, which are likely to contain a much greater volume of fuel, should always be constructed over pads made of noncombustible material, such as concrete.
For the same reason, the insulation on the exterior of all liquid hydrogen storage tanks and lines, both at the fuel station and on a vehicle, should be vapor sealed to ensure that air cannot contact the cold inner surface.
Leaking liquid hydrogen may spread on the ground for a short distance, but the liquid will quickly evaporate, creating a cloud of gaseous hydrogen over the liquid pool. The distance of spread and the rate of evaporation will depend on the size of the leak and on ambient conditions. The hydrogen cloud over the liquid pool will be very cold and dense, but will rise and dissipate as it is warmed by surrounding air. Often the cold hydrogen will condense water vapor in the air, creating a visible fog in the area of the leak. Frost or ice may also form on the storage vessel or lines in the area of the leak.
The cloud of cold gaseous hydrogen may move horizontally as it warms and rises, and may extend beyond the area of visible fog. This hydrogen cloud may be cold enough to cause frostbite to exposed skin and should be avoided.
Leaking liquid hydrogen is so cold that it can liquefy the oxygen and nitrogen in surrounding air. If liquid oxygen drips onto combustible material (for example, asphalt), it will significantly increase the fire hazard. This is not of major concern for the volumes of liquid hydrogen likely to be carried on a typical commercial vehicle. However, liquid hydrogen storage tanks at fuel stations, which are likely to contain a much greater volume of fuel, should always be constructed over pads made of noncombustible material, such as concrete.
For the same reason, the insulation on the exterior of all liquid hydrogen storage tanks and lines, both at the fuel station and on a vehicle, should be vapor sealed to ensure that air cannot contact the cold inner surface.
2.2.2 Effect on Materials
The extremely low temperature of liquid hydrogen makes many materials brittle and easily broken. This includes most metals, including many grades of carbon steel and low alloy steels, and many of the materials typically used for sealing joints in liquid storage systems, such as rubber and many plastics. Liquid hydrogen storage systems must be constructed from very specific materials that can maintain their strength at low temperatures. Liquid hydrogen storage tanks and lines are usually constructed from stainless steel, and nonwelded joints may be sealed with o-rings made from specialized materials specifically tested to be compatible with the extremely low temperature of liquid hydrogen.
The extremely low temperature of liquid hydrogen makes many materials brittle and easily broken. This includes most metals, including many grades of carbon steel and low alloy steels, and many of the materials typically used for sealing joints in liquid storage systems, such as rubber and many plastics. Liquid hydrogen storage systems must be constructed from very specific materials that can maintain their strength at low temperatures. Liquid hydrogen storage tanks and lines are usually constructed from stainless steel, and nonwelded joints may be sealed with o-rings made from specialized materials specifically tested to be compatible with the extremely low temperature of liquid hydrogen.
Monday, May 5, 2014
2.2.1 Low Temperature Storage
Liquid hydrogen storage vessels must also be well-insulated to maintain the temperature of liquid hydrogen over long periods. No matter how well-insulated, however, it is inevitable that eventually some heat will be absorbed through the vessel walls, which will cause some hydrogen in the tank to vaporize. As hydrogen vaporizes, it raises the vapor pressure inside the tank. If not relieved, the increased pressure might eventually cause the tank to rupture. 26
The extremely low temperature of liquid hydrogen poses a severe frostbite hazard to exposed skin. All vessels, hoses, and lines that carry liquid hydrogen, either on a vehicle or at a fueling station, should be protected to avoid casual contact by people.
Liquid hydrogen tanks are always equipped with pressure relief valves to relieve pressure in the tank as necessary. These valves are different from the PRD/TRDs used with gaseous hydrogen storage tanks. Liquid hydrogen pressure relief valves are designed to open when the vapor pressure inside the tank rises above a set pressure, and then to close again when the pressure inside the tank falls below a different set pressure. PRD/TRDs open as required to relieve pressure, but do not close again. Once a PRD/TRD opens, it must be replaced before the tank can be filled again.
Liquid hydrogen storage vessels must also be well-insulated to maintain the temperature of liquid hydrogen over long periods. No matter how well-insulated, however, it is inevitable that eventually some heat will be absorbed through the vessel walls, which will cause some hydrogen in the tank to vaporize. As hydrogen vaporizes, it raises the vapor pressure inside the tank. If not relieved, the increased pressure might eventually cause the tank to rupture. 26
The extremely low temperature of liquid hydrogen poses a severe frostbite hazard to exposed skin. All vessels, hoses, and lines that carry liquid hydrogen, either on a vehicle or at a fueling station, should be protected to avoid casual contact by people.
Liquid hydrogen tanks are always equipped with pressure relief valves to relieve pressure in the tank as necessary. These valves are different from the PRD/TRDs used with gaseous hydrogen storage tanks. Liquid hydrogen pressure relief valves are designed to open when the vapor pressure inside the tank rises above a set pressure, and then to close again when the pressure inside the tank falls below a different set pressure. PRD/TRDs open as required to relieve pressure, but do not close again. Once a PRD/TRD opens, it must be replaced before the tank can be filled again.
2.2 LIQUID HYDROGEN
Liquid hydrogen is gaseous hydrogen that has been cooled to the point that it condenses. To get hydrogen to condense to a liquid it must be cooled to −423ºF—just a few degrees warmer than “absolute zero,” which is as cold as anything can get. Liquid hydrogen is referred to as a “cryogenic liquid” because it is so cold, and must be stored in specially insulated containers.
As liquid hydrogen absorbs heat, some of the liquid “boils off” and vaporizes. Once it has vaporized, the resultant hydrogen gas has the same properties and presents the same hazards as discussed above.
However, while in liquid form, it presents additional hazards related to the extreme cold of the liquid. In addition, liquid hydrogen leaks behave somewhat differently than gaseous hydrogen leaks.
Liquid hydrogen is gaseous hydrogen that has been cooled to the point that it condenses. To get hydrogen to condense to a liquid it must be cooled to −423ºF—just a few degrees warmer than “absolute zero,” which is as cold as anything can get. Liquid hydrogen is referred to as a “cryogenic liquid” because it is so cold, and must be stored in specially insulated containers.
As liquid hydrogen absorbs heat, some of the liquid “boils off” and vaporizes. Once it has vaporized, the resultant hydrogen gas has the same properties and presents the same hazards as discussed above.
However, while in liquid form, it presents additional hazards related to the extreme cold of the liquid. In addition, liquid hydrogen leaks behave somewhat differently than gaseous hydrogen leaks.
2.1.6 Gaseous Hydrogen Leaks
Because hydrogen molecules are so small, leaking hydrogen gas rises and diffuses quickly. A gaseous hydrogen leak tends to create a very vertical, fairly narrow area in which the hydrogen-air mix is flammable. Outside of this region, the hydrogen concentration is too low for the mixture to burn.
Depending on size and flow rate, the leak may produce a hissing sound similar to other gas leaks.
Gaseous hydrogen leaks may self-ignite due to static electricity or heating created by the flow of hydrogen. If the leak does ignite, the flame will be virtually invisible in daylight.
Because hydrogen molecules are so small, leaking hydrogen gas rises and diffuses quickly. A gaseous hydrogen leak tends to create a very vertical, fairly narrow area in which the hydrogen-air mix is flammable. Outside of this region, the hydrogen concentration is too low for the mixture to burn.
Depending on size and flow rate, the leak may produce a hissing sound similar to other gas leaks.
Gaseous hydrogen leaks may self-ignite due to static electricity or heating created by the flow of hydrogen. If the leak does ignite, the flame will be virtually invisible in daylight.
2.1.5 High-Pressure Storage
Hydrogen gas contains a lot of energy per pound, but like all gases it is difficult to compress. In order to get enough hydrogen fuel on a vehicle to operate for several hundred miles or more between fill ups, it must be stored at very high pressures—typically between 5,000 and 10,000 pounds per square inch (psi). Gaseous hydrogen fuel with the same amount of energy as one gallon of diesel fuel would only weigh about one third as much, but would occupy almost seven times the volume if stored at 10,000 psi.8 If stored at only 3,000 psi, the hydrogen would occupy almost seventeen times the volume of the diesel fuel.
The use of high-pressure storage does introduce some potential hazards due to the large amount of mechanical energy in the compressed gas. However, the high-pressure storage tanks used to hold compressed hydrogen are designed with a high margin of safety, and designs are verified with extensive qualification testing. See Figure 14 for photos of the types of tests required for certification of a high-pressure storage tank design. High-pressure storage tanks are also protected from excessive pressure build-up inside the tank using pressure relief devices (PRD) and/or temperature relief devices (TRD). These devices act to vent hydrogen to relieve pressure in the tank if it gets so high that there will be danger of a rupture.
High-pressure storage tanks must be protected from abrasion or damage by road debris, and both the tanks and lines should be adequately protected from vibration to minimize the possibility of leaks.
Hydrogen gas contains a lot of energy per pound, but like all gases it is difficult to compress. In order to get enough hydrogen fuel on a vehicle to operate for several hundred miles or more between fill ups, it must be stored at very high pressures—typically between 5,000 and 10,000 pounds per square inch (psi). Gaseous hydrogen fuel with the same amount of energy as one gallon of diesel fuel would only weigh about one third as much, but would occupy almost seven times the volume if stored at 10,000 psi.8 If stored at only 3,000 psi, the hydrogen would occupy almost seventeen times the volume of the diesel fuel.
The use of high-pressure storage does introduce some potential hazards due to the large amount of mechanical energy in the compressed gas. However, the high-pressure storage tanks used to hold compressed hydrogen are designed with a high margin of safety, and designs are verified with extensive qualification testing. See Figure 14 for photos of the types of tests required for certification of a high-pressure storage tank design. High-pressure storage tanks are also protected from excessive pressure build-up inside the tank using pressure relief devices (PRD) and/or temperature relief devices (TRD). These devices act to vent hydrogen to relieve pressure in the tank if it gets so high that there will be danger of a rupture.
High-pressure storage tanks must be protected from abrasion or damage by road debris, and both the tanks and lines should be adequately protected from vibration to minimize the possibility of leaks.
2.1.4 Effect on Materials
Hydrogen is the smallest of all molecules, and it can diffuse through materials that other gases cannot. Seals and connections in high-pressure hydrogen storage systems must, therefore, be designed very carefully, with attention to both the materials used and the geometry of the mating surfaces of joints.
Over time, constant exposure to hydrogen can cause many materials to loose strength and develop small cracks. This phenomenon is called hydrogen embrittlement, and it can cause leakage or catastrophic failure in hydrogen tanks and lines. The mechanisms of hydrogen embrittlement are not well-understood, but certain factors are known to effect the rate of embrittlement, including hydrogen concentration, pressure, and temperature.
Stain less steel is more resistant to hydrogen embrittlement than ordinary steels, and both pure aluminum and many aluminum alloys are even more resistant than stainless steel if the gas is dry (Ringland, 1994). All components of hydrogen fuel systems must be constructed of materials known to be compatible with hydrogen.
During maintenance and overhaul, only manufacturer-approved replacement parts specifically designed for use in hydrogen systems must be used. Parts that “look the same,” even if they fit properly, could result in failures over time if they are made from incompatible materials.
Hydrogen is the smallest of all molecules, and it can diffuse through materials that other gases cannot. Seals and connections in high-pressure hydrogen storage systems must, therefore, be designed very carefully, with attention to both the materials used and the geometry of the mating surfaces of joints.
Over time, constant exposure to hydrogen can cause many materials to loose strength and develop small cracks. This phenomenon is called hydrogen embrittlement, and it can cause leakage or catastrophic failure in hydrogen tanks and lines. The mechanisms of hydrogen embrittlement are not well-understood, but certain factors are known to effect the rate of embrittlement, including hydrogen concentration, pressure, and temperature.
Stain less steel is more resistant to hydrogen embrittlement than ordinary steels, and both pure aluminum and many aluminum alloys are even more resistant than stainless steel if the gas is dry (Ringland, 1994). All components of hydrogen fuel systems must be constructed of materials known to be compatible with hydrogen.
During maintenance and overhaul, only manufacturer-approved replacement parts specifically designed for use in hydrogen systems must be used. Parts that “look the same,” even if they fit properly, could result in failures over time if they are made from incompatible materials.
2.1.3 Buoyancy and Diffusivity
Hydrogen is the smallest and lightest known molecule, and therefore is the “lightest” gas. Hydrogen has only 7 percent of the density of air, which means that a given volume of air will weigh fourteen times as much as the same volume of hydrogen gas at atmospheric pressure.
Because it is so light and the molecules are so small, hydrogen leaking from a vessel rises and diffuses very quickly in air. The rate of diffusion for hydrogen in air is over ten times the rate for gasoline and other fuel vapors (Raj, 1998). This means that leaked hydrogen will quickly dissipate in open air to the point that the mixture is no longer flammable.
Hydrogen is the smallest and lightest known molecule, and therefore is the “lightest” gas. Hydrogen has only 7 percent of the density of air, which means that a given volume of air will weigh fourteen times as much as the same volume of hydrogen gas at atmospheric pressure.
Because it is so light and the molecules are so small, hydrogen leaking from a vessel rises and diffuses very quickly in air. The rate of diffusion for hydrogen in air is over ten times the rate for gasoline and other fuel vapors (Raj, 1998). This means that leaked hydrogen will quickly dissipate in open air to the point that the mixture is no longer flammable.
2.1.2 Odor and Toxicity
Hydrogen gas has no color, taste, or smell. Therefore, a gaseous hydrogen leak cannot generally be detected by human senses alone, except perhaps by human hearing.6 Other gaseous fuels, such as methane (natural gas), are also naturally colorless and odorless. However, sulfur-baodorants7 are usually added to pipeline natural gas specifically to aid in detecting leaks. The sulfur in these odorants can poison the catalysts used in automotive fuel cell systems, and satisfactory substitute odorants compatible with fuel cells have not yet been developed.
Hydrogen sensors must be used to detect hydrogen leaks. There are a number of technologies used to sense hydrogen, but many sensors rely on a catalyst that contains palladium, which breaks the chemical bonds between the atoms in a hydrogen molecule. The hydrogen atoms then diffuse into the catalyst, changing its electrical properties (e.g., resistance, capacitance). This change is proportional to the hydrogen concentration and can, therefore, be used to measure hydrogen levels in the air (Speer, 2004).
Hydrogen is not toxic to humans or animals. However, if leaking into an enclosed space, hydrogen gas can displace oxygen in the air and would pose an asphyxiation hazard in high enough concentrations. The risk of asphyxiation from hydrogen leaking into an open area is virtually non-existent because hydrogen is so buoyant that it will rise and diffuse to very low concentrations quickly. Even in an enclosed area the danger from a small leak is slight, but may be greater from a large leak that releases a significant volume of hydrogen relative to the size of the space.
Hydrogen gas has no color, taste, or smell. Therefore, a gaseous hydrogen leak cannot generally be detected by human senses alone, except perhaps by human hearing.6 Other gaseous fuels, such as methane (natural gas), are also naturally colorless and odorless. However, sulfur-baodorants7 are usually added to pipeline natural gas specifically to aid in detecting leaks. The sulfur in these odorants can poison the catalysts used in automotive fuel cell systems, and satisfactory substitute odorants compatible with fuel cells have not yet been developed.
Hydrogen sensors must be used to detect hydrogen leaks. There are a number of technologies used to sense hydrogen, but many sensors rely on a catalyst that contains palladium, which breaks the chemical bonds between the atoms in a hydrogen molecule. The hydrogen atoms then diffuse into the catalyst, changing its electrical properties (e.g., resistance, capacitance). This change is proportional to the hydrogen concentration and can, therefore, be used to measure hydrogen levels in the air (Speer, 2004).
Hydrogen is not toxic to humans or animals. However, if leaking into an enclosed space, hydrogen gas can displace oxygen in the air and would pose an asphyxiation hazard in high enough concentrations. The risk of asphyxiation from hydrogen leaking into an open area is virtually non-existent because hydrogen is so buoyant that it will rise and diffuse to very low concentrations quickly. Even in an enclosed area the danger from a small leak is slight, but may be greater from a large leak that releases a significant volume of hydrogen relative to the size of the space.
Friday, May 2, 2014
2.1.1 Flammability, Ignition, and Luminosity
A mixture of hydrogen and air will burn when there is as little as 4 percent hydrogen or as much as 75 percent hydrogen in the mix4 This is a very wide flammability range.
In comparison diesel fuel vapors in air will burn over a range of 0.6 percent to 5.5 percent. With less than 0.6 percent diesel in the mixture it is too lean to ignite, and with more than 5.5 percent diesel in the mixture it is too rich. Natural gas will burn over a range of 5 percent to 15 percent.
It takes very little energy to ignite a hydrogen-air mixture—a common static electric spark may be sufficient.
As shown in Table 4, it takes less than one tenth of the energy to ignite a hydrogen air mixture as it does to ignite a mixture of gasoline vapors in air. Over much of its flammable range, common static electricity would be enough to ignite a hydrogen-air mixture. In some cases, the electrostatic charges or heating created by the flow of hydrogen from a leaking vessel would be enough to ignite the leaking hydrogen (Murphy, et al., 1995; Argonne, 2003).
Hydrogen flames burn very cleanly, producing virtually no soot. It is the soot created by most fuel that makes a flame visible. In addition, much of the energy radiated by a hydrogen flame is in the ultraviolet range, rather than the infrared or visible ranges of the light spectrum. Therefore, a hydrogen flame is virtually invisible to the human eye in day light, though the energy being released by the flame may create a visible “shimmer” in surrounding air due to changes in the air density. At night, hydrogen flames are visible to the unaided human eye, and in daylight, they can be “seen” by an ultraviolet light sensor.5 If a hydrogen flame ignites other nearby materials, flames or smoke may also be visible from them. See Figure 13.
A mixture of hydrogen and air will burn when there is as little as 4 percent hydrogen or as much as 75 percent hydrogen in the mix4 This is a very wide flammability range.
In comparison diesel fuel vapors in air will burn over a range of 0.6 percent to 5.5 percent. With less than 0.6 percent diesel in the mixture it is too lean to ignite, and with more than 5.5 percent diesel in the mixture it is too rich. Natural gas will burn over a range of 5 percent to 15 percent.
It takes very little energy to ignite a hydrogen-air mixture—a common static electric spark may be sufficient.
As shown in Table 4, it takes less than one tenth of the energy to ignite a hydrogen air mixture as it does to ignite a mixture of gasoline vapors in air. Over much of its flammable range, common static electricity would be enough to ignite a hydrogen-air mixture. In some cases, the electrostatic charges or heating created by the flow of hydrogen from a leaking vessel would be enough to ignite the leaking hydrogen (Murphy, et al., 1995; Argonne, 2003).
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Hydrogen flames burn very cleanly, producing virtually no soot. It is the soot created by most fuel that makes a flame visible. In addition, much of the energy radiated by a hydrogen flame is in the ultraviolet range, rather than the infrared or visible ranges of the light spectrum. Therefore, a hydrogen flame is virtually invisible to the human eye in day light, though the energy being released by the flame may create a visible “shimmer” in surrounding air due to changes in the air density. At night, hydrogen flames are visible to the unaided human eye, and in daylight, they can be “seen” by an ultraviolet light sensor.5 If a hydrogen flame ignites other nearby materials, flames or smoke may also be visible from them. See Figure 13.
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.
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.
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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