Dazo Water Fuel Technology

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).