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.
No comments:
Post a Comment