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.
No comments:
Post a Comment