ZBT tests 500-bar trailer tank
Filling and emptying large tanks for hydrogen transportation is a highly complex process. To ensure these activities are safe, they have to be carried out within permitted pressure and temperature windows. A global energy corporation undertook analyses and thermodynamic modeling for its vessels and asked the fuel cell technology center ZBT to validate the results in a series of physical tests.
Hydrogen refueling stations that are used to fill up fuel cell electric vehicles or FCEVs need to either produce the required hydrogen on site themselves or have it delivered via various distribution pathways. One way of conveying hydrogen to a filling station is by high-capacity trailers that are fitted with several high-pressure tanks for transporting gaseous hydrogen.
It must be ensured that these tanks meet the permitted temperature and pressure ranges while they are being both filled and discharged and that their operation is, above all, safe. To make certain this is the case, an international energy company devised analyses and thermodynamic modeling for the filling and discharging processes of its custom hydrogen trailer for type IV composite cylinders.
ZBT was commissioned to carry out physical tests on an individual tank to validate these analyses. The 2-cubic-meter (70-cubic-foot) trailer tank, which has an operating pressure of over 500 bar and a hydrogen storage capacity of around 70 kilograms, was tested successfully at the center’s testing area in Duisburg, Germany. The type IV tank, made from composite materials, is 6 meters (20 feet) long, has a diameter of approximately 80 centimeters (31 inches) and weighs just over one metric ton. The experimental investigations involved both filling and discharging under varying operating parameters.
The examined tank was produced especially for these series of tests, having been fitted with a range of thermocouples on and in the carbon fiber matrix. The multitude of measuring points provided information about the heating behavior during fill-up and the cooling behavior when the tank was emptied.
Expansion under pressure
The tank was additionally fitted with strain sensors for the investigations. The aim was to assess the axial and radial expansion in relation to pressure in order to check the design for the placement and arrangement of storage cylinders on the trailer. Tank expansion was predominantly identified along the axial plane. Radial expansion was minimal.
A recirculation system was installed in the hydrogen testing area so that hydrogen consumption could be kept as low as possible during the investigations. Thanks to this system, most of the hydrogen could be returned following each test to its original storage vessel in the testing area and thereby avoid unnecessarily high emissions of hydrogen into the atmosphere.
For the filling tests, the various operating parameters comprised the precooling temperature of the incoming hydrogen, the starting pressure of the tank and the filling speed. Here the focus was not on filling the tank at the usual controlled pressure ramping rates but on filling at a constant mass flow rate.
The discharging tests were in turn carried out at constant pressure ramping rates. This led to interesting results in relation to the temperature behavior of the gas both in the tank as well as in the gas flow coming from the tank.
The figures show examples of graphical analyses for a filling and discharging operation. Both representations show the measured tank pressure and the measured values of various embedded thermocouples during the test.
Parameters for safe operation identified
Assessment of the measured values revealed that safe and efficient operation is possible within the given temperature and pressure ranges. Variation of the filling parameters did not produce a configuration resulting in a state close to the thresholds. In any case, the temperature reached of the exiting gas as the tank discharged was under -40 °C, which in turn led to meeting the shutdown thresholds that were defined to protect downstream assemblies.
It was observed that the tank cooled down considerably even when discharged at low mass flow rates, evident from the temperature of the exiting gas. The tests were conducted at ambient temperatures of around 10 °C to 15 °C and gave rise to significant restrictions on maximum discharging speeds in some cases, particularly for the discharging of such tanks in colder environmental conditions.
ZBT is one of Europe’s leading research organizations for fuel cells, hydrogen technologies and energy storage. It is a sought-after R&D partner for top-level European and German research and industry projects, specializing in automotive applications, distribution, storage and stationary energy conversion. The around 170 staff members at ZBT have access to extensive technical resources that include production and testing facilities, chemical labs and high-tech analytics.