Bringing precision to hydrogen filling stations
On the road to climate-friendlier mobility, hydrogen is gaining importance as an energy source for fuel cells. However, hydrogen fuelling is more complex than that of gasoline and diesel.
Unlike other gases, hydrogen heats up as it expands, causing pressure and temperature changes during fueling that need to be precisely monitored. In response to this challenge, Heinrichs Messtechnik GmbH has developed the TMU-W 004 mass flow meter.
It is the first of its kind to be certified to international standard OIML R 139 2018 and approved for hydrogen filling stations. This newly developed device is already in use at filling stations of the listed company Nel Hydrogen.
“For passenger cars, fuelling the hydrogen is typically done at 700 bar. This high pressure is necessary to generate the energy density needed for useful ranges,” explains Guido Thometzki, CEO of Heinrichs Messtechnik GmbH. “During this process, the temperature must not exceed +85 °C, otherwise the tank lining can be compromised.”
Since the tank expands and heats up during filling, the hydrogen is cooled before it gets to the tank. This high-pressure environment, combined with the small size of the hydrogen molecule, makes special demands on the accuracy of flow monitoring.”
Heinrichs Messtechnik GmbH optimized its proven Coriolis Mass Flow Meter for such high-pressure applications, and now offers the TMU-W 004. The meter ensures that during filling the amount of hydrogen transferred to the vehicle tank is measured accurately. This enables correct billing of the amount filled, which is a prerequisite for fuelling at public hydrogen filling stations. It covers a measurement range of 0.133 kg/min H2 to 4 kg/min H2.
Special arrangement of sensors delivers precise results
For the meter to fit into Nel’s narrow fuel dispensers, it was designed substantially narrower than its predecessor models. Inside the compact and deformation-resistant housing are two U-shaped measuring tubes arranged in parallel. An exciter system causes the measuring tubes to vibrate at their natural frequency.
“As the media flows through these tubes, the Coriolis effect causes additional phase-shifted deviations of the tubes,” says Thometzki. “Sensors positioned at the optimum points on the measuring tubes pick up these phase-shifted deviations and send them as signals to the transducer (also called the transmitter) for further analysis. From them, the current mass flow is calculated.”
The special device architecture – that is, the shape of the measuring tubes and the ideal positioning of the sensors on the tubes – along with the further harmonizing of the other components such as the gusset plates, make this a particularly sensitive and accurate meter, despite the thick walls of the measuring tubes.
This robustness on the part of the measuring tubes is necessary in order to withstand the high 1,000 bar pressure (the test pressure is actually 1,500 bar). The tough, fully welded housing with corrugations for added stiffness gives ample protection for the sensitive measurement equipment within.
“The flow amount can be evaluated using the pulse or status output of the transducer. The user also receives information for the temperature flow and optionally the density as well. This can be used to control the compressor pumping the hydrogen, for example switching to a higher volume or conversely interrupting pumping,” says Thometzki.
The transducer has a two-line LCD display that shows the values directly. The unit is easy to configure using the four keys provided on it. Among other functions, the user can define outputs and do fault analysis with the keys. The transducer can be set up as a round version or installed in a switchgear cabinet as a rack version.
Both the TMU-W 004 housing and the field housing of the transducer are pressure-resistant and intrinsically safe, which helps prevent fire or explosion in case gas escapes.
Simulations and flow measurements ensure measurement precision
In the development, manufacture and calibration of its meters, Heinrichs Messtechnik GmbH makes use of many simulation and optimization methods to ensure the precision and reliability of its devices under all conceivable conditions.
“Devices for handling high-pressure gases like hydrogen, such as the new TMU-W 004 for hydrogen filling stations, naturally cannot be an exception,” says Thometzki. “On the contrary, in the development of the TMU-W 004 in particular we used specially developed high-precision simulation methods for the flow and structural design, like FEM (finite element method), CFD (computational fluid dynamics), and FSI (fluid structure interaction).”
Only in this way was it possible in the TMU-W 004 to meet the seemingly contradictory requirements of the highest measurement precision and extreme reliability in handling high-pressure gases. The measurement precision also had to remain high despite pressure and temperature fluctuations during filling.
“The fact that the TMU-W 004 not only meets the requirements of OIML R 139 2018, but significantly exceeds them with its precision class rating of 1.5 for hydrogen, shows, especially in international comparison, how much the TMU-W 004 development team has accomplished,” says Dr Thomas Chatzikonstantinou, technical director at Heinrichs Messtechnik GmbH.