Good checkout at hydrogen fueling

Different variants of hydrogen refueling

Hydrogen is delivered at 350 bar and 700 bar pressure as standard. On average, 4 to 5 kg of hydrogen is delivered to passenger cars and 20 to 40 kg to buses and trucks.

For delivery of hydrogen at 350 or 700 bar to passenger cars, filling guns with an internal diameter of 4 mm in combination with a delivery hose with an internal diameter of 6 mm are used. For delivery of hydrogen at 350 bar to buses and trucks, filling guns are used with an internal diameter of 8 and 12 mm, respectively, in combination with a delivery hose with an internal diameter of 6 mm.

Within the standard one distinguishes 3 filling rates:

• Slow refuelling: 30 g/s (1,8 kg/min)
• Normal refuelling: 60 g/s (3,6 kg/min)
• Fast refuelling: 120 g/s (7,2 kg/min)

The actual average filling speed is usually lower because it depends on several parameters including the volume of medium pressure buffers and high pressure buffers at the station and the frequency with which vehicles come to refuel. For fuel cell vehicles, refueling at a supply of 700 bar and -40 °C pre-cooling takes less than 3 minutes with a tank fill of 3 - 4 kg.

Photo: Delivery of hydrogen at 350 and 700 bar. On the left with the blue label a high flow variant for trucks at 350 bar.

Photo: engineer working on a Resato hydrogen refueling system

Pre cooling of hydrogen

Because hydrogen heats up during expansion in the vehicle's tank, the hydrogen to be delivered during rapid vehicle filling must first be cooled so as not to exceed the maximum allowable temperature of 85°C in the vehicle's fuel tank. The cooling required for this purpose depends on the delivery pressure (350 or 700 bar) and the intended filling rate, and amounts to a maximum of -40°C.

Several concepts exist for the gas coolers employed at a hydrogen fueling station:

• A spiral tube in which gaseous hydrogen is passed in counterflow with a refrigerant.
• A pipe heat exchanger in which the hydrogen passes through the pipes at high pressure and the refrigerant through the jacket.

Coriolis flow meter for hydrogen

Hydrogen is transferred from storage tank to fuel cell vehicle at very high pressures, high speeds and varying temperatures. When refueling starts, the pressure difference across the flow meter is large; when the tank is nearly full, the pressure difference is nearly zero. The hydrogen may pass the flow meter at -40 °C, but warms to 80 °C in the tank. So the temperature also fluctuates enormously over a refueling period. Finally, the flow rate starts high, only to drop to 0 over the refueling run. If all parameters fluctuate enormously then flow measurement is enormously difficult. Moreover, what needs to be taken into account is actually individual molecules and not the volume at a given pressure. This drops all measurement principles and leaves the coriolis flow measurement technique.

Even for a coriolis flowmeter, this application is not an everyday one. This is mainly due to the high pressures. But the fluctuation of the parameters also requires an incredibly good zero-point stability of the meter. At the lowest end of the measurement range, uncertainty is even dominated by zero-point stability.
While the absolute accuracy and repeateability must also be good because legislation requires it.

Finally, applying the meter is not easy because the amount of gas between the flow meter and the vehicle is not known because the pressure and temperature of that gas and its volume are not exactly known. So that volume must be kept as small as possible. That is the reason the flow meter is built in the dispenser, as close to the vehicle as possible.

Illustration: Coriolis mass flow meters from Tricor we apply in H2 dispensing stations for vehicles, both at 350 bar and 700 bar.

Photo: tubetrailer for hydrogen refueling. Sometimes the tubetrailer stays on location and refueling takes place directly from the tubetrailer. This happens mostly at temporary locations such as construction sites.

Placement of the flowmeter in the system

The placement in the system of mass flow meter largely determines the accuracy.

1. Installed after the compressor and before the heat exchanger: This scenario clearly shows challenges during metering. Reason: the differential pressure across the meter varies and this creates uncertainty about the hydrogen amount in the pressure line at the end of the refueling.
2. Use of the flow meter after the heat exchanger to solve these problems. The measurement distance between the flow meter and the dispenser is shorter, the pressure is constant, and therefore the refueling accuracy is better. However, this significantly increases the challenge for the flowmeter. Both the high pressure and temperature gradient become a special challenge for the zero-point stability of the flowmeter. The results are good but not entirely satisfactory for the application requirements.
3. Flowmeter directly in the dispenser, upstream of the heat exchanger and at constant pressure. In this case, there is no more undefined gas volume in the piping system. The flow meter is not exposed to the temperature gradient. This achieves the best accuracy during refueling.

Recommendations for coriolis custody transfer measurementsAanbevelingen voor coriolis custody transfer metingen

When using coriolis flowmeters in hydrogen refueling applications, there are many aspects to consider. For example, the sensor must be protected wherever possible from valves, manifolds and similar fittings that cause turbulence. It is recommended that valves be placed downstream.

The selection of the flowmeter should also take into account that it can handle the challenge of cooling to -33 °C in less than 30 seconds.

In addition, vibration is a possible problem. Homologated coriolis flow meters can withstand a defined amount of vibration. That does not mean they are insensitive to vibration. Therefore, it is wise to mount the coriolis flowmeter supported (with brackets) and the flowmeter should not support any other piping, because it is a sensitive measuring instrument which measures pipe movement. Transmission of vibrations from the system to the flowmeter should be prevented as much as possible. Mounting a coriolis flow meter freely suspended from pipework is therefore also out of the question. Also be aware that many coriolis flowmeters cannot be mounted in every orientation: often only horizontal mounting is allowed.

Furthermore, some general recommendations:

• Established flow profile present
• Inlet section must match mounting length
• Control valves always placed downstream
• Flow tubes must always be kept clean
• Process temperature must be controlled as specified Process pressure must be controlled as specified
• Ambient temperature must range from + 10 °C to + 30 °C
• Coriolis flowmeter has a warm-up/stabilization time Standard calibration must be performed at 20%, 50% and 100%
• High-frequency interference must be controlled according to EMC standards.

Illustration: a WEH tank nozzle and receptacle.

Photo: engineer working on a Resato hydrogen refueling system

Permeability of hydrogen and embrittlement

When selecting components for the system and the coriolis flowmeter, the permeability of hydrogen plays a role. Under high pressure, the small hydrogen molecules can penetrate all kinds of substances. This changes the physical properties of these substances. This is the main lifetime limiting factor of fillnozzles, recepticles, hoses and the flowmeter. Plastics can react chemically with hydrogen, or acquire abnormal physical properties due to absorption or swelling. The sensitivity of the material is affected by factors such as pressure, temperature, duration, and gas composition. The quality of metals can deteriorate due to interaction with hydrogen - hydrogen embrittlement is the phenomenon that a metal becomes brittle as a result of hydrogen absorption. This leads to a change in tensile strength, fracture and metal fatigue. For applications where hydrogen absorption occurs while a part is in use, using lower strength steels and reducing residual and applied stress are ways to prevent fracture due to hydrogen embrittlement.

Measurement of hydrogen set in standards

The accuracy requirement for the flow measurement technology used is legally defined in EU Directive 2014/32/EU Annex IV (MI 002) "Gas meters and volume correction devices." Applied harmonized standards or normative documents: OIML R 137 (component level) and OIML R139:2018 (system level).