Hydrogen Flow Standard for Vehicle Refueling

Today, hydrogen-fueled demonstration vehicles are refueled from dispensers in 3 to 5 minutes using sequential releases from a bank of pressurized cylinders. This results in large gas flow (0 to 10 kg/min), pressure (0 to 70 MPa), and temperature transients at the device metering the total hydrogen dispensed. To ensure equity in retail trade, regulators seek < 2 % uncertainty in the total hydrogen quantity delivered. However, hydrogen dispenser manufacturers have found errors of 8 % to 20 % in point-of-sale flow meters. NIST is constructing a flow facility that will mimic the refueling process, including transients, as a basis for evaluating the performance of dispensing flow meter technologies and that of prototype field standards used by state weights and measures inspectors to periodically field test retail hydrogen dispensers.

Impact

Provide metrological tools and standards supporting the fair trade of hydrogen gas as a vehicle fuel of the future.

Objective(s)

Design, construct, and utilize a facility that mimics retail hydrogen fueling station conditions to quantify uncertainties of flow measurement standards and to evaluate the performance of flow meter technologies used in dispensers.

Goals

Provide flow measurement uncertainty < 1 % and with a 100 ms or lower response time.

Technical Approach

NIST will use critical flow nozzles as working standard flow meters in a hydrogen flow facility that mimics the transient conditions occurring during vehicle refueling. The critical nozzles will be calibrated against existing flow standards using nitrogen and helium (< 0.05 % uncertainty). Since the physics of flow through critical nozzles is well understood, their calibration in hydrogen (and other gases) under rapidly changing conditions can be accurately predicted. By using fast pressure and temperature sensors, flow response times of 100 ms or less can be achieved with the critical nozzles. As the high pressure cylinders are sequentially discharged to low pressure cylinders (see Figure 1), instantaneous and totalized flows from reference critical nozzles are the basis for performance evaluation of a meter under test.

A diaphragm compressor will be used to recycle the gas and recharge the high pressure cylinders. The facility will be compatible with gases other than hydrogen (nitrogen, helium, or methane) to permit research on species effects for various flow meter types.

Figure 1. A schematic of the Hydrogen Transient Flow Standard. By sequentially opening valves on the high pressure tanks, the vehicle refueling process can be simulated while collecting data from the meter under test and from reference flow meters (three 1 mm critical nozzles).

• Thermodynamic model for the cylinder discharge and preliminary design completed
• System design completed
• 70 MPa compressor and critical nozzles procured
• Evaluation of sensor time response started