Transient 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 < 1.5 % 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 has constructed a transient flow facility (the TFF) that mimics the refueling process 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.

Objectives

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 is using critical flow nozzles as working standard flow meters in the TFF.  The facility is capable of simulating gaseous refueling processes, blow-down calibrations and a wide variety of tests dealing with transient pressure, temperature, and flow conditions.  A critical nozzle has been calibrated using nitrogen and helium against existing flow standards at pressures up to 8 MPa (< 0.05 % uncertainty) and in situ in the TFF via a transfer standard at pressures up to 30 MPa (< 0.25 % 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 above), instantaneous and totalized flows from reference critical nozzles are the basis for performance evaluation of a meter under test.

A 3-stage diaphragm compressor is used to recycle the gas and recharge the facility’s set of four 40 L “high” pressure cylinders to 41MPa in approximately 90 minutes.  The facility is compatible with nitrogen and helium to permit research on species effects for various flow meter types.

• Facility design and construction completed
• 70 MPa compressor commissioned and operational
• Temperature and pressure sensors calibrated and installed
• Data acquisition hardware installed and software complete
• Critical nozzles calibrated, instrumented and evaluated under quasi steady state and transient flows in the facility
• Evaluation of coriolis meter performance under quasi steady state and transient conditions underway