1. Introduction
As interest in developing eco-friendly cars has grown due to global warming and air pollution, hydrogen fuel cell electric vehicles are emerging as a measure of renewable and carbon-free energy in the world. Hydrogen vehicles are currently under development and are commercially available from several manufacturers, mostly in South Korea and Japan. In connection with the supply and distribution of hydrogen vehicles, there is increasing national and policy support for the development of the hydrogen infrastructure required, specifically hydrogen refueling stations, in many countries. To usher in the hydrogen economy and future energy transition, South Korea promulgated the Hydrogen Economy Promotion and Hydrogen Safety Management Act in 2020. The Korean government has formulated and implemented various policies to build 6.2 million hydrogen fuel cell electric vehicles and 1200 hydrogen refueling stations by 2040 according to the national hydrogen economy roadmap, announced to revitalize the hydrogen economy [
1].
A typical hydrogen refueling station consists of high-pressure compressors, storage tanks, a pressure ramp regulator, a pre-cooling system, and a hydrogen dispenser to dispense high-pressure hydrogen through a fueling nozzle that connects to a receptacle on the hydrogen fuel cell electric vehicle; an example of the layout of the hydrogen refueling station is shown in
Figure 1. Hydrogen refueling stations typically dispense high-pressure hydrogen gas of around 5 kg into the tanks of hydrogen vehicles, with wide span ranges of pressure and temperature [
2,
3]. A pre-cooling process to lower the temperature to −40 ℃ is done to prevent any substantial increase in the temperature of the compressed hydrogen gas during the fueling process, given that the pressure is as high as 70 MPa in the tank of the hydrogen vehicle. This process of fueling a hydrogen vehicle at a hydrogen fueling station is in accordance with the worldwide accepted fueling protocol SAE J2601 [
4].
To measure the amount of hydrogen gas dispensed into the hydrogen vehicle for customer billing, a Coriolis mass flowmeter is typically installed inside the dispenser at a hydrogen refueling station. Because the hydrogen fueling process operates under a condition in which the flowrate, pressure, and temperature vary widely, a Coriolis mass flowmeter, which can directly measure the mass flow, is used to determine the hydrogen filling amount at a hydrogen refueling station. In order to ascertain the measurement reliability of hydrogen gas transferred to the hydrogen vehicle for customer billing, the international recommendation OIML R 139-1 requires certain accuracy levels for the flowmeters used at hydrogen fueling stations [
5].
Table 1 shows the two classes of the maximum permissible error (MPE), which are defined as class 2 and class 4 to represent corresponding accuracy rate limits of 1.5% and 2% for these flowmeters. The MPE values are 2% to 5% for the two accuracy classes to ensure a complete measuring system during the type evaluation, verification, and in-service inspection processes that must take place when the stations are operating.
Several studies have investigated how to inspect and verify such accuracies of flow measurements under hydrogen-filling operating conditions at hydrogen refueling stations. It should also be noted that several portable gravimetric standards in national metrology institutes and the manufacturing companies of hydrogen dispensers in several countries have been developed for field verification according to the international recommendation for hydrogen refueling stations [
6,
7]. In a cooperative project for hydrogen vehicles by several European national metrology institutes, mobile gravimetric standards for verifying the measurements by the flowmeters inside the hydrogen dispensers were developed [
8]. Several hydrogen refueling stations were tested in Europe using the gravimetric standard developed during this project for the purpose of research related to the MPE acceptance criteria as defined in OIML R 139-1, which is the international recommendation [
6,
7,
8].
The Korea Research Institute of Standards and Science (KRISS) also developed a Hydrogen Field Test Standard (HFTS) that can be used for the field verification and calibration of the measurement accuracy of the mass flowmeters used at hydrogen refueling stations, as shown in
Figure 2. The KRISS HFTS consists of three 52 L pressure cylinder tanks (type IV), a 300 kg weighing scale with a 0.5 g resolution, the Coriolis mass flowmeter, needle valves, temperature and pressure sensors, and a receptacle. The testing method is based on the gravimetric principle. The hydrogen delivered from the receptacle is passed through the Coriolis mass flowmeter, and the amount is determined by the weighing of the hydrogen gas collected in the pressure tank on the weighing scale.
However, only hydrogen vehicles are allowed access to hydrogen gas at hydrogen refueling stations in Korea under the Korean High-Pressure Gas Safety Control Act for all high-pressure gas, including hydrogen gas. A field test at a hydrogen refueling station using the KRISS HFTS could not be carried out due to legal and safety restrictions in Korea. Recently, regulations in the High-Pressure Gas Safety Control Act have been revised to enable filling processes for verification and research purposes at hydrogen refueling stations. Because administrative evaluations and certification are still required, actual field experiments with the KRISS HFTS have not been conducted thus far.
Currently, there is no traceable method by which to verify and calibrate the Coriolis mass flowmeters to be used at hydrogen refueling stations, except for a water calibration process as a conventional method for mass flowrate calibration. Typically, the Coriolis mass flowmeters in the hydrogen dispenser in Korea are calibrated with water by their manufacturers. In order to address these measurement challenges, several approaches to calibrate mass flowmeters for hydrogen refueling stations have been devised using alternative fluids and matched operating conditions, such as the pressures and temperatures [
9,
10]. The performance and the measuring behavior of Coriolis mass flowmeters designed for use in hydrogen refueling stations were investigated with different fluids, including air and water with different pressure and temperature range conditions to simulate the fueling conditions of hydrogen gas when it is transferred to a hydrogen vehicle.
The aim of the present study is to develop the necessary methodologies and calibration facilities to allow a hydrogen refueling station to verify hydrogen flow metering to a suitable accuracy level under the challenging conditions of high pressure and a wide range of temperatures. In order to do this, the performance of Coriolis mass flowmeters under density conditions identical to those of hydrogen gas for refueling was assessed using compressed air as an alternative fluid at the high-pressure gas flow standard system of the Korea Research Institute of Standards and Science (KRISS). There is no facility or infrastructure to calibrate and verify Coriolis mass flowmeters with operating pressure conditions of up to 700 bar at hydrogen refueling stations. In the present study, we developed a high-pressure water flow test facility at KRISS to investigate the accuracy of Coriolis mass flowmeters with regard to operating pressure conditions using a substitute fluid such as water. The characteristics of the measurements of the Coriolis mass flowmeters to examine the pressure dependence of the supplied fluids were assessed in the pressure range of 2 bar to 700 bar.
4. Conclusions
Hydrogen fuel cell electric vehicles are emerging as a means of transportation using renewable and carbon-free energy, especially considering global warming and air pollution problems in many places of the world. In relation to the supply and distribution of hydrogen vehicles, there is increasing national support for the development of the required hydrogen infrastructure, specifically hydrogen refueling stations, in many countries. Hydrogen fuel cell electric vehicles are typically refueled at a wide range of temperatures (−40 °C to 85 °C) and at a high pressure (up to 875 bar) at hydrogen stations in accordance with globally accepted standards. Currently, there is no traceable method by which to verify and calibrate the Coriolis mass flowmeters to be used at hydrogen refueling stations, except for a water calibration process as a conventional method for mass flowrate calibrations.
In the present study, we developed the necessary methodologies and calibration facilities to allow a hydrogen refueling station to verify hydrogen flow metering to a suitable accuracy level under the challenging conditions of high pressure and a wide range of temperatures. In order to do this, the performance of Coriolis mass flowmeters under density conditions identical to those of hydrogen gas for refueling was assessed using compressed air as an alternative fluid in the high-pressure gas flow standard system of KRISS. The results of tests of a Coriolis mass flowmeter at 40 bar and at an ambient temperature (around 20 °C) with the density set to approximately 46 kg/m3, equivalent to hydrogen gas at 700 bar and −40 °C, showed that the average errors were within ±0.2%, from 1 kg/min to 4.5 kg/min.
We also developed a high-pressure water flow test facility at KRISS to investigate the accuracy of Coriolis mass flowmeters with regard to typical operating pressure conditions using a substitute fluid, in this case, water. The measurement characteristics of the Coriolis mass flowmeter during this pressure-matching approach were investigated, showing an error of less than −0.2% in a pressure range of 2 bar to 700 bar at flowrates in the range of 1.0 kg/min to 5.0 kg/min, with no detectable dependence of the pressure on the accuracy of the measurement. It can be assumed that the pressure corrections implemented by the manufacturer of the Coriolis mass flowmeter work well in the pressure and flowrate ranges utilized in this study. The calibration test results between the density-matching approach and the pressure-matching approach were within the corresponding measurement uncertainty rates. The difference in the errors between the results of the two approaches is due to the speed of sound being different in the air and water. As the first study in Korea, a comparison of the calibration test results of the Coriolis mass flowmeters by the density-matching approach and the pressure-matching approach under vehicle hydrogen refueling conditions shows that these approaches can be presented as a potential means of calibrating Coriolis mass flowmeters for hydrogen refueling stations in the absence of a facility to calibrate them with hydrogen gas, at 700 bar and −40 °C, if temperature dependency for the operating condition of the Coriolis mass flowmeters were further verified. In addition, to apply these approaches, the field performance test for the Coriolis mass flowmeter tested in the calibration laboratory should be conducted at the hydrogen refueling station first. The type evaluation and subsequent verification with these density and pressure-matching approaches should be performed after more parameters, including the temperature dependency of operating conditions, were clearly addressed.
Finally, KRISS developed a Hydrogen Field Test Standard (HFTS) that can be used for field verifications and calibrations of the measurement accuracy of the mass flowmeters used at hydrogen refueling stations. According to the newly enforced regulation of the High-Pressure Gas Safety Control Act, KRISS is developing a new facility in the form of a trailer that will maintain safe and strict explosion safety rules. Field experiments with the KRISS HFTS are planned at hydrogen refueling stations for the first time in Korea in the near future. The Coriolis mass flowmeter tested in the high-pressure gas flow standard system and high-pressure water flow facility will be installed and tested by KRISS HFTS at the hydrogen refueling station.