Mobile Hydrogen Refuelling Station Flow Meter Standard

There are plans for partial or full replacement of natural gas with hydrogen in grids as well as ambitious targets to enhance the production of fuel cell vehicles and the development of hydrogen refuelling stations. These will form the infrastructure of a future hydrogen network. Accurate metering of hydrogen at different points of this network is particularly important, especially when hydrogen is transferred from one party (a seller) to another (a buyer).

Fuel cell electric vehicles (FCEV) along with battery electric vehicles (BEV) are currently considered the most promising candidates for the future of transportation. FCEVs are electric vehicles that use hydrogen as their fuel. Hydrogen reacts with oxygen in a reverse electrolysis reaction in their fuel cells to generate the required electricity. This process is free of carbon emissions as the only product of the reaction is water. FCEVs offer significant advantages, especially for larger vehicles such as buses and heavy goods vehicles (HGV). The hydrogen tank of an FCEV (small or large) can be filled in a few minutes compared to the hours needed to charge a BEV. However, increasing the use of FCEVs requires the development of relevant infrastructures such as hydrogen refuelling stations (HRS), technologies such as accurate hydrogen flow meters, and regulations. All of these are in their preliminary stages of development, although growing at a fast pace.

Hydrogen is sold based on mass (in kilograms) in Hydrogen Refuelling Stations (HRSs). Accurate billing, however, needs accurate metering of hydrogen which is a challenge at the present time. Liquid fuels, such as petrol and diesel, need to be measured to 0.5% accuracy in the refuelling stations based on the recommendations of the International Organisation of Legal Metrology (Accuracy Class 0.5 based on the document OIML R117). The required accuracy for the measuring system of gaseous fuels, such as compressed natural gas (CNG), is 1.5% (Class 1.5 based on OIML R 119). However, OIML R 119, separates hydrogen from all other types of gaseous fuels and recommends Class 2 and Class 4 (2% and 4% accuracy of the measuring system, respectively) for its measurements. It is expected that many countries will enforce Class 2 of OIML R 119 in the coming years.

There are several factors that make the metering of hydrogen challenging at HRSs. Hydrogen has an extremely high gravimetric energy density of 140 MJ/kg. This means that it stores a lot of energy relative to its weight, much more than natural gas (53.6 MJ/kg), diesel (45.6 MJ/kg), and lithium-ion batteries (<5 MJ/kg). In volumetric terms, hydrogen is the least dense of any gas and takes up more space than natural gas and diesel. To improve its efficiency as an energy carrier, hydrogen is compressed to pressures as high as 700 bar in hydrogen vehicles. In this compressed state, hydrogen occupies about the same space as a battery, for much less weight. Another advantage of hydrogen vehicles is the fast-refuelling time. However, when hydrogen is rapidly compressed to 700 bar, a lot of heat is generated. To stay within safe operating limits, the quickest fuelling protocols pre-cool the gas to -40°C. 

Hydrogen refuelling stations are therefore required to operate across a wide range of pressures (up to 875 bar) and temperatures (-40 to 60 °C). This is particularly challenging from a measurement perspective since the accuracy of most flow meter technologies is adversely affected by extreme pressure and temperature conditions, as well as the transient flow encountered for vehicle filling.

To be able to assess the accuracy of hydrogen refuelling stations it is necessary to develop mobile hydrogen standards, that can be transferred to the site of the refuelling station to test the accuracy of the hydrogen dispenser. 

When considering hydrogen refuelling standards, it is important to consider the measurement accuracies of the mobile standards. For example, personal vehicles typically have fuel tanks ranging from (0.5 to 6kg) total capacity and are classed as light duty. Whilst heavy goods vehicles have much larger tanks (10 to 40 kg) and are classed as heavy duty. Therefore, to provide the necessary measurement accuracy for the mobile standards being developed, two systems have been developed. This is required as the typical way of measuring the flow in the hydrogen mobile standard is to use a gravimetric approach and weigh the mass of the hydrogen being added to the mobile standard. Due to the differences in fuel tank size in the different vehicles, it is difficult to design a system that can provide the necessary measurement accuracy over the full range of possible hydrogen masses required. 

TÜV SÜD National Engineering Laboratory is part of the National Measurement System and holder of the national standard for flow. We also operate a Flow Programme Project for the Department of Business, Energy, and Industrial Strategy (BEIS). As part of this project, TÜV SÜD undertakes flow metrology research and development aimed at keeping the United Kingdom at the forefront of the flow metrology arena, as well as determining the future trends and requirements for flow metrology. TÜV SÜD is carrying out the design, build and testing of two mobile hydrogen refuelling station standards. The light-duty vehicle mobile standard is in the process of being commissioned at TÜV SÜD whilst the heavy-duty standard is undergoing the detailed design stage. The table below shows the differences in hydrogen refuelling for both light and heavy-duty applications. 

Figures 1 and 2 show photographs of the light-duty standard developed by TÜV SÜD. Figure 3 shows a schematic of the mobile standard. The schematic shows the hydrogen storage tanks that are used to measure the hydrogen output from the refuelling station. These tanks are located on weigh scales that enable a very accurate measurement of the hydrogen to allow the flow from the refuelling station to be assessed.

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The light-duty mobile standard is currently being commissioned and validated so that it can be used to determine and verify the performance of hydrogen refuelling stations in the UK. Currently, there is no capability in the UK to confirm that hydrogen refuelling dispensers meet the regulatory requirement of OIML R-139, so the principal benefit will be to enable this.

Figure 4 shows the approach being used for the development of the heavy-duty mobile hydrogen refuelling station standard. Due to the increased requirement for hydrogen storage, as shown in the table, the heavy-duty standard is larger than the light-duty standard. In the heavy-duty standard, three large hydrogen storage tanks are used. Therefore, the system has been designed to operate in a separate container that can be disconnected from the transport vehicle. This has the added advantage that all the necessary power supply, data acquisition equipment, etc. do not have to be ATEX-rated, as they can be located outside of any potentially explosive atmosphere. The detailed design of the heavy-duty system is currently being performed and the system is expected to be ready for commissioning in early 2023.

Providing both station operators and consumers with confidence in the measurement of the dispensed quantity of hydrogen will help enable the widespread deployment of hydrogen refuelling stations within the UK. This would provide a pathway towards zero emissions at vehicle tailpipes, thus helping to meet the net zero target of the UK Government and ultimately helping to combat climate change.