M. SADRI, TÜV SÜD National Engineering Laboratory, Glasgow, Scotland, UK
H2 is recognized as playing a crucial role in reducing global carbon dioxide (CO2) emissions. From transportation to heating homes, H2 is already expected to play a significant part in replacing fossil fuels in net-zero policies in the UK and around the world. There are plans for partial or full replacement of natural gas with H2 in natural gas grids, as well as ambitious targets to enhance the production of fuel cell vehicles and the development of H2 refueling stations (FIG. 1). These will form the infrastructure of a future H2 network. Accurate metering of H2 at different points of this network is crucial, especially when H2 is transferred from one party (a seller) to another (a buyer).
Fuel cell electric vehicles (FCEVs) and battery electric vehicles (BEV) are considered the most promising candidates for the future of transportation. FCEVs are EVs that use H2 as fuel. H2 reacts with oxygen in a reverse electrolysis in their fuel cells to generate the required electricity. This process is free of carbon emissions, with the only byproduct of the reaction being water. FCEVs offer significant advantages, especially for larger vehicles such as buses and heavy goods vehicles. The H2 tank of an FCEV (small or large) can be filled in a few minutes vs. hours to charge a BEV. However, increasing the use of FCEVs requires the development of relevant infrastructure such as H2 refueling stations, and technologies such as accurate H2 flowmeters and regulations. All these aspects are in their early stages of development but growing at a fast pace.
H2 is sold based on mass (in kilograms) in H2 vehicle refueling stations. However, accurate billing needs accurate metering of H2, which is a challenge. Liquid fuels such as petrol (gasoline) and diesel must be measured to 0.5% accuracy in the refueling stations based on the recommendations of the International Organization of Legal Metrology (OIML) (Accuracy Class 0.5 in the document OIML R117). The required accuracy for the measuring system of gaseous fuels such as compressed natural gas is 1.5% (Class 1.5 in OIML R139). However, OIML R139 separates H2 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 counties will enforce Class 2 of OIML R139 in the coming years.
There are several factors that make H2 metering challenging at H2 refueling stations. H2 has a very 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, H2 is the least dense of any gas and takes up more space than both natural gas and diesel.
To improve its efficiency as an energy carrier, H2 is compressed to pressures as high as 700 bar in H2 vehicles. In this compressed state, H2 occupies about the same space as a battery, for much less weight. Another advantage to H2 vehicles is the fast refueling time. However, when H2 is rapidly compressed to 700 bar, a lot of heat is generated. To stay within safe operating limits, the quickest fueling protocols pre-cool the gas to –40°C.
H2 refueling stations are therefore required to operate across a wide range of pressures (up to 875 bar) and temperatures (–40°C–60°C). This is very challenging from a measurement perspective, since the accuracy of most flowmeter technologies is adversely affected by extreme pressure and temperature conditions, as well as the transient flow encountered for vehicle filling.
Coriolis meters have dominated the market of H2 dispensers. They have several advantages, but the most important one might be their capability of measuring the mass flowrate directly. The author’s company has extensive experience and knowledge in the application of flowmeters for H2 measurements, as well as other types of gas. In an ongoing joint research project, the author’s company is involved in the European project of Metrology for Hydrogen Vehicles II (MetroHyVe II), along with several national or designated measurement institutes and companies from the industry.
As a part of the first MetroHyVe project, several experiments were undertaken on commercially available Coriolis meters for the application in H2 dispensers (produced by various manufacturers). Results showed that these meters are not sensitive to pressure effects but can be significantly affected by temperature changes, especially before thermal equilibrium is reached between the flowmeter body and the incoming gas—i.e., when pre-cooled gas is suddenly introduced to a meter that is at ambient temperature.
The project highlighted other major sources of error in H2 dispenser billing. When a customer finishes the refueling of an FCEV, some H2 is trapped between the meter and the head of the dispenser that connects to the vehicle. This amount of H2 is measured by the meter but not received by the customer. Some of the trapped H2 that is in the hose of the dispenser must be vented for safety reasons. The rest remains trapped between the meter and the cutoff valve until the next customer starts refueling. Therefore, each customer receives some H2 metered for the previous customer and leaves some for the next. However, these two amounts are not always the same as different people might refill their vehicles to different pressures. Hence, a different amount of H2 is trapped each time. This effect is not related to the flowmeter accuracy but can introduce a significant error in the metering and billing of H2 for each customer, particularly when there is a large distance between the flowmeter outlet and the dispenser.
H2 refueling station design considerations and appropriate corrections can be employed to mitigate the uncertainties caused by the aforementioned factors. Finding the right location for the installation of the meter, optimizing the dispenser to reduce the dead volume, and developing and using proper correlations to compensate for the remaining dead volume and the vented H2 are some solutions. Results of the MetroHyVe project and the author’s company’s research suggest that, if these considerations are in place, available flowmeters produced for the application in H2 dispensers can achieve OIML R139 Accuracy Class 2. The author’s company has also developed the UK’s first mobile primary standard for field evaluation of H2 refueling station dispensers. This primary standard can be taken to a H2 refueling station to test its dispensers and determine if the metering systems can meet the requirements of OIML R139.H2T
MAHDI SADRI is a Clean Fuels Consultant at TÜV SÜD National Engineering Laboratory, a world-class provider of technical consultancy, research, testing and program management services. Part of the TÜV SÜD Group, the organization is also a global center of excellence for flow measurements and fluid-flow systems and is the UK’s designated institute for flow measurement.