Articles
Thermal-mass measurement principle: The best choice for H2-ready gas meters
Special Focus: H2 Equipment and Services
M. MONTINARO and K. DOMANSKI, Sensirion, Stäfa ZH, Switzerland
The European Green Deal is the strategic plan to make the European Union carbon neutral by 2050.1 Decarbonizing the energy sector will be vital to achieving this goal, as this sector contributes significantly to Europe’s carbon dioxide (CO2) emissions.2 A promising strategy is gradually replacing natural gas with renewable gases, such as sustainably produced hydrogen (H2) or biogas. The use of H2, in particular, is a potentially favorable course of action and preparations are already underway to switch the entire gas infrastructure in line with this planned transition (e.g., starting in 2025, all new gas boilers sold in the UK must be compatible with pure H2). Gas meters are an essential part of gas infrastructure, and are indispensable in reliably and fairly billing gas consumption.
This article will examine the metrological behavior of thermal-mass gas meters operating with natural gas mixtures containing significant amounts of H2 and pure H2. The authors will present measurement data, discuss measurement accuracy and operational safety, and explain why it will still be possible to maintain thermal-mass gas meters’ compact size regardless of the H2 content—this is the key advantage that H2-ready solutions have over other metering technologies.
An introduction to the thermal-mass measurement principle. A calorimetric sensor element based on a micro-electromechanical system (MEMS), as shown in FIG. 1a, is the core element of any thermal-mass flow sensor, such as those used in gas meters. The sensor element can be found on a membrane on a silicon chip and consists of a micro-heating element and temperature sensors that are integrated upstream and downstream. When an electric current flows through the micro-heating element, it generates a temperature profile on the membrane. If no gas is flowing, the temperature is identical at both the upstream and the downstream temperature sensors (FIG. 1b). If gas flows across the membrane, it generates a heat flow (i.e., it causes the temperature profile on the membrane to change), resulting in a temperature change between the upstream and downstream temperature sensors (FIG. 1c). The resulting difference in temperature between the two sensors creates a precisely measurable sensor signal that is a function of the flow velocity: the greater the difference in temperature, the greater the gas flow velocity over the sensor element.
The thermal-mass flow measurement principle is based on the physical effect of heat convection, so the sensor’s signal depends on both the gas flow velocity across the membrane and the measured gas mixture’s thermal properties.
So, a thermal flow sensor provides accurate measurement data if it has either been pre-calibrated for a particular gas mixture, or if it has a routine that dynamically considers varying gas mixtures when measuring flow.
Gas meters can be used for a large number of potential gas mixtures, and their composition can vary over time. In practice, it is infeasible to perform individual pre-calibrations for all the possible natural gas mixtures, which is why the authors’ company’s thermal-mass sensors for gas meters have a proprietary, dynamic natural gas and H2 recognition routine to ensure accurate flow measurements, even when gas compositions vary.
Test setup. The measurement data presented here was recorded using thermal-mass flow sensors with a dynamic gas recognition routine. The routine is optimized for H, L and E gases according to EN 437:2018 that contain up to 23% H2, as well as for pure and near-pure H2. The flow sensors’ output signal is temperature- and pressure-compensated in standard cubic meter per hour (m3/hr).
The flow sensors were tested in a generic gas meter prototype housing, with the flow sensor positioned at the gas meter housing outlet (FIG. 2a). An external gas supplier3 mixed the tested gas mixtures (TABLE 1). Sonic nozzles4 were used as flow references and the measurements were conducted at room temperature. The measurement setup for the flow measurements is shown as a schematic diagram in FIG. 2b.
Flow measurements in natural gas/H2 mixtures. FIG. 3 shows the relative measurement errors at the reference gas flow of up to 6 m3/hr (G4 meter) for five flow sensors in air, methane and natural gas mixtures containing 5%, 10% and 23% H2. The error limits of ± 3.5% and ± 2% shown in black are the maximum permissible error limits according to the European Directive 2016/32/EC on measuring instruments (MID) and the recommendations made by the International Organization of Legal Metrology OIML R 137 regarding accuracy temperature-compensated gas meters of accuracy Class 1.5.
All error curves of each of the five measured flow sensors are well within the maximum permissible error limits and also comply with the permitted air-gas relationship of 3% and 1.5%, respectively, according to the European Standard for thermal-mass gas meters, EN 17526, as well as EN 14236 for ultrasonic domestic gas meters.
The test gas containing 23% H2 is the test gas G 222 as per EN 437. G 222 is described as the “limit test gas”: a gas mixture with the maximum H2 content used for testing gas appliances for second-family natural gas mixtures, according to EN 437.
Flow measurements in H2 gases. Since H2 has a lower calorific value—lower than that of natural gas by a factor of three—significantly higher H2 flowrates are required to maintain the same energy flow through the meter. FIG. 4 shows the relative measurement errors at the reference gas flow of up to 20 m3/hr for five flow sensors in 100% H2 and 98% H2 + 2% CO2. Two percent of impurities is the maximum allowed for Class A H2 as defined by ISO 14687. Again, the error limits for Class 1.5 meters of ± 3.5% and ± 2% are indicated in black (for a meter with Qmax of 20 m3/hr).
It can be seen that the meters perform very well when working both with pure H2 and with H2 with 2% of CO2 contamination. It is important to mention that the size of the sensor (and, consequently, the meter) can stay the same whether it operates with a 6 m3/hr flow of natural gas or 20 m3/hr of H2.
Operational safety. The authors’ company’s thermal-mass flow sensors do not have any safety-related limitations when operated with natural gas or H2. Both the maximum temperature and the maximum thermal energy stored on the micro-sensor element are significantly below H2/air mixtures’ ignition temperature or ignition energy, even if the flow sensor’s voltage regulator malfunctions. This is why the company’s thermal-mass flow measurement technology has been successfully used for years in challenging gas analysis applications with pure H2.
Consistently compact size for any H2 content. When substituting natural gas with H2, it is important to consider that H2’s calorific value by volume is about three times lower than that of typical natural gas mixtures. In practice, this means that if a gas appliance is operated with pure H2 rather than natural gas, a gas volume approximately three times greater must be supplied to achieve comparable heating power. In this case, gas meters originally designed for operation with natural gas must be capable of measuring an increased gas volume due to the admixture of H2. Consequently, it may be necessary to choose purely volumetric natural gas meters in a larger size that also have an extended dynamic range (if they must be compatible with both natural gas and pure H2). Larger meter designs can be more costly and require additional installation space. If larger gas volumes flow through a meter than it was originally designed for during operation with H2-free natural gas, this can increase the wear on the meter mechanics, thus shortening the service life.
Similarly, ultrasonic gas meters face a challenge due to H2 having an increased speed of sound compared to that in natural gas (approximately by a factor of three). This means that the sound flow path must be physically lengthened and the electronics used for the measurement must become significantly faster. Consequently, such a meter is expected to be larger, more complex and more expensive.
In contrast, thermal-mass flow measurement technology is a static measurement principle that has no moving parts and directly measures mass flow. Consequently, increased volume flow causes additional wear and has no influence on a thermal-mass gas meter’s service life. Unlike volumetric or ultrasonic gas meters, thermal-mass gas meters can remain the same size and are just as simple whether operated with natural gas or with any optional H2 content.
In the thermal-mass measurement principle, the key parameter to consider is not the gas volume flowing through the meter; rather, it is the relevant gas mixture’s Reynolds number (Re). This is a fluid dynamics parameter that indicates whether turbulent (high Re number) or laminar (low Re number) flow conditions are formed in a system. Comparing the Re numbers for pure methane ReCH4 (representing a natural gas mixture) and for pure H2 ReH2 shows that ReH2 is lower than ReCH4 by a factor of more than six for the same meter housing geometry. Assuming that H2 flow increases by a factor of three (to compensate for H2’s lower calorific value, which is three times lower than natural gas), ReH2 is still about two times lower than ReCH4. Compared to methane, the lower Re number for H2 means that measuring conditions remain stable with the same meter housing geometry, even if the volume flow increases by a factor of three.
Similarly, the pressure drop across the meter will not increase with the higher H2 flow required. The pressure drop across the meter is proportional to gas density × velocity.2 Since the density of H2 is a factor of 14 lower than that of methane, the pressure drop when flowing H2 at three times a higher rate than methane will actually be lower.
Consequently, the same meter size can be readily used for both natural gas and for operation with up to 100% H2. A typical G4 thermal-mass meter can handle a Qmax of 6 m3/hr during operation with natural gas and > 20 m3/hr when used with H2. Moreover, no recalibration or setting change is needed to switch between different gases, and the same sensor can seamlessly adjust itself to the gas supplied.
Takeaways. The measurement data presented here demonstrates that the thermal-mass measurement principle complies with the error limits for measurement accuracy and the air-gas relationship as stipulated by the MID for various natural gas/H2 mixtures and pure H2. There are no limitations regarding operational safety, even during operation with 100% H2. The size of thermal-mass gas meters, which is already very compact, can be maintained regardless of gas H2 content. This is a distinct advantage when compared to mechanical and ultrasonic meters, eliminates the need for expensive, large meter designs, and keeps the logistics and installation of thermal-mass gas meters simple and affordable.
In recent years, technological advancements in gas meters have mainly involved enabling them to communicate as smart meters. H2 admixtures will drive further modernization in the gas meter industry, resulting in a move away from old, mechanical and volumetric measurement principles toward modern technologies that can offer significant advantages for operation with H2. More than 6 MM gas customers worldwide are already benefiting from reliable and fair billing thanks to their thermal-mass gas meters. Going forward, H2 admixtures will promote the ever-faster uptake of this compact, static metering technology.H2T
LITERATURE CITED
1 European Commission, “The European Green Deal,” online: https://eur-lex.europa.eu/resource.html?uri=cellar:b828d165-1c22-11ea-8c1f-01aa75ed71a1.0002.02/DOC_1&format=PDF
2 European Parliament, “Greenhouse gas emissions by country and sector,” online: https://www.europarl.europa.eu/news/en/headlines/society/20180301STO98928/greenhouse-gas-emissions-by-country-and-sector-infographic
3 Messer Schweiz AG, online: https://www.messer.ch
4 TetraTec Instruments, online: https://www.tetratec.de
About the authors
MICHELE MONTINARO is Director, Industrial Markets, at Sensirion. Responsible for the smart energy and industrial markets, he supports customers worldwide, helping them create value for all kinds of applications. Dr. Montinaro draws on a broad range of experience in sensor technology. He holds a PhD from the University of Basel.
KONRAD DOMANSKI is a Product Manager at Sensirion. He is responsible for the definition, product lifecycle management and certification of micro-thermal gas meter sensor modules used in smart gas metering products worldwide. Dr. Domanski brings broad experience in international project management and holds a PhD from the Ecole Polytechnique Fédérale de Lausanne (EPFL).
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