Calibration
planning on building two FTS, one for dispensers filling light duty vehicles and the other for heavy duty road transport vehicles. The MetroHyVe project partners, which includes NEL, have completed a test programme to assess the potential sources of error from the flow meter. Effects of pressure and temperature were investigated, as well as the potential to use alternative fluids to calibrate the flow meters prior to installation. As CFMs are based on mass flow measurement, use of alternative fluids that such as air, nitrogen and water would allow calibrations to be undertaken at significantly lower pressures and ambient temperatures whilst still achieving the same mass flow rate. Some positive results were seen in these
exact location of these dependent on the design of the refuelling station. Two common locations are upstream of the pre-cooler and near the compressor, and in the dispenser unit downstream of the pre-cooler, both locations are shown in Figure 1. CFMs perform best when calibrated at temperatures and pressures close to field operating conditions, and ideally using the same fluid. However, vehicles are filled at up to 700 bar and the temperature within the refuelling station can reach up to 60 °C, from pre-cooling at -40 °C. There are currently no independent flow calibration laboratories which can operate with hydrogen at these conditions to enable calibrations of the CFMs. There are also sources of flow measurement
error which are unrelated to the accuracy of the flow meter. For example, for safety reasons, the dispenser hose must be vented after use. As the hose is located downstream of the flow meter, this represents a quantity of hydrogen which has been measured but not delivered to the customer. A correction must therefore be applied for the vented hydrogen, otherwise it is an additional source of measurement error. In addition, once the dispenser hose is
vented, there may still be a large volume of pressurised hydrogen in piping further upstream, particularly if the flow meter is installed upstream of the pre-cooler. This ‘dead volume’ contains gas which has been measured by the flow meter, but not delivered to the receiving vehicle. If the current user fills their vehicle to a higher pressure than the previous user, there will be a positive error and the user will be over-charged, and vice versa. Once
50 September 2020 Instrumentation Monthly
again, corrections must be applied to ensure that the user has been billed correctly. To assess the magnitude of error at the dispenser, several field test standards (FTSs) have been developed, with the USA’s National Institute for Standards and Technology (NIST) being the first to develop one. Four more have been developed through the EMPIR Joint Research project (JRP) “Metrology for Hydrogen Vehicles” (MetroHyVe) by several National Metrology Institutes and Designated Institutes: METAS, Justervesenet, CESAME- EXADEBIT and VSL. Korea Research Institute of Standard and Science (KRISS) have also developed a FTS for use in South Korea. TÜV SÜD National Engineering Laboratory are
trials; the flow meters tested performed well over a range of constant temperatures and pressures representative of those found in a refuelling station. Also, the use of alternative fluids showed that they could be suitable for calibration of flow meters, which would negate the need to calibrate using hydrogen in challenging operating conditions and thereby reducing the cost of calibration. Another positive result was that pressures of up to 850 bar had an insignificant influence on flow meter performance. The JRP MetroHvVe II is due to start this September and it aims to further develop a metrological traceable framework for testing hydrogen dispensing meters. Capabilities developed would help enable fuel quality to comply with legislation, a crucial step to unlock the potential for wider deployment of hydrogen in road transport within Europe. Other activities related to the European Green Deal being undertaken by NEL include a new facility to evaluate domestic gas meters with natural gas and hydrogen blends and as well as pure hydrogen gas flow; one of the first facilities developed for decarbonisation of the gas grid.
TÜV SÜD National Engineering Laboratory
www.tuvsud.com/en-gb/nel
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