Development of a humid gas generator for sensors calibration in hydrogen

The urgent need to mitigate climate change and global warming to 1.5 °C requires an international effort toward decarbonization. In response, the European Green Deal aims to achieve climate neutrality with a net-zero emissions economy by 2050, highlighting the importance of renewable and low-carbon energy sources. Hydrogen is a key element to enable such a transition in the transport and energy sectors, as a zero-emission energy carrier that facilitates long-term energy storage and the decarbonization of hard-to-electrify sectors, such as heavy industry, aviation, and marine transport. Hydrogen as a fuel necessitates strict quality control (QC) across its supply chain. For instance, the ISO 14687 standard defines the hydrogen purity grade when it is used as a fuel in PEM fuel cell electric vehicles. Water vapour is one of the key impurities to control and monitor in the field, i.e. at a hydrogen refuelling station (HRS), to ensure efficiency and reliability of HRS operation and safety of personnel and asset. For instance, hydrogen grade-D fuel must contain less than 5 mol/mol of water. Trace water measurements are challenging when carried out in the field because of the large amount of water in the surrounding ambient. On the other hands, onsite hydrogen QC demands safe, low-cost, humidity sensors deployed in the field and integrated within the HRS infrastructure. There is a lack of calibration and testing facilities to evaluate humidity sensors performance at conditions representative of their use in a hydrogen matrix and to support research and development of novel, more advanced, trace water sensors and analysers. To address this challenge, a novel precision humid gas generator (PHG) for sensor calibration in hydrogen has been developed at INRiM in the framework of the EU Metrology Partnership project “Metrology for the Hydrogen Supply Chain (Met4H2).” The PHG is a high-pressure, single-pass, humidity generator capable of operating within a frost point temperature range of -55 °C to -10 °C in a pressure range from 0.1 MPa to 5.5 MPa, corresponding to a water vapour amount fraction approximately from 0.5 µmol/mol to 50 µmol/mol. The PHG works by temperature conditioning the incoming hydrogen gas as in a 6 m long heat exchanger coil and then saturating it with water vapour by allowing it to flow over an uniform ice layer in a 3-m long isothermal saturator. The PHG was designed to operate in two modes: single-temperature, single-pressure (1T-1P) and single-temperature, two-pressure (1T-2P). In this work the PHG design, commissioning, testing and characterization in both hydrogen and nitrogen carrier gases will be presented, and an estimate of the measurement uncertainty will be discussed. This work has received funding from the European Partnership on Metrology programme under the grant EPM 21GRD05 Met4H2 co-financed by the Participating States and the European Union.