A transportable precision humidity generator for operation with hydrogen

To address climate change and limit the global temperature rise to 1.5 °C, many actions are being taken at the international level. Europe, through the Green Deal, aims for climate neutrality by 2050 with a net-zero emissions economy. Hydrogen is a promising energy vector for transport, industry, heat, and energy storage. It stores energy from renewable sources like wind and solar, converting it into electricity on demand, ensuring a continuous energy supply even when these sources are not active. However, to promote its use, hydrogen must meet strict quality requirements across its value chain, from production to storage, distribution and end-use. Whether it is produced by current "brown" steam reforming of natural gas (SMR) or by “green” electrolysis of water, one of the most challenging impurities for hydrogen quality control is water vapour. Reliable and traceable standards are thus required to underpin accurate water vapour measurements under operational conditions relevant to the hydrogen supply chain. Further, in many circumstances, such reference standards need to be field-deployable to provide on-site measurement traceability and perform sensor calibration near the point of use. To meet these challenges, a novel transportable Precision Humidity Generator (t-PHG) able to generate traceable amounts of water vapour in hydrogen has been designed, constructed and commissioned at INRiM. The t-PHG is a saturation-based single-pass standard humidity generator developed within the EU Metrology Partnership project Met4H2 “Metrology for the hydrogen supply chain”. It can generate a moist hydrogen stream with an amount of water fraction between 0.5 µmol/mol and 50 µmol/mol in a wide absolute pressure range from 0.2 MPa to 5.5 MPa. These conditions encompass an equivalent pressure dew-point range between approximately -55 °C and -10 °C. In this work, the testing and characterisation of the humidity generator both in nitrogen and hydrogen will be reported and a preliminary estimate of the measurement uncertainty discussed.