From May 1, 2008, a hydrogen fuelling station is operating in Zaragoza as part of the celebration of the International Exposition (Zaragoza-EXPO 2008). The on-site hydrogen production, developed in collaboration with Air Products, will be generated via water electrolysis, since the thematic key of this EXPO is “Water and Sustainable Development”. The selected electrolyser produces a minimum of 15 kg of hydrogen per day and the station is designed to supply hydrogen to a fleet of “Fuel Cell Midibus” (Hydrogenics) with a nominal filling pressure of 250 bar and a hydrogen capacity of 6 kg. The station is prepared to carry out quick-filling in a maximum time of 10 minutes. Supporting supply is provided if necessary by tank trucks in the event that on-site hydrogen production can’t match the demand. The compression is provided by two diaphragm compressors. The first step is connected to the on-site electrolyser and boosts the pressure from 7 bar to 40 bar where the hydrogen is stored in a low pressure buffer tank. The second compressor (two stages), increases the pressure up to 350 bar, the high pressure storage. This last consists of three separate tanks operating in cascade with a total capacity of 77 kg of hydrogen. The main goal of the project is to simulate with HYSYS software, the behaviour of the hydrogen fuelling station in transient mode and be able to gain more insight about the strategies and the optimized process variables at the different scenarios that will be regular in a foreseen near future.