Zaragoza EXPO 2008 Hydrogen Fuelling Station: Simulation and optimization of process variables and strategies in different scenarios

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1 Zaragoza EXPO 28 Hydrogen Fuelling Station: Simulation and optimization of process variables and strategies in different scenarios P. Marcuello a, L. Correas b, I. Aso b,*, L. Romero b J.A. Peña a a Aragon Institute for Engineering Research, University of Zaragoza, Zaragoza (Spain) (jap@unizar.es) b Aragon Hydrogen Foundation, Parque Tecnológico Walqa, Huesca (Spain) (iaso@hidrogenoaragon.org) * Presenting author Keywords: Hydrogen fuelling station, Hydrogen supply, Hydrogen vehicle, Hydrogen fleet, optimization, inertization Abstract The strategies for supplying hydrogen in the new Zaragoza-EXPO Hydrogen fuelling station (Zaragoza Saragossa- Spain), has been simulated by means of ASPEN Hysys simulation software [1]. This is the first commercial fuelling station that has been built up in Spain. Departing from the specification sheet, a model has been developed which runs in transient mode, and attempts to gain insight over the most probable scenarios which will afford in its daily schedule. Introduction From 14 th June 28, a commercial hydrogen fuelling station will operate in Zaragoza (Spain), as part of the celebration of the International Exposition (Zaragoza-EXPO 28) [2-4]. The onsite hydrogen production will be generated via water electrolysis, since the thematic key of this EXPO is Water and Sustainable Development (See specifications sheet in Table I). The PFD is shown in Figure 1. Figure 1.- ASPEN Hysys flowsheet of the simulated hydrogen fuelling station. The selected electrolyzer 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 with a nominal filling pressure of 3 bar and a hydrogen capacity of 6 kg. The station is prepared to carry out quick-filling in a maximum time of 1 minutes. Extra 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 electrolyzer and boosts the pressure from 7 bar to up to bar where the hydrogen is

2 stored in a medium pressure (up to bar) buffer tank. The second compressor (two stages) increases the pressure up to 4 bar, the highest pressure storage. This last consists of three separate tanks operating in cascade with a total capacity of 77 kg of hydrogen. Table I.- Main specifications sheet Hydrogen production method: Water electrolysis (renewable energy source) Supply pressure to vehicle tank: 3 bar Pressure of vehicle tank at the beginning of filling up 3 bar operation Nominal production rate of Hydrogen 4 kg/day Minimum production rate of Hydrogen 15 kg/day Maximum mass of Hydrogen per filling up 6 kg Maximum mass of Hydrogen for two consecutive fillings 12 kg Maximum time between two consecutive fillings 2 hours 3 minutes Hydrogen purity % (mol) The main goal of the project is to simulate 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. Results and discussion Several scenarios have been simulated in order to gain knowledge on the possible schedules that should adopt the station along every working day, and on which one of them should be more appropriate or convenient. To name a few, the most interesting have been: Scenario A.- Continuous electrolyser production at a mass flow of 4 kg/day Scenario B.- Filling of a vehicle tank (6 3 bar) Scenario C.- Filling two consecutive vehicle tanks in less than 2 minutes Scenario D.- Inertization ( ) prior to hydrogen introduction in the lines Figure 2 shows the behaviour of pressures at different positions in the station for Scenario A. It represents a continuous mass flow of 4 kg/day of hydrogen is produced. The discontinuity of fuelling or not fuelling is achieved by means of an intermediate buffer vessel (.45 m 3 ) which receives the hydrogen produced by the electrolyser. Its pressure is represented by a blue line (right hand y axis). Every time the pressure achieves a nominal value of bar, it discharges to a second compressor (two stages), which compresses the gas up to 435 bar in three different stages (cascade). The three top lines show the pressure of the three vessel pressure cascade stages which transfer in turn hydrogen to the vehicle tank. The sequence is from high to low pressure filling for the cascade, and from low to high pressure for the vehicle tank. Finally, the vehicle tank has also been plotted (magenta) in two consecutive filling ups. The noisy segments correspond to releasing valves. 4 4 mid pressure low pressure filling up of cascade Low Pressure Vesel Medium Pressure Vessel Vehicle Tank High Pressure Vessel Intermediate Buffer vehicle tank Intermediate Buffer T im e (m in ) Figure 2.- Pressure profiles along time for different vessels in Scenario A.

3 Figure 3 shows in detail a filling up of the vehicle tank. It corresponds to the above mentioned Scenario B low pressure m id pressure high pressure Vehicle Tank Pressure High Pressure Vessel Medium Pressure Vessel Low Pressure Vessel Vehicle Tank Temperature vehicle tank pressure H ig h L o w P re ss u re M e d iu m P re ss u re P re s su re 2 2 V e s s e l V e s s e l V e s se l Temperature ( ºC) T im e (s) Figure 3.- Filling up of a vehicle tank from the pressure cascade: Scenario B. In this scenario (B), the discharge of hydrogen is produced in turn from the lower to the higher pressure. The hydrogen is directly transferred to the vehicle tank, increasing its pressure gradually and almost at a constant rate. The temperature inside the vehicle vessel increases too, but in this case, abruptly (up to ~39 ºC) as consequence of the compression effort at the beginning of the operation. Afterwards, the temperature begins to get lower because of a dilution effect with cooler hydrogen coming from the pressure cascade. The complete filling up process is completed in less than 1 minutes, which had been stipulated by the specifications sheet. Figure 4 shows two consecutive refilling of vehicle tanks whit no need of extra hydrogen coming from outside of the plant (not produced by the on-site electrolyzer) (Scenario C). Using this schedule, the pressure cascade is forced to go further than it was with the previous scenario (B) [5,6], but it is still able to get it. Both filling ups are completed just one after the other in less than 2 minutes, which was another of the specifications that should fulfil the installation H igh Pressure Mid Pressure 3 Low Pressure Vehicle tank Vehicle Tank H ig h P re s su re M e d iu m P re s su re Lo w P re ssure Vessel L o w P re s su re T im e (s ) M edium P ressu re V esse l H ig h P ressure V esse l Figure 4.- Two consecutive refilling of vehicle tanks from cascade; Scenario C. Figure 5 shows a suitable strategy for inertization of the lines to allow the purge of oxygen, prior to the entrance of hydrogen. The main goal is that no one point within the installation has a hydrogen content in air higher than 4% (vol) which corresponds to the Lower Explosion Limit, and lower than 75% (vol), which corresponds to the Higher Explosion Limit [7,8].

4 Two schedules have been plotted. In the first one (1 st option), nitrogen is used to inert the system, and hydrogen is not entered until the content of oxygen at the exit of the fuelling station is lower than 5.3% (vol). This fact implies, being very conservative, that hydrogen cannot enter the system until more than 17 hours later. The second option consists of introducing hydrogen at around 15 hours later than nitrogen did. Bearing in mind this strategy, which shortens the inertization something more than 2 hours, no one point in the installation is within the explosion interval. In both cases, the mass rates of nitrogen / hydrogen that have been tested correspond to the ones that were specified in the specifications sheet Molar fraction H 2 H 2 2 n d o p tio n O 2 H 2-2 n d o p tio n H 2-1 s t o p tio n T im e (h ) Figure 5.- Inertization and start up of the fuelling station; Scenario D Conclusions 1. ASPEN Hysys has proved to be a reliable tool to simulate the following topics: a. Fuelling station topologies (number of compressor, presence of buffer vessel, presence of booster compressor,...). b. Compression strategies (pressure at different stages, distribution of the number of effects in the pressure cascade if any-). c. Schedules depending on the average amount of vehicles, its tank capacity and the allowed length of refilling. 2. If pressure at the exit of the electrolizer is lower than 15 bar, a first compressor (one stage) which raise the pressure up to ~ bar could be convenient in order to shorten the temperature increase. This hot gas should be cooled down to ambient temperature because of safety considerations. 3. Inertization scenarios have proved to be extremely interesting in order to program startups or shut downs. A proper strategy to follow is needed in order to shorten the unproductive periods. 4. A direct consequence derived from the previous conclusion (no. 3) is that a steady regime must be maintained along time (e.g. keeping enough pressure of hydrogen in the station even when the electrolizer is switch off / out of order, or there are no vehicles refuelling). 5. More effort must be stressed in the following points [9]: a. In this model, only vessels have been taken into account. An approach to the pipe network should be bore in mind. b. Design aspects have not been taken into account related to the costs of the equipments. Economic balance should include these chapters. c. The results provided by this model must be contrasted against the real Zaragoza EXPO Hydrogen fuelling station.

5 Bibliography 1. ASPEN Hysys Aspen Technology Inc., Burlington MA ( 2. Montaner P. Montaje de una hidrogenera: Experiencia práctica EXPO Zaragoza 28. Diploma de Especialización en Tecnologías del Hidrógeno y Pilas de Combustible. March HyApproval (Proyecto HyApproval). Handbook for Hydrogen Refuelling Station. December Brey J.J., Brey R., Carazo A.F., Contreras I., Hernández-Díaz A.G., Castro A. Planning the transition to a hydrogen economy in Spain. International Journal of Hydrogen Energy Thomas S., Reardon J.P., Lomax F.D., Pinyan J., Kuhn I.F. Distributed Hydrogen Fueling Systems analysis. Proceedings of de DOE (Department Of Energy) Hydrogen Program Review H2-Review-VII-Framework-27. European funded research on Fuel Cells and Hydrogen: review, assessment and future outlook. Report from the conference Hydrogen and Fuel Cell Review Days 27. Bruxelles, Campbell K., Cohen J., Eichelberger E., Hansel J. Hydrogen Fuelling Safety Advances. Air Products and Chemicals. November EIGA (European Industrial Gases Association) Gaseous Hydrogen Stations. IGC Doc 15/6/E Revision of Doc 15/96 and Doc 15/ Ulleberg O., Ito H., Maack M.H., Ridell B., Miles S., Kelly N., Iacobazzi A., Argumosa, M., Schoenung S. Task 18: Integrated Systems Evaluation. Subtask B: Demostration Project Evaluations. Final Report for IEA-International Energy Agency- and HIA-Hydrogen Implementing Agreement-.27.