Scientists from the Fraunhofer Institute for Chemical Technologies (Germany) created a model allowing for precise calculation of hydrogen dehydration process using the absorption method at the operating off-shore wind-driven power plants. This instrument provides for designing reliable systems of receiving “green” hydrogen right on the continental shelf of the Northern Sea. The model takes into account real wind velocity fluctuations directly impacting the amount of produced hydrogen, and helps to optimize the purification system operations. The level of hydrogen purity, in its turn, determines if hydrogen may be safely stored, transported and used in industry or as transport fuel.
The key problem of modern hydrogen recovery technology is associated with high quality standards. The ISO 14687 International Standard sets the maximum permissible content of water in hydrogen for filling stations and storage systems – no more than 5 micromoles per mole. This is approximately one drop of water in the big tank-car of hydrogen. Overrunning of this limit implies the risk of ice formation on the valves when the ambient temperature is below zero degrees Celsius, the risk of corrosion and the risk of entire infrastructure failure. Standard cooling and moisture condensation are not instrumental here, hence, the adsorption is used – the process, when water molecules are captured by the surface of especially selected material, e.g., zeolite.
Sea platforms usually have the following specific feature: the amount of produced hydrogen directly depends on the wind intensity, and it changes practically every minute. Due to this the gas flow at the absorbing agent column inlet varies all the time and remains unstable. The traditional methods of the absorption systems calculation imply stable smooth flow; hence, they do not fit for operating in such conditions. To solve this problem, the scientists proposed a new model taking into account the fluctuations in the real-time mode. It allows for precise identifying the moment when the absorbing agent is completely saturated with moisture, and the column needs to be put into the regeneration mode – either by heating, or by decreasing the pressure.
In their study, the scientists measured experimentally, how much water a specific type of zeolite (13X BFK) could absorb under different temperatures and pressures, and then described these data using the Langmuir-Freundlich equation, which fits very well for modelling the absorption processes. On top of that, the researchers showed that hydrogen per se practically does not hold on the zeolite surface. For that purpose, they used the ideal absorption solutions theory, which confirmed: interaction between hydrogen and absorbing agent is hardly material, and this simplifies the model drastically meaning that the main focus may be on describing the retainment of water molecules.
The simulation results demonstrated the dependency of the dehydration process on weather conditions. When the wind velocity was medium or high (10 and 16 m/s), the absorbing agent got saturated with water in 18-20 minutes only, but when the wind was weak (6 m/s) the column remined unfilled even after one hour. It means that the control system cannot operate according to a fixed time-table and should flexibly adjust the operating cycles to the current weather. For this purpose, the scientists propose to equip the columns with temperature or humidity gauges, which will chow the absorbing agent loading, as well as connect the purification module with the data about the performance of the electrolysis units in order for the system to respond automatically to the changes in hydrogen flow.
Now, the model created by the German researchers allows for checking the system’s operations already at the initial stage of designing the off-shore wind farms given very different scenarios – from cutting blasts of wind to complete windless calm. The model can help to calculate the optimal sizes of absorption columns, to identify the need for buffer tanks for gas or for additional batteries. The future plan is to use this model for comparing the efficiency of different methods of absorbing agent regeneration, as well as for testing new materials, which may be really capable of substituting for zeolite in the dehydration systems.
