Exploring Hydrogen Compression Techniques for Hydrogen Refueling Stations
The key to integrating reciprocating compressors in hydrogen filling stations is selecting the appropriate working pressure, flow rate, and electric motor power consumption. The compressor must be able to achieve the appropriate hydrogen pressure while utilizing low pressure hydrogen from the first stage. Hydrogen production methods determine input pressure, which for electrolyzers, a key component in green hydrogen refueling stations, ranges from 8-30 bar. No matter the hydrogen generation process, the compressor must have a high compression rate to meet the pressure needed for the filled vehicles.
The compressor should match the hydrogen supply flow rate to ensure optimal station operation and facilitate the pre-specified daily vehicle capacity specified by the station designer. Finally, consider the station's ability to power the compressor's electric motor. While not a concern for grid-connected installations, this could be a concern for stand-alone stations using renewable energy sources. To ensure uninterrupted compressor function, it is crucial to properly size the power source and any energy storage equipment.
A Few Glimpses on Metal Hydride Hydrogen Compressor:
In recent years, metal hydride compressors?have gained attention for their ability to provide high-pressure hydrogen, consume less power, and integrate with renewable energy systems or industrial processes that generate waste heat. Metal hydride compressors, which have no moving components, run quietly and can be installed and used in residential areas at any time. Eliminating moving parts reduces component failures and maintenance costs, as maintenance activities are less frequent than with moving parts.
Metal hydride compressors operate through thermally induced reversible hydrogenation and dehydrogenation cycles. Following dehydrogenation at higher temperatures and pressures, the process involves hydrogenation at low temperatures and pressures. Unlike typical mechanical compressors, metal hydride compressors are thermally powered, accomplishing compression through heating and cooling instead of gas displacement. To make metal hydrides, metal hydride compressors use intermetallic compounds, hydride-forming metals or alloys, and hydrogen interaction. Most commonly utilized materials include AB5 (LaNi5, La0.5Ce0.5Ni5) and AB2 (Zr-V-Mn-Nb) alloys. To comprehend metal hydride compressor compression, it is necessary to first grasp the hydrogenation (absorption) and dehydrogenation (desorption) processes. Both reactions are efficient within a specific temperature and pressure range, determined on the metal hydride composition.
Design Principle & How to integrate..?
A compressor designer should sustain temperatures by giving sufficient heat to the vessel containing metal hydrides. Designing an efficient compressor stage requires considering the exothermic/endothermic characteristics of absorption/desorption processes. To maintain a consistent temperature during the hydrogenation cycle, the designer should continuously remove heat from the metal hydride tank, as absorption is an exothermic reaction. External flow of a cooling liquid eliminates produced heat by heat transfer in the vessel wall. The opposite is true for desorption.
Integrating a metal hydride compressor in a hydrogen refueling station requires careful consideration of key factors, including the compressor's ability to achieve the desired pressure and flow rate (based on installation needs). For stations that support large vehicles, the ideal pressure is 350 bar, whereas for passenger vehicles, it is 700 bar. The flow rate of the compressor is determined by the number of cars the station accommodates daily/weekly. To optimize the flow rate of the compressor, it is important to match system components such the hydrogen generating unit and compressor. A restriction exists on the amount of hydrogen produced per unit of time, particularly for green hydrogen refueling stations that use electrolyzers for hydrogen production. To avoid potential bottlenecks and maintain efficiency, the compression flow rate should match the electrolyzer output rate. A simple remedy for prospective bottlenecks is to use a buffer storage space to store surplus hydrogen. However, depending on the amount of the mismatch, this may only delay the bottleneck. It is important to match the hydrogen production pressure with the suction pressure of the first stage of the compressor when selecting a hydrogen producing unit, independent of compression type. Integrating the metal hydride compressor requires sufficient heating/cooling medium to ensure continuous operation . Calculate the medium flow rate and temperature ranges needed for metal hydride compressor operation. By estimating these values, one can determine the necessary heat flow and choose the mechanism for supplying it to the system.
Use solar heaters/chillers, resistances, heat pumps, or industrial waste heat as options. The latter way is better for economics and sector coupling, benefiting both heat-rejecting industries and metal hydride compressor operators due to abundant low-grade thermal energy. Their capacity to use renewable energy sources and excess heat is a major advantage. Other benefits include noiseless operation, low CRM usage, and low maintenance costs. Their main downsides are immature technology, low efficiency, and limited market readiness (which may not matter if a plentiful waste heat source is available.)
Developing hydrogen compression methods for hydrogen filling stations is essential to sustainable transportation. Innovation in compression technology enables wider deployment of hydrogen fuel cell vehicles, lowering greenhouse gas emissions and climate change.Adopting these innovations advances sustainable energy and creates new economic and career prospects. We can enable our communities to embrace a greener, more sustainable future by supporting hydrogen compression research and development.