The system used to produce hydrogen via electrolysis consists of more than just an electrolyzer stack. A typical electrolysis process diagram is shown in Fig. 6.45 The primary feedstock for electrolysis is water. Water provided to the system may be stored before or after the water purification unit to ensure that the process has adequate feedstock in storage in case the water system is interrupted.
Water quality requirements differ between electrolyzers. Some units include water purification inside their hydrogen generation unit, while others require an external purification unit, such as a deionizer or reverse osmosis unit, before water is fed to the cell stacks. The high purity water will be mixed with KOH if the system is an alkaline system before being introduced to the hydrogen generation unit. Note that PEM units will not a KOH feed, as no electrolytic solution is needed. Each system has a hydrogen generation unit that integrates the electrolysis stack, gas purification
and dryer, and heat removal. Electrolyte circulation is also included in the hydrogen generation unit in alkaline systems. The hydrogen generation system is usually en closed in a container or is installed as a complete package. Oxygen and purified hydrogen are produced from the hydrogen generation unit. If desired, a compressor, hydrogen storage, and oxygen storage can be added to the system.
A second feedstock needed for electrolysis is electricity. Typically electricity is not considered a feedstock but a utility; however it is a critical component in the splitting of the water molecule into hydrogen and oxygen. An electrolyzer typically will convert supplied AC to DC, as the stack requires DC to split water.
Typical utilities that the electrolysis systems need include electricity for other peripheral equipment; cooling water for the hydrogen generation unit; pre-pressurization gas; and instrumentation gas (Fig. 6).
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