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5 Hydrogen Production

5.2.2 Alkaline Electrolysis

Alkaline electrolysis is a well-established technique widely used for hydrogen production. Its key features include:

• Electrolyte: Typically, a 20-40% potassium hydroxide aqueous solution is used.

• Diaphragm: Historically made from asbestos, now replaced by polysulphone polymers or nickel oxide.

• Operating Temperatures: Generally range between 80 and 100 °C.

• Electrode Material: Commonly utilizes Raney-nickel.

Types of Connections

1. Unipolar Connection:

   • Single cells connected electrically in parallel, open to ambient conditions.

   • Handles high currents, limited by the inability to operate at elevated pressures.

   • Now offered by only a few manufacturers.

2. Bipolar Connection:

   • Cells are arranged in stacks where electrodes function as both anode and cathode.

   • Compact design facilitates lower currents and higher voltage handling.

   • Introduction of a "zero gap" design minimizes ohmic losses by placing electrodes directly onto the diaphragm.

Peripheral Units

An electrolyser plant includes peripheral units essential to its operation:

• Deionization: Ensures quality of feed water to prevent fouling.

• Process Control System: Maintains alkaline solution concentration by balancing water addition and removal.

• Lye Management Unit: Controls alkaline electrolyte concentration affected by gas production.

• Transformation Station: Converts AC to the required direct current voltage for operation.

Manufacturers and Capacity

Table 5.1 summarizes prominent manufacturers of alkaline electrolyser systems along with their hydrogen production capacities, reflecting a range from 0.05 to 500 Nm3/hr.

Investment Costs and Efficiencies

• Investment Costs: Vary based on the capacity of production; larger units typically exhibit lower costs per production unit.

• Efficiency: Relatively consistent across different system sizes. Figures illustrate the cost and efficiency trends depending on hydrogen production capacity.