The PEM electrolyzer

Four electrolysis processes are currently known, which are at different stages of technical development. 
Polymer electrolyte membrane electrolysis (also known as "proton exchange membrane electrolysis" or PEM electrolysis for short) is considered to have the potential to overtake the currently most mature and more powerful alkaline electrolysis (AEL) in the long term in terms of performance and cost, and thus to achieve the highest growth rates in production in the medium term.

The PEM electrolysis system operates with treated water, which is broken down into its components hydrogen and oxygen with the aid of electrical energy. The hydrogen generated in the process is treated and delivered to the interface ("delivery line") at a system pressure of 30 to 40 bar.
Ideally, the heat generated during electrolysis is used to increase the efficiency of the overall process.

Diagram – General electrolysis arrangement

Water treatment

In PEM electrolysis, the water used for decomposition is required as an ultrapure product, as also used for the pharmaceutical and semiconductor industries. It must be free of particles and salts, so that pure deionization is not sufficient for this purpose. 

A mixed bed filter is often used for this purpose as a fine purification stage, which performs the residual desalination of desalinated water in modern water treatment plants. It removes the residual ionogenic compounds present as slack in a deionization plant by a combination of cation and anion exchange resin.

Closed electrolysis circuit

Electrolysis is divided into the anode circuit (shown in green in the figure) and the cathode circuit (red section).

In the anode circuit (green), the oxygen side of the process, the water is provided downstream of a water treatment plant. From our experience, it is (at least partially) designed as a combined process and cooling water circuit.
Since a pressure of 1 – 1.5 bar is required in this circuit, a pressure reducing valve type DM 505 is connected upstream of the closed circuit. Additional filters may be required upstream and downstream of the valve. The electrolysis shown here works with two electrolysis stacks in which the separation of the ultrapure water takes place. The semi-permeable polymer electrolyte membrane allows the protons to pass in only one direction, which enables clean separation of the circuits.
The generated oxygen gas must be separated from the resulting water/oxygen two-phase mixture. As the water content is higher than the gas content, a Mankenberg gas separator type AS 5 is used after the mixture has left the electrolysis stacks. The remaining pure water, which has not been decomposed, is returned to the electrolysis circuit and flows back, for example, through a heat exchanger, which can be equipped with an EB 1.12 bleeding and venting valve for residual gas separation. A condensate trap KA 2 is arranged upstream of the heat exchanger to discharge residual liquids or any condensate sludge without pressure.

Cathode circuit (red): On the other side of the semi-permeable membrane, the raw hydrogen is released. It has left the electrolysis cell stack at about 30 to 40 bar pressure. This pressure level is maintained so as not to waste energy.
The hydrogen gas produced is saturated with water vapor and must be processed for the consumer. Drying can be carried out by means of a centrifugal separator type AS 2. If heat exchangers are used and the pressure level of the condensate to be discharged is reduced, post-separation processes may also be required, with separators type AS 2 or steam traps type KA 2. The centrifugal separators offer a separation rate of 99 % and feed the accumulated condensate back upstream of the treatment section, while the dry hydrogen gas is directed to the transfer section.
The H2 transfer section consists of two back pressure regulators and a buffer tank. The first back pressure regulator UV 8.2 upstream of the tank is set to 30 bar and opens as soon as the system pressure is reached after start-up of the system. The buffer tank is pressurized to 30 bar. As soon as this process is finished, the pressure continues to rise so that the back pressure regulator and buffer tank are overrun and the second valve opens at 37.5 to 40 bar and delivers the hydrogen at a defined pressure.

Hydrogen processing

The hydrogen generated through electricity is stored in existing gas infrastructures, transported and made available again as needed. Due to its relatively low volumetric storage density, the gas can only be stored and transported in compressed form.

Hydrogen compressors have been used for decades for compression, providing the required quantities and pressures for industrial applications. The compressed hydrogen gas is filled into a compressed gas container via a valve. Modular compressor units are an effective solution for growing demand, e.g. for end-use at hydrogen refueling stations for passenger cars and heavy-duty vehicles. Various new compressor technologies are being researched to increase the availability and long-term stability of the gas.

Liquefaction increases the storage density by a multiple. This is advantageous for the transport and storage of hydrogen. The gas is cryogenic at -253 °C and can be vaporized by heat input from outside. Liquid hydrogen has been used in the space industry since the 1950s. This technology requires the constant input of electricity for liquefaction, which significantly limits the use of renewable energies for this process.