Electrolyser structure and functioning

The unit is designed for indoor operation only, thus located in a container outside right next to the laboratory. A picture of the electrolyser cabin is visible in Figure 19.

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Fig. 19 – Picture of the electrolyser container at the Entrance test site

The container consists of two separate rooms. One side houses the PLC and the water preparation system, where tap water is treated by means of a Reverse Osmosis (RO) unit and ion exchange filters to create suitable water for electrolysis (Figure 20). The deionized water then flows to the room on the other side where the electrolyser itself is situated (Figure 21).

This room also houses a system of valves and two 5L storage cylinders for storing the produced hydrogen. The tanks can be emptied afterwards by venting the contained hydrogen outside.

Produced hydrogen can also be directly vented during operation, without filling the tanks. The former production method will be further referred to as tanks filling mode, the latter as venting mode. A flow controller has been added at last to the setup to control the flow of hydrogen while vented to the outside air. For the sake of completeness, Figure 23 shows the PFD of the hydrogen production process as well as the position of the various elements along the pathway.

Fig. 20 – Picture of one side of the electrolyser cabin

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Fig. 21 – Picture of the other side of the electrolyser cabin

The electrolyser unit contains an electrolysis cell stack and auxiliary equipment, required for regulating electrolysis operations (circulating water, drying hydrogen, pressurizing hydrogen, performing safety operations and shutting down the system). The internal part of the unit and the functional elements included therein are illustrated in Figure 22.

Fig. 22 – Inside of the Proton’s Hogen40 PEM electrolyser

A control board and on-board sensors are also inside the unit for the sake of monitoring performance and automating operations⁶. Specifically, this control system deals with the automation of normal and security operations, by turning on and off auxiliary equipment (such as ventilation fans, circulation pumps etc.) when needed or shutting off the entire system if something goes wrong during the process.

6 The sensors monitor system and process conditions whereas the regulator, along with proper transducers, controls the system operations accordingly.

41 The control also allows to regulate the H2 production in accordance to the pressure set-points.

The two set-points that can be set are:

- System pressure, which represents the H2 pressure in the H2 separator inside the unit;

- Product pressure, which represents the pressure of the H2 flowing out of the system from the H2 port.

For the sake of security, both of them are fixed and properly preset by a specialized technician to approximately 15 bar.

Briefly, the electrolysis unit is internally controlled and fully automated in every state and process. The operator only has to start the system up at the beginning and shut it down at the end of operation. Instructions on how to carry out these tasks are provided in the brief user’s manual in Appendix B. Throughout the electrolyser start-up and functioning described therein, various operating states can be distinguished. The states definitions are given in the Proton’s electrolyser manual [19] and, since an explanation is useful for the purpose of the electrolyser characterization, these are shortly introduced below:

 PRE-START: The state is initiated as soon as the main power switch is turned on. The system flushes itself out of water if water quality is below the required conductivity. Water levels in the two separators (oxygen/water and hydrogen/water) are adjusted if they are low. When the PRE-START operation is completed, it is possible to let the generator maintain this idle state or it can be started up bringing the system into next state.

 GENERATE-VENT: In this mode, the circulation pump is started up and the system verifies the process values of water flow rate and water quality. Full current is applied to the stack after 120 seconds and hydrogen generation is started. Now the system checks both separator levels, voltage conditions in the stack and rectifier operation, venting hydrogen and oxygen outside during this process. If the process values pass all checkpoints, the electrolyser will stop venting and start to build up pressure, leading to next state.

 PRESSURIZE STORAGE: In this state, the system pressure is increased up to the system pressure set-point while the hydrogen lines are pressurized up to the product pressure set-point. Once the set-point levels are reached, the process goes into steady-state.

 STEADY-STATE: The generator produces hydrogen that flows out of the system through the H2 product port. Full flow of gas is produced when the system pressure is below its set-point and, similarly, full flow of gas is delivered when the product pressure is below the respective set-point. When the set-points are reached, the power used for hydrogen generation is switched off and the electrolyser remains in idle state. When the pressure drops below the set-points, the generator will deliver full gas production until it reaches the pressure set-points again.

A comment on the last operating state seems necessary. During steady-state, in fact, it has been observed that the described control influences the electrolyser behavior both in tanks filling mode and venting mode.

42 When producing in tanks filling mode, it has been noted that the tanks are not capable of storing H2 for a long time but, due to leaks, the H2 is continuously lost and dispersed into the air. It follows that, when the tanks are filled up completely, the product pressure reaches its set-point and the electrolyser stops the production. When the H2 is lost, the product pressure drops and the electrolyser starts back to generate again.

During the performed measurements for the purpose of the electrolyser modelling, the H2

was instead blown out to the outside air in order to simulate the H2 load supply. In order to get the reduced load operation, the desired H2 flow was obtained by manually acting on the valves on the tubes leading to the outside air. By closing the valves, the output pressure increases so that the product pressure is constantly kept around its set-point. It follows that during this state the electrolyser supply is switched on and off intermittently.

For more information about the electrolyser construction, specifications, installation, operation and maintenance refer to the manuals [19-21].