In today’s article from the H2 Safety series, we focus on an extremely important topic – electrolyzers. These devices produce hydrogen by electrolysis. If we use energy from renewable sources to carry out the process, we obtain fully zero-emission green hydrogen, dubbed the fuel of the future. Do you want to learn more about hydrogen production methods, types of electrolyzers, and the safety of equipment and installations? We invite you to read the text by Aleksandra Tracz-Gburzyńska, Head of Safety at SES Hydrogen!
Hydrogen production
Hydrogen in the Earth’s atmosphere practically does not exist as a free element – we already know this from the previous article. To obtain it in its pure form, it must be produced from the compounds in which it is naturally found, such as water, methane, methanol, ammonia, ethanol, biomass, etc. Hydrogen production can be divided into two categories: centralized large-scale production and decentralized small- to medium-scale production. Centralized production refers to existing, large-scale chemical plants that mass-produce hydrogen, which is then transported to customers. Examples include large steam reformers owned by major gas companies.
Several established technologies for industrial hydrogen production are currently available on the market. There are two commercial production routes – water electrolysis (dating to the late 1920s) and reforming technologies (introduced in 1960). In this publication, we will focus on electrolysis. Let us first define what kind of process we are dealing with.
Electrolysis [per ISO/TR 15916:2015(E)] in its simplest terms is a process in which an electric current is used to cause a chemical reaction. In the case of water, an example is the separation of hydrogen from oxygen.
What is a hydrogen generator using water electrolysis reactions and how does it work?
The electric current flows in the electrochemical cell of a hydrogen generator, which causes water to dissociate into hydrogen and oxygen molecules. The electric current flows between two electrodes separated by a conductive electrolyte or “ion transport medium,” producing hydrogen at the negative electrode (cathode) and oxygen at the positive electrode (anode). Since the chemical formula of water is H₂O, electrolysis produces twice as much hydrogen as oxygen by volume.
Hydrogen gas produced by electrolysis technology can be used immediately or stored for later use.
The described process takes place in a device with a commonly used name – Electrolyzer (according to ISO 22734:2019 – A Hydrogen Generator Using Electrolysis of Water); which converts electrical energy into chemical energy and can also be seen as a device with the reverse operation of a fuel cell.
The electricity required to operate the electrolyzer can come from a variety of sources and, depending on the source of electricity, the total hydrogen production may involve CO₂ emissions or be completely CO₂ free. If the electricity is generated from renewable sources (wind, hydro, solar, or tidal energy), then no CO₂ will be emitted; if it is generated from fossil fuels, we are talking about associated CO₂ emissions.
Popular water electrolysis technologies and examples of electrolyzer types
Alkaline electrolysis is a mature hydrogen production technology, at the same time the most widely used in industry. Alkaline electrolysis uses the same principle as PEM electrolysis, which is the conversion of electrical energy into chemical energy. An alkaline electrolyzer has two electrodes immersed in a liquid alkaline electrolyte. The most commonly used is a potassium hydroxide aqueous solution with a concentration level of 25% at 80°C to 40% at 160°C.
The use of KOH solution is preferable to the use of sodium hydroxide (NaOH) solution due to its higher ionic conductivity, lower chloride impurity content, and lower saturated vapor pressure.
The electrodes (cathode and anode) are separated by a diaphragm (membrane). The diaphragm has two functions: first, it separates the gaseous products (i.e., hydrogen and oxygen), and second, it allows hydroxide ions (OH-) and water molecules to pass through. The membrane allows the ions, but not the hydrogen, to pass through. A typical alkaline electrolyzer consists of:
- A power, control, and associated instrumentation system;
- An electrolysis system includes a water purification node, a hydrogen purification node, a gas dryer, and a separator.
- Compressor.
Safety
The main risk associated with alkaline electrolysis systems arises from the possibility of an explosive atmosphere forming in the system in the form of a mixture of hydrogen and oxygen, which can lead to an internal explosion in the electrolyzer. For this reason, sensors are implemented in the unit to monitor operating parameters to detect electrolyzer failure. These include:
- measurement of hydrogen concentration in the oxygen line;
- Voltage and current measurement;
- temperature measurement at the input and output of the electrolyzer;
- measurement of electrolyte ion concentration.
Another type of risk is associated with exposure to an alkaline electrolyte solution in case of leakage. In the case of potassium hydroxide, for example, a leaking tank is recommended to avoid contact between the electrolyte and the environment.
PEM electrolyzers
When electrolysis takes place in two chambers, which are separated by a proton exchange membrane (PEM), we are dealing with PEM electrolyzers. In these, when direct current is applied, the water dissociates into hydrogen (H2) at the negative electrode and oxygen (O2) at the positive electrode. The electrodes and membrane usually form a membrane electrode assembly (MEA-Membrane Electrode Assembly) and an arrangement similar to a fuel cell (FC) stack.
PEM electrolyzers operate at low temperatures, and the PEM membrane serves as the electrolyte. The devices consist of the following components:
- a process cabin containing all process components such as valves, piping, pressure vessels, pumps, etc,
- an electrical cabin containing all electrical components such as instrumentation, controls, wiring, power conditioning, etc,
- a cooling system dedicated to removing heat from the electrolysis process,
- a weatherproof enclosure.
Safety
As with an alkaline electrolyzer, the main risk associated with PEM electrolysis systems stems from the potential for an explosive atmosphere to form in the system in the form of a hydrogen/oxygen mixture, which can lead to an internal explosion in the process chamber of the equipment or the separator (the separator is used to separate gaseous H2 and O2 from traces of water). To avoid hydrogen accumulation in the process chamber, the following measures are taken:
- control of pressure and the pressure difference between hydrogen and oxygen lines;
- controlling the hydrogen concentration in the chamber (< 0.4 vol. % H2);
- reducing the amount of hydrogen in the gas layer of the separator as much as possible, so that in the event of a catastrophic leak, an explosive atmosphere in the form of a hydrogen-air mixture does not form in the vessel.
The formation of the hydrogen-oxygen mixture in the separator can be caused by a malfunction of the water transfer line or, for example, membrane perforation. To avoid hydrogen build-up in the separator, the following measures are taken:
- maintaining the minimum water level in the gas separator above 55% of its height,
- control of water level in H2 and O2 gas separators,
- control of pressure and the pressure difference between H2 and O2 gas lines,
- control of H2 concentration at the outlet of the O₂ gas separator.
When the above safety functions are activated, the electrolyzer will shut down, which includes not only closing the shut-off solenoid valves connected to the storage tanks but also reducing the system pressure through the normally open solenoid valves.
Response of electrolyzers to varying operating conditions
When electricity is made available from solar or wind power plants, the electrolyzer must respond to changes due to fluctuations in the day and night cycle, as well as short-term fluctuations between full load and partial load, which can occur within seconds. This means that the operating conditions of such electrolyzers are very different from the optimal conditions required for long life and reliability at constant power.
Electrolysis and wind power in a nutshell
Among the most important criteria in designing a plant where electrolytic hydrogen is produced from wind energy are meteorological conditions, location, technology for converting wind flow energy into electricity, and how the hydrogen is used.
A wind power plant can be described as the area of land covered by wind turbines, the aerodynamically determined effective power factor, and the power generator rating. Together with the wind speed, these values determine the power output and annual energy production. The efficiency of the electrolyzer, the hydrogen storage capacity, and the power of the service components are other parameters that characterize the entire system.
When designing a power plant, the following parameters are identified as the most important: capacity, locations, and wind converter technologies. In many cases, the ability to adjust the rotor blades is required, especially in large power plants, to improve start-up characteristics and limit output at high wind speeds. The operating system of such a power plant must ensure that different sections of the plant are switched back and forth according to the different modes of operation of the turbines and electrolyzer plant. Wind speed and distribution also affect other parameters beyond those described above.
Therefore, the lower and upper operating speeds of the plant should be set at values that avoid excessive switching on and off at low wind speeds or are within acceptable safety ranges. Together with daily and annual wind speeds, these aspects determine the number, duration, and distribution of outages. Temperature, humidity, rain, hail, ice, sand, and dust must also be considered in power plant design.
Electrolysis and photovoltaic solar power plants in a nutshell
Solar generators can generally be divided into concentrating and non-concentrating systems. A further distinction is made between stationary, uniaxial, and biaxial “solar tracking” power plants based on the type of orientation concerning the radiation source. The simplest and most widely used system consists of non-centering, fixed structures mounted as flat modules side by side on a support frame and oriented in a southerly direction in several consecutive rows at a preset tilt angle (Northern Hemisphere).
The materials used for the substructure can be e.g. steel, aluminum, concrete. The angle of inclination for maximum plant efficiency depends on the latitude of the plant location as well as the prevailing climatic conditions.
The integration of a photovoltaic solar power plant and electrolyzers into a hydrogen generation system consists primarily of matching components in such a way that the photovoltaically generated electricity can be transferred to the electrolyzer with the highest possible efficiency.
For this reason, care must be taken at the design stage to ensure that the entire system follows the solar radiation dynamically avoiding any kind of loss. Electrolyzers are almost ideal consumers of photovoltaic energy, due to the described characteristics and the possible modular design of both system components.
Technical knowledge and safety issues in the design of hydrogen generators
Many international technical standards for electrolyzers are currently available. One of them is: ISO 22734:2019 Hydrogen generators using water electrolysis – Industrial, commercial, and residential applications.
The above standard specifies the design, safety, and performance requirements for modular or factory-fitted hydrogen generating equipment, called hydrogen GENERATORS, using electrochemical reactions to electrolyze water to produce hydrogen.
This standard applies to hydrogen generators intended for industrial and commercial applications and indoor and outdoor residential use in sheltered areas such as carports, garages, utility rooms, and similar residential areas. Importantly, its revision is currently being prepared due to advances in technology and engineering:
ISO/AWI 22734-1. Hydrogen generators using water electrolysis – Industrial, commercial, and residential applications – Part 1: General requirements, test protocols, and safety requirements
ISO/AWI TR 22734-2. Hydrogen generators using water electrolysis – Part 2: Testing guidance for performing electricity grid service
Literature:
[1] ISO 22734:2019. Hydrogen generators using water electrolysis — Industrial, commercial, and residential applications
[2] ISO/TR 15916:2015(E) – Basic considerations for the safety of hydrogen systems
[3] Hydrogen as an Energy Carrier. Technologies, Systems, Economy. Author: Carl-Jochen Winter, Joachim Nitsch (Eds.) ISBN-13: 978-3-642-64872-4
[4] HyResponse. Deliverable D2.1-Description of selected FCH systems and infrastructure, relevant safety features, and concepts (2014).
[5] HyResponse. Introduction to FCH applications and hydrogen safety. Compiled by S. Tretsiakova-McNally; reviewed by D. Makarov (Ulster University)
