Hydrogen
Glossary
Hydrogen plays a central role in the energy transition and is considered a key element for a climate-neutral future. However, with the increasing importance of this versatile energy source, the need for comprehensible information is also growing. Our hydrogen glossary provides a compact overview of the most important terms, technologies and contexts relating to hydrogen – from A for “alkaline electrolysis” to Z for “ignition limit”.
Whether you work in the industry, are interested in new energy solutions or simply want to expand your knowledge – this glossary will help you to navigate the technical language of the hydrogen economy with confidence.
A
Agri-PV
Agri-photovoltaics are integrated photovoltaics that make it possible to use an area for agriculture and electricity generation at the same time.
Alkaline electrolysis (AEL)
A classic process for hydrogen production that has been tried and tested industrially for decades. In this process, water is split into hydrogen and oxygen in an alkaline solution (usually potassium hydroxide, KOH) using an electric current. AEL systems are considered robust, durable and cost-effective, but react slowly to dynamic load changes – a disadvantage when using fluctuating PV electricity.
Anode
The anode is the electrode at which oxidation takes place during electrolysis – in other words, this is where water is split into oxygen, protons and electrons. The oxygen molecules rise as a gas at the anode. The precise design and choice of material for the anode has a significant influence on the efficiency and service life of the electrolysis process.
Self-sufficient energy supply
A system is considered self-sufficient if it is operated independently of the public power grid. In the context of hydrogen production, this means that the electrolysis is operated exclusively with locally generated PV electricity. Such systems are particularly interesting for remote locations, but require a precise design and possibly intermediate storage (electricity or hydrogen) to compensate for fluctuations in supply.
B
Balancing of Plant (BoP)
This term covers all supporting systems that enable the actual electrolysis process but are not directly involved in the electrochemical reaction. These include, for example, cooling, rectifiers, gas treatment, water treatment, safety and control equipment. The BoP often accounts for a significant proportion of the investment and operating costs and is central to reliable operation.
Bipolar plate
Bipolar plates are located between the cells of an electrolyser stack and perform several functions simultaneously: they conduct electrical current, separate the gas compartments from each other, distribute reaction gases and contribute to heat dissipation. Their materials (usually graphite or coated metals) and structure (e.g. flow channels) are decisive for efficiency, gas tightness and service life.
Calorific value
The calorific value describes the amount of energy released during the complete combustion of a substance, including the heat of condensation of the water vapor produced during combustion. For hydrogen, the calorific value is around 39.4 kWh/kg. The term is important for energy assessments and the dimensioning of systems for using the hydrogen produced.
D
Deionized water
For electrolysis to work efficiently and last a long time, the water fed in must be very pure. Deionized water is free of salts and minerals, as these can lead to deposits and electrochemical corrosion. Water treatment is therefore an integral part of every electrolysis system, especially in stand-alone operation with PV power.
Diffusion layer
A porous layer between the catalyst and the flow channel that ensures even gas distribution in the PEM cell, drains water and improves electrical conductivity. Its structure is crucial for reaction kinetics and cell performance.
Direct coupling
With direct coupling, a PV system is connected directly to the electrolyser – often without the electricity grid being connected in between. This saves grid usage fees and reduces losses, but requires very precise interaction between electricity generation and consumption, as well as a dynamic electrolysis system (e.g. PEM electrolysis).
Pressure electrolyzer
A pressurized electrolyser produces hydrogen directly at a higher pressure (e.g. 30-80 bar), so that subsequent compression is often not necessary. This saves energy, compressors and costs for transportation or storage. Such systems are particularly attractive for PV-coupled applications that store or feed in hydrogen on site.
E
Electrolyte
The electrolyte is the substance through which ions flow in an electrolysis cell. Depending on the technology, a distinction is made between liquid (AEL: potassium hydroxide solution) and solid electrolytes (PEM: membrane). The choice of electrolyte determines important properties such as efficiency, operating temperature, material requirements and flexibility in operation.
Electrolysis
The basic process for producing hydrogen using electrical energy: H₂O is split into hydrogen (H₂) and oxygen (O₂) using electricity. The purer the water and the more consistent the electricity, the more efficiently the chemical reaction takes place. With PV electricity, however, the fluctuating output and intermittent availability pose particular challenges for electrolysis technology.
Energy management system (EMS)
An EMS monitors and controls all energy flows within a system – for example between the PV system, electrolyser, storage system, grid and consumers. In hydrogen containers with PV coupling, the EMS decides dynamically whether electricity is used directly, stored or converted into hydrogen. It makes a significant contribution to efficiency, grid compatibility and cost-effectiveness.
F
Faraday’s law
A fundamental physical principle that describes how much substance (e.g. hydrogen) can be produced with a certain amount of electrical charge. It forms the basis for calculating the theoretical efficiency of an electrolysis process and is particularly important for the design and control of systems.
Fluctuating feed-in
Photovoltaic systems do not provide a constant supply of electricity, but are dependent on solar radiation, weather and time of day. This fluctuating feed-in requires electrolysers that can react quickly to changes in output – something that PEM technology, for example, does better than AEL.
G
Gas warning system
A central safety module in hydrogen-bearing systems. Sensors continuously record the gas concentration in the environment. If defined limit values are exceeded (e.g. 0.4% H₂ by volume in air), ventilation, shutdown or emergency measures are automatically initiated. Gas warning systems often have a redundant design and are integrated into safety concepts (e.g. ATEX).
Rectifier
An electronic device that converts the alternating current (AC) generated by the PV system into the direct current (DC) required for the electrolyser. The quality of the conversion (efficiency, signal stability) directly influences the efficiency and service life of the electrolyser.
Green hydrogen
Hydrogen produced exclusively with electricity from renewable energies (e.g. PV, wind). It does not cause any CO₂ emissions during production and is therefore a central pillar for climate-neutral industry, mobility and energy storage. The designation is important to distinguish it from gray, blue or turquoise hydrogen.
H
H₂ compressor
Technical system for increasing the pressure of hydrogen produced, e.g. for storage in pressurized tanks, distribution via pipelines or feeding into industrial processes. Compression is energy-intensive and, depending on the application, represents a significant operating cost component.
H₂ memory
Systems for the staggered use of the hydrogen produced. Frequently used forms of storage are pressure tanks (350-700 bar), underground caverns or innovative concepts such as metal hydride storage. In combination with PV electricity, they enable a decoupling between electricity generation and hydrogen demand.
Hybrid operation
An operating concept in which the electrolyser can draw electricity from several sources – such as photovoltaics, wind power and the public grid. The advantage lies in higher availability and more consistent operation, which is particularly important in the event of weather-related PV fluctuations. An intelligent energy management system decides dynamically on the power source.
I
Island operation
In stand-alone mode, a system operates independently of the public power grid. This means that all the energy required is generated locally – e.g. via PV. With an electrolysis system in stand-alone operation, the integration of buffer storage or intelligent load management is particularly important in order to enable continuous hydrogen production.
Ionic conductivity
A measure of how well a material (e.g. the electrolyte or membrane) can transport ions. High ion conductivity is crucial for the efficiency of electrolysis. Low conductivity leads to voltage losses, reduced performance and higher energy consumption.
K
Cathode
Counterpart to the anode: Reduction takes place at the cathode during electrolysis, in which hydrogen ions (H⁺) take up electrons and combine to form hydrogen gas (H₂). The efficiency of the cathode depends on its structure, the catalyst material and the gas distribution.
L
Load profile
The load profile describes the temporal progression of the electrical power that is available or required. For PV electricity, the load profile is dependent on the day and the weather – with a typical “midday peak”. Knowledge of the load profile is essential for the design and control of electrolysis systems in PV operation.
Power control
The ability of an electrolyser to flexibly adapt its hydrogen production to the amount of electricity available. This is particularly important for PV electricity, as it fluctuates greatly depending on the weather. PEM electrolysers are considered to be particularly well controllable and quickly adaptable, even at one-second intervals.
Power density
Refers to the power generated per unit area or unit volume of an electrolyser or individual cell. A high power density means a more compact design with the same production volume. This is a particularly important factor in mobile or space-critical applications.
M
Membrane electrode assembly (MEA)
The MEA is the heart of a PEM electrolysis cell. It consists of an ion-conducting membrane (usually Nafion) and two applied catalyst layers (anode and cathode). The quality and processing of the MEA have a significant influence on the efficiency, operating time and costs of a PEM system.
Modular design
Electrolysers are often modular, i.e. they consist of several stacks or system units that can be expanded or serviced individually as required. This enables flexible scaling depending on the available PV power or the desired amount of hydrogen.
N
Nominal power (PV)
The nominal power of a photovoltaic system indicates how much electrical power it can generate under standard test conditions (STC), e.g. 1 kW_peak. For hydrogen production, the effective output over the course of the day is decisive, i.e. also the annual energy production and its distribution over time.Mains parallel operation
An operating mode in which a hydrogen system is simultaneously connected to the public power grid and local power generation (e.g. PV). This enables higher availability and possibly additional revenue through grid feed-in. In parallel grid operation, electrolysers can use surplus electricity or contribute to grid stabilization (e.g. through load management).Mains support
A concept in which flexible consumers such as electrolysers can actively stabilize the electricity grid – for example by adjusting the load in the event of oversupply or short-term demand. In combination with PV, an intelligently controlled electrolyser can, for example, absorb surplus electricity and thus relieve the load on the grid.Emergency shutdown
A safety-relevant mechanism that automatically or manually transfers all systems to a safe state in the event of critical conditions (e.g. leakage, overpressure, fire). This includes power isolation, gas shut-off, ventilation and alarms. In container solutions with H₂, the emergency shutdown is part of the safety concept in accordance with applicable standards.O
Ohmic losses
Energy losses caused by electrical resistance in cables, membranes or contact points. These losses lead to additional heating and reduce system efficiency. These losses can be minimized through careful material selection and good cable routing.
Oxidation
A sub-process of electrolysis in which electrons are released at the anode. In the case of water electrolysis, this means Water (H₂O) is split into oxygen (O₂), protons (H⁺) and electrons (e-). This step also determines how much voltage is required for splitting.
p
Partial load
An operating point at which an electrolyser is operated at less than its maximum output – typically with low PV generation. It is important that the system remains efficient even at partial load and does not suffer disproportionate wear. PEM systems are particularly suitable for this.Peak shaving (peak load capping)
A strategy for reducing short-term high electricity consumption (peak loads), which can lead to high grid charges. Load peaks are avoided by using buffer solutions (e.g. battery storage) or temporarily reducing the electrolysis output. In combination with PV systems, peak shaving helps to reduce the load on the grid and optimize costs.PEM electrolysis (Proton Exchange Membrane)
A modern electrolysis technology that works through a solid polymer-based membrane. PEM systems are characterized by high power density, short response times and good controllability. They are particularly suitable for dynamic PV applications and enable compact system layouts.Buffer power supply
A system (e.g. battery storage or supercapacitors) that compensates for short-term fluctuations in PV power. This allows the electrolyser to be operated more evenly, even when there are clouds or short gaps in the power supply. Buffer solutions improve operating efficiency and extend the service life of components.R
Redox reaction
The central chemical reaction in electrolysis is a combination of reduction (electron uptake) and oxidation (electron release). These reactions take place at the two electrodes of the cell. A deep understanding of these processes is important for the design and optimization of cell chemistry and materials.
Reduction
Reduction is the second part of the redox reaction in electrolysis: hydrogen ions (H⁺) take up electrons at the cathode and form hydrogen gas (H₂). The efficiency of this process depends on the catalysts used, the cell architecture and the operating voltage. A well-optimized cathode reaction minimizes energy losses.
Reversible electrolyzer
A device that not only splits water into hydrogen and oxygen (electrolysis), but can also convert the hydrogen produced back into electricity (fuel cell mode). These systems enable flexible energy applications, especially in combination with PV systems in self-sufficient or off-grid regions.
S
Oxygen (O₂)
Oxygen gas is produced at the anode as a by-product of water electrolysis. In many applications, it is discharged unused into the atmosphere, but can be used, for example, in sewage treatment plants, the chemical industry or medicine. The economic use of oxygen can contribute to the amortization of electrolysis.Sector coupling
Sector coupling describes the connection between the electricity, heating, gas and mobility sectors, for example by using green hydrogen as a link. One example: PV electricity is electrolyzed into hydrogen, which powers fuel cell vehicles or supplies industrial furnaces. In this way, surplus PV electricity can be made usable across sector boundaries.Sensors
Includes all sensors required for safe and efficient operation: Pressure, temperature, humidity, conductivity, flow and gas concentration sensors. They provide real-time data for system control and fault detection. Robust, calibrated sensor technology is indispensable, especially for dynamic PV operation and island grids.Stack
A stack is the central unit of an electrolyser. It consists of a large number of individual cells that are connected in series and produce hydrogen together. The quality, service life and performance of the stack play a key role in determining the efficiency and cost-effectiveness of an electrolysis system. Modern systems allow easy maintenance and replacement.System efficiency
The system efficiency describes the ratio of usable hydrogen energy (usually as calorific value) to the electrical energy used. In practice, the efficiency of typical electrolysis systems is between 60-75%. Losses are caused by heat generation, electrical resistances and auxiliary units, among other things.T
Thermal management
Electrolysis processes generate heat. To ensure efficiency and material conservation, the temperature must be kept within narrow limits. Thermal management includes cooling, heat utilization or heat recovery. Stable temperature control is particularly important for PV-coupled systems with load changes.
U
Inverter
An inverter converts forms of electrical energy – e.g. from direct current (PV) to alternating current or vice versa. In electrolysis systems, a DC/DC or AC/DC converter is usually required to provide the optimum voltage and current for the cells. The quality of the conversion influences the system efficiency.
V
Availability factor
This characteristic value describes the proportion of time in which a system is technically able to operate – regardless of the electricity supply. High availability (e.g. > 95 %) is a quality feature of electrolysers and directly influences the annual production of hydrogen.
Full load hours (PV system)
A measure of how many hours per year a PV system would have to run at its rated output in order to achieve the amount of energy actually generated. In Germany, for example, approx. 900-1,100 h/year. For hydrogen production, this value indicates how much “electricity potential” is available per year.
W
Water treatment
Ions, particles and organic impurities must be removed from water that is used for electrolysis. Reverse osmosis, ion exchangers or UV disinfection are used for this purpose. The water quality not only influences the efficiency, but also the service life of the electrolysis cells.
Hydrogen (H₂)
An energy-rich, colorless and odorless gas that is produced from water during electrolysis. Hydrogen can be stored, transported and used in various applications – for example to generate electricity, as a fuel or as a reducing agent in industry. Its high energy content (33.3 kWh/kg) makes it a versatile energy source.
Hydrogen compressor
Hydrogen often has to be compressed for storage and distribution. Compressors increase the pressure to 350 or 700 bar for filling stations, for example, or to 200 bar for industrial gas storage facilities. The energy required for compression is typically 5-15% of the energy content of the hydrogen.
Hydrogen storage
Hydrogen is stored in tanks for delayed use – usually as a compressed gas, liquid (at -253 °C) or chemically bound (e.g. metal hydrides). The form of storage depends on the application, available space and cost-effectiveness. In PV systems, they enable energy to be available regardless of the weather.
Z
Cell voltage
The voltage applied across a single electrolytic cell. It results from the theoretical decomposition voltage of water (1.23 V) plus additional losses (overvoltages, ohmic losses, etc.). Typical cell voltages in real systems are between 1.8-2.2 V. Low cell voltages are an indicator of high efficiency.
Ignition limit (hydrogen)
Indicates the concentration range in which a hydrogen-air mixture is explosive. For H₂, the lower ignition limit is approx. 4 % by volume and the upper limit is approx. 75 % by volume. These properties make hydrogen challenging in terms of safety. Venting, sensors and structural measures serve to avoid dangerous concentrations.