SIST EN ISO 24078:2025

SLOVENSKI STANDARD
01-september-2025
Vodik v energijskih sistemih - Slovar (ISO 24078:2025)
Hydrogen in energy systems - Vocabulary (ISO 24078:2025)
Wasserstoff in Energiesystemen - Vokabular (ISO 24078:2025)
Hydrogène dans les systèmes énergétiques - Vocabulaire (ISO 24078:2025)
Ta slovenski standard je istoveten z: EN ISO 24078:2025
ICS:
01.040.27 Prenos energije in toplote Energy and heat transfer
(Slovarji) engineering (Vocabularies)
27.075 Tehnologija vodika Hydrogen technologies
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 24078
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2025
EUROPÄISCHE NORM
ICS 27.075; 01.040.27
English Version
Hydrogen in energy systems - Vocabulary (ISO
24078:2025)
Hydrogène dans les systèmes énergétiques - Wasserstoff in Energiesystemen - Vokabular (ISO
Vocabulaire (ISO 24078:2025) 24078:2025)
This European Standard was approved by CEN on 24 June 2025.

CEN and CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for
giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical
references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to
any CEN and CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN and CENELEC member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.

CEN and CENELEC members are the national standards bodies and national electrotechnical committees of Austria, Belgium,
Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia,
Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 24078:2025 E
CEN/CENELEC worldwide for CEN national Members and for CENELEC Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 24078:2025) has been prepared by Technical Committee ISO/TC 197
"Hydrogen technologies" in collaboration with Technical Committee CEN-CENELEC/ JTC 6 “Hydrogen in
energy systems” the secretariat of which is held by NEN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by January 2026, and conflicting national standards shall
be withdrawn at the latest by January 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN-CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN and CENELEC
websites.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 24078:2025 has been approved by CEN-CENELEC as EN ISO 24078:2025 without any
modification.
International
Standard
ISO 24078
First edition
Hydrogen in energy systems —
2025-06
Vocabulary
Hydrogène dans les systèmes énergétiques — Vocabulaire
Reference number
ISO 24078:2025(en) © ISO 2025
ISO 24078:2025(en)
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 24078:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Energy .1
3.2 Energy system and market .5
3.3 Electric Power Network .9
3.4 Hydrogen production system .10
3.5 Hydrogen production equipment . 13
3.6 Hydrogen infrastructure .14
3.6.1 General .14
3.6.2 Components . 15
3.6.3 Stations and plants .16
3.7 Hydrogen storage .17
3.8 Hydrogen fuelled heat and power generation devices .19
3.9 Hydrogen-to-X . 22
3.10 Gas mixture . 23
3.11 Safety. 25
3.12 Risk reduction measure .27
3.13 Hydrogen detection .31
3.14 Metrology .31
3.15 Quality of energy carriers .32
3.16 Testing . 34
3.17 Certification . 35
3.18 Materials compatibility . . 36
Bibliography .38
Index .42

iii
ISO 24078:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by ISO TC 197, Hydrogen Technologies, in collaboration with Technical
Committee CEN-CENELEC/JTC 6, Hydrogen in Energy Systems, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO 24078:2025(en)
Introduction
In this document, terms and definitions have been identified, reviewed and proposed to cover technical
aspects for hydrogen in energy systems, with input from sources such as ISO/IEC Standards, European
Standards from CEN and CENELEC, national standards, and existing definitions from the dictionaries
relevant to particular industries.
This document only contains terms used to describe hydrogen in energy systems within the scope of CEN/
CLC/JTC 6.
This document aims to present the basics of the concepts that are subjected to standardisation in the fields
related to hydrogen in energy systems. Therefore, this document consists of high-level terms and definitions,
and guides the reader to more specific standards/documents, where more technical details can be found.
NOTE In particular, for 3.6, the following applies. Definitions in the existing scopes are mostly specific for the
scope of the standard they are used in. Therefore, general definitions are drafted, complemented by more available
and useful definitions from European and International standards (CEN, CENELEC, ISO, IEC) and exceptionally also by
industry standards, such as ASME, where no European or International standards' definition is available.
Terms and definitions are categorized in the following structure:
— energy carriers,
— energy system, energy infrastructure, smart grid and energy systems integration,
— electric power network and electrical energy storage,
— hydrogen production from electricity and other methods for hydrogen production,
— hydrogen production equipment,
— transmission, distribution and storage in dedicated hydrogen infrastructure and gas network, as well as
hydrogen admixture into natural gas and separation,
— hydrogen heat and power generation devices,
— power-to-hydrogen, hydrogen-to-X and energy storage,
— cross cutting items such as: hydrogen safety issues, metrology, quality of energy carriers, certification
and materials compatibility.
v
International Standard ISO 24078:2025(en)
Hydrogen in energy systems — Vocabulary
1 Scope
This document establishes the terms, definitions, symbols and abbreviations used in the fields related to
hydrogen in energy systems.
This document is not applicable to the following fields:
— biological methanation,
— reactors for hydrogen production from other sources,
— road, maritime and aviation transport,
— aeronautics and space.
Note These fields are foreseen to be covered in future editions of this document.
This document does not apply to carbon capture, storage and utilisation, as well as services.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
NOTE 1 The sources for the following terms and definitions in this document include documents with different
scopes and different application areas. They can therefore be based on premises in the respective sources that are not
listed here.
NOTE 2 The following terms and definitions are intended to stand on their own or in the context of this document.
This document generally excludes any requirements beyond the use of the terms. Any procedures, tests material
selection, or other aspects that play a role separately in the sources must be specified separately in the standards that
reference this document.
NOTE 3 In this document, the term ‘gas’ refers - in its physical sense - to fluids in a gaseous state. If specification of
the gaseous fluid is needed, the specific term of the gaseous energy carrier is used, such as biomethane, hydrogen and
natural gas.
3.1 Energy
3.1.1
energy carrier
substance or medium that can transport energy
Note 1 to entry: For example, electricity (3.1.15), hydrogen (3.1.2), natural gas (3.1.6), fuels.
[SOURCE: ISO/IEC 13273-1:2015, 3.1.2, modified — added note 1 to entry]

ISO 24078:2025(en)
3.1.2
hydrogen
chemical element, H with atomic number 1, usually occurring as a diatomic molecule, H which is a highly
flammable, colourless, odourless and tasteless gas at standard ambient temperature and pressure
Note 1 to entry: Hydrogen in energy systems is usually in gaseous or liquid form.
[SOURCE: JRC Report EUR 30324 EN 326, modified — added a note 1 to entry.]
3.1.3
hydrogen-based fuel
gaseous hydrogen or a synthetic fuel which can be used directly (i.e. without external reforming) as a fuel
for a hydrogen turbine, fuel cell (3.8.1) or combustion engine
Note 1 to entry: More specifications in ISO 14687:2019.
3.1.4
liquid hydrogen
hydrogen (3.2.1) that has been liquefied, i.e. brought to a liquid state
[SOURCE: ISO 14687: 2019, 3.15]
3.1.5
slush hydrogen
hydrogen (3.2.1) that is a mixture of solid and liquid at the eutectic (triple-point) temperature
[SOURCE: ISO 14687: 2019, 3.18]
3.1.6
natural gas
NG
complex gaseous mixture of hydrocarbons, primarily methane, but generally includes ethane, propane and
higher hydrocarbons, and some non-combustible gases such as nitrogen and carbon dioxide
Note 1 to entry: Natural gas can also contain components or contaminants such as sulfur compounds and/or other
chemical species.
[SOURCE: EN ISO 14532:2017 2.1.1.1]
3.1.7
biomethane
gas comprising principally methane, obtained from either upgrading of biogas (3.1.8) or methanation (3.10.7)
of biosyngas (3.1.9)
Note 1 to entry: See EN 16723-1 and EN ISO 14532 for further vocabulary relating to biomethane.
[SOURCE: EN 16723-1:2016, modified — added Note 1 to entry.]
3.1.8
biogas
generic term used to refer to gases produced by anaerobic fermentation or digestion of organic matter, and
without further upgrading or purification
Note 1 to entry: This can take place in a landfill site to produce landfill gas or in an anaerobic digester to produce
biogas. Sewage gas is biogas produced by the digestion of sewage sludge. Biogases comprise mainly methane and
carbon dioxide
Note 2 to entry: See EN 16723-1 and EN ISO 14532 for further vocabulary relating to biogas, biomass, biological
material from living, or recently living organism, typically this can be plants or plant-derived materials
[SOURCE: EN ISO 14532:2017, 2.1.1.14, — added Note 2 to entry, which is sourced from EN 16723-1:2016, 3.2]

ISO 24078:2025(en)
3.1.9
biosyngas
gas, comprising principally carbon monoxide and hydrogen, obtained from gasification of biomass
[SOURCE: EN 16723-1:2016, 3.4]
3.1.10
syngas
gas, comprising principally of carbon monoxide and hydrogen, obtained from gasification of fossil fuel
[SOURCE: EN 16723-1:2016, 3.13]
3.1.11
synthetically produced methane
synthetic methane
SM
methane, which has been produced by subsequent methanation of hydrogen with carbon oxides
3.1.12
substitute natural gas
SNG
gas from non-fossil origin, which is interchangeable in its properties with natural gas (3.1.6)
[SOURCE: ISO 14532:2014, 2.1.1.3 ]
3.1.13
manufactured gas
synthetic gas
gas, which has been treated and can contain components that are not typical of natural gas (3.1.6)
Note 1 to entry: Manufactured (synthetic) gases can contain substantial amounts of chemical species that are not
typical of natural gases or common species found in atypical proportions as in the case of wet and sour gases.
Note 2 to entry: Manufactured gases fall into two categories, as follows:
a) those that are intended as synthetic or substitute natural gases, and that closely match true natural gases in both
composition and properties;
b) those that, whether or not intended to replace or enhance natural gas in service, do not closely match natural
gases in composition.
Case b) includes gases such as town gas, coke oven gas (undiluted), and LPG/air mixtures. None of which is
compositionally similar to a true natural gas (even though, in the latter case, it can be operationally interchangeable
with natural gas).
[SOURCE: ISO 14532:2014, 2.1.1.4]
3.1.14
interchangeability
measure of the degree to which properties of one gas are more compatible with those of another gas
Note 1 to entry: Two gases are said to be interchangeable when one gas can be substituted for the other gas without
interfering with the operation of appliances or equipment
[SOURCE: EN ISO 14532:2014, modified — generalised by exchanging “combustion characteristics” with
“properties” and removing “gas-burning” in note 1 to entry.]
3.1.15
electricity
set of phenomena associated with electric charges and electric currents
Note 1 to entry: Examples of usage of this concept: static electricity, biological effects of electricity.
[SOURCE: IEV 151-11-01, modified — deleted note 2 to entry.]

ISO 24078:2025(en)
3.1.16
electric power
rate at which electric energy is transferred in an electric circuit
Note 1 to entry: The coherent SI unit of electric power is watt, W.
[SOURCE: IATE 1697301, modified — added note 1 to entry.]
3.1.17
heat
energy transferred from one body or system to another, as well as within one system, due to a difference in
temperature
Note 1 to entry: The coherent SI unit of heat is joule, J.
[SOURCE: ISO 14934-1:2010 3.1.2, modified — added note 1 to entry.]
3.1.18
combined heat and power generation
CHP
system consisting of modules for the simultaneous generation of electricity (3.1.15) and heat (3.1.17)
Note 1 to entry: Simultaneous generation of electricity (3.1.15) and heat (3.1.17) is based on the block heat and power
plant definition.
3.1.19
energy from renewable sources
primary energy, the source of which is replenished and will not become depleted upon use
Note 1 to entry: Examples of renewable energy are: wind, solar (solar thermal and solar photovoltaic) and geothermal
energy, ambient energy, tide, wave and other ocean energy, hydropower, biomass, and biogas.
[SOURCE: IEV 617-04-11, modified — added wind, solar (solar thermal and solar photovoltaic) and
geothermal energy, ambient energy, tide, wave and other ocean energy, hydropower, biomass, and biogas.]
3.1.20
variable renewable energy
VRE
energy source characterized by output that is dependent on the natural variability of the source rather than
[2]
the requirements of consumers
3.1.21
non-renewable energy sources
[3]
energy from non-renewable sources, for example oil, natural gas (3.1.6), coal and nuclear energy
Note 1 to entry: Inverse of renewable energy sources.
3.1.22
renewable hydrogen
hydrogen produced through processes using renewable sources (3.1.19)
EXAMPLE Possible examples are water electrolysis using renewable electricity, reforming of biomethane, biogas
or biomass.
3.1.23
low carbon hydrogen
hydrogen produced in processes with significantly lower life-cycle greenhouse gas (GHG) emissions than
the fossil fuel benchmark, which is compliant with a defined GHG threshold
EXAMPLE Possible examples are hydrogen from natural gas reforming with carbon capture and storage, methane
pyrolysis and water electrolysis using nuclear power.
Note 1 to entry: For Life-cycle emission calculation, see ISO 14067:2018 and/or ISO/TS 19870.

ISO 24078:2025(en)
Note 2 to entry: The fossil fuel benchmark is steam methane reforming process using natural gas.
Note 3 to entry: The full life cycle is calculated using ISO 14040.
Note 4 to entry: Threshold for low carbon can be introduced by national or regional legislation.
3.1.24
natural hydrogen
hydrogen produced through natural, often geological, processes
[4]
EXAMPLE Hydrogen liberated by the reaction of water with subterranean minerals .
3.2 Energy system and market
3.2.1
energy system
system primarily designed to produce, convert, synthetize, transform, process and/or store an energy
carrier and transport or distribute it to the end-user
3.2.2
energy infrastructure
collective term for network for energy carriers, including ancillary equipment and facilities for their physical
transmission
Note 1 to entry: In the context of this document, energy carriers are listed in 3.1.1.
3.2.3
gas system
any gas transmission networks, gas distribution networks, liquified gas facilities and/or storage facilities
owned and/or operated by a gas undertaking, including line pack and its facilities supplying ancillary
services and those of related undertakings necessary for providing access to transmission, distribution and
liquified gas
Note 1 to entry: The physical term gas is used here. It refers to fluids in gaseous state, such as hydrogen, natural gas,
biogases, synthetic gases, irrespective of their different chemical and/or safety characteristics.
3.2.4
electric power system
all installations and plant provided for the purpose of generating, transmitting and distributing electricity
[SOURCE: IEV 601-01-01]
3.2.5
bulk power system
BPS
bulk electricity system
BES
portion of the electric power system comprising the facilities used for the generation and transmission of
electric energy
Note 1 to entry: The extent of the bulk power system is usually limited to the means for production and transmission
of electric energy to major industrial and distribution centres.
Note 2 to entry: In English, the term "composite system" is also used for this concept.
[SOURCE: IEV 692-01-04]
3.2.6
heating/cooling system
set of devices and circuits ensuring the flow of heating/cooling medium
Note 1 to entry: The heating/cooling medium can be a gas or a liquid.

ISO 24078:2025(en)
[SOURCE: IEV 841-27-63, modified — included heating and cooling, as well as other media besides air and water.]
3.2.7
hybrid energy system
system that builds on infrastructure synergies and efficiencies between the electricity and gases’ sectors
Note 1 to entry: These aspects include energy transport, short and long-term energy storage, security of supply and
[5]
resilience of having two or more energy carriers.
Note 2 to entry: The physical term gas is used here. It refers to fluids in gaseous state, such as hydrogen, natural gas,
biogases, synthetic gases, irrespective of their different chemical and/or safety characteristics, which, however, need
to be considered in the hybrid system.
3.2.8
smart grid
intelligent grid
system that utilizes information exchange and control technologies, distributed computing and associated
sensors and actuators
Note 1 to entry: System applied for purposes such as:
— integrate the behaviour and actions of the network users and other stakeholders,
— efficiently deliver sustainable, economic and secure electricity supplies
[SOURCE: IEV 617-04-13, modified — generalized by deleting “electric power” (system) and text describing
applications moved to Note 1 to entry.]
3.2.9
energy systems integration
process of coordinating the operation and planning of energy systems across multiple pathways and/or
[6]
geographical scales to deliver reliable, cost-effective energy services with minimal impact on environment
Note 1 to entry: Coordinated planning and operation of the energy system ‘as a whole’, across multiple energy carriers,
[6]
infrastructures, and consumption sectors .
Note 2 to entry: Energy system integration is connected with the concept of sector coupling, which envision creating a
[6]
link between the power and gas sectors .
Note 3 to entry: In Note 2 to entry, the physical term gas is used. It refers to fluids in gaseous state, such as hydrogen,
natural gas, biogases, synthetic gases, irrespective of their different chemical and/or safety characteristics, which,
[6]
however, need to be considered in the energy systems integration .
3.2.10
interoperability
property permitting diverse systems or components to work together for a specified purpose
[7,8]
Note 1 to entry: There are three main types of interoperability :
— Syntactic Interoperability: Where two or more systems are able to communicate and exchange data. It allows
different software components to cooperate, even if the interface and the programming language are different.
— Semantic Interoperability: Where the data exchanged between two or more systems is understandable to each
system. The information exchanged should be meaningful, since semantic interoperability requires useful results
defined by the users of the systems involved in the exchange.
— Cross-domain or cross-organization interoperability: This refers to the standardization of practices, policies,
foundations and requirements of disparate systems. Rather than relating to the mechanisms behind data exchange,
this type only focuses on the non-technical aspects of an interoperable organization.
[SOURCE: IEC 80001-1:2010, 2.11]

ISO 24078:2025(en)
3.2.11
synergy
solutions that connect energy systems between energy domains and across spatial scales to take advantage
of benefits in efficiency and performance
[6]
EXAMPLE coupling of heat and electricity sectors for fuel-saving purposes .
3.2.12
energy markets
commodity markets that deal specifically with the trade and supply of energy, generally electricity, natural
[9]
gas (3.1.6), hydrogen (3.1.2) and liquid fuels
Note 1 to entry: Energy systems includes energy markets and energy supply networks.
3.2.13
demand response
action resulting from management of the electricity demand in response to supply conditions
[SOURCE: IEV 617-04-16]
3.2.14
flexibility of energy systems
[6]
ability to adjust supply and demand by integrating various energy systems :
— by physically linking energy vectors, namely electricity, thermal and fuels;
— by coordinating these vectors across other infrastructures, namely water, data and transport;
— by institutionally coordinating energy markets; and
— spatially, by increasing market footprint with granularity all the way down to customer level
3.2.15
energy demand management
actions, such as education and financial incentives, to reduce customer demand for a particular form of
[10]
energy and/or to shift demand from peak to off-peak times or to other energy systems
3.2.16
energy management system
EMS
system operating and controlling energy resources and loads of the grid
[SOURCE: IEV 617-04-25, modified — referred to “grid”, instead of “microgrid”.]
3.2.17
security of energy supply
uninterrupted ability of an energy system to provide energy to end-users with evaluation of existing
standards and contractual agreements at the point of supply
[SOURCE: IEV 617-01-06, modified — deleted “electricity” and “uninterrupted”]
3.2.18
seasonal storage
technologies that store energy during one seasonal condition and discharging the stored energy in another
seasonal condition, to meet demand
EXAMPLE Hydrogen, natural gas.

ISO 24078:2025(en)
3.2.19
virtual storage
action/service/utility in the energy system, where the flexibility in one part of the system (e.g. heat,
transport, water, etc.) can be integrated with, for example electricity system, and used in similar manner to
[6]
electrical energy storage, EES (3.3.11) or energy storage (3.10.16)
Note 1 to entry: Demand management (e.g. controlling heating and cooling loads) technologies currently being
[6]
deployed are in part leveraging this virtual storage .
Note 2 to entry: Energy Systems Integration proposes that it is at a grand scale where fuel, thermal, water, and
transport systems will be systematically planned, designed, and operated as flexible “virtual storage” resources for
the electricity grid (and vice versa). There is also potential to use the natural gas fuel grid to create energy storage
[6]
through the “chemical-gas” concept .
Note 3 to entry: Virtual storage can be significantly cheaper than dedicated storage, as it does not require large capital
[6]
investment – but it does require a more integrated energy system .
3.2.20
interconnection point
physical point connecting adjacent entry-exit systems or connecting an entry-exit system with an
interconnector
[SOURCE: EN 16726:2015+A1:2018, 3.3]
3.2.21
interface point
measurement point at the boundary of a fuel cell power system at which material and/or energy either
enters or leaves
Note 1 to entry: This boundary is intentionally selected to accurately measure the performance of the system. If
necessary, the boundary or the interface points of the fuel cell power system to be assessed can be determined by
agreement of the parties.
[SOURCE: IEV 485-09-12, modified — used “and/or” instead of “or both”.]
3.2.22
entry/exit point
point at which gas enters/leaves the energy system
Note 1 to entry: The physical term gas is used here. It refers to fluids in gaseous state, such as hydrogen, natural gas,
biogases, synthetic gases, irrespective of their different chemical and/or safety characteristics.
[SOURCE: EN 16726:2015 +A1:2018, 3.2, modified — included both “entry” and “exit” and Note 1 to entry added.]
3.2.23
grid connected
energy delivery method, where the energy is supplied via transmission or distribution energy system(s) (3.2.1)
3.2.24
directly connected
energy delivery method, where the energy is supplied via a direct line (3.2.25)
3.2.25
direct line
either an electricity line or a gas pipeline linking an isolated energy generation site with an isolated consumer
or with an energy supply undertaking to supply directly their own premises, subsidiaries and customers
Note 1 to entry: The physical term gas is used here. It refers to fluids in gaseous state, such as hydrogen, natural gas,
biogases, synthetic gases, irrespective of their different chemical and/or safety characteristics.

ISO 24078:2025(en)
3.2.26
value chain
[11]
entire sequence of activities or parties that provide or receive value in the form of products or services
Note 1 to entry: Parties that provide value include suppliers, outsourced workers, contractors and others.
Note 2 to entry: Parties that receive value include customers, consumers, clients, members and other users.
3.3 Electric Power Network
3.3.1
electric power network
installations, substations, lines and cables provided for the transmission and distribution of electricity
Note 1 to entry: The boundaries of the different parts of this network are defined by appropriate criteria, such as
geographical situation, ownership, voltage, etc.
[SOURCE: IEV 692-01-03]
3.3.2
transmission of electricity
transfer in bulk of electricity, from generating stations to areas of consumption
[SOURCE: IEV 601-01-09]
3.3.3
distribution of electricity
transfer of electricity to consumers within an area of consumption
[SOURCE: IEV 601-01-10]
3.3.4
interconnection
single or multiple transmission link between transmission systems enabling electricity to
be exchanged between these systems by means of circuits and/or transformers
[SOURCE: IEV 601-01-11]
3.3.5
interconnection
single or multiple transmission link between transmission systems enabling
electric power and energy to be exchanged between these networks by means of electric circuits and/or
transformers
[SOURCE: IEV 617-03-08]
3.3.6
point of connection
reference point on the electric power system where the user’s electrical facility is connected
[SOURCE: IEV 617-04-01]
3.3.7
distributed generation
embedded generation
dispersed generation
generation of electric energy by multiple sources which are connected to the power distribution system
[SOURCE: IEV 617-04-09]
ISO 24078:2025(en)
3.3.8
security
ability of an electric power system to operate in such a way that credible events
do not give rise to loss of load, stresses of system components beyond their ratings, bus voltages or system
frequency outside tolerances, instability, voltage collapse, or cascading
Note 1 to entry: This ability can be measured by one or several appropriate indices.
Note 2 to entry: This concept is normally applied to bulk power systems.
Note 3 to entry: In North America, this concept is usually defined with reference to instability, voltage collapse and
cascading only.
[SOURCE: IEV 617-01-02]
3.3.9
reliability
probability that an electric power system can perform a required function under
given conditions for a given time interval
Note 1 to entry: Reliability quantifies the ability of an electric power system to supply adequate electric service on a
nearly continuous basis with few interruptions over an extended period of time.
Note 2 to entry: Reliability is the overall objective in electric power system design and operation.
[SOURCE: IEV 617-01-01]
3.3.10
service reliability
ability of a power system to meet its supply function under stated conditions for a specified period of time
[SOURCE: IEV 603-05-02]
3.3.11
electrical energy storage
EES
electrical installation able to absorb electrical energy, to store it for a certain duration, and to release it
EXAMPLE An installation that absorbs electrical energy to produce hydrogen (3.1.2) by electrolysis (3.4.2), stores
the hydrogen, and uses that gas to produce electrical energy.
Note 1 to entry: The term electrical energy storage can be also used to indicate the activity that an installation,
described in the definition, carries out when performing its functions.
Note 2 to entry: The term electrical energy storage does not refer to designate a grid-connected installation, for which
electrical energy storage system (IEV 631-01-02) is the appropriate term.
Note 3 to entry: Energy conversion processes can be included during energy absorption, storage or release.
[SOURCE: IEC 60050-631:2023, 631-01-01, modified — minor editorial changes in the definition and in
the notes.]
3.3.12
control reserve
energy stock used to control the frequency of the electricity grid in case of unintended variations in demand
[12]
and supply
3.4 Hydrogen production system
3.4.1
hydrogen production system
system that produces hydrogen from a hydrogen-carrying feedstock

ISO 24078:2025(en)
3.4.2
electrolysis
process in which electric current is used to promote a chemical reaction
Note 1 to entry: In the case of water, an example is the separation of hydrogen from oxygen.
[SOURCE: ISO/TR 15916:2015, 3.34]
3.4.3
photo-electrolysis
photo-electrochemical process which uses optical (light) radiation as source of energy to generate a photo-
[13]
current to eventually split, for example, water into hydrogen and oxygen by electrolysis
3.4.4
co-electrolysis
[13]
intended simultaneous electrolysis of water (steam) and another reducible substance
3.4.5
power-to-X
collective for processes using electricity (and heat) to generate primarily a hydrogen intermediate for
producing a useful substance (chemical, fuel, syngas) as a final product in power-to-X applications such as
power-to-fuel, power-to-syngas, and power-to-chemical with the latter subdivided into power-to-ammonia,
[13]
power-to-ethanol, power-to-methane and power-to-methanol
3.4.6
electric energy storage system using hydrogen
EES system using hydrogen
EES (3.3.11) system comprising at least one EES using hydrogen, whose purpose is to extract electric energy
from the electric power system, store this energy as hydrogen and inject electric energy into the electric
power system, using hydrogen as a fuel
Note 1 to entry: The conceptual configurations of the EES system using hydrogen are referred to in Clause 1 of
IEC 62282-8-201:2020.
[SOURCE: IEC 62282-8-201:2020, 3.1.3]
3.4.7
power-to-hydrogen
PtH
[14]
concept meaning that hydrogen is produced via water electrolysis
Note 1 to entry: Electricity supply can be either grid, off-grid or mixed systems.
Note 2 to entry: Power-to-hydrogen, in the context of sectoral integration, means the production of hydrogen using
water electrolysis that can be (later) used as an energy carrier, fuel and/or a feedstock.
Note 3 to entry: conversion of electric power - typically surplus electric power generated from renewable energy
[13]
sources during periods when generation exceeds load - to hydrogen gas .
3.4.8
thermochemical cycle
cyclical process whereby water is split, using high-temperature heat, into hydrogen and oxygen in a series
of reactions; all participating chemical compounds, except for water, are returned to their original state and
[15]
recycled
Note 1 to entry: Examples of heat sources are nuclear and solar.
Note 2 to entry: These processes are termed hybrid thermochemical cycles when an electrolysis step is included.
Note 3 to entry: Also known as thermochemical water splitting, thermochemical water decomposition cycle and
thermochemical hydrogen production.

ISO 24078:2025(en)
3.4.9
gasification
reaction in which various types of feedstocks (most commonly hydrocarbons) are converted into a syngas
composed mainly of carbon monoxide (CO) and hydrogen (H )
Note 1 to entry: This is achieved by ex
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