The Open Energy Ontology (OEO) is a BFO-based ontology for the domain of energy system analysis. Core of this domain is the concept of energy. Since the concept of energy is ambiguous and has not (yet) been introduced into BFO, we describe why the OEO classifies energy as a quality. We present how energy is diferentiated from energy carriers, fuels and other related concepts. The process of energy conversion makes it possible to depict many diferent types of its generation and usage. Further, we describe the concept of origin of matter and energy. In this article, we define the core concepts of the OEO according to the BFO. We thus demonstrate how the OEO enables the consistent use of core concepts throughout the energy system analysis domain.
Researchers in the domain of energy system analysis (ESA) use energy system models to explore
current and future energy systems. Such models are often applied to questions involving the
expansion of the use of renewable energy sources, energy storage and to investigate how energy
and climate policy impact the energy system. ESA is a heterogeneous and multidisciplinary
domain. Experts in engineering, natural and social sciences, physics, mathematics, computer
science, economics, meteorology, and geography developed diferent nomenclatures and
conceptualisations for similar or the same things. These diferent terminologies are reflected in the
respective documentation of research data, models, and results. Due to this heterogeneity,
challenges arise regarding transparency and reproducibility. For example, time needs to be invested
to identify where and if a nomenclature is documented and how it difers from one’s own. To
address these challenges, the Open Energy Ontology (OEO) was created and we maintain and
develop it openly and collaboratively across institutes and research projects [
Although energy is everywhere and we use it versatilely in our daily lifes, energy cannot be
grasped intuitively. Neither is it easy to represent in an ontology. But, as the name of the OEO
suggests, we consider energy as one of the ontology’s core topics. Thus, we have to properly
represent, classify and define the concept of energy and related subjects in the OEO and pay
special attention to it. Based on the book Building Ontologies with Basic Formal Ontology (BFO)
by Arp et.al. [
In the following, we describe the ontological representations and definitions of energy (section 2), energy carriers and fuels (section 3), energy transformations (section 4). Implications of the design choices are discussed in section 5.
Energy appears in various forms and humans use and transform it in many diferent ways. In physics, energy is described both as matter-bound and immaterial. Energy can occur in diferent states and can be transformed from one state into another. Examples for matter-bound energies used in the domain are the chemical energy in wood or diesel, the kinetic energy of wind (moving air) and the thermal energy of steam. In contrast, radiative energy that moves through space without a material carrier is an example of immaterial character of energy in the energy system analysis domain. These diferent forms of appearance of energy make the classification of energy in an ontology neither intuitive nor self-explaining.
In the OEO development process, described in [
Energy is a quality of material entity which manifests as a capacity to perform work (such as causing motion or the interaction of molecules).4 Our design choice underlines the material-bound perspective to energy, which plays an important role. To also depict the perceived immaterial aspects of energy, we introduced as a vehicle the concept of photons5, understood as light particles. We axiomatised photons as bearer of those aspects of energy, that are not perceived as matter-bound. The description of the BFO 1https://github.com/OpenEnergyPlatform/ontology/wiki/Best-Practice-Principles 2https://github.com/OpenEnergyPlatform/ontology/releases/tag/v1.10.1 3material entity, BFO_0000040, http://purl.obolibrary.org/obo/BFO_0000040 4https://openenergy-platform.org/ontology/oeo/OEO_00000150 5A photon is a portion of matter that is a light particle. https://openenergy-platform.org/ontology/oeo/OEO_ 00230021 class “material entity”, states the concept “photon” explicitly as material entity in its examples of usage. Due to the nature of the OEO – focusing on matter-bound aspects of energy – we thus decided to ignore the wave nature of photons. If we classified a photon as a wave in addition, this may pose a challenge to upper-level ontologies such as the BFO: it would break the distinction between continuants and occurrents.
The intention of BFO2 is, that matter could encompass both mass and energy. It is planned
to introduce “portion of energy” in a later release of the BFO [
In the OEO we diferentiate energy into several subclasses. The subclasses categorise common physical types of energy, i.e. chemical energy, electrical energy, kinetic energy, nuclear binding energy, potential energy, radiative energy and thermal energy. From these, we derive further energy subclasses with relevance for energy systems analysis (ESA). Subclasses are, for example, wind energy7 and solar energy8. Other energy types depicted in the OEO are distinguished by their origin9 (see also section 5), i.e. renewable energy and fossil energy. Renewable energy sums up all energies that replenish on a human time scale, like solar energy, wind energy, etc. Fossil energy is chemical energy that is stored in fossil combustion fuels, e.g. natural gas or gasoline. We denfied both classes, renewable energy and fossil energy, by using an equivalence of concepts. Since we classify energy and its subclasses as qualities of material entities, we have to consider their relation with their bearer, which we describe in section 3.
We extensively reviewed literature, analysed other energy-related ontologies and compiled the result:10
OEO refers to the capacity to perform work, when defining energy. This fundamental physical
interpretation of energy is also chosen by PATO (the Phenotype And Trait Ontology)11 [
12A disposition which is realized during the execution of work, the emission of heat, or the possession of mass., http://purl.obolibrary.org/obo/ENVO_2000015 on phenotypic qualities (properties, attributes or characteristics)13, hence, the classification as physical quality is not surprising. For OEO, we do not see the need to distinguish between physical and other qualities, though. In ENVO, energy is defined as a disposition, which is also a specifically dependent continuant, yet its realisation needs to get triggered by a process. A comment in ENVO indicates that it is considered to align with PATO in the future.14 Although PATO and ENVO chose similar definitions and classification for the concept of energy, we decided to not re-use them, but create an OEO-owned class energy, mostly because of the central relevance for the ESA domain.
Other ontologies also have concepts related to energy, but do not strictly apply an upper-level
ontology that allows a comparison of classification. The enArgus ontology [
The Saref4ener16 ontology defines saref:Energy as A saref:Property related to some
measurements that are characterized by a certain value measured in an energy unit (such as Kilowatt_Hour
or Watt_hour). Other energy-related ontologies to not explicitly classify it. For example the
OEMA (Ontology network for Energy Management Applications) [
Every material entity is bearer of energy. Only a part of that energy is useful energy (exergy),
while the other part (anergy) cannot be exploited by any technical apparatus [
13Quality is defined as A dependent entity that inheres in a bearer by virtue of how the bearer is related to other entities http://purl.obolibrary.org/obo/PATO_0000001, subclass of BFOs specifically dependent continuant 14Comment in ENVO_2000015: “Consider replacing with PATO class; note that PATO class is not a disposition. This should likely go into OBO Core and also have a physicist look at it.”
15https://www.enargus.de/pub/bscw.cgi/d3048-2/*/*/Energie.html?op=Wiki.getwiki Translation: Energy is a physical quantity with the unit Joule (J) or Watt second (Ws). It has the formula sign E. Energy can be present in diferent forms of energy and is sometimes also called work.
16https://saref.etsi.org/core/Energy 17https://github.com/smart-data-models/dataModel.EnergyCIM/ 18https://openenergy-platform.org/ontology/oeo/OEO_00000151 19https://openenergy-platform.org/ontology/oeo/OEO_00020039 'energy carrier' EquivalentTo: 'material entity' and
('has disposition' some 'energy carrier disposition') How exactly are material entities related to energy? Let us take water as an example: Water can be hot (contain thermal energy), can be moving (contain kinetic energy) or be elevated (contain potential energy), but its energy value can also be zero. How can hot water and other heated material entities be grouped and related to thermal energy? How can moving water and other accelerated material entities be related to kinetic energy? In the OEO, we created “anonymous classes” (general class axioms) to establish these relations. For example, the fact that all “moving/accelerated things” have kinetic energy is axiomatised as: 'material entity' and ('bearer of' some 'kinetic energy')
SubClassOf: 'has disposition' some 'energy carrier disposition' Similarly, we created anonymous classes for all “heated things” that contain thermal energy, all “electrically charged things” that contain electrical energy, all “combustible things” that contain chemical energy, all “elevated things” that contain potential energy.
Radiative energy is a special case, since it represents the wave-like characteristics of energy. To conform to the scheme of material entities being the bearer of the quality energy, we defined photons as light particles. Photons are the only (relevant) energy carriers for radiative energy, thus we axiomatised as follows: photon SubClassOf: 'has quality' some 'radiative energy' photon SubClassOf: 'has disposition' some 'energy carrier disposition' A photon can also behave like a wave. This is in almost all cases irrelevant to the domain of ESA. Thus, we excluded this aspect from the definition and axiomatisation of photon.
An important concept, we introduced in the OEO, is portion of matter20. This class encompasses
materials like coal, gasoline, water, carbon dioxide, methane, air, uranium, steel and minerals
[
A fuel role is a role of a portion of matter that has the disposition to be an energy carrier and is used in a process that releases the carried energy by transforming the portion of matter into a diferent kind of portion of matter in a way that releases heat or does work.21 20A portion of matter is an aggregate of material entities that have a state of matter. http://openenergy-platform. org/ontology/oeo/OEO_00000331 21http://openenergy-platform.org/ontology/oeo/OEO_00000001
We call portions of matter which have a fuel role fuels22, implemented as equivalent class. As a consequence, all portions of matter which have a fuel role can be inferred as subclass of fuel. But not all material entities which have the energy carrier disposition are portions of matter with a fuel role. This answers our initial question of this section: All fuels are energy carriers, but not all energy carriers are fuels.
independent continuant specifically dependent
continuant material entity realizable entity
entity continuant
occurent has output has input object aggregate role disposition quality
process has bearer
origin has origin
has origin portion of matter energy transformation has bearer fuel role
= fuel has role has disposition
= energy carrier energy carrier disposition has disposition has energy input has energy output has participant energy transformation
22A fuel is a portion of matter that has the disposition to be an energy carrier and which has a fuel role. http: //openenergy-platform.org/ontology/oeo/OEO_00000001
To further diferentiate between diferent types of fuels, we distinguish fuels depending on whether they release their energy content by using some kind of chemical reaction (e.g. combustion)23 or in a nuclear reaction, notably nuclear fission (nuclear fusion does not (yet) play any role in the ESA domain)25.
Figure 1 summarises how the OEO classes we described relate to each other and how they fit into the upper hierarchical structure of the BFO using object properties from both the Relation ontology (RO) and newly defined in the OEO. We introduce the energy transformation class shown in the graph in the next section.
While our everyday language describes how energy is produced (generated) or consumed (used), the laws of thermodynamics tell us that it is all about changes in form. Energy possesses the ability to be converted from one type into another. In the OEO, we defined a transformation as a process that transforms one or more inputs into at least one output.27 For our domain, a relevant subclass is energy transformation
Energy transformation is a process in which one or more certain types of energy as input result in certain types of energy as output.28 We defined this broadly so as to allow any kind of energy transformation sub-process. This also includes the so called “energy losses”, e.g. in the form of waste heat.
We created an object property called has energy participant: A relation between an artificial object or a process and an energy, where the energy is used in the artificial object or process. 29 with the sub-properties has energy input and has energy output. They relate the energy transformation processes and the involved energies, see Figure 1.
The relevant facilities where the energy transformations take place are grouped in the class energy transformation units30. Examples for energy transformation units are power plants or power-to-gas systems. Their internal components which are performing the transformation, like turbines or electrolysers, are grouped into the class energy converting component31.
23A combustion fuel is a fuel that realises its fuel role in processes that release energy in the form of heat or work by chemical reaction with other substances.24
25A nuclear fuel is a fuel that realises its fuel role in processes that release energy in the form of heat or work by undergoing nuclear fission. 26 27https://openenergy-platform.org/ontology/oeo/OEO_00000429 28https://openenergy-platform.org/ontology/oeo/OEO_00020003 29https://openenergy-platform.org/ontology/oeo/OEO_00010238 30An energy transformation unit is an artificial object that transforms, changes or transfers a certain type of energy. , https://openenergy-platform.org/ontology/oeo/OEO_00020102
31An energy converting component is an artificial object that is usually a discrete part of an energy transformation unit with the function of transforming, transferring or changing a certain type of energy. http://openenergy-platform. org/ontology/oeo/OEO_00000011
We decided to define energy as quality, which is a specifically dependent continuant in BFO. But, as described in Section 2, classifying energy is ambiguous. Therefore, and because energy plays such an important role for the ESA domain, it becomes necessary that, unlike other specifically dependent continuants, energy itself needs to be bearer of qualities and roles. This is a special case that is not foreseen in BFO. But for the central concept of energy, we decided to make an exception here. This exception entails some implications, we describe in the following.
One of the most important ongoing research topics in the ESA domain is to find solutions for the climate crisis. A central aspect here is the question of where the energy comes from, e.g. if it is renewable or not. The questions of origin is also asked for the portion of matter carrying the energy. Thus we introduced origin as a quality of both energy and a portion of matter: Origin is a quality of a portion of matter or energy based on where it comes from. It is inherited from its primary sources. 32
To relate a portion of matter or energy to its origin we created the object property has origin33 with the domain energy or 'portion of matter'. To distinguish between several types of origins, we defined several subclasses. Some subclasses are limited to being a quality of either portion of matter or energy.
We mentioned in section 2 two important subclasses of origin: renewable and fossil. Renewable is an origin of energies only, defined as: Renewable is an origin of energies that replenish on a human time scale.34 As counterpart to renewable, there is conventional35 as further origin of energies.
As fossil in the ESA domain is ultimately connected to some energy carrying materials like coal, oil or natural gas extracted from the ground, it is thus an origin of portions of matter. Fossil is a geogenic origin of portions of matter created from organic material by geological processes lasting thousands or millions of years.
The origin quality is transferred if the portion of matter or energy is transformed and the output of the process inherits the origin: Crude oil has clearly a fossil origin, but also gasoline or heating oil are fossil as they are produced from crude oil. Similarly, the kinetic energy of moving air (usually called wind energy) has a renewable origin. When transforming wind energy into electrical energy, the latter inherits the renewable quality and thus electrical energy generated in a wind turbine has renewable origin, in contrast to electrical energy generated in a coal power plant. Other subclasses of origin that inhere in portions of matter are: anthropogenic36, 32http://openenergy-platform.org/ontology/oeo/OEO_00000316 33x has the origin of y http://openenergy-platform.org/ontology/oeo/OEO_00000530 34http://openenergy-platform.org/ontology/oeo/OEO_00030004 35Conventional is an origin of energies that don’t replenish when transformed / consumed. http:// openenergy-platform.org/ontology/oeo/OEO_00020147
36Anthropogenic is an origin of portions of matter or energies created by human activity. http://openenergy-platform. org/ontology/oeo/OEO_00030000 synthetic37, biogenic38, and geogenic39. We currently discuss an origin sustainable for portions of matter.40
Apart from origin, energies can also bear roles, i.e. the commodity role. Electrical energy is commonly traded at energy market exchanges as a commodity, similarly to fuels like oil. Thus we needed to weaken the domain of the object properties bearer of 41 and has role42. In the OEO, their range has been extended to energy or 'independent continuant'.
We introduced the object property has energy participant and its subproperties in section 4 to axiomatise artificial objects or processes to energies. These special object properties reduce the need to weaken object properties like has participant43 and subproperties, imported from RO.
Energy is challenging to classify ontologically due to its dual character. Some ontologies in the energy domain avoid to classify energy, either by not using an upper-level ontology or by avoiding the concept at all. For the use cases of the OEO and due to its design, an appropriate classification is worthwhile. With this paper we demonstrate why it is useful for the domain of energy systems analysis to classify energy as a quality of material entities. We showed that the current version of BFO is not optimal for classifying energy. We were able to depict the diferent aspects of energy and related concepts like energy carriers and transformations, though.
In the ESA domain there is often no clear distinction between a material entity, its capability to carry energy and the energy itself. This means that a lump of coal and the energy contained in that lump of coal is often treated as the same: An energy systems modeller requires to depict the energy contained in that lump of coal and thus does not quantify that lump of coal other than directly in an energy unit (J). But to calculate CO2 emissions from its combustion, the properties calculated in a mass unit (kg) or a volumetric unit (m3) get relevant.
The OEO does not yet cover all aspects of energy. For example the distinction between usable energy (exergy) and non-usable energy (anergy) is currently under discussion.44 Despite classifying energy as a quality, energy itself can be bearer of roles. In the OEO we covered this so far for electrical energy having a commodity role as it is traded at energy market exchanges. However, there are likely more roles of diferent types of energy to explore.
The OEO also distinguishes between energy and power, as well as the concepts of installation capacities. However, this goes beyond the scope of this paper and may be content of a later publication.
37Synthetic is an anthropogenic origin of portions of matter created artificially by a chemical process. http:// openenergy-platform.org/ontology/oeo/OEO_00030005
38Biogenic is an origin of portions of matter made by or produced from life forms. http://openenergy-platform.org/ ontology/oeo/OEO_00030001
39Geogenic is an origin of portions of matter or energies that are the result of geological processes. http: //openenergy-platform.org/ontology/oeo/OEO_00030003 40https://github.com/OpenEnergyPlatform/ontology/issues/872 41http://purl.obolibrary.org/obo/RO_0000053 42http://purl.obolibrary.org/obo/RO_0000087 43http://purl.obolibrary.org/obo/RO_0000057 44https://github.com/OpenEnergyPlatform/ontology/issues/1166
With clear definitions of energy and related classes as well as axioms describing the relations between these classes, the OEO not only represents the knowledge of the domain. It will function as the foundation of the Open Energy Knowledge Graph45 which is currently under development as part of the Open Energy Family in the SIROP project. We see further applications of the OEO in data annotation and in facilitating comparisons of energy scenario studies.
We wrote this paper as part of the research project SIROP – Towards Scenario Interoperability (grant number 03EI1035), performed by Fraunhofer IEE, Öko-Institut (Institute for Applied Ecology), Otto von Guericke University Magdeburg and Reiner Lemoine Institute, and funded by the 7th Energy Research Programme of the German Federal Ministry for Economic Afairs and Climate Action (BMWK). We thank all members of the OEO development team. Without their contribution to the ontology this paper would have been impossible.
45https://github.com/OpenEnergyPlatform/oekg
The Open Energy Ontology (OEO) can be downloaded via the Open Energy Platform (OEP).46 For easy access, the OEP provides an ontology class viewer.47 The OEO is mainly developed on GitHub48 and in developper meetings49 that take place online usually every fortnight.