Anenergy system is asystem primarily designed to supplyenergy-services toend-users.^[1]: 941 The intent behind energy systems is to minimise energy losses to a negligible level, as well as to ensure the efficient use of energy.^[2] TheIPCC Fifth Assessment Report defines an energy system as "all components related to the production, conversion, delivery, and use ofenergy".^[3]: 1261

The first two definitions allow for demand-side measures, includingdaylighting, retrofittedbuilding insulation, andpassive solar building design, as well as socio-economic factors, such as aspects ofenergy demand management andremote work, while the third does not. Neither does the third account for theinformal economy in traditionalbiomass that is significant in manydeveloping countries.^[4]

The analysis of energy systems thus spans the disciplines ofengineering andeconomics.^[5]: 1 Merging ideas from both areas to form a coherent description, particularly wheremacroeconomic dynamics are involved, is challenging.^[6][7]

The concept of an energy system is evolving as new regulations, technologies, and practices enter into service – for example,emissions trading, the development ofsmart grids, and the greater use ofenergy demand management, respectively.

From a structural perspective, an energy system is like anysystem and is made up of a set of interacting component parts, located within an environment.^[8] These components derive from ideas found inengineering andeconomics. Taking a process view, an energy system "consists of an integrated set of technical and economic activities operating within a complex societal framework".^[5]: 423 The identification of the components and behaviors of an energy system depends on the circumstances, the purpose of the analysis, and the questions under investigation. The concept of an energy system is therefore anabstraction which usually precedes some form of computer-based investigation, such as the construction and use of a suitableenergy model.^[9]

Viewed in engineering terms, an energy system lends itself to representation as aflow network: thevertices map to engineering components likepower stations andpipelines and theedges map to the interfaces between these components. This approach allows collections of similar or adjacent components to be aggregated and treated as one to simplify the model. Once described thus, flow network algorithms, such asminimum cost flow, may be applied.^[10] The components themselves can be treated as simpledynamical systems in their own right.^[1]

Conversely, relatively pure economic modeling may adopt a sectoral approach with only limited engineering detail present. The sector and sub-sector categories published by theInternational Energy Agency are often used as a basis for this analysis. A 2009 study of the UK residential energy sector contrasts the use of the technology-richMarkal model with several UK sectoral housing stock models.^[11]

Internationalenergy statistics are typically broken down by carrier, sector and sub-sector, and country.^[12]Energy carriers (aka energy products) are further classified asprimary energy andsecondary (or intermediate) energy and sometimes final (or end-use) energy. Published energy datasets are normally adjusted so that they are internally consistent, meaning that all energy stocks and flows mustbalance. The IEA regularly publishes energy statistics and energy balances with varying levels of detail and cost and also offers mid-term projections based on this data.^[13][14] The notion of an energy carrier, as used inenergy economics, is distinct and different from the definition ofenergy used in physics.

Energy systems can range in scope, from local, municipal, national, and regional, to global, depending on issues under investigation. Researchers may or may not include demand side measures within their definition of an energy system. TheIntergovernmental Panel on Climate Change (IPCC) does so, for instance, but covers these measures in separate chapters on transport, buildings, industry, and agriculture.^{[a][3]: 1261 [15]: 516}

Household consumption and investment decisions may also be included within the ambit of an energy system. Such considerations are not common because consumer behavior is difficult to characterize, but the trend is to include human factors in models. Household decision-taking may be represented using techniques frombounded rationality andagent-based behavior.^[16] TheAmerican Association for the Advancement of Science (AAAS) specifically advocates that "more attention should be paid to incorporating behavioral considerations other than price- and income-driven behavior into economic models [of the energy system]".^[17]: 6

The concept of an energy-service is central, particularly when defining the purpose of an energy system:

It is important to realize that the use of energy is no end in itself but is always directed to satisfy human needs and desires. Energy services are the ends for which the energy system provides the means.^[1]: 941

Energy-services can be defined as amenities that are either furnished through energy consumption or could have been thus supplied.^[18]: 2 More explicitly:

Demand should, where possible, be defined in terms of energy-service provision, as characterized by an appropriate intensity^[b] – for example, airtemperature in the case of space-heating orlux levels forilluminance. This approach facilitates a much greater set of potential responses to the question of supply, including the use of energetically-passive techniques – for instance, retrofittedinsulation anddaylighting.^[19]: 156

A consideration of energy-services per capita and how such services contribute to human welfare and individual quality of life is paramount to the debate onsustainable energy. People living in poor regions with low levels of energy-services consumption would clearly benefit from greater consumption, but the same is not generally true for those with high levels of consumption.^[20]

The notion of energy-services has given rise toenergy-service companies (ESCo) who contract to provide energy-services to a client for an extended period. The ESCo is then free to choose the best means to do so, including investments in the thermal performance andHVAC equipment of the buildings in question.^[21]

ISO 13600, ISO 13601, and ISO 13602 form a set ofinternational standards covering technical energy systems (TES).^{[22][23][24][25]} Although withdrawn prior to 2016, these documents provide useful definitions and a framework for formalizing such systems. The standards depict an energy system broken down into supply and demand sectors, linked by the flow of tradable energy commodities (or energywares). Each sector has a set of inputs and outputs, some intentional and some harmful byproducts. Sectors may be further divided into subsectors, each fulfilling a dedicated purpose. The demand sector is ultimately present to supply energyware-based services to consumers (seeenergy-services).

Energy systemdesign includes the redesigning of energy systems to ensuresustainability of the system and its dependents and for meeting requirements of theParis Agreement forclimate change mitigation. Researchers are designing energy systems models and transformational pathways forrenewable energy transitions towards100% renewable energy, often in the form of peer-reviewed text documents created once by small teams of scientists and published in ajournal.

Considerations include the system'sintermittency management,air pollution, various risks (such as for human safety, environmental risks, cost risks and feasibility risks), stability for prevention ofpower outages (including grid dependence or grid-design), resource requirements (including water and rare minerals and recyclability of components), technology/development requirements, costs,feasibility, other affected systems (such as land-use that affectsfood systems), carbon emissions, available energy quantity and transition-concerning factors (including costs, labor-related issues and speed of deployment).^{[26][27][28][29][30]}

Energy system design can also considerenergy consumption, such as in terms of absolute energy demand,^[31] waste and consumption reduction (e.g. via reduced energy-use, increased efficiency and flexible timing), process efficiency enhancement andwaste heat recovery.^[32] A study noted significant potential for a type of energy systems modelling to "move beyond single disciplinary approaches towards a sophisticated integrated perspective".^[33]

• Control volume – a concept from mechanics and thermodynamics
• Electric power system – a network of electrical components used to generate, transfer, and use electric power
• Energy development – the effort to provide societies with sufficient energy under the reduced social and environmental impact
• Energy modeling – the process of building computer models of energy systems
• Energy industry – the supply-side of the energy sector
• Mathematical model – the representation of a system using mathematics and often solved using computers
• Object-oriented programming – a computer programming paradigm suited to the representation of energy systems as networks
• Network science – the study of complex networks
• Open energy system databases – database projects which collect, clean, and republish energy-related datasets
• Open energy system models – a review of energy system models that are alsoopen source
• Sankey diagram – used to show energy flows through a system

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• Energy
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