Exergy · Glossary

Exergy is the portion of an energy that can at most be converted into work under given ambient conditions. It refers to the work that could be obtained if a system or material stream were brought into thermodynamic equilibrium with its environment without losses. Exergy therefore describes not only the quantity of an energy but above all its quality. The opposite term is anergy. This is the portion of energy that can no longer be converted into work relative to the environment, such as heat at ambient temperature. For heat above ambient temperature, the simplified rule therefore applies: energy is the sum of exergy and anergy.

One kilowatt-hour of electricity and one kilowatt-hour of space heat are equal in terms of the quantity of energy. Their capacity to perform work, however, is very different. In thermodynamics, mechanical and electrical energy are regarded as pure exergy. Electricity can therefore be used in many ways, for example in motors, for lighting, in information technology or for heat at any temperature level. Real devices nevertheless have losses. Heat just above ambient temperature, by contrast, can be converted back into work only to a very small extent.

Unit, reference environment and Carnot factor

Exergy has the same units as energy, such as joules or kilowatt-hours. The difference lies not in the unit but in the reference point. Exergy is always determined relative to a defined environment, usually via its temperature, pressure and chemical composition. A substance in the ambient state has no exergy. A hot steam stream has more exergy in a cold environment than in a warm one. Compressed gas has exergy because it can perform work when it expands. Differences in concentration also contain exergy, for example between fresh water from a river and salty seawater. Exergy figures are only comparable if the chosen ambient state is stated as well.

For heat, the exergy fraction can be estimated with the Carnot factor: 1 minus ambient temperature divided by heat temperature. Both temperatures are entered in kelvin. For an environment of 0 degrees Celsius (273 K), the following fractions result roughly: heat at 30 degrees Celsius contains about 10 percent exergy, space heating heat at 50 degrees Celsius about 15 percent, heat at 90 degrees Celsius about 25 percent and heat at 800 degrees Celsius about 75 percent. With a warmer environment, these values decrease. Heat exactly at ambient temperature is pure anergy. Even very hot heat can never be converted completely into work, because this would require total entropy to decrease, which is physically impossible.

Cold, that is, a reservoir below ambient temperature, also contains exergy. A heat engine could in fact operate between the cold and the environment. Conversely, a refrigeration machine needs exergy as its drive. Cold, however, cannot simply be split into exergy and anergy the way heat can. At very low temperatures, the work that can be obtained can even be greater than the associated quantity of heat. The simplified formula "exergy as a fraction of a quantity of energy" therefore applies mainly to heat above ambient temperature.

Energy is conserved, exergy is destroyed

According to the first law of thermodynamics, energy is conserved. Exergy, by contrast, is not a conserved quantity. It is partly destroyed in every real, that is, irreversible process. Causes include friction, electrical resistance, mixing, throttling, combustion and other chemical reactions, and heat transfer across finite temperature differences. How much exergy is lost is directly related to the entropy generated. According to the Gouy–Stodola theorem, the destroyed exergy equals the ambient temperature times the entropy generated in the process. An ideal, reversible process destroys no exergy.

A vivid example is the mixing of hot and cold water. The quantity of energy is conserved, but two separate temperature levels become one intermediate level, from which less work could be obtained. The same applies when heat flows from a hot body to a colder one: no energy is lost, but the heat ends up at a lower temperature level. Exergy makes this loss of quality visible.

Distinction from efficiency, useful energy and energy service

Exergy is often confused with energy efficiency. A high energy efficiency means that little energy is lost within the chosen system boundary. It does not say, however, whether a high-quality form of energy is being used for a simple task. An electric resistance heater converts electricity almost completely into heat, so in energy terms it reaches nearly 100 percent. Its exergy efficiency for space heating, however, is only a few percent. The situation is similar with a boiler: it can reach a high utilization rate and still destroys most of the fuel's exergy, because only low-temperature heat results. Exergy efficiency therefore relates the exergy used to the exergy supplied.

Useful energy and energy service must also be distinguished from exergy. Useful energy is the energy available for the desired purpose after a conversion, such as heat in a radiator, light from a lamp or motion at a shaft. The energy service is the actual benefit, such as a heated room, a distance travelled or an industrial process at a particular temperature. Exergy adds to these concepts the question of what minimum energy quality a service requires.

For heat, the separation of the quantity of energy from the temperature level is particularly important. One kilowatt-hour of heat at 30 degrees Celsius is sufficient for surface heating systems but hardly suitable for generating electricity. One kilowatt-hour at 800 degrees Celsius, by contrast, can be used in high-temperature processes or in a heat engine with a considerably higher share of work. Energy statistics treat both quantities the same, an exergy analysis does not. This also applies to heating: radiators often need about as much heating energy as underfloor heating, but at a higher flow temperature. Their exergy demand is therefore higher.

Why exergy is relevant in the electricity system

Electrical energy is a particularly high-quality form of energy. It can be controlled precisely, transported over grids and used in motors with high efficiency. If one only counts kilowatt-hours, this quality advantage remains invisible. With electrification it becomes more important. Heat pumps, electric vehicles and electric industrial processes increase electricity demand but often reduce total final energy demand. The reason lies in the conversion chains. An electric motor, for example, uses electricity considerably more efficiently than an internal combustion engine uses the chemical energy of a fuel.

A heat pump does not simply burn electricity for heat on a one-to-one basis. It lifts ambient heat, that is, anergy, to a usable temperature level. The electricity provides the drive work of the cycle for this. With a coefficient of performance of 4, 1 kWh of electricity and 3 kWh of ambient heat yield 4 kWh of heat, that is, four times as much as with direct heating. The smaller the temperature lift between the heat source and the heating system, the less exergy is needed per kilowatt-hour of heat. A low heating temperature level therefore increases the coefficient of performance. Even real heat pumps, however, remain well below what is physically possible in exergy terms.

For fuels, the exergy perspective shows why combined generation can make sense. A boiler converts an exergy-rich fuel directly into low-temperature heat. If the same fuel is instead first converted to electricity as efficiently as possible, the remaining heat is used (combined heat and power, CHP) and the electricity drives a heat pump, more heat can result overall than the fuel contained in energy. Whether this is more favourable in an individual case depends on efficiencies, temperatures, costs and emissions.

Chemical energy in fuels is predominantly exergy, because it can be converted into heat at a very high temperature level. This also applies to hydrogen. Hydrogen can be valuable where high temperatures, chemical reducing agents or storable energy carriers are needed. For low-temperature heat in buildings, by contrast, an exergy-rich and costly-to-produce energy carrier competes with technologies that provide the same service with considerably less exergy. A cost calculation does not replace the exergy perspective. It does show, however, which energy quality is consumed in which application.

Typical misinterpretations

"Energy is consumed": Physically, energy is not destroyed; what is lost is its usability. When coal is burned in a power plant, the energy afterwards resides in electricity, waste heat and flue gases. The portion usable for work, however, decreases with every irreversible conversion. The everyday term energy consumption therefore usually means the consumption of an energy carrier or a loss of exergy.
"Every kilowatt-hour is the same": For billing and statistics, this equal treatment is often necessary, but it is not sufficient for system decisions. Exergy captures neither the time of supply nor the load on the grid. It does explain, however, why some conversion chains are unfavourable despite high energy efficiencies.
"Waste heat is always valuable": The usefulness of waste heat depends on its temperature, its availability over time, the distance to consumers and the existing infrastructure. Waste heat at 90 degrees Celsius can feed a district heating network directly. Waste heat at 25 degrees Celsius can usually be used sensibly only with a heat pump. With an environment of 20 degrees Celsius, it contains only just under 2 percent exergy.
"Heat is the same as anergy": Only heat at ambient temperature is pure anergy. Heat above or below ambient temperature contains exergy.
"Efficiencies are directly comparable": A heat pump can reach a seasonal performance factor of three or four because it uses ambient heat in addition to electricity. A boiler reaches a utilization rate close to 100 percent. Such figures are comparable only if the system boundary is clear: final energy, primary energy, greenhouse gas emissions, costs, peak load or exergy input.

Exergy, flexibility and system costs

Exergy replaces neither grid planning nor market design nor analyses of security of supply. It does not answer whether enough power is available at a particular time, whether a grid connection is sufficient or whether an investment is worthwhile. For that, one needs quantities such as power, load profile, flexibility and residual load. Exergy provides different information: it shows whether the quality of the energy used matches the quality of the service demanded.

This information nevertheless affects system costs. If many applications convert electricity directly into low-temperature heat at times of high grid load, the requirements for generation, grids and firm capacity rise. If the same applications are supplied via efficient heat pumps, heat storage, load shifting or suitable waste heat sources, the need for electricity, power and infrastructure changes. Exergy-appropriate use is not cheaper in every individual case. It does avoid unnecessary quality losses in the conversion chains and creates room for efficient solutions. Economically, one essentially pays for exergy. Anergy in the form of ambient heat is available almost without limit, but can be used only together with exergy.

In energy statistics, tariffs and policy targets, exergy usually remains invisible. There, mainly quantities of energy, emissions or costs are accounted for. Measures can therefore look similar even though they affect the conversion chains very differently. Anyone who looks only at electricity volumes overlooks, for example, that one kilowatt-hour of electricity in a heat pump can deliver several kilowatt-hours of low-temperature heat. How strongly electrification actually reduces primary energy demand additionally depends on the electricity mix and on the chosen primary energy assessment.

Exergy sharpens the question of which form of energy should be used for which task. Electricity is suited to applications with a high capacity for work. Heat sources should match the required temperature level. Chemical energy carriers belong where their storability or their suitability for processes is needed. The concept does not explain all technical and economic decisions. It does, however, prevent a central confusion: as a calculation quantity every kilowatt-hour is the same, as usable quality in the energy system it is not.

Sources

Exergie – Glossar — Bundesministerium für Wirtschaft und Energie (energieforschung.de); unbekannt. Glossareintrag Exergie
Exergie — RP-Energie-Lexikon (Dr. Rüdiger Paschotta, RP Photonics AG); 2025-04-16 (letzte Änderung). Abschnitte zur Exergie von mechanischer, elektrischer und Wärmeenergie sowie zu Exergie und Energieeffizienz
Technische Thermodynamik (Leseprobe), Kapitel 5.7 Exergie und Anergie — Pearson Deutschland; unbekannt. Abschnitte 5.7.1 und 5.7.2
Exergy — Wikipedia (englischsprachig); unbekannt. Abschnitte zur Definition und zur Exergievernichtung (Gouy-Stodola-Theorem)

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Edited by eCMS on 11.11.2026 19:47