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Glossary · Energy industry & energy transition

Anergy

Anergy exergy heat low-temperature heat heat pump Energy Quality

Anergy

Anergy denotes the portion of a given energy that, under specified ambient conditions, cannot be converted into useful work even in the theoretically best, reversible process. The complementary portion is called exergy: it describes the maximum obtainable useful work. In this split, energy = exergy + anergy. Like energy, both quantities are expressed in joules or kilowatt-hours. The crucial qualification is "under specified ambient conditions": anergy is not a fixed property of a substance, nor is it a special energy source.

The distinction concerns the capacity to do work, not every conceivable use. Heat with a high anergy share can be well suited for heating even though hardly any mechanical work can be obtained from it. Conversely, electrical energy is, in the thermodynamic ideal, entirely exergy. The fact that a real motor has losses does not change this classification: exergy describes a theoretical upper limit, not the efficiency of a specific device.

Energy content and energy quality

The first law of thermodynamics describes the conservation of energy. The second law limits its conversion: heat passes spontaneously from a higher to a lower temperature level. A heat engine therefore cannot convert the heat of a hot reservoir completely into work; it must release part of it to a colder reservoir. If this reservoir is at ambient temperature, the remaining heat in the reversible limiting case corresponds to the anergy share of the supplied heat.

In real processes such as friction, mixing or heat transfer across finite temperature differences, exergy is destroyed: energy is conserved, but its capacity to do work decreases. "Energy loss" therefore often means that energy leaves the technical system under consideration or is no longer available for its purpose. It does not mean that energy physically disappears. Anergy, in turn, is not simply all the energy that a particular device leaves unused; that energy may still contain exergy.

The reference environment must suit the question at hand and must be specified explicitly. For heat, its temperature is the main factor; for general states of matter, ambient pressure and chemical composition also play a role. The application, by contrast, determines whether an available temperature level is appropriate. If the same heat flow is considered for an industrial furnace instead of for space heating, this alone does not change its exergy share relative to the same reference environment. It changes its suitability for the desired purpose.

A numerical example with clear limits

For a quantity of heat Q transferred at a constant absolute temperature T above the ambient temperature T₀, the following applies: exergy = Q × (1 − T₀/T) and anergy = Q × T₀/T. Both temperatures must be entered in kelvin. This relationship describes a reversible limiting case; technical machines achieve less work. If a finite volume of water cools down while releasing heat, the calculation must be carried out over the changing temperature.

Own sample calculation: at 25 °C, i.e. 298.15 K, and an environment of 20 °C, i.e. 293.15 K, the exergy share is about 1.68 percent. Of one kilowatt-hour of heat transferred at a constant 25 °C, at most about 0.017 kWh could thus be converted into work; about 0.983 kWh would be anergy. At a constant 40 °C and the same environment, the exergy share rises to about 6.39 percent. These are not figures for the electricity yield of a real plant.

The rule of thumb "the colder, the less exergy" applies only to the heat above ambient temperature considered here. A body below ambient temperature also has a capacity to do work owing to its temperature difference from the environment: it can serve as the cold side of a hypothetical heat engine process. A low Celsius figure alone is therefore not sufficient to classify a state as pure anergy.

Distinction from waste heat, environmental heat and useful energy

Low-temperature heat and waste heat are not synonyms for anergy. Waste heat denotes the origin or previous use of a quantity of heat, for example as a by-product of an industrial process. Depending on its temperature, it can contain considerable exergy. Low-temperature heat, by contrast, describes a temperature level. Whether its anergy share is high only emerges in relation to the specified environment. An unused heat source is therefore not anergy by definition.

Environmental heat from air, water or ground is often close to the chosen ambient temperature. Its capacity to do work is then low or, in idealised equilibrium, zero. Nevertheless, a heat pump can make it usable for buildings. To do so, it requires high-grade drive energy, usually electricity. The capacity to do work of the heat delivered does not come from a free conversion of anergy into exergy, but from this additional input.

Final energy and useful energy also lie on a different conceptual level. Final energy denotes the energy provided at the consumer, for example electricity or supplied district heating. Useful energy is the energy available for the purpose after the last conversion, such as space heat or mechanical work. Anergy and exergy, by contrast, assess thermodynamic quality. Even heat that is used entirely as intended can consist predominantly of anergy.

Heat pumps: moving heat instead of generating energy

A simplified balance example illustrates the principle: if a heat pump takes 3 kWh of heat from its source and requires 1 kWh of electricity to do so, it delivers 4 kWh of heating heat across the system boundary, with losses neglected. The ratio of heating heat to electricity is four. This is not an energy generation process and not a thermal efficiency of 400 percent. Over a whole year, the corresponding ratio is called the seasonal performance factor; in this case it must be stated which auxiliary drives and supplementary heaters are included.

The greater the temperature lift required between the heat source and the heat delivery, the more demanding the task becomes. Under otherwise comparable conditions, electricity demand rises. Low heating water temperatures therefore help to save high-grade drive energy. In addition to building insulation, heating surfaces, heat source temperature, hydraulics and controls are important. A poorly insulated or old building does not necessarily require unsuitably high heating temperatures. In the Fraunhofer field test WP-QS im Bestand, no correlation between building age and seasonal performance factor was found; the decisive factors were in particular the temperatures actually required and the system.

Heat networks and the electricity system

Cold local heating networks are often referred to as anergy networks. They distribute thermal energy close to the ambient level and connect heat sources, storage and consumers. Heat pumps in buildings or in shared energy centres provide the required heating temperatures. However, not every low-temperature heat network is such a cold network; with sufficiently high network temperatures, direct heat supply is also possible. Moreover, the term anergy network does not mean that every transported heat flow has exactly zero exergy.

One implemented example is the Hönggerberg campus of ETH Zurich. There, a low-temperature ring main connects energy centres and ground storage. Heat surpluses from cooling processes can be matched with heat demand. The example also shows that the usable resource does not consist of a quantity of heat alone: heat sources, sinks, storage and controls must fit together. The benefit arises from the coordinated use of low temperatures and the targeted deployment of drive energy.

The electrification of heat supply links these questions with the electrical load. Particularly with air-source heat pumps, high heating demand and unfavourable source temperatures can coincide. Buildings and heat storage at the same time offer flexibility: heat can be stored within technical and comfort-related limits and electricity consumption can be temporarily shifted. Prerequisites are suitable storage or building characteristics and appropriate control. Load shifting eliminates neither the heat demand nor, automatically, winter peak loads; under suitable conditions, it can mitigate them.

Benefits and limits of the anergy perspective

Large quantities of waste heat do not automatically replace fuels. Temperature, distance, availability over time and demand must match. Heat at 40 °C can be useful for preheating or suitable heating systems, but cannot directly supply a process at several hundred degrees. If consumers or infrastructure are lacking, even a large resource remains unused. Conversely, heat with very little capacity to do work can be economically valuable if the application requires precisely this low temperature level.

The exergy perspective also explains why direct electric heating and a heat pump require different electricity inputs despite delivering the same space heat. However, it does not alone decide an investment. Purchase cost, operating hours, space, noise, connection conditions and security of supply can influence the choice. Likewise, a high anergy share does not imply a low price, a particular environmental benefit or an entitlement to funding. Energy quality, costs and legal classification must be examined separately.

Anergy is therefore better understood as the "portion unable to do work under a specified environment" than as "worthless energy". The concept does not replace an energy or cost balance. It supplements it with the question of what capacity to do work is present and how much high-grade energy the desired use additionally requires. For heat supply in particular, this perspective makes visible why identical kilowatt-hours can be used in technically very different ways.

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Edited by eCMS on 11.11.2026 13:50

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