Why cannot energy be created or destroyed

Exergy

Lexicon> Letter E> Exergy

Definition: Energy in high quality, suitable for performing mechanical work

More general term: energy

Counter-term: anergy

English: exergy

Categories: Basic Concepts, Physical Basics

Formula symbol: E.

Unit: Joule (J), kilowatt hour (kWh)

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 03/17/2010; last change: 03/15/2020

URL: https://www.energie-lexikon.info/exergie.html

Why do we need the term Exergy in addition to that of the energy?

In the following, it should first be explained why it makes sense to use the term in addition to the well-known term of energy Exergy to introduce. Energy occurs in various forms that can only be converted into one another with restrictions (→Laws of Thermodynamics). The “highest quality” (most flexible) forms of energy are mechanical and electrical energy. You can e.g. B. can easily be completely converted into heat, while the conversion of heat into mechanical or electrical energy is less complete, the lower the available temperature gradients. Different types of energy can therefore have a different “quality”. That is why the consideration of energy quantities alone often falls short, while the concept of Exergy, which is explained in more detail below, takes these circumstances into account.

Exergy from mechanical, electrical and thermal energy

Mechanical and electrical power are pure exergy while warmthenergy Depending on the temperature level, only a smaller or larger part counts as exergy. This assessment is made in relation to an ambient temperature level, which z. B. can correspond to the temperature of the outside air or that of a nearby body of water:

  • High temperature heat At temperatures far above the ambient temperature, it can be converted into mechanical energy with relatively good efficiency with the help of suitable heat engines. The physically maximum possible efficiency of this conversion is called Carnot efficiency designated. The amount of maximum mechanical energy generated is called the exergy, which corresponds to the respective amount of heat. Exergy can therefore also be seen as the theoretically “workable” part of energy.
  • Low temperature heat, d. H. Heat at temperatures close to the ambient temperature has only a small exergy share because the Carnot efficiency is low. It can be seen from Figure 1 that z. B. Heat at 50 ° C only contains approx. 15% exergy.
  • cold, d. H. a reservoir with a temperature well below the ambient temperature, in turn, contains exergy, because a heat engine could perform mechanical work between this reservoir and the environment. Conversely, a refrigeration machine needs exergy to drive it.

There is no exergy in the Ambient heat contain the z. B. has the outside air. The outside air on a cold winter's day can indeed be cooled with the help of a heat pump. B. feeds into a heating system. However, this is only possible by supplying exergy to drive the heat pump. “Voluntarily” and free of charge, no heat from the cold outside air would flow into a house to be heated (and warmer). This warmth of the environment, which can only be used for heating with the use of additional exergy, is also called Anergy designated. In a sense, anergy is the opposite of Exergy: Energy that cannot do any work. You can usually get them for free, but you usually need additional exergy to use them.

In the case of heat conduction from hot to colder bodies, no energy is lost, but the exergy decreases because heat is brought to a lower temperature level.

Exergy of other forms of energy

Chemical energy, contain e.g. B. in fuels and fuels, is for the most part exergy because it can be converted into heat at a very high temperature level if required. However, e.g. B. Heating oil when burned in a boiler usually only low-temperature heat. Even if the heat recovery is almost lossless, i. H. If the amount of heat gained is almost the same as the chemical energy used, most of the exergy is lost in the boiler.

Something similar to that for chemical energy applies to nuclear energy in nuclear fuels such as uranium.

Visible light has a high exergy share; its energy could in principle be converted into electrical energy with the help of photovoltaics with a very high degree of efficiency - although the actual efficiencies in photovoltaics are often not very high up to now. Infrared light (invisible light with longer wavelengths, Thermal radiation) has a lower proportion of exergy. The exergy portion of electromagnetic radiation also depends on its intensity: If the intensity is only as high as that of thermal radiation from a black body at the ambient temperature, there is no exergy at all.

Differences in concentration also contain exergy. If, for example, fresh water from a river flows into the sea, which has a much higher salt content, exergy can be used via osmosis processes. In the future, osmotic power plants at river mouths could possibly use this potential.

Relationship between exergy and entropy

Exergy can also be understood as energy without entropy. The entropy of a system is not reduced by the release of pure exergy, unlike the release of heat. (The release of a quantity of heat Q at the temperatureT reversible leads to a decrease in entropy Q / T.) Therefore, for example, the conversion of pure exergy into heat is always physically possible, even at a very high temperature level. On the other hand, even heat cannot be completely converted into pure exergy at a very high temperature, because this would lead to a decrease in the total entropy, which is fundamentally physically impossible.

Processes in which a lot of exergy is lost are typically those in which the total entropy increases sharply. These are highly irreversible processes that often also affect energy efficiency (see below).

Economic value of exergy and anergy

You actually only pay for exergy, not anergy. The latter is available in unlimited quantities, but can only be used to a very limited extent.

Only exergy can be sold, not anergy, because only exergy opens up many possibilities of use, while anergy is available in almost any quantity in the area, but cannot be used without exergy. Against this background, z. For example, an electricity bill can be viewed as a bill for exergy, not energy. It also becomes clear why low-temperature heat also has a lower value per kilowatt hour than electrical energy in economic terms. The following section shows that the efficient use of exergy is necessary - especially in connection with energy efficiency.

Relationship between exergy and energy efficiency

Exergy is different from energy no Conservation size: It is lost in many processes, but can never increase, except through an external supply.

Examining exergy often makes it clearer why certain energy conversions are inefficient: degrees of efficiency do not tell the whole truth.

A particularly large amount of exergy is lost when low-temperature heat is generated from electrical energy in an electric heater or from chemical energy in a boiler (see above). This often has an impact on the energy efficiency of the overall system, even if hardly any energy is lost during the respective energy conversion. The most efficient overall system is often achieved by keeping the losses of exergy (and not just energy) as small as possible at every step. In other words, every step should be as high as possible exergetic efficiency can be achieved. This is because a loss of exergy means a loss of opportunities for further transformations or energy uses. Examples for this are:

Electric heaters are terrible exergy destroyers. However, boilers are far from ideal.
  • The conversion of electrical energy into low-temperature heat in an electric heater is possible with 100% energetic efficiency, but the exergetic efficiency is very low (at a few percent). The overall energetic efficiency of the system used (power plant plus electric heating) is also low, because the extraction of exergy from heat in a power plant entails large losses. It is inefficient to gain exergy with high losses without using its quality later. The greatest loss of exergy occurs in the electric heating, not in the power plant.
  • If you only consider energy and not exergy, you might think that electric heating with electricity from hydropower is quite efficient, as the overall energy efficiency can be over 80%. However, the following example shows that this is wrong:
The article about thermodynamically optimized heating shows how to use the exergy optimally in heating.
  • A heat pump can be operated with electrical energy from any source. This allows z. B. win with a COP of 4 low temperature heat. This means that 4 kWh of low-temperature heat from 1 kWh of electrical energy and 3 kWh of ambient heat (free Anergy) be won. So you get four times more heat than with direct electric heating. This is despite the fact that the exergetic efficiency of a heat pump is usually well below what is physically possible in principle.
Cogeneration makes better use of the exergy of fuels, even if the overall efficiency is lower than that of a boiler.
  • As already mentioned, a boiler destroys a large part of the exergy of the fuel (e.g. natural gas), even if it has hardly any energy losses. More efficient heat generation is possible if the exergy of the fuel is used in the best possible way: ideally to generate electricity with the highest possible electrical efficiency and with additional use of the remaining heat (→Combined heat and power). A heat pump can then be operated with the electricity, and the total thermal energy obtained is significantly greater than the energy used in the fuel - especially if the flow temperature of the heating system is minimized so that the exergy requirement of the heating system is low. Here, too, the overall degree of utilization is not yet at the limit of what is physically possible because various exergy losses still occur, especially in the power plant (where the gas or steam temperature is lower than the maximum possible combustion temperature for technical reasons) and in the heat pump. Nevertheless this can thermodynamically optimized heating save a lot of energy compared to other solutions.
  • Central heating with radiators needs about the same amount of heating energy as underfloor heating (or other surface heating), but with a higher flow temperature. Consumption on Exergy so is higher. When using a heat pump heating system, this results in a lower coefficient of performance and thus a higher primary energy consumption.

From these examples one can learn that considering exergy losses in addition to energy losses helps to find an overall system that is as efficient as possible. It is true that one could simply compare the overall energetic efficiency of different systems without knowledge of exergy. However, the exergy consideration often shows more quickly and clearly which step is the biggest problem - e.g. B. in the electric radiator and not in the power plant.

Technologies that reduce primary energy consumption with special attention to the aspect of exergy are also called LowEx technologies designated. One example of this is thermodynamically optimized heating.

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See also: energy, entropy, thermodynamics, main principles of thermodynamics, cooling, energy efficiency, energy depreciation, thermodynamically optimized heating
as well as other articles in the categories basic concepts, physical fundamentals