In
thermodynamicsIn physics, thermodynamics is the study of the conversion of energy into work and heat and its relation to macroscopic variables such as temperature, volume and pressure...
, the
exergy of a system is the maximum
workIn physics, mechanical work is the amount of energy transferred by a force acting through a distance. Like energy, it is a scalar quantity, with SI units of joules...
possible during a process that brings the system into
equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
with a
heat reservoirIn thermodynamics a heat reservoir is considered as a constant temperature source. The temperature of the reservoir does not change irrespective of whether heat is added or extracted to or from it....
. When the surroundings are the reservoir, exergy is the potential of a system to cause a
changeChange can mean:* The process of becoming different:** Social change** Biological metamorphosis** The mathematical study of change* Small denominations of money given in exchange for a larger denominationChange may also refer to:...
as it achieves equilibrium with its environment. Exergy is then the
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
that is available to be used. After the system and surroundings reach equilibrium, the exergy is zero. Determining exergy was also the first goal of
thermodynamicsIn physics, thermodynamics is the study of the conversion of energy into work and heat and its relation to macroscopic variables such as temperature, volume and pressure...
.
Energy is never destroyed during a process; it changes from one form to another (
see First Law of ThermodynamicsThe first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed , but cannot be created or destroyed. Alternatively:-Description:...
). In contrast, exergy accounts for the
irreversibilityIn science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics, as applied to processes...
of a process due to increase in
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
(
see Second Law of ThermodynamicsThe second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
). Exergy is always destroyed when a process involves a
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
change. This destruction is proportional to the entropy increase of the system together with its surroundings. The destroyed exergy has been called
anergy. For an
isothermal processAn isothermal process is a change in which the temperature of the system stays constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir , and the change occurs slowly enough to allow the system to continually adjust to the temperature of the...
, exergy and energy are interchangeable terms, and there is no anergy.
Exergy analysis is performed in the field of
industrial ecologyIndustrial Ecology is an interdisciplinary field that focuses on the sustainable combination of environment, economy and technology. The central idea is the analogy between natural and socio-technical systems. The word 'industrial' does not only refer to industrial complexes but more generally to...
to use energy more efficiently. The term was coined by
Zoran RantZoran Rant was a Slovene mechanical engineer, scientist and professor, associate member of SAZU. He invented terms known today as "exergy and anergy"....
in 1956, but the concept was developed by
J. Willard GibbsJosiah Willard Gibbs was an American theoretical physicist, chemist, and mathematician. He devised much of the theoretical foundation for chemical thermodynamics as well as physical chemistry. As a mathematician, he invented vector analysis...
in 1873. Ecologists and design engineers often choose a
reference state for the reservoir that may be different from the actual surroundings of the system.
Exergy is a combination property of a system and its environment because unlike energy it depends on the state of both the system and environment. The exergy of a system in equilibrium with the environment is zero. Exergy is neither a
thermodynamic property of matter nor a thermodynamic potential of a system. Exergy and energy both have units of
jouleThe joule , named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by a force of one newton acting to move an object through a distance of one metre...
s. The
Internal EnergyIn thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational and electric energy of atoms...
of a system is always measured from a fixed reference state and is therefore always a
state functionIn thermodynamics, a state function, state quantity, or a function of state, is a property of a system that depends only on the current state of the system, not on the way in which the system acquired that state. A state function describes the equilibrium state of a system...
. Some authors define the exergy of the system to be changed when the environment changes, in which case it is not a state function. Other writers prefer a slightly alternate definition of the available energy or exergy of a system where the environment is firmly defined, as an unchangeable absolute reference state, and in this alternate definition exergy becomes a property of the state of the system alone.
The term exergy is also used, by analogy with its physical definition, in
information theoryInformation theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Historically, information theory was developed by Claude E. Shannon to find fundamental limits on compressing and reliably storing and communicating data...
related to
reversible computingReversible computing, sometimes called non-destructive computing includes any computational process that is reversible; i.e., time-invertible, meaning that a time-reversed version of the process could exist within the same general dynamical framework as the original process...
. Exergy is also synonymous with:
availability,
available energy,
exergic energy,
essergy (considered archaic),
utilizable energy,
available useful work,
maximum (or minimum) work,
maximum (or minimum) work content,
reversibleIn thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy. Due to these infinitesimal changes, the system is at rest throughout the...
work, and
ideal work.
Carnot
In 1824,
Sadi CarnotNicolas Léonard Sadi Carnot was a French physicist and military engineer who, in his 1824 Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines, now known as the Carnot cycle, thereby laying the foundations of the second law of thermodynamics...
studied the improvements developed for steam engines by James Watt and others. Carnot utilized a purely theoretical perspective for these engines and developed new ideas. He wrote:
The question has often been raised whether the motive power of heat is unbounded, whether the possible improvements in steam engines have an assignable limit—a limit by which the nature of things will not allow to be passed by any means whatever… In order to consider in the most general way the principle of the production of motion by heat, it must be considered independently of any mechanism or any particular agent. It is necessary to establish principles applicable not only to steam-engines but to all imaginable heat-engines… The production of motion in steam-engines is always accompanied by a circumstance on which we should fix our attention. This circumstance is the re-establishing of equilibrium… Imagine two bodies A and B, kept each at a constant temperature, that of A being higher than that of B. These two bodies, to which we can give or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited reservoirs…
Carnot next described what is now called the
Carnot engineA Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824...
, and proved by a
thought experimentA thought experiment, sometimes called a gedankenexperiment in German, is a proposal for an experiment that would test or illuminate a hypothesis or theory....
that any heat engine performing better than this engine would be a
perpetual motionThe term perpetual motion, taken literally, refers to movement that goes on forever. However, the term more commonly refers to any device or system that perpetually produces more energy than it consumes, resulting in a net output of energy for indefinite time...
machine. Even in the 1820s, there was a long history of science forbidding such devices. According to Carnot, "Such a creation is entirely contrary to ideas now accepted, to the
laws of mechanicsIn the fields of physics, classical mechanics is one of the two major sub-fields of study in the science of mechanics, which is concerned with the set of physical laws governing and mathematically describing the motions of bodies and aggregates of bodies geometrically distributed within a certain...
and of sound
physicsPhysics is a natural science; it is the study of matter and its motion through spacetime and all that derives from these, such as energy and force...
. It is inadmissible."
This description of an upper bound to the work that may be done by an engine was the earliest modern formulation of the
second law of thermodynamicsThe second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
. Because it involves no mathematics, it still often serves as the entry point for a modern understanding of both the second law and
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
. Carnot's focus on
heat engineA heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle...
s,
equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
, and
heat reservoirIn thermodynamics a heat reservoir is considered as a constant temperature source. The temperature of the reservoir does not change irrespective of whether heat is added or extracted to or from it....
s is also the best entry point for understanding the closely related concept of exergy.
Carnot believed in the incorrect
caloric theoryThe caloric theory is an obsolete scientific theory that heat consists of a fluid called caloric that flows from hotter to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores in solids and liquids...
of heat that was popular during his time, but his thought experiment nevertheless described a fundamental limit of nature. As
kinetic theoryKinetic theory is the theory that gases are made up of a large number of small particles , all of which are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container...
replaced caloric theory through the early and mid-1800s (
see timeline- Before 1800 :* 1650 – Otto von Guericke builds the first vacuum pump* 1660 – Robert Boyle experimentally discovers Boyle's Law, relating the pressure and volume of a gas...
), several scientists added mathematical precision to the first and second
laws of thermodynamicsThe laws of thermodynamics, in principle, describe the specifics for the transport of heat and work in thermodynamic processes. Since their inception, however, these laws have become some of the most important in all of physics and other types of science associated with thermodynamics.It is wise to...
and developed the concept of
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
. Carnot's focus on processes at the human scale (above the
thermodynamic limitIn physics and physical chemistry, the thermodynamic limit is reached as the number of particles in a system, N, approaches infinity — or in practical terms, one mole or Avogadro's number ≈ 6 x 1023...
) led to the most universally applicable concepts in
physicsPhysics is a natural science; it is the study of matter and its motion through spacetime and all that derives from these, such as energy and force...
. Entropy and the second-law are applied today in fields ranging from
quantum mechanicsQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
to
physical cosmologyPhysical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of our universe and is concerned with fundamental questions about its formation and evolution. Cosmology involves itself with studying the motions of the celestial bodies and the first cause....
.
Gibbs
In the 1870s,
Josiah Willard GibbsJosiah Willard Gibbs was an American theoretical physicist, chemist, and mathematician. He devised much of the theoretical foundation for chemical thermodynamics as well as physical chemistry. As a mathematician, he invented vector analysis...
unified a large quantity of 19th century
thermochemistryIn thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling...
into one compact theory. Gibbs's theory incorporated the new concept of a
chemical potentialChemical potential, symbolized by μ, is a quantity first described by the American engineer, chemist and mathematical physicist Josiah Williard Gibbs...
to cause change when distant from a
chemical equilibriumIn a chemical process, chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction...
into the older work begun by Carnot in describing thermal and
mechanical equilibriumA standard definition of static equilibrium is:This is a strict definition, and often the term "static equilibrium" is used in a more relaxed manner interchangeably with "mechanical equilibrium", as defined next....
and their potentials for change. Gibbs's unifying theory resulted in the thermodynamic potential
state functionIn thermodynamics, a state function, state quantity, or a function of state, is a property of a system that depends only on the current state of the system, not on the way in which the system acquired that state. A state function describes the equilibrium state of a system...
s describing differences from
thermodynamic equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
.
In 1873, Gibbs derived the mathematics of "available energy of the body and medium" into the form it has today. (See the equations below). The physics describing exergy has changed little since that time. The term
exergy was suggested in 1956 by
Zoran RantZoran Rant was a Slovene mechanical engineer, scientist and professor, associate member of SAZU. He invented terms known today as "exergy and anergy"....
(1904–1972) by using the Greek
ex and
ergon meaning "from
workIn thermodynamics, work performed by a system is the quantity of energy transferred by the system to another due to changes in the external parameters of the system. If these changes happen in a reversible way, then the performed work does not lead to a change of the entropy. It is a...
."
exergy is the energy that can be use form the sourse of energy. In case of solar energy exergy is the avaialble energy from solar radiation to the earth
An application of the second law of thermodynamics
- See Also: Second law of thermodynamics
The second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
Exergy uses system boundaries in a way that is unfamiliar to many. We imagine the presence of a
Carnot engineA Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824...
between the system and its reference environment even though this engine does not exist in the real world. Its only purpose is to measure the results of a "what-if" scenario to represent the most efficient work interaction possible between the system and its surroundings.
If a real-world reference environment is chosen that behaves like an unlimited reservoir that remains unaltered by the system, then Carnot's speculation about the consequences of a system heading towards equilibrium with time is addressed by two equivalent mathematical statements.
B, the exergy or available work, will decrease with time, and
Stotal, the entropy of the system and its reference environment enclosed together in a larger
isolated systemIn the natural sciences an isolated system, as contrasted with a open system, is a physical system that does not interact with its surroundings. It obeys a number of conservation laws: its total energy and mass stay constant. They cannot enter or exit, but can only move around inside...
, will increase with time:
For macroscopic systems (above the
thermodynamic limitIn physics and physical chemistry, the thermodynamic limit is reached as the number of particles in a system, N, approaches infinity — or in practical terms, one mole or Avogadro's number ≈ 6 x 1023...
), these statements are both expressions of the
second law of thermodynamicsThe second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
if the following expression is used for exergy:
where the extensive quantities for the system are
U =
Internal energyIn thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational and electric energy of atoms...
,
V =
VolumeThe volume of any solid, liquid, gas, plasma, theoretical object, or vacuum is how much three-dimensional space it occupies, often quantified numerically. One-dimensional figures and two-dimensional shapes are assigned zero volume in the three-dimensional space...
, and
Ni =
MolesThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
of component
i. The intensive quantities for the surroundings are
PR =
PressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and =
Chemical potentialChemical potential, symbolized by μ, is a quantity first described by the American engineer, chemist and mathematical physicist Josiah Williard Gibbs...
of component
i. Individual terms also often have names attached to them: is called "available PV work", is called "entropic loss" or "heat loss" and the final term is called "available chemical energy."
Other thermodynamic potentials may be used to replace internal energy so long as proper care is taken in recognizing which natural variables correspond to which potential. For the recommended nomenclature of these potentials, see (Alberty, 2001). Equation (2) is useful for processes where system volume, entropy, and number of moles of various components change because internal energy is also a function of these variables and no others.
An alternative definition of internal energy does not separate available chemical potential from
U. This expression is useful (when substituted into equation (1)) for processes where system volume and entropy change, but no chemical reaction occurs:
In this case a given set of chemicals at a given entropy and volume will have a single numerical value for this thermodynamic potential. A
multi-stateIn the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
system may complicate or simplify the problem because the Gibbs phase rule predicts that intensive quantities will no longer be completely independent from each other.
A historical and cultural tangent
In 1848,
William Thomson, 1st Baron KelvinWilliam Thomson, 1st Baron Kelvin , OM, GCVO, PC, PRS, FRSE, was a British mathematical physicist and engineer...
asked (and immediately answered) the question:
- Is there any principle on which an absolute thermometric scale can be founded? It appears to me that Carnot’s theory of the motive power of heat enables us to give an affirmative answer.
With the benefit of the hindsight contained in equation (3), we are able to understand the historical impact of Kelvin's idea on physics. Kelvin suggested that the best temperature scale would describe a constant ability for a unit of temperature in the surroundings to alter the available work from Carnot's engine. From equation (3):
Rudolf ClausiusRudolf Julius Emanuel Clausius , was a German physicist and mathematician and is considered one of the central founders of the science of thermodynamics. By his restatement of Sadi Carnot's principle known as the Carnot cycle, he put the theory of heat on a truer and sounder basis...
recognized the presence of a
proportionalityIn mathematics, two quantities are said to be proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio.Proportion also refers to the equality of two ratios....
constant in Kelvin's analysis and gave it the name
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
in 1865 from the Greek for "transformation" because it describes the available energy lost during transformation from heat to work. The available work from a Carnot engine is at its maximum when the surroundings are at a temperature of
absolute zeroAbsolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible and cannot be reached by artificial or natural means, because it is impossible to decouple a system fully from the rest of the universe...
.
Physicists then, as now, often look at a property with the word "available" or "utilizable" in its name with a certain unease. The idea of what is available raises the question of "available to what?" and raises a concern about whether such a property is
anthropocentricAnthropocentrism or anthrocentrism is the belief that humans must be considered at the center of, and above any other aspect of, reality. This concept is sometimes known as humanocentrism or human supremacy...
. Laws derived using such a property may not describe the universe but instead describe what people wish to see.
The field of
statistical mechanicsStatistical mechanics is the application of probability theory, which includes mathematical tools for dealing with large populations, to the field of mechanics, which is concerned with the motion of particles or objects when subjected to a force...
(beginning with the work of
Ludwig BoltzmannLudwig Eduard Boltzmann was an Austrian physicist famous for his founding contributions in the fields of statistical mechanics and statistical thermodynamics...
in developing the
Boltzmann equationThe Boltzmann equation, also often known as the Boltzmann transport equation, devised by Ludwig Boltzmann, describes the statistical distribution of one particle in a fluid...
) relieved many physicists of this concern. From this discipline, we now know that macroscopic properties may all be determined from properties on a microscopic scale where entropy is more "real" than temperature itself (
see thermodynamic temperatureThermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is an “absolute” scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the...
). Microscopic kinetic fluctuations among particles cause entropic loss, and this energy is unavailable for work because these fluctuations occur randomly in all directions. The anthropocentric act is taken, in the eyes of some physicists and engineers today, when one draws a hypothetical boundary and in effect says, "This is my system. What occurs beyond it is surroundings." In this context, exergy is sometimes described as an anthropocentric property, both by those who use it and those who don't. Entropy is viewed as a more fundamental property of matter.
In the field of
ecologyEcology is the interdisciplinary scientific study of the interactions between organisms and the interactions of these organisms with their environment....
, the interactions among systems (mostly
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
s) and their manipulation of exergy resources is of primary concern. With this perspective, the answer to, "available to what?" is simply, "available to the system" because ecosystems appear to exist in the real world. With the viewpoint of
systems ecologySystems ecology is an interdisciplinary field of ecology, taking a holistic approach to the study of ecological systems, especially ecosystems. Systems ecology can be seen as an application of general systems theory to ecology. Central to the systems ecology approach is the idea that an ecosystem...
, a property of matter like absolute entropy is seen as anthropocentric because it is defined relative to an unobtainable hypothetical reference system in isolation at absolute zero temperature. With this emphasis on systems rather than matter, exergy is viewed as a more fundamental property of a system, and it is entropy that may be viewed as a co-property of a system with an idealized reference system.
A potential for every thermodynamic situation
In addition to and , the other thermodynamic potentials are frequently used to determine exergy. For a given set of chemicals at a given entropy and pressure,
enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
H is used in the expression:
For a given set of chemicals at a given temperature and volume,
Helmholtz free energyIn thermodynamics, the Helmholtz free energy is a thermodynamic potential which measures the “useful” work obtainable from a closed thermodynamic system at a constant temperature and volume...
A is used in the expression:
For a given set of chemicals at a given temperature and pressure,
Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
G is used in the expression:
The potentials
A and
G are utilized for a constant temperature process. In these cases, all energy is
free to perform useful workIn thermodynamics, the term thermodynamic free energy refers to the amount of work that can be extracted from a system, and is helpful in engineering applications...
because there is no entropic loss. A chemical reaction that generates electricity with no associated change in temperature will also experience no entropic loss. (
See fuel cellA fuel cell is an electrochemical cell that produces electricity from a replenishable fuel tank. The electricity is generated through the reaction, triggered in the presence of an electrolyte, between the fuel and an oxidant . The reactants flow into the cell, and the reaction products flow out of...
.) This is true of every isothermal process. Examples are gravitational potential energy,
kinetic energyThe kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its...
(on a macroscopic scale), solar energy, electrical energy, and many others. If
frictionFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties:...
, absorption,
electrical resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
or a similar energy conversion takes place that releases heat, the impact of that heat on thermodynamic potentials must be considered, and it is this impact that decreases the available energy.
Applications
Applying equation (1) to a subsystem yields:
This expression applies equally well for theoretical ideals in a wide variety of applications:
electrolysisIn chemistry and manufacturing, electrolysis is a method of using an electric current to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally-occurring sources such as ores using an electrolytic...
(decrease in
G),
galvanic cellThe Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells...
s and
fuel cellA fuel cell is an electrochemical cell that produces electricity from a replenishable fuel tank. The electricity is generated through the reaction, triggered in the presence of an electrolyte, between the fuel and an oxidant . The reactants flow into the cell, and the reaction products flow out of...
s (increase in
G), explosives (increase in
A),
heating and refrigerationHVAC is an acronym that stands for the closely related functions of "Heating, Ventilating, and Air Conditioning"-the technology of indoor environmental comfort...
(exchange of
H),
motorsA heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle...
(decrease in
U) and
generatorsIn electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities...
(increase in
U).
Utilization of the exergy concept often requires careful consideration of the choice of reference environment because, as Carnot knew, unlimited reservoirs do not exist in the real world. A system may be maintained at a constant temperature to simulate an unlimited reservoir in the lab or in a factory, but those systems cannot then be isolated from a larger surrounding environment. However, with a proper choice of system boundaries, a reasonable constant reservoir can be imagined. A process sometimes must be compared to "the most realistic impossibility," and this invariably involves a certain amount of guesswork.
Engineering applications
Application of exergy to
unit operationIn chemical engineering and related fields, a unit operation is a basic step in a process. For example in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process...
s in
chemical plantA chemical plant is an industrial process plant that manufactures chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials. Chemical plants use special equipment,...
s was partially responsible for the huge growth of the
chemical industryThe chemical industry comprises the companies that produce industrial chemicals. It is central to modern world economy, converting raw materials into more than 70,000 different products.-Products:...
during the 1900s. During this time it was usually called
availability or
available work.
As a simple example of exergy, air at atmospheric conditions of temperature, pressure,
and composition contains energy but no exergy when it is chosen as the thermodynamic reference state known as
ambientIn physical sciences, standard conditions for temperature and pressure are standard sets of conditions for experimental measurements, to allow comparisons to be made between different sets of data...
. Individual processes on Earth like combustion in a power plant often eventually result in products that are incorporated into a large atmosphere, so defining this reference state for exergy is useful even though the atmosphere itself is not at equilibrium and is full of long and short term variations.
If standard ambient conditions are used for calculations during plant operation when the actual weather is very cold or hot, then certain parts of a chemical plant might seem to have an exergy efficiency of greater than 100% and appear on paper to be a perpetual motion machine! Using actual conditions will give actual values, but standard ambient conditions are useful for initial design calculations.
One goal of energy and exergy methods in engineering is to compute what comes into and out of several possible designs before a factory is built. Energy input and output will always
balanceBalance may refer to:*Analytical balance, a weighing scale.*Balance , an ability to maintain balance .*Balance *Media balance, the presentation of multiple viewpoints ....
according to the
First Law of ThermodynamicsThe first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed , but cannot be created or destroyed. Alternatively:-Description:...
or the energy conservation principle. Exergy output will not balance the exergy input for real processes since a part of the exergy input is always destroyed according to the
Second Law of ThermodynamicsThe second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
for real processes. After the input and output are completed, the engineer will often want to select the most efficient process. An energy efficiency or
first law efficiency will determine the most efficient process based on wasting as little energy as possible relative to energy inputs. An
exergy efficiencyExergy efficiency computes the efficiency of a process taking the second law of thermodynamics into account.-Motivation:...
or
second-law efficiency will determine the most efficient process based on wasting
and destroying as little available work as possible from a given input of available work.
Design engineerDesign engineer is a general term that covers multiple engineering disciplines including electrical, mechanical and civil engineering, architectural engineers in the U.S. and building engineers in the U.K....
s have recognized that a higher exergy efficiency involves building a more expensive plant, and a balance between capital investment and operating efficiency must be determined in the context of economic competition.
Applications in natural resource utilization
In recent decades, utilization of exergy has spread outside of physics and engineering to the fields of
industrial ecologyIndustrial Ecology is an interdisciplinary field that focuses on the sustainable combination of environment, economy and technology. The central idea is the analogy between natural and socio-technical systems. The word 'industrial' does not only refer to industrial complexes but more generally to...
,
ecological economicsImage:Sustainable development.svg|right|The three pillars of sustainability. Clickable.|275px|thumbpoly 138 194 148 219 164 240 182 257 219 277 263 291 261 311 264 331 272 351 283 366 300 383 316 394 287 408 261 417 224 424 182 426 154 423 119 415 87 403 58 385 40 368 24 347 17 328 13 309...
,
systems ecologySystems ecology is an interdisciplinary field of ecology, taking a holistic approach to the study of ecological systems, especially ecosystems. Systems ecology can be seen as an application of general systems theory to ecology. Central to the systems ecology approach is the idea that an ecosystem...
, and
energeticsEnergetics is the scientific study of energy flows and storages under transformation. Because energy flows at all scales, from the quantum level, to the biosphere and cosmos, energetics is therefore a very broad discipline, encompassing for example thermodynamics, chemistry, biological energetics,...
. Defining where one field ends and the next begins is a matter of semantics, but applications of exergy can be placed into rigid categories.
Researchers in ecological economics and environmental accounting perform exergy-cost analyses in order to evaluate the impact of human activity on the current
natural environmentThe natural environment, commonly referred to simply as the environment, is a term that encompasses all living and non-living things occurring naturally on Earth or some region thereof....
. As with ambient air, this often requires the unrealistic substitution of properties from a natural environment in place of the
reference stateIn thermodynamics a heat reservoir is considered as a constant temperature source. The temperature of the reservoir does not change irrespective of whether heat is added or extracted to or from it....
environment of Carnot. For example, ecologists and others have developed reference conditions for the
oceanAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
and for the Earth's crust. Exergy values for human activity using this information can be useful for comparing policy alternatives based on the efficiency of utilizing
natural resourcesNatural Resources is a soul album released by Motown girl group Martha Reeves and the Vandellas in 1970 on the Gordy label. The album is significant for the Vietnam War ballad "I Should Be Proud" and the slow jam, "Love Guess Who"...
to perform work. Typical questions that may be answered are:
- Does the human production of one unit of an economic good by method A utilize more of a resource's exergy than by method B?
- Does the human production of economic good A utilize more of a resource's exergy than the producution of good B?
- Does the human production of economic good A utilize a resource's exergy more efficiently than the production of good B?
There has been some progress in standardizing and applying these methods.
Measuring exergy requires the evaluation of a system’s reference state environment. With respect to the applications of exergy on natural resource utilization, the process of quantifying a system requires the assignment of value (both utilized and potential) to resources that are not always easily dissected into typical cost-benefit terms. However, to fully realize the potential of a system to do work, it is becoming increasingly imperative to understand exergetic potential of natural resources, and how human interference alters this potential.
Referencing the inherent qualities of a system in place of a reference state environment is the most direct way that ecologists determine the exergy of a natural resource. Specifically, it is easiest to examine the thermodynamic properties of a system, and the reference substances that are acceptable within the reference environment. This determination allows for the assumption of qualities in a natural state: deviation from these levels may indicate a change in the environment caused by outside sources. There are three kinds of reference substances that are acceptable, due to their proliferation on the planet: gases within the
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
, solids within the Earth’s crust, and molecules or ions in seawater. By understanding these basic models, it’s possible to determine the exergy of multiple earth systems interacting, like the effects of solar radiation on plant life. These basic categories are utilized as the main components of a reference environment when examining how exergy can be defined through natural resources.
Other qualities within a reference state environment include temperature, pressure, and any number of combinations of substances within a defined area. Again, the exergy of a system is determined by the potential of that system to do work, so it is necessary to determine the baseline qualities of a system before it is possible to understand the potential of that system. The thermodynamic value of a resource can be found by multiplying the exergy of the resource by the cost of obtaining the resource and processing it .
Today, it is becoming increasingly popular to analyze the environmental impacts of natural resource utilization, especially for energy usage. To understand the ramifications of these practices, exergy is utilized as a tool for determining the impact potential of emissions, fuels, and other sources of energy.
CombustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
of fossil fuels, for example, is examined with respect to assessing the environmental impacts of burning
coalCoal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
,
oilAn oil is any substance that is liquid at ambient temperatures and is hydrophobic but soluble in organic solvents. Oils have a high carbon and hydrogen content and are nonpolar substances. The general definition above includes compound classes with otherwise unrelated chemical structures,...
, and
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
. The current methods for analyzing the emissions from these three products can be compared to the process of determining the exergy of the systems affected; specifically, it is useful to examine these with regard to the reference state environment of gases within the
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
. In this way, it is easier to determine how human action is affecting the natural environment.
Applications in sustainability
In
systems ecologySystems ecology is an interdisciplinary field of ecology, taking a holistic approach to the study of ecological systems, especially ecosystems. Systems ecology can be seen as an application of general systems theory to ecology. Central to the systems ecology approach is the idea that an ecosystem...
, researchers sometimes consider the exergy of the current formation of natural resources from a small number of exergy inputs (usually solar radiation,
tidal forceThe tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted on one body by a second body is not constant across its diameter...
s, and geothermal heat). This application not only requires assumptions about reference states, but it also requires assumptions about the real environments of the past that might have been close to those reference states. Can we decide which is the most "realistic impossibility" over such a long period of time when we are only speculating about the reality?
For instance, comparing oil exergy to coal exergy using a common reference state would require geothermal exergy inputs to describe the transition from biological material to fossil fuels during millions of years in the Earth's crust, and solar radiation exergy inputs to describe the material's history before then when it was part of the biosphere. This would need to be carried out mathematically backwards through time, to a presumed era when the oil and coal could be assumed to be receiving the same exergy inputs from these sources. A speculation about a past environment is different from assigning a reference state with respect to known environments today. Reasonable guesses about real ancient environments may be made, but they are untestable guesses, and so some regard this application as
pseudosciencePseudoscience is a methodology, belief, or practice that is claimed to be scientific, or that is made to appear to be scientific, but which does not adhere to an appropriate scientific methodology, lacks supporting evidence or plausibility, or otherwise lacks scientific status...
or pseudo-engineering.
The field describes this accumulated exergy in a natural resource over time as
embodied energyEmbodied energy is defined as the available energy that was used in the work of making a product. Embodied energy is an accounting methodology which aims to find the sum total of the energy necessary for an entire product lifecycle...
with units of the "embodied joule" or "emjoule".
The important application of this research is to address
sustainabilitySustainability, in a broad sense, is the capacity to endure. In ecology, the word describes how biological systems remain diverse and productive over time...
issues in a quantitative fashion through a sustainability measurement:
- Does the human production of an economic good deplete the exergy of Earth's natural resources
Natural Resources is a soul album released by Motown girl group Martha Reeves and the Vandellas in 1970 on the Gordy label. The album is significant for the Vietnam War ballad "I Should Be Proud" and the slow jam, "Love Guess Who"...
more quickly than those resources are able to receive exergy?
- If so, how does this compare to the depletion caused by producing the same good (or a different one) using a different set of natural resources?
Assigning one thermodynamically obtained value to an economic good
A technique proposed by systems ecologists is to consolidate the three exergy inputs described in the last section into the single exergy input of solar radiation, and to express the total input of exergy into an economic good as a
solar embodied joule or
sej. (
See emergy) Exergy inputs from solar , tidal, and geothermal forces all at one time had their origins at the beginning of the solar system under conditions which could be chosen as an initial reference state, and other speculative reference states could in theory be traced back to that time. With this tool we would be able to answer:
- What fraction of the total human depletion of the Earth's exergy is caused by the production of a particular economic good?
- What fraction of the total human and non-human depletion of the Earth's exergy is caused by the production of a particular economic good?
No additional thermodynamic laws are required for this idea, and the principles of
energeticsEnergetics is the scientific study of energy flows and storages under transformation. Because energy flows at all scales, from the quantum level, to the biosphere and cosmos, energetics is therefore a very broad discipline, encompassing for example thermodynamics, chemistry, biological energetics,...
may confuse many issues for those outside the field. The combination of untestable hypotheses, unfamiliar jargon that contradicts accepted jargon, intense advocacy among its supporters, and some degree of isolation from other disciplines have contributed to this
protoscienceProtoscience refers to historical philosophical disciplines which existed prior to the development of scientific method, which allowed them to develop into science proper...
being regarded by many as a
pseudosciencePseudoscience is a methodology, belief, or practice that is claimed to be scientific, or that is made to appear to be scientific, but which does not adhere to an appropriate scientific methodology, lacks supporting evidence or plausibility, or otherwise lacks scientific status...
. However, its basic tenets are only a further utilization of the exergy concept.
Implications in the development of complex physical systems
A common hypothesis in systems ecology is that the design engineer's observation that a greater capital investment is needed to create a process with increased exergy efficiency is actually the economic result of a fundamental law of nature. By this view, exergy is the analogue of economic currency in the natural world. The analogy to capital investment is the accumulation of exergy into a system over long periods of time resulting in
embodied energyEmbodied energy is defined as the available energy that was used in the work of making a product. Embodied energy is an accounting methodology which aims to find the sum total of the energy necessary for an entire product lifecycle...
. The analogy of capital investment resulting in a factory with high exergy efficiency is an increase in natural organizational structures with high exergy efficiency. (
See maximum powerMaximum power can refer to different concepts:* In electronics, the maximum power theorem* In systems theory, the maximum power principle...
). Researchers in these fields describe biological
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
in terms of increases in organism complexity due to the requirement for increased exergy efficiency because of competition for limited sources of exergy.
Some biologists have a similar hypothesis. A biological system (or a chemical plant) with a number of intermediate compartments and intermediate reactions is more efficient because the process is divided up into many small substeps, and this is closer to the
reversible idealIn thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy. Due to these infinitesimal changes, the system is at rest throughout the...
of an infinite number of
infinitesimalInfinitesimals have been used to express the idea of objects so small that there is no way to see them or to measure them. The word infinitesimal comes from a 17th century Modern Latin coinage infinitesimus, which originally referred to the "infinite-th" item in a series.In common speech, an...
substeps. Of course, an excessively large number of intermediate compartments comes at a capital cost that may be too high.
Testing this idea in living organisms or ecosystems is impossible for all practical purposes because of the large time scales and small exergy inputs involved for changes to take place. However, if this idea is correct, it would not be a new fundamental law of nature. It would simply be living systems and ecosystems maximizing their exergy efficiency by utilizing laws of thermodynamics developed in the 19th century.
Philosophical and cosmological implications
Some proponents of utilizing exergy concepts describe them as a biocentric or ecocentric alternative for terms like
qualityQuality may refer to:Concepts:* Quality * Quality , an attribute or a property* Quality , which has separate meanings in thermodynamics and harmonics...
and
valueValue theory encompasses a range of approaches to understanding how, why, and to what degree humans should value things, whether the thing is a person, idea, object, or anything else. This investigation began in ancient philosophy, where it is called axiology or ethics. Early philosophical...
. The "
deep ecologyDeep ecology is a somewhat recent branch of ecological philosophy that considers humankind as an integral part of its environment. The philosophy emphasizes the equal value of human and non-human life as well as the importance of the ecosystem and natural processes...
" movement views economic usage of these terms as an anthropocentric
philosophyPhilosophy is the study of general and fundamental problems concerning matters such as existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing these questions by its critical, generally systematic approach and its reliance on reasoned...
which should be discarded. A possible universal thermodynamic concept of value or utility appeals to those with an interest in
monismMonism is any philosophical view which holds that there is unity in a given field of inquiry, where this is not to be expected. Thus, some philosophers may hold that the Universe is really just one thing, despite its many appearances and diversities; or theology may support the view that there is...
.
For some, the end result of this line of thinking about tracking exergy into the deep past is a restatement of the
cosmological argumentThe cosmological argument is an argument for the existence of a First Cause to the universe, and by extension is often used as an argument for the existence of an "unconditioned" or "supreme" being, usually then identified as God...
that the universe was once at
equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
and an input of exergy from some First Cause created a universe full of available work. Current science is unable to describe the first 10
–43 seconds of the universe (
See Timeline of the Big BangThis timeline of the Big Bang describes the events according to the widely accepted scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates....
). An external reference state is not able to be defined for such an event, and (regardless of its merits), such an arguments may be better expressed in terms of
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
.
Exergy is highly multidisciplinary
(This section will probably be shortened and added to the "Utilization" section as a table if possible)
The
cumulative exergy consumption of a good is a
sumSUM can refer to:* The State University of Management* Soccer United Marketing* Society for the Establishment of Useful Manufactures* StartUp-Manager* Software User’s Manual,as from DOD-STD-2 167A, and MIL-STD-498...
of the exergy decreases that occurred in order to create it. An initial state for an analysis might consist of exergy contributions from:
- 1) the material entering individual reactors or other unit
- 2) the material delivered to the industry and used for all the units in the industrial process.
- 3) the material purchased from other industries and all the associated indirect exergy decreases involved in transport and administration to get the material and process it.
- 4) all the initial natural resource
Natural resources occur naturally within environments that exist relatively undisturbed by mankind, in a natural form. A natural resource is often characterized by amounts of biodiversity existent in various ecosystems.Natural resources are derived from the environment...
s used directly or indirectly to make the good.
- 5) all the initial ecological inputs (such as solar radiation, tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted on one body by a second body is not constant across its diameter...
s, and geothermal heat) that created the natural resources.
- 6) These multiple inputs related to a single reference input (such as solar radiation).
- 7) This reference input is related to a single input of exergy to the universe from some external source at a time in the past when the entire universe was at equilibrium
In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
.
Choice 1 (if there are few components) is a tricky undergraduate homework problem in chemical engineering if a chemical reaction occurs in an open system. The worked example above utilizes choice 1 for a closed system with no reaction.
Choice 2 would require an in-house exergy accounting analogous to a process
mass balanceA mass balance is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique...
or
energy balanceEnergy balance has the following meanings in several fields:* In physics, energy balance is a systematic presentation of energy flows and transformations in a system. Theoretical basis for an energy balance is the first law of thermodynamics according to which energy cannot be created or destroyed,...
. If there are a reasonable number of unit operations, a professional chemical engineer could do this in a short period of time with a good software package to determine exergy flows in the plant.
Choice 3 would require all of choice 2 and converting multiple items usually thought of as economic overhead into terms of exergy. This could be a challenging task requiring considerable thought, but with several assumptions here and there (and more software to keep track of the accounting), it could be done.
Choice 4 would require a repetition of choice 3 for multiple industries, governmental agencies, and all other human activity to convert raw materials to the product. It seems unlikely that many producers would take the time to determine the complete exergy history of their product, but if we ever live in a world where producers were required to perform choice 3, we might be able to get a reasonable estimate of choice 4.
Choice 5 in combination with choice 4 is the only option that is relevant to environmental
sustainabilitySustainability, in a broad sense, is the capacity to endure. In ecology, the word describes how biological systems remain diverse and productive over time...
. Choice 5 requires exergy information from the field of
systems ecologySystems ecology is an interdisciplinary field of ecology, taking a holistic approach to the study of ecological systems, especially ecosystems. Systems ecology can be seen as an application of general systems theory to ecology. Central to the systems ecology approach is the idea that an ecosystem...
and many additional assumptions. However, with this information, we may address the questions:
- Does the human production of this item deplete the Earth's natural resources
Natural Resources is a soul album released by Motown girl group Martha Reeves and the Vandellas in 1970 on the Gordy label. The album is significant for the Vietnam War ballad "I Should Be Proud" and the slow jam, "Love Guess Who"...
more quickly than those resources are able to regenerate themselves?
- If so, how does this numerically compare to the depletion caused by producing an entirely different item using an entirely different set of natural resources?
Choice 6 represents all exergy changes on Earth in terms of one "currency" that may be used to estimate the
relative valueRelative value is the attractiveness measured in terms of risk, liquidity, and return of one instrument relative to another, or for a given instrument, of one maturity relative to another...
of different natural resources, but this value appraisal would not be on a time scale relevant to human activity. Choice 6 is useful for systems ecologists to consider exergy concepts as a driving force for the
emergenceIn philosophy, systems theory and science, emergence is the way complex systems and patterns arise out of a multiplicity of relatively simple interactions. Emergence is central to the theories of integrative levels and of complex systems.-Definitions:...
structures in nature using a concept like emergy.
Choice 7 is the consequence of this line of thinking carried out to its fullest extent. It is a
thought experimentA thought experiment, sometimes called a gedankenexperiment in German, is a proposal for an experiment that would test or illuminate a hypothesis or theory....
to restate the
cosmological argumentThe cosmological argument is an argument for the existence of a First Cause to the universe, and by extension is often used as an argument for the existence of an "unconditioned" or "supreme" being, usually then identified as God...
.
Quality of energy types
(exergy-to-energy ratio will be in this article. exergy-to-exergy will be moved to Exergy efficiencyExergy efficiency computes the efficiency of a process taking the second law of thermodynamics into account.-Motivation:...
. Some will be removed.)
The ratio of exergy to energy in a substance can be considered a measure of
energy qualityEnergy quality is the contrast between different forms of energy, the different trophic levels in ecological systems and the propensity of energy to convert from one form to another. The concept refers to the empirical experience of the characteristics, or qualia, of different energy forms as they...
. Forms of energy such as macroscopic kinetic energy, electrical energy, and chemical
Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
are 100% recoverable as work, and therefore have an exergy equal to their energy. However, forms of energy such as radiation and thermal energy can not be converted completely to work, and have exergy content less than their energy content. The exact proportion of exergy in a substance depends on the amount of entropy relative to the surrounding environment as determined by the
Second Law of ThermodynamicsIn physics, thermodynamics is the study of the conversion of energy into work and heat and its relation to macroscopic variables such as temperature, volume and pressure...
.
Exergy is useful when measuring the efficiency of an energy conversion process. The exergetic, or 2nd Law, efficiency is a ratio of the exergy output divided by the exergy input. This formulation takes into account the quality of the energy, often offering a more accurate and useful analysis than efficiency estimates only using the
First Law of ThermodynamicsIn physics, thermodynamics is the study of the conversion of energy into work and heat and its relation to macroscopic variables such as temperature, volume and pressure...
.
Work can be extracted also from bodies colder than the surroundings. When the flow of energy is coming into the body, work is performed by this energy obtained from the large reservoir, the surrounding. A quantitative treatment of the notion of energy quality rests on the definition of energy. According to the standard definition,
EnergyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
is a measure of the ability to do work. Work can involve the movement of a mass by a force that results from a transformation of energy. If there is an energy transformation, the
second principle of energy flow transformationsEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
says that this process must involve the dissipation of some energy as heat. Measuring the amount of heat released is one way of quantifying the energy, or ability to do work and apply a force over a distance.
However, it appears that the ability to do work is relative to the energy transforming mechanism that applies a force. This is to say that some forms of energy perform no work with respects to some mechanisms, but perform work with respects to others. For example, water does not have a propensity to combust in an internal combustion engine, whereas gasoline does. Relative to the internal combustion engine, water has little ability to do work that provides a motive force. If “energy” is defined as the ability to do work then a consequence of this simple example is that water has no energy — according to this definition. Nevertheless, water, raised to a height, does have the ability to do work like driving a turbine, and so does have energy.
This example means to demonstrate that the ability to do work can be considered relative to the mechanism that transforms energy, and through such a conversion applies a force. From this observation we might wish to use the word “quality”, and the term “energy quality” to characterise the energetic differences between substances and their propensities to perform work given a specific mechanism. That is the abilities of different energy forms to flow and be transformed in certain mechanisms. With this lexicon, we can say that energy quality is mechanism-relative, and the energy efficiency of a mechanism is energy quality-relative – an internal combustion engine running on water has nearly 0% efficiency since it has the propensity to transform little or no water-energy into thermal-energy. In order to clarify things here we might think of this as the “water-efficiency”. The mechanism of interest is also our system of reference, such that the choice of energy quality specifies a certain system of reference. Thus with respects to the internal combustion system of reference, it has a low “water-efficiency”.
Exergy of heat available at a temperature
Maximal possible conversion of heat to work, or exergy content of heat, depends on the
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
at which heat is available and the temperature level at which the reject heat can be disposed, that is the temperature of the surrounding. The upper limit for conversion is known as Carnot efficiency and was discovered by
Nicolas Léonard Sadi CarnotNicolas Léonard Sadi Carnot was a French physicist and military engineer who, in his 1824 Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines, now known as the Carnot cycle, thereby laying the foundations of the second law of thermodynamics...
in 1824. See also
Carnot heat engineA Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824...
.
Carnot efficiency is
where
TH is the higher temperature and
TC is the lower temperature, both as absolute temperature. From Equation 1 it is clear that in order to maximize efficiency one should maximize
TH and minimize
TC.
For calculation of exergy of heat available at a temperature there are two cases: the body releasing heat is higher than the surrounding, or, the temperature of the body is lower than the surrounding.
Exergy exchanged is then:
where
Tsource is the temperature of the heat source, and
To is the temperature of the surrounding.
External links
Exergy and Rankine cycle - http://twt.mpei.ac.ru/MAS/Worksheets/Rankine3D.mcd