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8/18/2019 Exergy Termo III
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
Exergy
Ing. José Luis Palacios E., M.Sc.
Quito, 08 de Enero 2014 1
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Termodinámica III
Exergy
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Energy is conserved in every device or process.
However, the energy conservation idea alone is inadequate for
depicting some important aspects of resource utilization.[2]
[2]
A large enclosure with adiabatic boundaries
containing a lot of air @Ti
is heated by afurnace.
In the final state the fuel is totally consumed
and there is a slightly warm misxture of
combustion products and air @ Ti + dT .
The total quantity of energy is the same.
The combination of fuel and air in the initial state
has a greater potential than the warm mixture in
the final state.
The fuel can be used to generate electricity, do
work or heat room.
The slightly warm combustion products are muchmore limited. [3]
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Exergy
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100 kJ electricity stored in a 12 V/2.3 Ah car-battery is more useful than
the same amount of energy stored in 1 kg water @ 43ºC in an ambient
temperature of 20ºC.
The electricity can be used for running a machine, operating a light bulb
of 40 W for 42 min or at leats heating 1 kg of water with 23ºC. The energy in the 1 kg water is only suitable for washing our hands or
doing the dishes.
The term exergy manifests the importance of energy´s quanti ty and
qual i ty . [3]
[3]
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Exergy definition
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An opportunity exists for doing work whenever two systems at different
states are brought into communication.
In principle, work can be developed as the systems are allowed to
come into equilibrium. When one of the two systems is a suitably
idealized system called environment and the other is some system of
interest.
Exergy is a property of a system-envi ronment combination.
In 1952, Rant (in Europe) introduced the name exergy, defined as
external useful work in opposition to energy (internal work).
The term availabi l i ty was made popular in USA by the M.I.T. School of
Engineering in the 1940´s.
[2]
[4]
Exergy represents the upper limit on the amount of work a device
can deliver withouth violating any thermodynamics laws.
[4]
[5]
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Termodinámica III
Exergy
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Termodinámica III
Modeling the envinronment
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Simplifications are made in a model to reduce the complexity of the
physical world.
The validity and utility of an analysis using any model are, of course
restricted by the idealizations made in formulating the model.
Environment is regarded to be a simple compressible system that islarge in extent and uniform in temperature T o and pressure po.
Values of temperature and pressure are normally taken as typical
environmental conditions, such as 1 atm and 25ºC.
Although its intensive properties do not change, the environmentl can
experience changes in its extensives properties as a result ofinteractions with other systems.
Changes in the extensive properties U e, Se and V e are related through
the Tds realtion, so that is it takes de form:
[2]
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Evaluating Exergy
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An energy balance for the combined system:
Where Wc is the work by the combined
system, and Ec is the energy change of the
combined system, equal to the sum of theenergy changes of the closed system and the
environment.
The energy of the closed system initially
is denoted by E.
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Evaluating Exergy
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[2]
irreversibilities
Exergy of a system
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Energy and Exergy analysis
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An energy analysis of an energy conversion system is essentially an
accounting of the energies entering and exiting (FLT).
The exiting energies can be broken down into products and wastes.
Efficiencies are often evaluated as ratios of energy quantities, and are
often used to assess and compare various systems. However, energy
efficiencies are often misleading in that they do not always provide a
measure of how nearly the performance of a system approaches
ideality.
The results of energy analysis can indicate the main inefficiencies to be
within the wrong sections of the systems, and a state of technological
efficiency different than actually exists.
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Energy and Exergy analysis
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Exergy analys is is based on SLT, and is useful in identifying the
causes, locations, and magntudes of process inefficiencies.
Exergy analysis acknowledges that, although energy cannot be created
or destroyed, it can be degraded in quality, eventually reaching a statein which it is complete equilibrium with the surroundings and hence no
further use for performing tasks.
Exergy analysis states the theoretical limitations imposed upon a
system, clearly pointing out that no real system can conserve exergyand that only a portion of the input exergy can be recovered.
Also, exergy analysis quatitatively specifies practical limitatios by
providing losses in a form in which they are a direct measure of lost
enxergy. [6]
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Exergy equations
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Closed Systems
Energy:
Exergy:
Contro l Volume
Energy:
Exergy:
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Example
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A cylinder of an internal combustion engine 2450 cm³ of gaseous
combustion products at a pressure of 7 bar and a temperature of 867ºC
just before the exhaust valve opens. Determine the specific exergy of the
gas, in kJ/kg. Ignore the effects of motion and gravity, and model the
combustion products as air as an ideal gasl. Take T o = 300 K (27ºC) and
po = 1.013 bar.
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Example
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Exergy transfer by heat
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Exergetic (Second Law) efficiency
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Tasks such as space heating, heating in industrial furnaces, and
process steam generation commonly involve the combustion of coal,
oil, or natural gas ( non-renewable resources).
When the products of combustion are at a temperature significantly
grater than required by a given task, the end use is not well matched to
the source and the result is inefficient use of the fuel burned.
source
use
lost
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Exergetic (Second Law) efficiency
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Exergetic Efficiency < 1[2]
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Fuel is used more effectively in the higher use-temperature industrial
applications than in the lower use-temperature space heating.
The importance of matching end use to source.
Ts = 220 K
η = 1
Tu = 320 K
Tu = 480 K
Tu = 700 K
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Energy Quality
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[7]
Low exergy (or LowEx) systems are defined as heating or cooling
systems that allow the use of low valued energy, which is delivered by
sustainable energy sources.
These systems practically provide heating and cooling energy at a
temperature close to room temperature.
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Exergy - Grassmann diagram
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“In daily life, exergy can also be viewed as the opportunities that
we have and the exergy destruction as the opportunities
wasted. Time is the biggest asset that we have, and the time
wasted is the wasted opportunitiy to do something useful.” Cengel& Boles ,2006
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Referencias
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1. F. Günther, Exergy, What we actually live for,
http://www.holon.se/folke/kurs/Distans/Ekofys/fysbas/exergy/exergybasics.shtml, último acceso enero 2015
2. Moran M.J. and Shapiro H.N. 1998, Fundamental of Engineering Thermodynamics, 3rd. Edi., John Wiley & Sons, New
York, US, p. 272-316
3. Guidebook to IEA ECBCS Annex 37, Low Exergy Systems for Heating and Cooling of Buildings, VTT, 2003. p.13-18
4. Yunus Cengel and Michael Boles, Thermodynamics an Engineering Approach, McGrawHill, 5th Ed., p. 424-465, 2006
5. Valero A. and Torres C., Exergy, Energy System Analysis and Optimization –
Vol. II., http://www.eolss.net/sample-
chapters/c08/e3-19-02-00.pdf, último acceso enero 2015
6. Dincer I. and Rosen M., Exergy, Energy, Environment and Sustainable Development, 2 nd Ed., ELSEVIER, UK, 2013
7. International Energy Agency, EBC Annual Report 2013, Energy in Buildings and Communities Programme, AECOM
8. Hepbasli A., Low exergy (LowEx) heating and cooling systems for sustainable buildings and societies, Science Direct,
Renewable and Sustainable Energy Reviews, Vol. 16, Issue 1, January 2012, Pages 73-104
9. S. de Oliveira Jr., Exer, Green Energy and Technology, Springer-Verlag, London 2013
http://www.sciencedirect.com/science/journal/13640321
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Gracias
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