Exergy Termo III

8/18/2019 Exergy Termo III

1/22

ESCUELA POLITÉCNICA NACIONAL

FACULTAD DE INGENIERÍA MECÁNICA

Termodinámica III

Exergy

Ing. José Luis Palacios E., M.Sc.

Quito, 08 de Enero 2014 1

[1]


2/22



Termodinámica III

Exergy

2

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]


3/22



Termodinámica III

Exergy

3

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]


4/22



Termodinámica III

Exergy definition

4

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]

[4]


5/22



Termodinámica III

Exergy

5

[2]


6/22



Termodinámica III

Modeling the envinronment

6[2]

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]


7/22



Termodinámica III

Evaluating Exergy

7

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.

[2]


8/22



Termodinámica III

Evaluating Exergy

8

[2]

irreversibilities

Exergy of a system

[4]


9/22



Termodinámica III

Energy and Exergy analysis

9

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.

[6]


10/22



Termodinámica III

Energy and Exergy analysis

10

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]


11/22



Termodinámica III

Exergy equations

11

Closed Systems

Energy:

Exergy:

Contro l Volume

Energy:

Exergy:


12/22



Termodinámica III

Example

12

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.


13/22



Termodinámica III

Example

13


14/22



Termodinámica III

Exergy transfer by heat

14[4]


15/22



Termodinámica III

Exergetic (Second Law) efficiency

15

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

[2]


16/22



Termodinámica III

Exergetic (Second Law) efficiency

16

Exergetic Efficiency < 1[2]


17/22



Termodinámica III

17

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

[2]


18/22



Termodinámica III

Energy Quality

18

[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.

[8]


19/22



Termodinámica III

Exergy - Grassmann diagram

19[9]


20/22



Termodinámica III

20

“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

[4]


21/22



Termodinámica III

Referencias

21

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


22/22



Termodinámica III

Gracias

22

Date post:	06-Jul-2018
Category:	Documents
Upload:	johnny-mauricio
View:	236 times
Download:	1 times

Exergy Termo III

Documents