Exergy Analysis of Renewable Energy Systems

17 January 2015

Afyon

Arif Hepbasli

Department of Energy Systems Engineering

Yasar University, Izmir, Turkey

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Outline (61 slides)

1. Objective

2. Introduction

3. Historical Development of Exergy Concept

4. What is exergy ?

5. Why exergy ?

6. Dead State

7. Driving Force

8. Exergy efficiency

9. Sankey and Grassmann diagrams

10. Modeling

11. An Illustrative Example

12. Conclusions

LET US COME TO AN AGREEMENT!!!

My speech will not be on the pure technical basis.

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1. Objective

A Sketch Showing the Meaning of RESs

RESs are money mints.

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2. Introduction

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ENERGY EXERGY Unit energy price - Unit exergy price

Specific energy production - Specific exergy

production

Energy analysis - Exergy analysis

Energy efficiency – Exergy efficiency

Energy management – Exergy management

Energy consultant - Exergy consultant Specific moisture extraction - Specific moisture

exergetic rate (SMER) index (SMEI) Number of publications ???????

Footprints…

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Carbon footprint

Water footprint

h-index

Impact-Citations-Exergy (ICEx)

Fractionalized exergy for evaluating research performance (X)

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Source: Prathap, G. JOURNAL OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE AND TECHNOLOGY, 62(11):2294–2295, 2011

By fractional counting we mean that instead of

counting each citation as unity, we consider it as weighted

(fractionated) in terms of the number of references in the citing

article.

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Exergy Consulting Services

Dr. Wall (in Sweden):www.exergy.se

Dr. Cornelissen (Holland):www.exergy.nl

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Engineers and scientists dealing with the design and

operation of an energy conversion plant want to improve

or optimize it by maximizing efficiency and minimizing

(a) product cost and (b) environmental impact

associated with this plant.

In order to effectively deal with these problems, we

must understand the real mechanisms according to

which thermodynamic inefficiencies, costs, and

environmental impacts are formed.

Source: G. Tsatsaronis 9

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In the improvement phase, the following questions arise:

● Where do real thermodynamic inefficiencies occur, how high are they, and what causes them ?

(Exergy analysis)

● What measures or what alternative designs would improve the efficiency of the overall plant ?

(Exergy analysis, but a conventional one is usually not sufficient)

● How high are the total investment and the equipment costs of the major plant components ?

(Economic analysis)

Source: G. Tsatsaronis

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● How high are the cost and the environmental impact

associated with thermodynamic inefficiencies ?

(Exergoeconomic and exergoenvironmental analyses )

●How high are (a) the environmental impacts associated

with the major plant components, and (b) the

overall environmental impact ?

(LCA and exergoenvironmental analysis)

Source: G. Tsatsaronis 11

A conventional exergetic, conventional exergoeconomic and conventional exergoenvironmental analysis does not evaluate the mutual interdependencies among the system components nor the potential for improving a component.

These issues are considered in advanced analyses, in which the exergy destruction, capital investment cost and construction-of-component-related environmental impact in each component are split into:

● endogenous and exogenous parts,

● unavoidable and avoidable parts, and

● the resulting combined parts.

12 Advanced (or enhanced) analyses

Source: G. Tsatsaronis 12

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Nicolas Léonard Sadi Carnot: Sadi Carnot In his only publication, the 1824 monograph Reflections on the Motive Power of Fire, Carnot gave the first successful theory

of the maximum efficiency of heat engines.

Source: http://en.wikipedia.org/wiki/Nicolas_L%C3%A9onard_Sadi_Carnot, Access date: 17 January 2015.

1824’de ısı makinaları üzerine çalışmasını yayınladı.

What is his birthday ? Doğum yılı nedir ?

What is the magic of number 28 ?

“28” sayısınin sihiri nedir ?

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Kaynak: Yantovski, E.

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Five Equations That Changed the World

http://en.wikipedia.org/wiki/Five_Equations_That_Changed_t

he_World

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Comparison and evaluation of the energy systems Energy analysis Exergy analysis Economic analysis Life cycle assessment Exergoeconomic analysis Exergoenvironmental analysis Exergetic life cycle assessment Advanced exergy analysis Advanced exergoeconomic analysis Advanced exergoenvironmental analysis

17 Our understanding

Energy analysis Conventional exergetic analysis

Sensitivity analysis Advanced exergetic analysis

Source: G. Tsatsaronis 17

3. Historical Development of Exergy Concept

In 1824, Carnot started that the extractable work of a heat engine is proportional to the temperature difference, which later led to the definition of the second law of thermodynamics.

In 1873, Gibbs introduced the nation of available work, including the diffusion term.

In 1953, Keenan interpreted exergy as “available” energy.

In 1953, Rant suggested the term exergy to denote

“technical working capacity”.

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Wall and Gong has reported the historical development of the concept “exergy” as follows:

In 1961, Tribus linked classical thermodynamics with information theory, through the entropy and the exergy concepts.

At a conference in Rome in 1987, it was agreed among the participants to encourage strongly the use of exergy for the general concept of the potential to cause change,

in lieu of terms such as

availability, available energy, essergy, utilizable

energy, work potential, available work, convertible energy, etc.

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The quality of energy The capacity of energy to cause change

The maximum work that can be obtained from a

given form of energy using the environmental parameters as the reference state

A measure of the departure of the state of the system from the state of the environment

Sources: Leskinen, M. Low Exergy Sources for Heating and Cooling & IEA Annex 37 Tsatsaronis, G and Cziesla, F. Thermoeconomics, 2003.

4. What is Exergy ?

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Where is Exergy ?

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It is in my pocket.

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What happens when we squeeze the tooth paste tube ?

Could we put the paste back in the tube again after coming out ?

The entropy of a system increases when exergy is lost.

Source: http://www.holon.se/folke/kurs/Distans/Ekofys/fysbas/exergy/exergybasics.shtml

To more easily understand the concept of exergy, you can consider the previous picture as an analogy: You buy is the (toothpaste) tube, but have to squeeze it to get at what you really need, the toothpaste. When the tube is empty of paste (exergy) the tube is still there, the same amount as when you bought it.

In these circumstances, the word entropy often comes up. In the picture this is represented as the depression in the tube. The depression increases as the amount of paste diminishes, but the depression is not a negative paste (You can not take the depression and unbrush your teeth !).

Entropy is not negative exergy, but another description of the system. Furthermore, it is not defined in far-from-equilibrium systems, as living systems and other organised systems.

26 Source: http://www.holon.se/folke/kurs/Distans/Ekofys/fysbas/LOT/LOT.shtml, Access date: 13 Feb. 2011.

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Differences between Energy and ExergyAnalysis

Source: LOWEX Guidbook

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Source: Helga Ferket, Ben Laenen & Peter Van Tongeren. Low-exergy applications of low-enthalpy

geothermal prospects . 12 Oct. 2009.

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Symbols for exergy and exergetic efficiency used in textbooks [initially compiled by Noam Lior]

E

exergy

[J]

the specific

exergy

function [J/kg]

exergy

destruction

exergetic

efficiency

Textbook

reference

b Irreversibility Keenan

Λ Φ, b I Hatsopoulos &

Keenan, 1965

Kotas, 1985

Moran

E Ed Moran and

Shapiro

Ex for open

systems, Ξ

for closed

b for open

systems, a for

closed

Wlost ηII Bejan

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(contnd.) Symbols for exergy and exergetic efficiency used in textbooks [initially

compiled by Noam Lior]

E

exergy

[J]

the specific exergy

function [J/kg]

exergy

destruction

exergetic

efficiency

Textbook

reference

E e ED Bejan, Tsatsaronis,

and Moran

B δb ηB, ηp Szargut et al.

X I, Xdestroyed ηII Cengel and Boles

Ψ, ф η2nd Anderson

Ψ, Ω ε Gyftopoulos and

Beretta

E Bosnjakovic

ηII Sussman

ф I η2nd law Sontag,

Borgnakke, van

Wylen

Ė (only

for

streams)

ĖV

Baehr 31

Exergy and Anergy

Exergy = “Useful” energy = Q [1-(T0/Th)]

Anergy = “Useless” energy = Q (T0/Th)

Given: an amount of material with temperature Th Energy content = Q

Q Energy = Exergy + Anergy

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5. Why Exergy ? An effective method using the conservation of mass and

conservation of energy principles together with the SLT for the design and analysis of energy systems.

A primary tool in best addressing the impact of energy resource utilization on the environment.

A suitable technique for furthering the goal of more efficient energy-resource use.

A crucial key for determining the locations, types, and true magnitudes of wastes and losses.

An efficient technique revealing whether or not and by how much it is possible to design more efficient energy systems by reducing the inefficiencies.

A key component in obtaining sustainable development.

Source: I. Dincer

Exergy analysis of the thermal systems allows the designer to: a) assess the influence of every process (component)

of the system on the overall efficiency;

b) eliminate the major processes (components) of the system that diminish its performance;

c) definite the maximum value of the system efficiency with respect to the best performance of the assumed process (component); and

d) identify methods for improving the effectiveness of a power plant.

Source: I. Dincer

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Energy and Exergy Flow Diagrams

Source: Wall, G; Zvolinschi, A.

6. Dead (Reference) State Definition

It should be noticed that exergy is always evaluated with respect to a reference environment (i.e. dead state).

When a system is in equilibrium with the environment, the state of the system is called the dead state due to the fact that the exergy is zero. At the dead state, the conditions of mechanical, thermal, and chemical equilibrium between the system and the environment are satisfied: the pressure, temperature, and chemical potentials of the system equal those of the environment, respectively.

In addition, the system has no motion or elevation relative to coordinates in the environment. Under these conditions, there is neither possibility of a spontaneous change within the system or the environment nor an interaction between them. The value of exergy is zero..

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Dead (Reference) State Definition (Cont’d)

Another type of equilibrium between the system and environment can be identified. This is a restricted form of equilibrium, where only the conditions of mechanical and thermal equilibrium (thermomechanical equilibrium) must be satisfied. Such state is called the restricted dead state.

At the restricted dead state, the fixed quantity of matter under consideration is imagined to be sealed in an envelope impervious to mass flow, at zero velocity and elevation relative to coordinates in the environment, and at the temperature T0 and pressure P0 taken often as 25 oC and 1 atm.

The selection of dead state conditions is arbitrary, but depends on some criteria.

Please note that we will call only the dead state throughout this lecture.

Source: Moran MJ. Availability analysis: a guide to efficiency energy use. Englewood Cliffs,

NJ: Prentice-Hall; 1982. 37

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7. Driving Force

The impetus, power, or energy behind something in motion, as in He was clearly the driving force in the new administration. This term transfers the force that sets in motion an engine or vehicle to other enterprises. Ralph Waldo Emerson was among the first to use it figuratively

(English Traits, 1856): “The ability of its journals is the driving force.”

Source: http://dictionary.reference.com/browse/driving+force

What is the driving force for the heat transfer, electric current and fluid flow ?

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7. Driving Force

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8. Exergy Efficiency

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Exergy Efficiency = Product/Fuel

Exergy Efficiency = Desired Effect/Fuel

Exergy Efficiency = Benefit /Fuel

Exergy Efficiency = Product /Source

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http://upload.wikimedia.org/wikipedia/commons/1/10/JIE_Sankey_V5_Fig1.png

The first

Sankey

diagram

9. Sankey and Grassmann diagrams

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http://www.sei.ie/uploadedfiles/CHP/Sankey%20diagram2.png

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In this diagram, called a Sankey Diagram, we can see that of 72 kW of power from the fuel, only 9 kW are used in actually driving a car along a road. The rest is lost as low grade heat.

http://www.antonine-education.co.uk/Physics_A2/Options/Module_7/Topic_5/Sankey.gif

Exergy balance diagram

(Grassmann diagram)

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Mass balance equation:

General energy balance:

General exergy balance:

Other balance equations ???

Exergy efficiency:

10. Modelling

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Total exergy of a system:

Exsys= ExPH+ ExKN+ ExPT+ ExCH

Total specific exergy on a mass basis:

exsys= exPH+ exKN+ exPT+ exCH

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11. An Illustrative Example

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12. Conclusions

Exergy analysis is an effective method, using the conservation of mass and conservation of energy principles together with the SLT, that can be employed for the design and analysis of thermal systems.

It is therefore an efficient technique revealing whether or not and by how much it is possible to design more efficient thermal systems by reducing the inefficiencies.

Illustrative example is presented to highlight the importance of understanding and considering exergy as a potential tool.

The potential usefulness of exergy analysis in addressing and solving environmental problems is substantial.

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THANK YOU VERY MUCH FOR YOUR STAYING HERE TILL THE END

QUESTIONS ???

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I QUESTIONS ???

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