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EXERGY-101

Prof. Dr. Birol I. KILKIS, Fellow ASHRAE

Birol Kilkis © 2006

Prof. Dr. Birol I. KILKIS, Fellow ASHRAEDistinguished Lecturer

birolkilkis@hotmail.com

May not be distributed or transmitted except intended recipients

WHAT IS EXERGY?

Exergy is a measure of the quality, diversity, and the usefulness of the maximum work potential of an energy

source relative to an equilibrium state in the environment.

Consider a fully charged electric battery and a pot of 10 lb of water at 110oF heated from 40oF. The battery and the pot have the same amount

The answer is battery!

There is a variety of useful work possibilities, like starting an engine, running a hybrid car or electric heat pump etc.

With the warm water you may just wash your hands, prepare tea, do dishes but not obtain a more useful work.

y pof energy (700 Btu), but different exergy. Which one has higher exergy?

700 Btu

700 Btu

High Exergy Low Exergy

Let us see what we mean by low-exergy and high-exergy source

THERMAL ENERGY AND EXERGY

Consider again a pot of 10 lb of well water heated to 110oF. The thermal energy Q of this heat source is 10 lb x (110oF- 40oF) = 700 Btu.

The maximum useful work potential W of this heat source using a Carnot Heat Engine is small:

Btu 8470012.07004601104604011

Q

TT

W reference

Tsource = 110oFQ = 700 Btu Treference= 40oC

460110 Tsource

That is to say, only 84 Btu may ideally be converted to useful work. This is the exergy of the pot of water. It is therefore, a low-exergy source. While water cools down to room temperature, part of the remaining thermal energy (Q - W ) may somehow heat that room. In other words, this energy source at its best is useful for space heating. Electric generation or mechanical useful work is quite unlikely.

Unlikely Q - W

WELL

W

EXERGY OF THE ELECTRIC BATTERY

Exergy of the battery is almost equal to the electric work potential, and almost all of it can be converted to useful work with minor energy losses. For example, we can drive an electric motor and use the mechanical

f f l k Th f thienergy for any useful work. Therefore, this is a high-exergy source.

Very Likely

But we should question where this exergy first came to the battery. The answer will later appear in this presentation!

Where did the electric energy come from ?

W

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source

referencesource T

TExergy 1

It is proportional to the ratio of environment reference temperature to the energy source temperature .

For example, exergy of natural gas combustion at 1700 R when referenced to a soil temperature of 500 R (40oF) is:

BACK TO THE EXERGY OF A HEAT SOURCE

706.017005001

sourceExergy

p ( )

In another example, exergy of waste heat source at 510 R (50oF) is:

019.05105001

sourceExergy

High-Exergy Source

Low-Exergy Source

This high-exergy source has a potential of diverse applications, like electric power generation, metallurgy, steam production etc.

For example, exergy required in a natural gas thermo-electric plant operating between 1700oR and 900oR:

11

2

TTExergyrequired

EXERGY REQUIRED BY A THERMAL PROCESSBased on a Carnot Cycle, it is a function of temperatures T1 and T2 that the application operates in between:

operating between 1700 R and 900 R:

High-exergy required

1700 R

900 R

500 R Was

ted

(des

troy

ed)

Exer

gy re

quire

d

Exer

gyso

urce Exergy efficiency is a measure of how the source

exergy matches with the required exergy in a given application:

665.0706.0470.0

source

required

ExergyExergy

iciencyExergy Eff

0.470 17009001

requiredExergy

From the exergy perspective, this is a good match

EXERGY REQUIRED FOR HEATING A BUILDING

In another example, minimum exergy required for heating a building to 528 R (68oF) when referenced to a soil temperature of 500 R (40oF):

053.05285001

requiredExergy Low-exergy

required

When we use natural gas just for heating the building, exergy efficiency is very low, because we waste most of the source exergy:

1700 R

500 R

Surp

lus

(des

troy

ed e

xerg

y)

Exer

gyso

urce

528 R

075.0706.0053.0

source

required

ExergyExergy

iciencyExergy Eff

supplied

From the exergy perspective, this is a poor match

THE EXERGY OF THE SOURCE AND THE EXERGY REQUIRED MUST MATCH

Otherwise exergy is wasted.

Source Exergy

Source Temp.

Required Exergy

Application Temp.

HIGHELECTRIC

METALURGY

Process Heat

STEAM

FOOD IND.

Exergy Waste (destroyed) Greenhouse.

HEATING.

Radiant Heat

Exer

gy S

urpl

us

(des

troy

ed)

INDIVIDUAL APPLICATIONS

HIGH

LOW

EXER

GY

Ambient Temp.0

HIGH

EXER

GY

LOW

0

For example, if we use natural gas just for heating a greenhouse, there is a large exergy surplus and it is eventually wasted.

In the same token, if we use natural gas just for electric power, a portion of available exergy is wasted.

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WE MUST MINIMIZE EXERGY WASTE

FOR THE RATIONAL UTILIZATION OF ENERGY RESOURCES

FOR SUSTAINABLE DEVELOPMENT

Source Exergy

Source Temp. Required Exergy

Application Temp.

FIRST STEP: GROUP AND CASCADE INDIVIDUAL APPLICATIONS

HIGHHIGH

ALL INDIVIDUAL APPLICATIONSBECOME COLLECTIVE APPLICATIONS…………………..

IDEALLY SPEAKING…

… NO EXERGY IS WASTED

EXER

GY

0

LOW

HIGH

EXER

GY

LOW

0

energy

environment

energy

environment

EXERGY FOR HARMONY AND SUSTAINABLE DEVELOPMENT

humankind economy

energy

humankind economy

ON OUR EXERGY CANVAS…THIS IS AN ONGOING QUEST. WE MUST KEEP EXPANDING THIS PUZZLE WITH NEW TECHNOLOGIES WHILE FACING NEW CHALLENGES.

WE MUST WEAVE PIECES OF THE SUSTAINABILITY PUZZLE TOGETHER

SECOND STEP: BRIDGE THE SOURCE AND APPLICATION

IN AN EXERGY AND ENERGY- WISE MANNER

Once the energy sources and applications are matched, grouped and cascaded, we must link them with exergy and energy efficient systems

and equipment.

Now lets follow a concluding example

Exergy Source Minimum

Exergy Required by Applications

System and Equipment

Exergy Transport

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COMPARE FLOOR HEATING USING THE BATTERY OR THE POT OF WARM WATER

Electric mat heating has lower exergy efficiency.

Warm water heating has high exergy efficiency because it matches with floor heating better.

EXAMPLE

It can be increased by using an electric heat pump.

BUT WAIT! WHERE DOES THE EXERGY ORIGINATE?

High Exergy

Efficiency

High Exergy

Efficiency

Very Low Exergy

EfficiencyVery Low Exergy

Efficiency

110oF

Q =700 Btu

SUMMARY

• Energy Efficiency is the ratio of useful work output to the energy input of a system. It does not recognize where the input energy comes from and where the energy waste goes to. It considers the thermodynamic path of energy flow in the system only. Energy efficiency checks the source just for its ability to do a required work.

• Exergy Efficiency is a measure of how exergy of the energy source matches with• Exergy Efficiency is a measure of how exergy of the energy source matches with the minimum exergy required by the application. It checks the quality and availability of the source for a given application and shows how environmentally rational this matching is. Temperatures of the energy source, the application, and the reference state in the environment are important to determine the exergy efficiency. Exergy Efficiency;

1. Helps us to select, match, group and cascade energy sources and applications for better sustainability.

2. Together with energy efficiency, helps us to determine the best manner to transfer exergy from the source to the application.

ILLUSTRATIVE CARTOONS

A single driver in SUV.

A bad habit of wasting exergy.

A school bus, full of children.

A good use of exergy.

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Hungry

professor

Useful knowledge is exergy.

Exergy of a full-time professor is properly utilized.

Unemployed professor’s knowledge is not utilized. Exergy is wasted.