EXERGY-101 usefulness of the maximum work potential of an ... 3/Exer… · Exergy is a measure of...
Transcript of EXERGY-101 usefulness of the maximum work potential of an ... 3/Exer… · Exergy is a measure of...
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EXERGY-101
Prof. Dr. Birol I. KILKIS, Fellow ASHRAE
Birol Kilkis © 2006
Prof. Dr. Birol I. KILKIS, Fellow ASHRAEDistinguished Lecturer
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.