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8.Generation of Electricity
9. Basic Thermodynamics
Maxine Narburgh
CSERGE
N.K. Tovey (杜伟贤 ) M.A, PhD, CEng, MICE, CEnv
Н.К.Тови М.А., д-р технических наук
Energy Science Director CRed Project
HSBC Director of Low Carbon Innovation
NBSLM01E Climate Change and Energy: Past, Present and Future
2010
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8. Generation of Electricity - Conventional
Diagram illustrates situation with coal, oil, or nuclear
Gas Generation is more efficient - overall ~ 45%
Overall efficiency ~ 35%
Largest loss in Power Station
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8. Generation of Electricity - Conventional.
Pump
Multi-stage Turbine
Generator
Boiler
Condenser
Simplified Diagram of a “generating set”
includes boiler, turbine, generator, and condenser
Superheated Steam 563oC 160 bar
Steam at ~ 0.03 bar
Why do we condense the steam to water only to heat it up again?.
Does this not waste energy?
NO!!
But we must wait until the Thermodynamics section to understand why?
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8. Generation of Electricity - Conventional
Chemical Energy
Coal / Oil / Gas
Electrical Energy
Heat EnergyBoiler
Turbine
GeneratorMechanical Energy
Electricity used in Station
Power Station100 units
38 units
90 units
3 units
90%
95%
48%
41 units
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Why not use the heat from power station? - it is typically at 30oC?
Too cold for space heating as radiators must be operated ~ 60+oC
What about fish farming - tomato growing?
- Yes, but this only represent about 0.005% of heat output.
Problem is that if we increase the output temperature of the heat from the power station we get less electricity.
Does this matter if overall energy supply is increased?
8. Generation of Electricity - Conventional.
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8. Generation of Electricity - CHP
Overall Efficiency - 73%
•Heat is rejected at ~ 90oC for supply to heat buildings.
•City Wide schemes are common in Eastern Europe
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1947 Electricity Act blinked our approach for 35 years into attempting to get as much electricity from fuel rather than as much energy.
Since Privatisation, opportunities for CHP have increased
on an individual complex basis (e.g. UEA), unlike Russia
A problem: need to always reject heat.
What happens in summer when heating is not required?
Need to understand basic thermodynamics
8. Generation of Electricity - Conventional.
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9. Introduction to Thermodynamics
N.K. Tovey (杜伟贤 ) M.A, PhD, CEng, MICE, CEnv
Н.К.Тови М.А., д-р технических наук
Energy Science Director CRed Project
HSBC Director of Low Carbon Innovation
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NBSLM01E Climate Change and Energy: Past, Present and Future
2010
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9. Elementary Thermodynamics - History.
Newcomen Engine
pushes piston up
3) At end of stroke, close steam value open injection valve
(and pumping rod down)
4) Water sprays in condenses steam in cylinder creating a vacuum and sucks piston down - and pumping rod up
2) Open steam valve
1) Boil Water > SteamProblem:
Cylinder continually is cooled and heated.
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9. Elementary Thermodynamics - Watt Engine.
Watt Engine
1) Cylinder is always warm
2) cold water is injected into condenser
3) vacuum is maintained in condenser so “suck” out exhaust steam.
4) steam pushes piston down pulling up pumping rod.
Higher pressure steam used in pumping part of cycle.
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9. Elementary Thermodynamics.
Thermodynamics is a subject involving logical reasoning.
Much of it was developed by intuitive reasoning.
• 1825 - 2nd Law of Thermodynamics - Carnot
• 1849 - 1st Law of Thermodynamics - Joule
• Zeroth Law - more fundamental - a statement about measurement of temperature
• Third Law - of limited relevance for this Course
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9. Elementary Thermodynamics.
Carnot’s reasoning
Water at top has potential energy
Water at bottom has lost potential energy but gained kinetic energy
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9. Elementary Thermodynamics.
Carnot’s reasoning
Water looses potential energy
Part converted into rotational energy of wheel
Potential Energy = mgh
• Theoretical Energy Available = m g (H1 - H2)
• Practically we can achieve 85 - 90% of this
H1
H2
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9. Elementary Thermodynamics.
Carnot’s reasoning
Temperature was analogous to Head of Water
• Energy Temperature Difference
• Energy (T1 - T2)
• T1 is inlet temperature
• T2 is outlet temperature
• Just as amount of water flowing in = water flowing out.
• Heat flowing in = heat flowing out.
• In this respect Carnot was wrong
• However, in his day the difference was < 1%14
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9. Elementary Thermodynamics.
Joule 1849
• Identified that “Lost” Heat = energy out as Work
• Use a paddle wheel to stir water - the water will heat up
• Mechanical Equivalent of Heat
Berlin Demonstration
Symbols
W - work Q - heat
Over a complete cycle
Q = W
Heat in +ve Heat out -ve
Work in -ve Work out +ve
FIRST LAW: “You can’t get something for nothing”15
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9. Elementary Thermodynamics.
Schematic Representation of a Power Unit
Heat Engine
Heat In Q1
Heat Out Q2
Work Out W
First Law:
W = Q1 - Q2
so efficiency
1
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Q
InHeat
OutWork
But Carnot saw that
Heat Temperature
1
21
T
TT
• What do we mean by temperature?
• Which should we use?
Kelvin?Rankine,Reamur,Fahrenheit,Celcius,
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9. Elementary Thermodynamics.
1
21
T
TT
Is this a sensible definition of efficiency?
%Q
QQQ
Q
QW
InHeat
OutHeatOutWorknotwhy
1001
221
1
2
If T1 = 527oC ( = 527 + 273 = 800K)
and T2 = 27oC ( = 300K)
%5.62800
300800
Note: This is a theoretical MAXIMUM efficiency
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9. Elementary Thermodynamics.
Second Law is more restrictive than First
“It is impossible to construct a device operating in a cycle which exchanges heat with a SINGLE reservoir and does an equal amount of work on the surrounds”
This means Heat must always be rejected
Second Law cannot be proved
- fail to disprove the Law
If heat is rejected at 87oC (360K)
%0.55800
360800
By keeping T2 at a potentially useful temperature, efficiency has fallen from 62.5%
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9. Elementary Thermodynamics.
The Practical efficiency will always be less than the Theoretical Carnot Efficiency.
To obtain the "real" efficiency we define the term Isentropic Efficiency as follows:-
EfficiencyCarnotfrompredictedwork
outworkactualisen
Thus "real" efficiency = carnot x isen
Typical values of isen are in range 75 - 80%
Hence in a normal turbine, actual efficiency = 48%
A power station involves several energy conversions. The overall efficiency is obtained from the product of the efficiencies of the respective stages.
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9. Elementary Thermodynamics.
EXAMPLE:
In a large coal fired power station like DRAX (4000MW), the steam inlet temperature is 566oC and the exhaust temperature to the condenser is around 30oC.
The combustion efficiency is around 90%, while the generator efficiency is 95% and the isentropic efficiency is 75%.
If 6% of the electricity generated is used on the station itself, and transmission losses amount to 5% and the primary energy ratio is 1.02, how much primary energy must be extracted to deliver 1 unit of electricity to the consumer?
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9. Elementary Thermodynamics.
(566 + 273) - (30 + 273)
Carnot efficiency = ------------------------------ = 63.9%
566 + 273
so overall efficiency in power station:-
= 0.9 x
|
combustion loss
0.639 x
|
Carnot efficiency
0.75 x
|
Isentropic efficiency
0.95 x
|
Generator efficiency
0.94
|
Station use
= 0.385
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10. Elementary Thermodynamics.
Transmission Loss ~ 91.5% efficient
Primary Energy Ratio for Coal ~ 1.02
Overall efficiency
1 x 0.385 x 0.915 = -------------------------- = 0.345 units of delivered energy 1.02
i.e. 1 / 0.345 = 2.90 units of primary energy are needed to deliver 1 unit of electricity.
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9. Elementary Thermodynamics.
How can we improve Carnot Efficiency?
Increase T1 or decrease T2
If T2 ~ 0 the efficiency approaches 100%
T2 cannot be lower than around 0 - 30oC i.e. 273 - 300 K
T1 can be increased, but properties of steam limit maximum temperature to around 600oC, (873K)
1
21
T
TT
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9. Elementary Thermodynamics.
In this part of the lecture we shall explore ways to improve efficiency
We need to work with thermodynamics rather than against it
The most important equation:
What if we could use Q2 effectively?
1
21
T
TT
1
21
Q
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9. Applications of Thermodynamics - CHP
Overall Efficiency - 73%
•Heat is rejected at ~ 90oC for supply to heat buildings.
•City Wide schemes are common in Eastern Europe
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• Pipes being laid in streets in Copenhagen
• Most towns in Denmark have city wide schemes such as these
• Pipes like these were recently laid in UEA to new Thomas Paine Building
Ways to Respond to the Challenge: Technical IssuesCombined Heat and Power
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9. Applications of Thermodynamics.
Combined Heat and Power
Engine Generator
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Working with Thermodynamics.
Heat Pumps
Schematic Representation of a Heat Pump. IT IS NOT A REVERSED REFRIGERATOR.
Schematic Representation of a Heat Pump
Heat Pump
Heat Out Q1
Heat In Q2
Work IN
W
A Heat Pump is a reversed Heat Engine: NOT a
reversed Refrigerator
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1
Q
InWork
OutHeatCOP
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1
TT
TCOP
If T1 = 323K (50oC)
and T2 = 273K (0oC)
46.6273323
323COP
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Working with Thermodynamics.
A heat pump refrigerator consists of four parts:-
Heat Pumps and Refrigerators
1) an evaporator (operating under low pressure and temperature)
3) a condenser (operating under high pressure and temperature)
4) a throttle value to reduce the pressure from high to low.
2) a compressor to raise the pressure of the working fluid
Throttle Valve
Compressor
Condenser
Evaporator
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Throttle Valve
Condenser
Heat supplied to house
Evaporator
Heat extracted from outside
Low TemperatureLow Pressure
High TemperatureHigh Pressure
Responding to the Challenge: Technical SolutionsThe Heat Pump
Any low grade source of heat may be used• Typically coils buried in garden• Bore holes• Example of roof solar panel
Compressor
A heat pump delivers 3, 4, or even 5 times as much heat as electricity put in. We are working with thermodynamics not against it.
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Types of Heat Pump
Heat Source
air water ground
HeatSink
air air to air water to air
ground to air
water air to water
water to water
ground to water
solid air to solid water to solid
ground to solid
For Space Heating Purposes: The heat source with water and the ground will involve laying coils of pipes in the relevant medium passing water, with anti-freeze to the heat exchanger. In air-source heat pumps, air can be passed directly through the heat exchanger.
For Process Heat Schemes: the source may be a heat exchanger in the effluent of one process
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Keith Tovey ( 杜伟贤 ) Н.К.Тови M.A., PhD, CEng, MICE, CEnvEnergy Science Director: Low Carbon Innovation Centre
School of Environmental Sciences, UEA. Rotary Club of Norwich
Recipient of James Watt Gold Medal
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