NBSLM01E Climate Change and Energy:
Past, Present and Future
2010
8.Generation of Electricity
9. Basic Thermodynamics
N.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnv
Н.К.Тови М.А., д-р технических наук
Maxine Narburgh
Energy Science Director CRed Project
1
HSBC Director of Low CarbonCSERGE
Innovation
8. Generation of Electricity - Conventional
Largest loss in
Power Station
Overall efficiency ~ 35%
Diagram illustrates situation with coal, oil, or nuclear
Gas Generation is more efficient - overall ~ 45%
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8. Generation of Electricity - Conventional.
Superheated Steam
563oC
160 bar
Multi-stage
Turbine
Generator
Boiler
Why do we condense the steam to water only
to heat it up again?.
Does this not waste energy?
Pump
NO!!
Steam at ~
0.03 bar
Condenser
But we must wait until
the Thermodynamics
section to understand why?
Simplified Diagram of a “generating set”
includes boiler, turbine, generator, and condenser
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8. Generation of Electricity - Conventional
Chemical
Energy
Coal / Oil /
Gas 100 units
Power Station
Heat Energy 90 units
Boiler
90%
Turbine
Generator 95%
Electrical
Energy
48%
41 units
Mechanical Energy
Electricity used in Station
3 units
38 units
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8. Generation of Electricity - Conventional.
 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?
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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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8. Generation of Electricity - Conventional.
 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
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NBSLM01E Climate Change and Energy:
Past, Present and Future
2010
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
8
HSBC Director of Low Carbon Innovation
9. Elementary Thermodynamics - History.
Problem:
Cylinder continually is
cooled and heated.
1) Boil Water > Steam
2) Open steam valve
pushes piston up
(and pumping rod down)
3) At end of stroke, close steam
value open injection valve
4) Water sprays in condenses steam
in cylinder creating a vacuum and
sucks piston down - and pumping
rod up
Newcomen Engine
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9. Elementary Thermodynamics - 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.
Watt Engine
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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
H1
Part converted into
rotational energy of
wheel
Potential Energy = mgh
H2
• Theoretical Energy Available = m g (H1 - H2)
• Practically we can achieve 85 - 90% of this
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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%
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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
Over a complete cycle
Q =
W
Heat in +ve
Q - heat
Heat out -ve
Work in -ve
Work out +ve
FIRST LAW: “You can’t get something for nothing”
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9. Elementary Thermodynamics.
First Law:
Heat In
Q1
W = Q 1 - Q2
so efficiency
Heat Engine
Work Out W

Heat Out Q2
Work Out
Heat In

Q1  Q 2
Q1
But Carnot saw that
Schematic Representation
of a Power Unit
Heat
• What do we mean by temperature?
Celcius,
Fahrenheit, Reamur,

Temperature
T1  T2

T1
Rankine,
Kelvin?
• Which should we use?
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9. Elementary Thermodynamics.
T1  T2

T1
Is this a sensible definition of efficiency?
If T1 = 527oC ( = 527 + 273 = 800K)
and T2 =
27oC ( = 300K)
800  300

 62.5%
800
Note: This is a theoretical MAXIMUM efficiency
Work Out  Heat Out W  Q 2
why not  

Heat In
Q1
Q1  Q 2  Q 2

 100%
Q1
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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)
By keeping T2 at a
potentially useful
temperature, efficiency
has fallen from 62.5%
800  360

 55.0%
800
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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:-
isen
actual work out

work predicted from Carnot Efficiency
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
0.639
|
x
0.75
|
x
0.95
x
|
combustion Carnot
Isentropic Generator
loss
efficiency efficiency efficiency
0.94
= 0.385
|
Station
use
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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.
T1  T2

T1
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)
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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:
T1  T2

T1
Q1  Q2

Q1
What if we could use Q2 effectively?
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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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Ways to Respond to the Challenge: Technical Issues
Combined Heat and Power
• 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
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9. Applications of Thermodynamics.
Combined Heat and Power
Engine
Generator
27
Working with Thermodynamics.
Heat Pumps
Heat
Out Q1
Heat
Pump
Work IN
W
Heat In
Q2
Schematic Representation
of a Heat Pump
A Heat Pump is a reversed
Heat Engine: NOT a
reversed Refrigerator
COP 
Heat Out
Work In
COP 

Q1
Q1  Q 2
T1
T1  T2
If T1 = 323K (50oC)
and T2 = 273K (0oC)
323
COP 
 6.46
323  273
Schematic Representation of a Heat Pump.
IT IS NOT A REVERSED REFRIGERATOR.
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Working with Thermodynamics.
Heat Pumps and Refrigerators
A heat pump refrigerator consists of four parts:-
Throttle
Valve
Condenser
Evaporator
Compressor
1) an evaporator (operating under low pressure and temperature)
2) a compressor to raise the pressure of the working fluid
3) a condenser (operating under high pressure and temperature)
4) a throttle value to reduce the pressure from high to low.
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Responding to the Challenge: Technical Solutions
The Heat Pump
Heat supplied
to house
High Temperature
High Pressure
Condenser
Throttle
Valve
Compressor
Evaporator
Heat extracted
from outside
Low Temperature
Low Pressure
Any low grade source of heat may be used
• Typically coils buried in garden
• Bore holes
• Example of roof solar panel
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
air
Heat
Sink
air
air to air
water
air to
water
solid
air to solid
Heat Source
water
water to
air
water to
water
water to
solid
ground
ground to air
ground to
water
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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Recipient of James Watt Gold Medal
Keith Tovey (杜伟贤) Н.К.Тови M.A., PhD, CEng, MICE, CEnv
Energy Science Director: Low Carbon Innovation Centre
School of Environmental Sciences, UEA. Rotary Club of Norwich
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