1. Generation of Electricity
NBSLM01E Climate Change and Energy: Past, Present and Future
2010
Generation of Electricity
Basic 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
Maxine Narburgh
CSERGE 1 1

2. 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% 2 2

3. NO!! 8. Generation of Electricity - Conventional. Multi-stage Turbine
Superheated Steam oC
160 bar
Generator
Boiler
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?
Pump
Steam at ~ 0.03 bar
Condenser
Simplified Diagram of a “generating set”
includes boiler, turbine, generator, and condenser 3 3

4. Power Station 8. Generation of Electricity - Conventional
Chemical Energy
Power Station
Coal / Oil / Gas
100 units
Heat Energy
90 units
Boiler
90%
Turbine
48%
Mechanical Energy
41 units
Generator
95%
Electrical Energy
Electricity used in Station
3 units
38 units 4 4

5. 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? 5 5

6. 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 6 6

7. 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 7 7

8. 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
HSBC Director of Low Carbon Innovation 8 8

9. 9. Elementary Thermodynamics - History.
1) Boil Water > Steam
Problem:
Cylinder continually is cooled and heated.
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 9 9

10. 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 10 10

11. 9. Elementary Thermodynamics.
 Thermodynamics is a subject involving logical reasoning.
Much of it was developed by intuitive reasoning.
nd Law of Thermodynamics - Carnot
st Law of Thermodynamics - Joule
Zeroth Law - more fundamental - a statement
about measurement of temperature
Third Law - of limited relevance for this Course 11 11

12. Carnot’s reasoning 9. Elementary Thermodynamics.
Water at top has potential energy
Water at bottom has lost potential energy but gained kinetic energy 12 12

13. 9. Elementary Thermodynamics.
Carnot’s reasoning H1
Water looses potential energy
Part converted into rotational energy of wheel
Potential Energy = mgh H2
Theoretical Energy Available = m g (H1 - H2)
Practically we can achieve % of this 13 13

14. 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 14

15. 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 15

16. Schematic Representation
9. Elementary Thermodynamics.
Schematic Representation
of a Power Unit
First Law:
W = Q Q2
so efficiency
Heat In Q1
Work Out W
Heat Engine
Heat Out Q2
But Carnot saw that
Heat  Temperature
What do we mean by temperature?
Fahrenheit,
Reamur,
Rankine,
Celcius,
Kelvin?
Which should we use? 16 16

17. 9. Elementary Thermodynamics.
Is this a sensible definition of efficiency?
If T1 = 527oC ( = = 800K)
and T2 = oC ( = 300K)
Note: This is a theoretical MAXIMUM efficiency 17 17

18. 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% 18 18

19. 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:-
Thus "real" efficiency = carnot x isen
Typical values of isen are in range %
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. 19 19

20. 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? 20 20

21. 9. Elementary Thermodynamics.
( ) - ( )
Carnot efficiency = = %
so overall efficiency in power station:-
= x |
combustion
loss x |
Carnot
efficiency x |
Isentropic
efficiency x |
Generator
efficiency
0.94 |
Station
use
= 0.385 21 21

22. 10. Elementary Thermodynamics.
Transmission Loss ~ 91.5% efficient
Primary Energy Ratio for Coal ~
Overall efficiency
1 x x
= = units of delivered energy
1.02
i.e. 1 / = units of primary energy are needed to deliver 1 unit of electricity. 22 22

23. 9. Elementary Thermodynamics.
How can we improve Carnot Efficiency?
Increase T1 or decrease T2
If T2 ~ the efficiency approaches 100%
T2 cannot be lower than around oC i.e K
T1 can be increased, but properties of steam limit maximum temperature to around 600oC, (873K) 23 23

24. 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? 24

25. 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 25 25

26. 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 26 26 26

27. 9. Applications of Thermodynamics.
Combined Heat and Power
Engine
Generator 27

28. Schematic Representation
Working with Thermodynamics.
Heat Pumps
A Heat Pump is a reversed Heat Engine: NOT a reversed Refrigerator
Schematic Representation
of a Heat Pump
Heat Out Q1
Work IN W
Heat Pump
Heat In Q2
If T1 = 323K (50oC)
and T2 = 273K (0oC)
Schematic Representation of a Heat Pump.
IT IS NOT A REVERSED REFRIGERATOR. 28

29. Working with Thermodynamics. Heat Pumps and Refrigerators
A heat pump refrigerator consists of four parts:-
Condenser
Throttle
Valve
Compressor
Evaporator
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. 29

30. Responding to the Challenge: Technical Solutions
The Heat Pump
High Temperature
High Pressure
Condenser
Heat supplied to house
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. 30 30 30

31. Types of Heat Pump Heat Source air water ground Heat Sink air to air
water to air
ground to air
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 31

33. 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 33 33