Aula de geração convencional de energia

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Conventional Methods of 
Centralised Generation of 
Electricity
Kevin Davis
© Kdavis CIT 2014
References
• Ref 1. www.cer.ie (see Annual reports)
• Ref 2 Energy System and Sustainability 
by Boyle, Everett, Ramage Pub: Oxford
Additional resources www.daviddarling.info
www.khanacademy.org (see Chemistry, 
Carnot cycle and Carnot engine, Carnot 
Efficiency etc.)
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Conventional Centralised Generation
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• Thermal Generating Stations
• Hydropower Generating Stations
• Nuclear Generating Stations
Thermal Generating Stations
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• Combustion of coal, oil or natural gas 
produces thermal energy (high pressure 
steam)
• Converted to mechanical energy in a 
turbine 
• The combined boiler+turbine is known as the Prime Mover
• Converted to electrical energy in a 
generator 
• Need large water resources to cool steam 
as it exhausts the turbines…why?
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Quick Maths Review
Energy Required to heat 1kg of water from Ts to Tf can be 
calculated using
Q = m x Cp x (Tf-Ts) [J]
Where m = mass [kg]
Cp = specific heat capacity of water
Ts = starting temperature
Tf = final temperature
This formula is valid provided Tf is at or below the boiling 
point of water. (0.336MJ/kg to heat water from 20oC to 
100oC)
The energy need to evaporate 1kg of any liquid is its Specific 
Latent heat of vaporisation (2.258MJ/kg for water)
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The principles of heat engines
(Generation using Thermal Power Stations)
External 
Combustion 
systems
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Input Energy
Waste Energy at 
end of cycle 
(condenser)
Reference 2
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The principles of Heat engines
Example : coal burning power station 
• The working fluid is water in a closed cycle; 
• the energy released from the coal, converts the 
water to high pressure steam at temperature T1; 
• Let the value of heat energy at the input of the 
boiler be Q1
• This high pressure steam is used to rotate a 
turbine creating useful work output 
(W)…mechanical energy output.
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The principles of Heat engines
• As the steam passes through the turbines it 
loses some of its heat energy and now has an 
heat energy value Q2 and a temperature of T2.
• This is effectively waste heat energy and is 
cooled in a condenser. (If the heat released in the 
condenser can be used as useful energy then Q2 and T2
can be referenced to the condenser output)
• T2 will also be higher than ambient temperature so 
minimum value is 20+273 = 293K
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Heat Engine -Carnot Cycle
Efficiency = Energy output = W
Energy Input Q1
W = Q1 – Q2 so efficiency = Q1-Q2
Q1
Carnot showed that Q2/Q1 = T2/T1
Max possible efficiency = 1 – T2/T1
(Low value of T2 and High value of T1)
(Remember all temperatures in Kelvin)
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Heat Engine -Carnot Cycle
Maximum Efficiency occurs with a Low value of T2 and High 
value of T1
Steel and other metals used in the boiler, turbine and pipe 
works limit the value of T1 to approximately 550oC = 
550+273 = 823K
Max possible efficiency = (1-293/823) = 64.4%
When other loses such as boiler efficiency, pumps 
etc are taken into account and the difficulty of 
optimising the turbine for maximum efficiency, the 
actual efficiencies can be as low as 30%
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Blades 
of a 
Steam 
Turbine
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Gas Turbine
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Heat Engine -Carnot Cycle
Maximum 
efficiency when 
(Q1-Q2 )
is high
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Reference 2
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Schematic of a 660MW Steam Turbine
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Reference 2
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Efficiency and Economical
• What determines the price that a licenced 
electricity generator can sell their output to 
the market?
• The cost of their fuel used to generate the 
steam (coal cheap, oil expensive, gas = 
depends on natural resources in your country)
– Fuel costs generally are influenced by global markets for 
these fuels, shipping costs etc.
• The efficiency of your power station
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REF: : UNDERSTANDING ELECTRIC POWER SYSTEMS By Jack Casazza Frank Delea
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Stages 1-2 of the cycle 
1. Fuel is burned in the boiler transferring heat to 
a series of pipes, containing water, in the 
furnace. Water is converted to steam. The 
water flow is maintained by pumps
2. The steam passes through another set of Pipes 
(called a superheater), which are also heated 
by the flames, raising the steam temperature by 
200oC, producing high energy dry (superheated 
steam at 600oC. Density of steam is 70kg/m3)
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Stages 3-5 of the cycle 
3. Turbines: As the High Pressure (HP) steam 
passes through the first turbine, pressure drops 
=> volume increases, turbine size increases. In 
some designs the steam passes through a 
reheater (another set of pipes in the boiler) 
before going onto the next turbine,
4.Condenser: removes latent heat from steam to 
convert it back to water which is pumped back 
into the boiler.
5. Fans: these are generally needed to provide the 
high levels of air required for combustion and to 
aid in the removal of the exhaust gases.
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REF: : UNDERSTANDING ELECTRIC POWER SYSTEMS By Jack Casazza Frank Delea
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Power station Turbines
• Boiler design is main factor in plant efficiency
• Condenser type depends on plant location
– near large river or sea (cooling water is 
returned to it’s source)
– Otherwise, Cooling tower needed.
• Safety equipment, such as backup power 
supplies for pumps and fans, high 
pressure release valves, are also 
required.
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Cooling Tower
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Uses the 
evaporation of 
water sprayed on 
the warm pipes to 
remove large 
quantities of heat 
with only 2% water 
loss if the vapour 
can be condensed 
again before it 
leaves the tower
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Cooling Towers
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Power Station Turbines
• Based on Newton’s 3rd Law of Motion
• Tandem Compound Arrangements of HP, IP 
and LP turbines on a single shaft
• Cross compound arrangement uses two shafts 
and two generators (for very large outputs, two 
generators can be operated in parallel)
• The range of rotation speed is 1500rpm –
3000rpm. Speed must be tightly controlled as 
output electrical supply frequency directly 
related to rotational speed. 
• Double Flow Design (to reduce forces along 
the turbine axis)
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Cross - Compound Turbine 
Arrangement
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Reference 2
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Tandem - Compound Turbine 
Arrangement
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Reference 2
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External or Internal Combustion 
Engines
Steam based power plants are external
combustion engines : 
– the combustion of the fuel and the mechanical force 
output are in two separate processes.
Diesel and Gas power plants :Internal Combustion 
engines
• Air & Fuel compressed into a small volume making the 
mixture highly inflammable. 
• Ignited by a Spark, fuel burns rapidly causing mixture to 
increase pressure.
• high pressure pushes down the piston to give mechanical 
motion. © Kdavis CIT 2014
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The Gas Turbine
http://education.rolls-royce.com/how-a-gas-turbine-works/
Modern gas turbines based on the jet engine.
Three Key Components
1.Axial compressor: banks of turbine blades that squeeze 
the incoming air into a smaller volume
2.Combustion Chamber: continuous ignitionprocess 
where the compressed air and gas are fed into a 
combustion chamber. Outputs are jets of very hot, high 
pressure gases.
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The Gas Turbine
3. Turbine: the kinetic energy in the stream of hot 
gases is extracted and used to rotate the shaft. 
(the temperature of the gases entering the 
turbine can be 1300oC)
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Gas turbine & Modern Jet Engine
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Open Cycle Gas Turbine (OCGT)
Very fast acting Gas turbine 
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Reference 2
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Combined Cycle Gas Turbine (CCGT)
Combination of a Open Cycle Gas Turbine and a Steam Turbine
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Reference 2
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CCGT Power Station
Gas Turbines; Inlet temperatures 1300oC and outlet 
temperatures of 600oC
Steam Turbines; inlet temperatures of 600oC and 
outlet temperatures of 25oC
Max ideal efficiency (carnot efficiency) = 1-(Tout/Tin)
CCGT using hot output gases of the gas turbine as 
the heat source for a boiler and using the steam 
generated in this boiler to turn a steam turbine
Result Efficiencies of over 50%
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Start-up of a CCGT Power Station
1. Run generator as a motor to get the gas 
turbine up to speed (used to compress air 
supply).
2. Fire the gas: Within 30 minutes gas stage is 
fully operational but low efficiency (same as 
OCGT)
3. Boiler starts to generate steam over the next 
two hours 
4. When the steam turns at the same speed as 
the gas turbine, the clutch is engaged and both 
turbines work in Tandem (high efficiency as 
the fuel input level has not changed)
(total start-up time approx 3 hours)
© Kdavis CIT 2014
Growth of CCGT’s In Ireland
• Expansion of the gas pipeline network
• High Efficiency 
– Lower fuel cost per kWh generated
– Lower CO2 emissions (Kyoto Protocol)
– Moderate start-up times
• Opening up of the electricity generation 
market
– New entrants using the latest technology
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Nuclear Generating Stations
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CO2 free electricity generation
Heat released by splitting the nucleus of an atom (fisson)
After this reaction, it is identical to a thermal generating 
station with an efficiency of 30-40%
Skip Chemistry Notes for 2014
(if interested Theodore Wildi book Chapter 24 is quite good)
Energy released is given by E= mc2
E= joules
m= loss of mass when atoms split
c= speed of light = 3x108 m/s
Nuclear Generating Stations
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Energy released from 1 gram loss = 3000 tons of coal
Problem: Chain reaction. Splitting one nucleus, 
releases two high speed neutrons causing further 
nuclei to split. 
The chain reaction is controlled by limiting the amount 
of fuel and providing large volumes of coolant to 
remove the heat released. 
The coolant is pumped to a heat exchanger 
(effectively a steam generator)
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Types of Nuclear Reactors
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1. Pressure water Reactor. High pressure water 
used as a coolant (high pressure maintains the water in 
liquid state and has high heat removal properties)
2. Boiling Water Reactors. High pressure water used 
but allowed to convert to steam which is circulated in the 
turbines, condenser etc. 
3. High Temperature Gas Reactors. Inert gas such as 
helium used as coolant. Very high temperatures developed 
so slightly higher efficiencies 
4. Fast Breeder reactor: Generates heat and additional 
nuclear fuel during operating
Revision Questions
1. What are CCGT power plants and explain the rapid 
growth in the use of CCGT power plants in Ireland 
during the last number of years.
2. Outline the operation of a typical CCGT plant and 
explain the impact of CCGT plants on meeting 
Irelands commitment to the Kyoto Protocol.
3. Describe the operation of a CCGT power plant.
4. Using a simple diagram of a thermal generation 
station for electricity, explain why the maximum 
efficiency of this type of plant is limited to 64%
5. Compare Nuclear power generating stations and 
thermal generating stations
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