1 8.Generation of Electricity 9. Basic Thermodynamics Maxine Narburgh CSERGE N.K. Tovey ( ) M.A, PhD, CEng, MICE, CEnv Н.К.Тови М.А., д-р технических наук.

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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?


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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


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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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Carnot’s reasoning

Water at top has potential energy

Water at bottom has lost potential energy but gained kinetic energy

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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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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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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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Schematic Representation of a Power Unit

Heat Engine

Heat In Q1

Heat Out Q2

Work Out W

First Law:

W = Q1 - Q2

so efficiency

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21

Q

QQ

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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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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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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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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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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(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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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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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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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

QQ

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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

QQ

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

1 8.Generation of Electricity 9. Basic Thermodynamics Maxine Narburgh CSERGE N.K. Tovey ( ) M.A, PhD, CEng, MICE, CEnv Н.К.Тови М.А., д-р технических наук.

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