Heat Engines

1
Heat Engines

• A Brief Review of Thermodynamics

2
Thermodynamics

• The science of thermodynamics deals with the
  relationship between heat and work.
• It is governed by two laws, neither of which have
  ever been proved.
• On the other hand no violations of either law
  have ever been observed.

3
First Law of Thermodynamics

• The energy that can be extracted from a process
  can never be more than the energy put into the
  process
• In other words
• Energy out Energy in
• This is essentially the law of conservation of
  energy, i.e.
• Energy can be neither created nor destroyed, it
  can only be converted from one form to another

4
The second law of thermodynamics

• The first law is concerned with the totality of
  energy in a process
• The second law tells us how much work we can
  extract from a given amount of heat.
• Carnots statement was to the effect that we
  cannot convert all the the available heat into
  work.
• The second law is also concerned with whether a
  process can occur at all. For example,
• Heat will always flow from a high to a low
  temperature
• A gas under pressure will expand compression
  does not occur naturally

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Heat Engines

• A heat engine is a device for extracting work
  from a hot fluid. For example
• A car engine extracts power from the combustion
  of fuel with air
• A steam steam turbine extracts power from steam
• Both of these function by allowing a hot fluid to
  expand so as to cause motion in a critical
  component of the engine.
• In the process, high grade energy is said to be
  degraded to lower grade energy.

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An ideal heat engine

• The diagram on the right represents an ideal heat
  engine
• Heat is added at constant temperature to the
  fluid at the high temperature source
• The fluid flows through an expansion device where
  work is done, and the temperature of the fluid
  falls from TH to TL
• Heat is then rejected at constant temperature at
  the low temperature source.

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Closed Cycle Heat Engine

• The cycle in the previous slide is known as an
  open cycle.
• The closed cycle here has four stages
• Isothermal heat addition
• Adiabatic expansion
• Isothermal heat removal
• Adiabatic compression
• Isothermal const. Temp
• Adiabatic perfectly insulated

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The Carnot Engine

• The cycles above are examples of the Carnot
  engine.
• In the Carnot cycle all processes are reversible.
• In a Carnot engine, the maximum work that can be
  done, and hence the efficiency of the ideal
  engine depends on the temperatures TH and TL
• The efficiency of a Carnot engine is given by

• The temperature is in the Kelvin or absolute
  scale
• This efficiency is called the Carnot efficiency

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Practical heat engines (1)

• The Carnot engine represents the theoretical
  limit and is not a practical engine.
• The main limitations of the Carnot engine are
• The processes in all four stages are reversible.
  For this to be the case they must all take place
  infinitely slowly
• The work extracted on expansion is equal to the
  work required for compression, so no net work is
  extracted.
• A practical heat engine has a lower efficiency
  than a Carnot engine, but can make more effective
  use of the energy in the hot fluid.

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Practical Heat Engines (2)

• Practical Heat Engines include
• The Rankine cycle basis of steam engines in
  power stations
• Otto and Diesel cycles internal combustion
  engines
• Gas turbine
• These have lower efficiencies than the Carnot
  cycle but are permit useful work to be extracted.

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The Rankine cycle

• This has two differences to the Carnot cycle
• There must be reasonable temperature differences
  in the boiler and condenser to ensure that heat
  addition and rejection occurs at an acceptable
  rate
• The turbine exhaust is completely condensed and
  returned to the boiler by a pump. This uses very
  much less energy than a compressor.
• These result in lower efficiencies than the
  Carnot cycle but permit useful work to be done.

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Other cycles

• Otto, Diesel and Gas turbines all involve an
  initial compression stage, but are otherwise open
  cycle processes.
• Combined cycle gas turbine
• This combines a gas turbine with a Rankine steam
  cycle to maximise the work extracted from the
  fuel.
• Efficiencies are much closer to Carnot
  efficiencies than in other practical cycle used
  to date.

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Example

• Steam from a geothermal well is expanded in a
  Carnot engine from a temperature of 150?C to
  50?C. How much work is extracted from 1kg of
  steam?
• If the steam is heated to 250?C before expansion,
  how much work is now extracted in relation to the
  extra heat added
• Heat capacity of steam 1.9 kJ kg-1 K-1
• 0?C 273 K

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Solution

• Energy extracted 1 ? 1.9 ? 100 190 kJ
• Efficiency

After heating to 250 Energy extracted 380
kJ Efficiency 38
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And Finally...

• Work is heat and heat is work
• and all the heat in the universe
• is gonna coooool down!
• Yeh! Thats entropy man.

Michael Flanders and Donald Swann