Transcript Heat Engines - Geoff Walker's Home Page

Heat Engines
A Brief Review of Thermodynamics
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.
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
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.
Carnot’s 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
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.
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.
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
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
TH  TL
TL

 1
TH
TH


The temperature is in the Kelvin or absolute scale
This efficiency is called the Carnot efficiency
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.
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.
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.
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.
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
Solution
Energy extracted = 1  1.9  100 = 190 kJ
Efficiency
50  273
  1
 24%
150  273
After heating to 250:
Energy extracted = 380 kJ
Efficiency = 38%
And Finally...
Work is heat and heat is work
and all the heat in the universe
is gonna coooool down!
Yeh! That’s entropy man.
Michael Flanders and Donald Swann