Lesson 2.11 Thermodynamics - 1
Overview
This lesson deals with the First Law of Thermodynamics,
external combustion engines and Carnot efficiency. On
completion of the lesson, you should be able to describe
the processes that occur in a heat engine, discuss the
differences between internal and external combustion
engines and estimate the ideal efficiency of a steam
turbine.
MINI LAB
Observe the operation of
an external combustion engine such as a steam engine.
Make a model steam turbine.
Thermodynamics
Thermodynamics is simply the study of the movement of
energy or the study of patterns of energy change. (Thermo
= energy) Dynamics = patterns of change.
Absolute Zero
Absolute zero is the lowest possible temperature. At
absolute zero, all the movement of particles that
contribute to the internal energy of a material ceases.
Absolute zero is
equivalent to -273ēC.
The absolute temperature
scale with the same sized units as the Celsius scale is
the Kelvin scale. The units are called Kelvins (K) (not
ēK)
The absolute temperature
scale with the same sized units as the Fahrenheit scale
is the Rankine scale. The units are called degrees
Rankine. (ēR)
First Law of
Thermodynamics
The First Law of Thermodynamics is essentially the law of
conservation of energy. Simply stated: Energy can neither
be created nor destroyed. When heat is added to a system,
it changes to an equal amount of some other form of
energy.
Heat Engines
Heat engines convert thermal energy into mechanical
energy. Examples include steam engines, steam and gas
turbines, spark-ignition and diesel engines. All heat
engines operate in a cycle of repeated sequences of
heating (or compressing) and pressurizing the working
fluid, the performance of mechanical work, and rejecting
unused or waste heat to a "sink." At the
beginning of each cycle, energy is added to a gas forcing
it to expand under high pressure so that the gas "performs"
mechanical work. The thermal energy contained in the
pressurized gas is converted to mechanical energy. The
gas then looses pressure - or liquefies, and after unused
energy (in the form of heat) is rejected, it must then be
reheated or recompressed to restore it to high pressure.
Heat engines are devices
that use thermal energy to do work. A basic heat engine
consists of a gas confined by a piston in a cylinder. If
the gas is heated, it expands, moving the piston. A
useful engine goes through cycles; the piston moves back
and forth. Once the gas is heated, moving the piston in
one direction, it can be cooled and the piston will move
back in the other direction.
A cycle of heating and
cooling will move the piston forward and back again - or
up and down.
Two temperatures are
needed. At one stage the system is heated, at another it
is cooled.
In a full cycle of a heat
engine, three things happen:
- Energy (heat) is
added - at a relatively high temperature.
- Some of the energy
from that input heat is used to perform work.
- Unused energy is
removed at a lower temperature.
External
Combustion Engines
Typical Steam
Engine
A steam engine is a type of heat engine. It takes heat
from the hot steam, converts some of this heat into
useful work and dumps the rest into the colder
surrounding air.
In a typical steam engine
a piston moves back and forth inside a cylinder. Hot,
high-pressure steam is produced in a boiler, and this
steam enters the cylinder through a valve. Once inside
the cylinder, the steam pushes outward on every surface,
including the piston. The piston moves. The steam does
mechanical work on the piston and the piston does
mechanical work on the machinery attached to it. The
expanding steam transfers some of its thermal energy to
this machinery, so the steam becomes cooler as the
machinery operates.
When the piston reaches
the end of its range, the valve stops the flow of steam
and opens the cylinder to the outside air. The piston can
then return easily. In many cases, steam is allowed to
enter the other end of the cylinder so that the steam
pushes the piston back to its original position. Once the
piston is back at its starting point, the valve again
admits high-pressure steam to the cylinder and the whole
cycle repeats.
Steam Turbines
Steam turbines are heat engines that use the thermal
energy of steam produced in a boiler to drive a turbine
at high speed. Jets of steam from nozzles around the
periphery of the turbine impinge on the turbine blades,
causing them to turn. Steam-turbines can work at high
rotational speeds and generate high powers from a
relatively small unit. Their major use is in the
generation of electricity.
Carnot Efficiency
Heat engines cannot convert all the input energy to
useful mechanical energy in the same cycle; some amount,
in the form of heat, is always not available for the
immediate performance of mechanical work. The fraction of
thermal energy that is converted to net mechanical work
is called the thermal efficiency of the heat engine. The
maximum possible efficiency of a heat engine is that of a
hypothetical (ideal) cycle, called the Carnot Cycle.
Practical heat engines operate on less efficient cycles
but in general, the highest thermal efficiency is
achieved when the input temperature is as high as
possible and the sink temperature is as low as possible.
The Carnot efficiency is
the maximum possible efficiency for a heat engine
operating between a high source with absolute temperature
Thot and a cold sink with absolute temperature Tcold is:
(1-Tcold/Thot) x 100%
Stirling Engine
One of the engines that experiences continuing interest
without commercial use is the hot air engine - or
Stirling engine. Some believe that it is obsolete but
others believe that it is an engine of the future because
it can use many different types of fuel and it is quiet.
There is controversy over
who invented the engine. British literature gives credit
to Sir George Cayley (1807) American literature gives
credit to Rev. Robert Stirling who presented his engine
when he was 26 years old in 1816.
The Stirling cycle engine
(also called an "external" combustion engine)
uses a gas, such as air, helium, or hydrogen, instead of
a liquid, as its working fluid. Concentrated sunlight,
biomass, or fossil fuels are sources potential fuels to
provide external heat to one cylinder. This causes the
gas to alternately expand and contract, moving a
displacer piston back and forth between a heated and an
unheated cylinder.
CARNOT EFFICIENCY
Ideal efficiency = (T(hot)
– T(cold) ) / T(hot) Where T is the absolute
temperature
Ideal Carnot Efficiency =
Difference in temperature between hot reservoir and cold
reservoir divided by the absolute temperature of the hot
reservoir.
Where: T(hot)
= absulute temperature of hot reservoir (K)
T(cold) =
absolute temperature of cold reservoir (K)
Example 2.11.1
Calculate the ideal
efficiency of a natural energy engine that uses cold
water pumped up from the ocean at 280 K as the heat sink
and warm surface water at 297 K as the heat source.
Solution
Ideal efficiency = (T(hot)
– T(cold) ) / T(hot) = (297 – 280) / 297 = 0.057
or 5.7%
Questions
- How are the Kelvin
and Celsius temperature scales related?
- State the First Law
of Thermodynamics.
- What happens to the
internal energy of a system when work is done on
it?
- What is a heat
engine?
- What three processes
occur in every heat engine?
- Label the following
engines as internal combustion or external
combustion:
a) Steam turbine
b) Diesel engine
c) Sterling engine
- How is the ideal (Carnot)
efficiency of a steam turbine related to the
temperatures of the steam entering and leaving
the turbine?
- What is the ideal
efficiency of a heat engine that has a hot
reservoir at 500 K and a cold sink
at 150 K?
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