Download 8.1 Energy Degradation and Power Generation

Energy, Power, and Climate Change
8.1 Energy Degradation and Power Generation
FUELS: INDIRECT ENERGY SOURCES
If you've ever camped out you've
probably used a campfire in these
two obvious ways:
(a) For cooking
(b) For warmth
In both cases, it is the heat
released during combustion (a
chemical reaction) that is used.
The reason the title of this
slide has the word "indirect" in
it is because if you want to
burn wood or coal, and get
useful work out of it, you have
to somehow convert the heat to
mechanical motion.
In general, the device
which converts this energy
into mechanical motion is
called an engine.
Energy, Power, and Climate Change
8.1 Energy Degradation and Power Generation
FUELS: INDIRECT ENERGY SOURCES
Here is a simple means of converting heat to
useable mechanical energy: A hot air balloon.
One form of mechanical energy we could "extract"
from the balloon is potential energy.
By using it to lift a quantity of water up high,
we could release the water in a stream, to turn a
turbine.
The turbine could be used directly to turn a
winch, or it could be used indirectly to run a
generator to produce electricity.
Eventually, the water will run out and the
balloon will have to descend to obtain more.
In other words, the balloon "engine" must go
through repeated cycles in order to keep
producing mechanical energy.
Getting the balloon back down is simple: Let
it cool down by releasing its heat to the
surrounding environment.
FYI: All the kinetic energy of the box is converted to heat via
friction. Energy,
Obviously we Power,
cannot convert
that Climate
heat back intoChange
and
mechanical
energy to Degradation
make the box travel
back
to where
it started:
8.1 Energy
and
Power
Generation
THE SECOND LAW OF THERMODYNAMICS
The second law of thermodynamics states that
although it is possible to convert mechanical
energy completely into thermal energy, it is NOT
possible to convert all heat energy into
mechanical energy.
More simply phrased we may say that "it is
impossible to take heat from a hot object and use
it all to do work without losing some of it to
the environment."
The hot air balloon example illustrates this
law: During its descent, all of its heat energy
is lost to the environment.
FYI: It is also lost during its ascent! How?
FYI: During
each stage
of the cycle,
energy
is being lostChange
to the
Energy,
Power,
and
Climate
environment
in a formDegradation
that cannot be recovered
for useful
work.
8.1 Energy
and Power
Generation
FYI: We call this imperfect conversion of energy from one form
ENERGY
into
anotherDEGRADATION
ENERGY DEGRADATION.
Suppose we are using wood to heat up the air to
FYI:
energy degradation
as a spreading out of our
makeThink
the of
balloon
rise.
source
wider surroundings.
original
Think energy
of the
woodinto
asever
a concentrated
form of
chemical
energy.
FYI: In terms
of ENTROPY, you may view energy degradation
as
Weanthen
burn
the wood,
heat
to universe
the
increase in entropy
(whichreleasing
is the tendency
of the
environment. This has the effect of spreading
toward disorganization). We begin with a very compact and
out the energy into the balloon and the
organized energy
source (the wood) and end up with a lot of
surrounding
air.
energy
spread
over largercauses
areas. the balloon to
thermal
Some of
this
heatout
actually
rise, increasing the potential energy of the
system. The rest is lost.
The stream of water converts potential to
kinetic energy (both forms are mechanical
energy).
The kinetic energy of the water is then
transferred to the turbine blades, with much of
it lost to the surroundings.
The kinetic energy of the turbine can then be
converted to electricity through use of a generator.
Energy, Power, and Climate Change
8.1 Energy Degradation and Power Generation
SANKEY DIAGRAMS
We can show energy degradation for the hot air
balloon engine with a flow chart called a Sankey
diagram.
High Water
Falling Water
Wood
Turbine
Chemical
Energy
100 MJ
Potential Energy
70 MJ
Heat
30 J
Kinetic Energy
60 MJ
Heat
10 J
Mechanical Energy
45 MJ
Heat
15 J
Note that each conversion has an efficiency
associated with it, which can be calculated using
Output
70
= 0.7 = 70%
=
Input
100
Conversion from wood to potential energy is, for
example...
Efficiency =
Energy, Power, and Climate Change
8.1 Energy Degradation and Power Generation
TYPICAL EFFICIENCIES
Here are some examples of efficiencies:
Machine
Efficiency (%)
Steam Locomotive
5 to 10
Human Muscle
20 to 25
Automobile
less than 25
Compressor
80
Electric Motor
70 to 95
FYI: You should be able to make Sankey diagrams for each of these
machines. The lost energy is unusable heat, given up to the
environment.
Energy, Power, and Climate Change
8.1 Energy Degradation and Power Generation
TYPICAL EFFICIENCIES
You may recall from
biology the
"efficiency" of
organisms - for example
a caterpillar.
This flow chart
illustrates energy
partitioning.
If we look at the caterpillar's
biomass, only 33 J/200 J = 16.5%
of the total energy consumed by
the caterpillar becomes
caterpillar.
Energy, Power, and Climate Change
8.1 Energy Degradation and Power Generation
TYPICAL EFFICIENCIES
In fact, the
higher up in the
food chain you go,
the less of the
original energy
you have left:
Energy,
Climate
Change
FYI: This
EMF can bePower,
produced byand
moving
the wire through
a stationary
magnetic
or by moving
a magneticand
field past
a stationary
wire.
8.1 field,
Energy
Degradation
Power
Generation
FYI: The polarity of the EMF can be reversed if the relative motion is
GENERATING ELECTRICITY
reversed (called ALTERNATING CURRENT).
Electricity is one of the most useable forms of
FYI:
Mostwe
of our
electricity
is produced
using
generators,
which are in
energy
have
because
it is so
easily
transportable.
turn
by turbines,
rotatedelectrons
by fluids such
as water a(dams),
Yourun
may
recall which
that are
moving
produce
magnetic
field.
wind,
or steam
(coal fired or nuclear power plants).
It turns out that the process is symmetric: A
FYI:
From the Sankey diagrams, we see that every energy conversion
moving magnetic field produces moving electrons
between the basic energy source and the useable form of energy results
(called an electromotive force or EMF).
in energy degradation.
Thus a coil of wire spinning in a magnetic field
will produce an EMF (aka a voltage) that can be used
to drive electrical devices.
B-Field
Electrons
Wire