This video will introduce Sankey diagrams, which are one of the more fun
parts of this unit in my opinion. Sankey diagrams are a way of using pictures to
show how much energy is going into a system, how much useful energy or power
is coming out, and how much energy or power is lost by the system. Here's an
example: in a car 100 Joules of chemical energy in the gas is converted to 40
Joules of kinetic energy and 60 joules of thermal energy. The goal of drawing a
Sankey Diagram is to draw a clear simple picture to show how much energy is going
into a system, how much useful energy is going out, and how much useless energy is
being lost by the system. These are the steps to drawing it: first you're going
to decide how much energy each square on your grid represents. I'm choosing each
box to be 20 only because I have a hundred Joules of chemical energy, and
the energies it's being split up into are both divisible by 20. So that will
probably make this easy without too many squares or too many details. Step number
2 is to draw a rectangle with a vertical height equal to the total
energy going into your system. Label the left side with the type of energy going
into the system. So if each box is 20 Joules of energy and I have a hundred
Joules of chemical energy going into the system, that means that I'm going to have
to draw a rectangle that is five boxes high. Because if each box is 25 boxes is
equal to a hundred Joules, so five boxes are the total energy going in. So I'm
going to draw a rectangle like that and I'm going to label this with the type of
energy that's going into the system. Here that's chemical energy. Step number 3
is to draw a horizontal arrow pointing right out of the right side of your
rectangle with a vertical height equal to the useful energy you are getting out
of the system. Label the arrow with the type of energy created. I'm specifically
going to draw this on the top of the right side of the rectangle like this
because the goal of using gas in a car is to get kinetic energy out of the
chemical energy. The useful good energy that we're trying to get out here is the
kinetic energy, so here the useful good energy is 40 joules because it says that
there are four joules of kinetic energy, and because
each box is 20 Joules, I'm going to make the arrow that represents how much energy is coming out
two boxes high to represent 40 joules altogether. Lastly I'm going to actually
draw the arrow at the end and label it kinetic energy. This is the useful good
energy coming out of my system. Step four is to draw a curving arrow out
of the rest of the rectangle aiming down with a width equal to the total energy
lost, and then we're going to label the type of energy lost here. We're losing
thermal energy. We don't really want the gas to create thermal energy. so because
40 Joules was converted into useful good kinetic energy, the rest 60 Joules or
three boxes was converted into bad thermal energy. So I'm going to make this
arrow aiming down like this, and this represents the lost energy, and I'm
labeling this thermal energy specifically. This is what a Sankey
diagram looks like for this situation described. You can see that it's good at
showing you the exact proportions of how much energy you're putting into a system
versus how much useful and useless energy you're getting out of the system.
Here's another situation with a car. This time the car is taking in a hundred
Joules of chemical energy and changing it to just 10 Joules of kinetic energy
and 90 Joules of thermal. I'm going to say again that each box is 20 Joules so
this is still going to be five boxes high, and chemical is going in this time
though only 10 joules is being turned into kinetic which is just going to be
one-half of one of the boxes in height. So that is the useful energy out and the
rest the other four and a half boxes or 90 joules is all being lost to thermal
energy like this. So that's exactly four and a half boxes high to represent 90
joules. So this Sankey diagram represents that
new situation, and you can see that it kind of intuitively shows you how
there's more energy being lost in this situation than in the previous situation
and exactly how much more specifically. Another advantage of Sankey diagrams is
that we can draw them for much more complicated situations where we're
losing all types of energy or the same type of
energy multiple times. In this example a coal power plant is taking in a hundred
and sixty Joules of chemical energy. As the coal is burned, 60 Joules of thermal
energy is lost to the environment and not used. The burning coal is used to
heat water which spins turbines and 40 Joules of the kinetic energy of the
water is unused, and as the electricity created is transported through a power
line 20 Joules of energy is lost to thermal energy and the rest of the
energy in the power plant arrives as electrical energy in homes. So that's a
lot, but we can use a Sankey diagram to make it look simple. Again I'm going to
say that each box is 20 Joules so my rectangle is going to have to be 8 boxes
high for 160 Joules and that's chemical going in because coal contains chemical
energy. So coal is burned and some heat is lost here and that's going to be
exactly three boxes high or three boxes long because it's 60 Joules of heat
energy being lost as thermal energy and the rest stays as good energy for now
but later on 40 Joules of the kinetic energy is lost to water. So that's lost
and you can see that that arrow has a height of 2 boxes for 40 joules and the
rest stays in the system and finally 20 Joules of energy is lost to thermal
energy in transmission and the rest of the energy arrives as electrical energy
in homes. So that's what a more complicated Sankey diagram would look
like and you can see that this still does a pretty good job of showing us the
proportion of how much energy is lost at each point versus how much useful energy
we end up getting out of the Sankey diagram. Sankey diagrams are used very
often to represent energy. As an example this is kind of like the ultimate Sankey
diagram for the United States. This shows the total US energy consumption, and you
can see different sources of energy going into different parts of the
economy, different parts of society, and where the energy goes after as well. As a
final note we can use Sankey diagram to calculate the efficiency of a system.
Even if we don't know the values of the boxes we can compare the amount of
useful energy out to the total energy in. As an example to find the efficiency of
the system, I can see that because efficiency is the useful work out over
the total work in here there are two boxes of kinetic energy, which is labeled
as the good energy that we want coming out, and there are five boxes of chemical
energy going in. So the efficiency is going to be a ratio of two to five it's
not necessarily true that there are two Joules of kinetic energy and five Joules
of chemical but the ratio between them is definitely going to be two to five. So
because efficiency is a ratio, that can tell us the efficiency is two-fifths or
0.4 or 40%, and that's what you need to know about drawing Sankey diagrams.