To succeed in engineering, you have to master
the art of repetition.
That applies to repetition in your own work
– so that you’re always designing, prototyping,
testing, and then designing again.
But repetition is also key to the processes
that you use as an engineer.
I’m talking here about cycles.
They can be pretty complex, but at their core,
each one is just a sequence of events or steps
that repeat themselves in the same order.
And that’s exactly what’s happening in some
of the most commonly used devices that have
ever been engineered,
like the heat pump that’s heating your home
and the refrigerator that’s keeping your food
from spoiling.
So, let’s learn, and then repeat.
[Theme Music]
In engineering, we say that a system has
undergone a cycle if it returns to its initial
state at the end of the process.
So, its initial and final states should be
the same.
And cycles are important because, they allow us
to run through the same process again and again,
instead of limiting us to doing something only once.
With the right resources, we can keep running
through cycles until we get enough of what we
want, like the distance traveled in a car.
We can also keep making cycles for as long
as we need, say, to keep food cold in a fridge
at a constant temperature.
The process that your fridge uses today is
based on the work of 19th century American
inventors Oliver Evans and Jacob Perkins.
In 1805, Evans came up with a closed
vapor-compression refrigeration cycle, but
he never actually built a refrigerator.
Then, in the 1830’s, Perkins used Evans’
ideas and actually did.
Perkins’ system didn’t succeed commercially
at first, but it was the first step towards the
modern refrigerators that we use today.
So how do refrigeration cycles work?
Well, it’s easier to understand if we start
with something similar that we already know
a bit about: heat engines.
Last time, we learned that heat engines are
machines or systems that convert heat into
other forms of energy.
A basic heat engine can do this by taking
in heat at a higher temperature, from say,
solar energy or a furnace,
and then converting part of that heat to work,
usually by rotating a shaft.
The engine then releases wasted heat at a lower
temperature, maybe into its surroundings or a water
supply, and then readies itself to start over again.
So, like so many other things, heat engines
operate on a cycle.
And cycles achieve some goal, like heating or
cooling a room, by circulating what’s known as
a working fluid through a series of operations.
This working fluid will absorb and release energy,
change from liquid to vapor and back again,
and continue to circulate through the
cycle as part of the system’s operation.
So, let’s look at a heat engine that uses
water as its working fluid.
It goes through four main stages.
In the first stage, we’ll add heat to our
system by bringing in an energy source, QH.
The water will absorb this heat through a boiler,
which will cause it to become compressed steam.
In stage two, that steam will enter a turbine,
expand, and cause the turbine shaft to turn,
which gives us an output of work, Wout,
which was converted from some of the heat
energy in our fluid.
Remember, not all of the heat energy will
convert to work.
We’re going to have excess heat, which needs
to be released from our system.
We do this in stage 3 by condensing the
steam in a condenser, which releases that
excess heat into an energy sink, QL.
For the fourth and final stage, our fluid
needs to be re-pressurized.
To make this happen, we’ll send it through
a pump, which will need work as an input.
We’ll then send the re-pressurized water
back to the boiler at the beginning to start
the process all over again!
That’s just one cycle.
For each one that we do, we should have an
output of work.
And if we look at the heat engine as a closed system,
then the total changes in the kinetic energy, potential
energy, and internal energy are all 0 through the cycle.
So, per the first law of thermodynamics, the changes
in work and heat should equal themselves out!
Now, another way of looking at this cycle
is by using a phase diagram.
Phase diagrams compare different properties
to show what state or phase a substance is in.
For this example, we’ll compare entropy to the
temperature of the heat engine’s fluid using the Rankine
cycle, which is the ideal cycle for vapor power plants.
If we take a look at the diagram, all the material to
the left of the curve is in a liquid phase, while all the
material to the right is in a gaseous state.
Everything under the curve is a mixture of
gas and liquid.
This plot lets us easily see what phase our
fluid is in as it goes through each stage of
the heat engine cycle.
Now, not only is the heat engine a great way to turn
heat energy into work, but with a few small changes,
we can turn it into a very different type of system.
Instead of trying to get work as our output,
what if we tried to get heat?
Well, we do this every day when we try to
heat or cool our homes!
We use heat pumps to add heat to a system when
we’re feeling chilly, and refrigerators to remove heat
from a system when we want to keep things cool.
In either case, we put work into the system,
rather than trying to get it out.
What’s interesting for refrigerators and
heat pumps is that, since we’re aiming for
an output of heat,
it’s possible to get a 100% conversion
from work, which we know from the second
law of thermodynamics.
However, it’s important to note that even though
all of our work can be converted into heat,
it may not all be the exact heat we want,
because we're still going to have two different
temperature levels.
Now, with all that in mind, let’s go back
to the refrigerator.
If we’re talking about the fridge in your
kitchen, the inside stays cool because of what’s
happening on its rear exterior wall.
This is where our cycle will take place.
Just like with the heat engine, we can break this
cycle down into four stages, again with a working
fluid that’s circulating through all the stages.
In your kitchen fridge, this working fluid
is a hydrochlorofluorocarbon chemical that’s
usually referred to generically as freon.
The first stage of the cycle is the evaporator,
which removes heat from the inside of the fridge.
It starts out with liquid fluid that’s colder
than the inside of the fridge, which is the result
of the last stage of the previous cycle.
But we’ll get to that.
So, this liquid is really cold, but its boiling
point is also really low.
In fact, the liquid is just about at the temperature
where it’s ready to boil.
And when a liquid changes to a gas, it absorbs
heat.
So when the liquid in the evaporator boils,
it absorbs heat from the refrigerator at the
same time.
But even though it’s absorbing heat, its
temperature doesn’t actually change.
All that heat energy is going into changing
the liquid into a gas, not raising its temperature.
Stage two is the compressor.
Its job is to raise the pressure of the gas,
which also raises its temperature — and
its boiling point.
After stage two is complete, the gas is really
hot — hotter than the air outside the fridge.
But because its boiling point increased too,
it’s still around the temperature where it’s ready
to condense into a liquid.
Which brings us to stage three: the condenser,
which is basically the opposite of the evaporator
in stage one.
In the condenser, the gas turns into a liquid,
a process that releases heat.
Since the refrigerant is now hotter than
the air outside, heat can flow from inside the
condenser to the surrounding air.
But, like in the evaporator, the temperature
of the refrigerant stays constant in the condenser.
Finally, in stage four, an expansion valve throttles
the liquid, lowering its pressure — and therefore,
lowering both its temperature and boiling point.
It’s the opposite of what happens in the
compressor.
You end up with cold liquid refrigerant at
a lower pressure, ready to enter the evaporator
and start the process all over again,
absorbing more heat from the fridge as it boils.
So basically, the food inside your fridge
stays cold because we’re taking heat out
from the inside of your fridge.
And we can see this all a bit more clearly
if we take another look at a phase diagram,
this time for a refrigeration cycle.
While similar to the phase diagram for the heat
engine, we’ll see a few differences in what phases
our fluid is in at the different stages.
The fluid spends more time in a gaseous
state and less time as a liquid than our fluid
did for the heat engine.
Now, full cycles like these are great, but sometimes
we’ll need to have an incomplete cycle with a little
outside help to get what we want.
That’s because sometimes we’re limited by
our environments and what we have available.
So, for example, refrigerators often need
electricity, but that kind of power isn’t
always available.
So, with a little bit of problem-solving,
we can design ones that don’t need it!
That was the idea behind the zeer pot.
The zeer pot is a simple refrigerator made
from one earthen pot set inside another, with
a layer of wet sand in between them.
It was made famous by Nigerian inventor Mohammed
Bah Abba in the 1990’s, but similar devices may
date back all the way to Egypt around 2500 BCE.
So how does the zeer pot work?
Well, as the moisture from the sand evaporates,
it cools the inner pot by “pulling” out heat.
It’s a great way to have a refrigeration
system in a hot climate when you have very
limited resources.
But while it’s pretty awesome, the zeer
pot isn’t quite a cycle.
Without recapturing the evaporated water, we’re going
to need some outside work to make our sand wet again
if we want to continue the cooling process.
So, sometimes we need to forgo a perfect cycle
for the sake of practically.
But all that being said, let’s say we do
have the resources for a refrigerator that
can run on a cycle.
How can we improve this process even more?
Well, one way is by using a renewable energy
resource to fuel our system and produce the
work that we need!
Solar energy is a great example.
Rather than taking electricity that was
made from a typical source,
we can use solar-powered photovoltaic panels
to convert the sun’s rays into electricity by exciting
the electrons within the panel’s cells.
The electricity that we’d get from this energy
could replace the work that we needed for the cycle,
thus making the cycle itself more reusable!
Which is great, because a big goal for us as
engineers is to find ways to improve our processes,
even when something is already working.
We can always improve on our designs in one
way or another.
We can always take another step forward.
The refrigerators that we make now are far
better than the ones that Perkins made back
in the 1800’s.
Our heat engines are getting more and more
efficient as time goes on.
You see, the journey of an engineer is both
discovery and optimization.
And we’re just getting started.
So today, we learned all about cycles, what
they are, and some of the systems that use them.
One of the biggest ones was heat engines.
Not only did we learn how heat engines work,
but we also saw that with a few small changes,
we can create other systems too, like refrigerators
and heat pumps.
We also learned about phase diagrams and the
power of using renewable energy resources.
I’ll see you next time, when we’ll learn
about fluid mechanics and momentum transfer.
Crash Course Engineering is produced in association
with PBS Digital Studios.
You can head over to their channel to check
out a playlist of their amazing shows, like
America from Scratch, Hot Mess, and Eons.
Crash Course is a Complexly production and this
episode was filmed in the Doctor Cheryl C. Kinney
Studio with the help of these wonderful people.
And our amazing graphics team is Thought Cafe.
