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If you put water in the freezer, you'll end
up with ice. If you leave ice on your countertop,

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you'll end up with liquid water. You've almost
certainly seen these phase changes in your

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everyday experience. But there's more to freezing
and melting than meets the eye, and we can

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use these seemingly simple phenomena to make
buildings significantly more energy efficient.

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In this video, we'll explain the concept of
"latent heat" and see how it can dramatically

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reduce heating and cooling costs in homes
and skyscrapers alike.

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This video is part of the Conservation video
series.

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In order to analyze or modify a system, it
is important to understand how the laws of

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conservation place constraints on that system.

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Hi. My name is Stephen Ray and I am a graduate
student in the Department of Mechanical Engineering

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at MIT. My research in the Building Technology
Lab under the guidance of Professor Leon Glicksman

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focuses on energy efficient buildings.

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In order to understand the topic of this video,
you should be familiar with the Law of Conservation

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of Energy. You should also be familiar with
the effects of intermolecular forces on phase

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transitions.

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After watching this video, you should be able
to describe the energy transformations that

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occur during a phase change and apply the
law of conservation of energy to phase changes.

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Let's start with a demonstration.

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Here we have a container of water to which
we are adding table salt.

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Quite a bit of table salt, actually.

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Let's stir that up.

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Next, we are going to crush up some ice to
add to the salt water.

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Our goal is to lower the temperature of the
ice bath.

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Let's add some more salt.

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Freezing point depression, yeah!

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Alright, minus 8 degrees Celcius. That's pretty
good.

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Let's place a smaller container of liquid
water into this ice bath. We've added green

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food coloring to the water so that it is easier
for you to see.

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After a few minutes, we'll insert a thermometer
and watch the temperature drop as the water

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is cooled by the ice bath.

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The temperature reads about -5 degrees Celsius.
The green water is still a liquid even though

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the temperature is below the normal freezing
point of water. This is called supercooling.

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In the next step of this demo, we are going
to add a couple of small pieces of ice to

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the green water. The supercooled water will
crystallize rapidly with the addition of the

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ice.

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When this happens, what do you predict will
happen to the temperature on the thermometer?

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Will it increase, decrease, or stay the same?

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What reasoning supports your prediction? Pause
the video here to discuss your ideas with

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the person beside you, then continue playing
the video to see what happens.

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Okay, ready to see what happens? Be sure to
keep your eye on the digital display.

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Now we'll drop a couple of small pieces of
ice into the container of green water.

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Watch what's happening.

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The green water froze. There is still a little
bit of liquid, but we can turn the container

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upside down and you can see that the ice remains
inside.

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So, when the liquid froze, what happened to
the temperature?

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The temperature went up! Is this what you
predicted? How can we explain what happened?

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Well, let's think about what happens at a
molecular level when water changes phase from

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a liquid to a solid. In the liquid state,
water molecules are moving around a lot. As

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it gets colder, the water molecules slow down.

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Generally speaking, as the water cools and
solidifies, there is an increase in hydrogen

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bonding, the dominant intermolecular force
amongst the water molecules.

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With this hint, can you now explain why we
observed a temperature increase when the water

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froze? Pause the video, take a moment to think
about it on your own and then discuss your

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idea with a classmate. Then, continue playing
the video for an explanation.

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As we said before, when water transitions
from liquid to solid, there is an increase

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in the number of hydrogen bonds that are formed
between water molecules. Does bond formation

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release energy or require energy?

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Bond formation releases energy.

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So then, is the process of ice forming exothermic
or endothermic?

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Ice formation is exothermic.

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If we were to do an energy balance on the
system and the surroundings, we would see

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that the energy that was released by the water
freezing is equivalent to the thermal energy

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that caused the temperature to increase. This
is the Law of Conservation of Energy.

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Generally speaking, when a substance transitions
between phases, intermolecular forces between

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neighboring molecules are either formed or
broken. When a substance transitions from

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a liquid to a solid, intermolecular forces,
or bonds, are formed and energy is released

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to the surroundings. This is called the latent
heat of fusion.

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Going in the other direction, when a substance
transitions from a solid to a liquid, energy

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is required to overcome intermolecular forces,
so this is an endothermic process. Energy

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is absorbed from the surroundings. This is
called the latent heat of melting.

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The latent heat of melting is equivalent in
magnitude to the latent heat of fusion, but

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opposite in sign.

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This phenomenon of latent heat is used in
a variety of ways to heat and cool buildings.

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Using melting ice to absorb thermal energy
from the surroundings saves some buildings

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thousands of dollars a year in cooling costs.

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Consider a large office building during the
middle of a hot summer day. In order to keep

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the building comfortable, the air conditioning
is running at full power, requiring a lot

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of electricity from the utility company. The
utility companies can't easily shut down their

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power plants, so nearly the same amount of
electricity is produced during the night as

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is during the day. However, nighttime demand
for electricity is very low, so utility companies

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sell this electricity at a lower price. This
is common practice by utility companies across

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the U.S.

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Engineers have thought of a way to buy the
electricity during the night when it is very

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cheap, but use it during the day when they
need to run the air conditioning. This strategy

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is called peak load shifting. Think about
what you have learned so far. There are a

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variety of ways this can be done. How do you
think they do this? Pause the video here,

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take a moment to think about it, and discuss
your idea with the person next to you. Continue

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playing the video to hear about one way they
do this.

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One way we can decrease the amount of electricity
needed in the daytime to cool a building is

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to use the energy storage capacity of ice,
or its latent heat of melting.

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Large tanks, such as these in the basement
of the Bank of America Tower in New York,

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store water that is frozen over night using
cheap electricity. During the day, the ice

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melts and absorbs energy from the cooling
fluid running through the building's air conditioning

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system. Each of the tanks in the Bank of America
building holds approximately 1600 gallons

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of water which translates to roughly 570 kilowatt
hours of cooling capacity. Bank of America

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reports that these ice tanks supply 25% of
their cooling energy annually.

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The desire to harness latent heat has led
to the development of a class of materials

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called phase change materials. These materials
have been specifically designed to change

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phase at desirable temperatures so that they
can store and release energy in a way that

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is useful to consumers. This slide shows some
examples of these materials. The materials

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on this slide fall into three classes of phase
change materials: inorganic salt hydrates,

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paraffinic hydrocarbons, and organic fatty
acids.

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Some ordinary building materials such as concrete,
dry wall, or insulation have been specially

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engineered over the past 50 years to contain
microscopic pellets of phase change materials.

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Cellulose insulation, shown here, is commonly
used to insulate attics and walls. Researchers

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at the Oak Ridge National Laboratories in
the United States have impregnated small paraffin

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pellets in this common type of insulation
to increase its performance.

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Because these pellets are microscopic, the
phase change insulation looks exactly like

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the ordinary insulation to our naked eye.
However, under a Scanning Electron Microscopic,

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the clusters of paraffinic pellets are easy
to spot.

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Now lets look at the measured performance
of a similar type of phase change insulation

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that is installed in a typical residential
roof. The chart here shows the heat transfer

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into a house on the vertical axis and time
on the horizontal axis. The two large black

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peaks, which occur during two consecutive
summer days, correspond to the large amount

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of heat transfer into a home that a conventional
roof allows. The smaller green peaks show

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how changing the roof surface material and
venting the attic significantly help lower

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the heat transfer into a home by approximately
70%. This reduction in heat transfer leads

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to less energy required for cooling the home.
The purple curve shows that if phase change

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insulation is used in addition to these roof
modifications, we can reduce the required

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cooling energy by 90%!

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How does phase change insulation help save
energy? Pause the video, take a moment to

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think about it, and share your thoughts with
a classmate.

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Consider two homes with and without PCM roof
insulation. Both homes are exposed to the

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same amount of solar energy from the sun.
Although not all of that energy enters the

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home without PCM insulation, a significant
portion is transferred through the roof into

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the house. The house with PCM insulation reduces
this amount because some of the energy that

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would otherwise enter the house is used to
melt the phase change material in the roof.

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In order for this effect to occur each day,
the phase change material must solidify every

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night before it can melt again the next day.

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Let's take a look at this chart of example
phase change materials again. What phase change

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material would you use in a home or building
in Singapore? Why? What additional information

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might you need to know to help make your decision?
Pause the video here and discuss your choice

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with the person beside you.

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On average, year-round temperatures in Singapore
hover between 23 and 32 degrees Celcius. This

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is important when making our decision. Some
of you may have selected the calcium chloride

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hydrate as a candidate phase change material
because its melting temperature falls within

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the range of daytime high temperatures in
Singapore. You may have selected octadecane

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for a similar reason. If you eliminated paraffin
wax and palmitic acid, it was probably because

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their melting points are so much higher than
the highest outdoor temperatures reached in

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the day. However, these phase change materials
could be used on a conventional dark-colored

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roof where temperatures can exceed 70 C because
of all the energy absorbed from the sun. Caprylic

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acid, on the other hand, has such a low melting
temperature, that few, if any, building components

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would ever drop below this temperature.

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So, let's go back to calcium chloride hydrate
and octadecane. Both would melt and solidify

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within the temperature range of the environment,
but octadecane has a higher heat of fusion,

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which leads to greater potential energy savings.
While it is beyond the scope of this video,

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we also need to think about the heat transfer
properties of the other building materials

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used in the roof and the cost and stability
of the phase change materials.

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Today, you learned about the concept of latent
heat and how the energy transformations that

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occur during a phase change are a consequence
of the law of conservation of energy. You

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also saw how engineers have used the concept
of latent heat to design phase change materials

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that can allow us to cool and heat homes more
efficiently. The incorporation of phase change

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materials in building materials is still an
active area of research. There are still engineering

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challenges to address, perhaps by you, in
order to make these technologies cost-effective

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and more widely used.