phase change materials by dhandabani,anna university,ceg,chennai
TRANSCRIPT
Phase change
materials
P.DHANDABANI,R&AC DEPT.
INTRODUCTION
The materials which stores and releases energy
during phase changes
It stores large amounts of heat or cold in a
definite volume
Mainly as a ‘latent heat’
It has high storage density with small
temperature swing.
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Thermal energy storage
Thermal energy can be stored and retrieved as a
change in internal energy
METHODS:
sensible heat storage
Latent heat storage
Since,amount of heat energy stored as the latent heat will
be higher than the sensible heat .so,we are going for LHS
system
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LATENT HEAT
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Heat
(energy) is
transferred
into the ice.
The heat is used
to break the
bonds between
molecules, not
to increase the
average kinetic
energy of the
molecules.
Since the bonds among
the ice molecules have
been broken, water is
formed. The water
molecules, at this
moment, have the same
average kinetic energy
as they did when they
were ice.
Since the ice and
water molecules
both have the
same average
kinetic energy,
they are at the
same Kelvin
temperature.
So, how could there be a change in heat during a state
change without a change in temperature?
Latent heat
When a substance changing its phase there will
be a release or absorption heat energy (latent
heat)
Example:-
when the molten wax is poured into our skin
,it will produces burning sensation.
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Heating and cooling in PCM
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Why solid/liquid?
Why not liquid/gaseous?
-larger changes in volume (gases are
having large specific volume)
-whereas ,smaller volume changes in
the solid/liquid conversion
-difficulty in incorporation
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Types of pcm materials
Water based gel - 0°C
-non flammable,non-toxic,ecofriendly
-application temperature is 0°C
Salt hydrates-15 to 80°C
-inorganic salts
-high LHS capacity
-fail to recrystallize
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Types of pcm materials
Paraffins-(-8°C to 40°C )
-overcomes supercooling
-chemical stability over no. of cycles
-varying cost,hazardous to skin
Bio pcm(-40°C to 151°C )
-Organic compounds from animal fats and
plant oil
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Selection factors
Latent heat- Higher latent heat of fusion
preferably above 180 kj/kg.
Sharpness of latent heat release & absorption -
melting point/freezing point
Stability to recycling & ageing
Non-corrosive to encapsulation
Safe to use-non-flammable,non-toxic
No supercooling during freezing
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How to incorporate pcm?
1. Direct incorporation
2. Immersion
3. Encapsulation
i) Microencapsulation
ii) Macro encapsulatiom
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PCM in buildings
Increasing the thermal storage capacity of a
building can increase human comfort by
decreasing the frequency of internal air
temperature swings
so that the indoor air temperature is closer to
the desired temperature for a longer period of
time.
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Contd…
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Microencapsulated PCMs are able to be
incorporated into almost any building material
economically
Pcm intergration can offer a larger surface area
for heat exchange
Example
gypsum boards (micro encapsulated
building material)
Micro encapsulation in building
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Benefits and Drawbacks of PCMBenefits: Higher storage density than sensible heat Smaller volume Smaller temperature change between storing and releasing
energy
limitation High cost Corrosiveness Density change Low thermal conductivity Phase separation Incongruent melting Supercooling
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Applications
Construction-building materials
Textiles
Food packaging industry
Medical packaging industry
Automobile, Transportation
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Pcm in building roofs visual
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THANK YOU!!!
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