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PHASE CHANGE MATERIAL AS A THERMAL ENERGY
STORAGE MATERIAL FOR COOLING OF BUILDING
1M.RAVIKUMAR, 2DR. PSS. SRINIVASAN
1Sr.Lecturer, Department of Mechanical Engineering, B I T, Sathyamangalam, Erode, India-648301
2Principal, K S R College of Technology, Tiruchengode, Erode, India-637209
E-mail: kumarmravi74@yahoo.co.in,drpss@yahoo.com
ABSTRACT
As the demand for refrigeration and Air conditioning has been increased during the last decade, the cool
storage systems can be used to the economic advantage over conventional cooling plants. Cool storage
system using phase change materials can be used for peak load shifting if they are installed in the building.
In the case of sensible heat storage system, energy is stored or extracted by heating or cooling a liquid or a
solid, which does not change its phase during the process. A variety of substances like water, heat transfer
oils and certain inorganic molten salts, and solids like rocks, pebbles, and refractory are used. The choice of
the substances used largely depends upon the temperature level of the application. Phase change material
(PCM) are one of the latent heat materials having low temperature range and high energy density of melting
– solidification compared to the sensible heat storage. The tests on transient heat transmissions across
different roof structures were conducted. It was found that when installing PCM in the withering course
(WC-mixture of broken bricks and lime mortar) region nearly uniform roof bottom surface temperature was
maintained.
Keywords:
Phase Change Material (PCM), Energy storage, Latent heat storage (LHS), Withering Course
(WC), Natural Cooling, Building roof, heat transmission.
1. INTRODUCTION
As a demand for air conditioning increased
greatly during the last decade, large demands of
electric power and limited reserves of fossil
fuels have led to a surge of interest with
efficient energy application. Electrical energy
consumption varies significantly during the day
and night according to the demand by the
industrial, commercial and residential activities.
In hot and cold climate countries, the major part
of the load variation is due to the air
conditioning and space heating respectively.
This variation leads to a differential pricing
system for peak and off peak periods of energy
use. Recent discussions on topics like global
warming and heat waves have brought attention
once again to energy efficient cooling systems
utilizing renewable energy sources. Cooling
demand has already been increasing due to the
evolving comfort expectations and technological
development around the world. Climate change
has brought additional challenges for cooling
systems designers. Significant economic benefit
can be achieved by thermal energy storage for
heating and cooling in residential and commercial
buildings. Buildings that will have large mass will
react slowly to changes in heating and cooling
demands.
Efficient and economical technology that
can be used to store large amounts of heat or cold
in a definite volume is the subject of research for
a long time. Thermal storage plays an important
role in building energy conservation, which is
greatly assisted by the incorporation of latent heat
storage in building products. Devices which store
heat during peak power operation and release the
same during reduced power operation. Phase
change material is one of the thermal storage
devices.
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504
Solid-liquid
Gas-liquid
Solid-gas
Solid-solid
Latent
heat
Eutectics
Single
temp.
Mixtures
Temp.
interval
Hydrated
salts
Eutectics
Single
temp.
Mix
Temp.
interval
Fatty Acids
Paraffins
Inorganics
Organics
Analytical
grade
Commercial
grade
LHS in a phase change material (PCM) is
very attractive because of its high storage
density with small temperature swing. It has
been demonstrated that for the development of a
latent heat storage system in a building fabric,
the choice of PCM plays an important role in
addition to heat transfer mechanism in the
PCM. Thermal energy storage in the walls,
ceiling and floor of the buildings may be
enhanced by encapsulating or embedding
suitable PCMs within these surfaces. They can
either capture solar energy directly or thermal
energy through natural convection. Increasing
the thermal storage capacity of building can
increase human comfort by decreasing the
frequency of internal air temperature swings so
that indoor air temperature is closer to the
desired temperature for a longer period of time.
This system provides a valuable solution for
correcting the difference between the supply
and demand of energy. Latent heat storage is a
new area of study and it received more attention
during early 1970s and 1980s.
Many phase change materials has been
studied and tested for different practical uses by
many scientists. This paper attempts to analyse
the information about application of PCM in the
building roofs for residential and commercial
establishments.
2. PCM CLASSIFICATION AND
PROPERTIES
In 1983, Abhat [1] gave the general
classification of energy storage material in Fig.1
and also by Lane [2, 3], Dinser and Rosen [4].
These papers gave the full detail like
classification and characteristics of PCM.
B.Zalba [5] listed the properties of different
PCM’s (Organic, Inorganic, Fatty acids) like
density, specific heat, thermal conductivity and
melting temperature.
Some of the important properties required
for PCM are
•
High latent heat of fusion per unit
mass, so that a lesser amount of material stores
a given amount of energy.
•
High specific heat that provides
additional sensible heat storage effect and also
avoid sub cooling.
•
High thermal conductivity so that the
temperature gradient required for charging
the storage material is small
•
High density, so that a smaller container
volume holds the material
•
A melting point in the desired operating
temperature range.
•
The phase change material should be
non-poisonous, non-flammable and non-
explosive.
•
No chemical decomposition, so that the
(LHTS) system life is assured.
•
No corrosiveness to construction
material
•
PCM should exhibit little or no super
cooling during freezing.
Figure 1. Classification of PCM
PCMs have not always resolidified properly,
because some PCMs get separated and stratify
when in their liquid state. When temperature
dropped, they did not completely solidify,
reducing their capacity to store latent heat. These
problems are overcome by packaging PCM in
containers and by adding thickening agents.
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To solve some of the problems inherent in
inorganic PCMs, an interest has turned towards
a new class of materials: low volatility,
anhydrous organic substances such as
paraffin’s, fatty acids and polyethylene glycol.
Those materials were more costly than common
salt hydrates and they have somewhat lower
heat storage capacity per unit volume. It has
now been realized that some of these materials
have good physical and chemical stability, good
thermal behavior and adjustable transition zone.
When salt hydrates are used as PCM they have
a tendency to super cool and do not melt
congruently so that segregation results. Even
though advances were made, some hurdles
remained towards the development of reliable
and practical storage systems utilizing salt
hydrates and similar inorganic substances.
Hydrated salts are attractive materials for use
in thermal energy storage due to high
volumetric storage density, relatively high
thermal conductivity and moderate costs
compared to paraffin waxes. Glauber salt
(Na2SO4. H2O) which contains 44% Na2SO4 and
56%H2O has been studied in 1952 [6,2], and it
has melting temperature of 32.4ºC, latent heat
of 254Kj/Kg. The selection of such material as
PCM for a specific application should be based
on thermodynamic properties, kinetic properties
and chemical properties. For low temperature
applications ranging from 0°C to 99°C, Salt
Hydrates would be the best option owing to
their availability in a less temperature range
with a reasonable specific heat capacity of
133.4(cal/deg.mol), thermal conductivity of
0.987 W/m-K, density of 1552 kg/m3 in the
solid phases respectively and phase transfer
temperature ranging from 35°C - 39°C.
3. DEVELOPMENT OF PCM FOR
COOLING OF BUILDINGS
The PCM can be used as natural heat and
cold sources or manmade heat or cold sources.
In any case, storage of heat or cold is necessary
to match availability and demand with respect
to time. There are three different ways to use
PCMs for heating and cooling of buildings
exist:
-PCMs in building walls;
-PCMs in building components other than
walls i.e in ceilings and floors;
-PCMs in separate heat or cold stores.
The first two are passive systems, where the
heat or cold stored is automatically released when
indoor or outdoor temperatures rise or fall beyond
the melting point. The third one is active system,
where the stored heat or cold is contained
thermally separated from the building by
insulation. Therefore, the heat or cold is used only
on demand and not automatically. In building
applications, only PCMs that have a phase
transition close to human comfort temperature
(20–28ºC) can be used. Some Commercial PCMs
have been also developed for building
application.
Hawes and Feldman [7] have considered the
means of PCM incorporation into the building by
direct
incorporation,
immersion
and
encapsulation. Arkar and Medved [8] designed
and tested a latent heat storage system (LHS)
used to provide ventilation of a building. Stritih
and Novak [9] designed an ‘experimental wall’
which contained black paraffin wax as the PCM
heat storage agent. The stored heat was used for
heating and ventilation of a house. Peippo et al.
[10] considered a PCM impregnated plasterboard
as a storage component in a lightweight passive
120m2 solar house with good insulation and a
large area of south facing glazing in Madison,
Wisconsin .The house could save up to 3GJ in a
year or 15% of the annual energy cost. Stetiu and
Feustel [11] used a thermal building simulation
program based on the finite difference approach
to numerically evaluate the LHS performance of
PCM wallboard in a building environment.
Feustel and Stetiu also investigated using double
PCM-wallboard to further increase the storage
capacity of a building so that the room
temperatures could be kept closer to the upper
comfort limits without using mechanical cooling.
Neeper [12] has examined the thermal dynamics
of a gypsum wallboard impregnated by fatty acids
and paraffin waxes as PCMs that are subjected to
the diurnal variation of room temperature but are
not directly illuminated by the sun. Salyer and
Sircar [13] defined a suitable low-cost linear alkyl
hydrocarbon PCM from petroleum refining and
developed methods of containing the PCM in
plasterboard to eliminate leakage and problems of
expansion in melting and freezing. Athienitis et
al. [14] conducted an extensive experimental and
one dimensional nonlinear numerical simulation
study in a full scale outdoor test room with PCM
gypsum board as inside wall lining. Lee et al. [15]
have studied and presented the results of macro-
scale tests that compare the thermal storage
performance of ordinary concrete blocks with
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506
those that have been impregnated with two
types of PCMs, BS and commercial paraffin.
Hawes et al. [16] presented the thermal
performance of PCM’s (BS, dodecanol,
paraffin, and tetradecanol) in different types of
concrete blocks. The presentation has covered
the effects of concrete alkalinity, temperature,
immersion time and PCM dilution on PCM
absorption during the impregnation process.
Hadjieva et al. [17] have applied the same
impregnation technique for concrete but with
sodium
thiosulphate
penta
hydrate
(Na2S2O3.5H2O) as a PCM. Mehling et al.
[18] were found that PCMs can be combined
with wood–lightweight concrete and that the
mechanical properties do not seem to change
significantly. It forwards a new kind of under-
.floor electric heating system with shape-
stabilized PCM plates. Different from
conventional PCM, shape-stabilized PCM can
keep the shape unchanged during phase change
process. Therefore, the PCM leakage problem
can be avoided. This system can charge heat by
using cheap night time electricity and discharge
the heat stored at daytime.
A major development in this area is to
develop a PCM which will maintain good heat
storage during the day and heat loss to the
environment during night time.
4. PROBLEM FORMULATION
4.1 Roof types and study area
Three roof structures are taken for studies
are as follows:
Roof -1(RCC): simple RCC roof
(150mm thick);
Roof -2 (WC): RCC roof (150mm thick)
covered with withering
course -WC (75 mm thick);
Roof -3(PCM): RCC roof (150mm thick)
covered with WC (75mm
thick) having PCM in the
WC region.
4.2 Assumptions made
To study the system the following
assumptions are made:
i.
The temperature variation is two
dimensional (across width and depth
directions only);
ii.
The ambient temperature Tamb and solar
heat flux qs are the functions of time over
the day;
iii.
The material properties are constant;
iv.
Inside and outside heat transfer
coefficients are constant;
v.
Radiation heat exchange within the room
is neglected;
Due to similar symmetry of all these structures,
width of all roofs were taken equally (150mm) for
the investigation. Boundary conditions were same
for all types of roof.
4.3 Boundary conditions
For right and Left
0
=
∂
∂
X
T
Bottom surface Convection, hi =10w/m2k,
T =25ºc
Top surface Convection, ho = 10w/m2k,
T =hourly values
Solar radiation flux, q = hourly values
5. MATERIALS AND METHODS
The temperature distribution inside the roof as
analyzed by using the Finite element analysis
software ANSYS 10. The parameters required for
the analyses are given below.
Table 1. Material property Data
Material
Density
(Kg/m3)
Thermal
Conductivity
(W/mK)
Specific
Heat
(J/Kg K)
Concrete
2300
1.279
1130
Withering
course
1300
0.25
800
Phase
Change
Material
(PCM)
941*
0.172*
2.35*
Latent heat
172
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507
0
100
200
300
400
500
600
700
800
900
1000
0 2 4 6 8 10 12 14 16 18 20 22 24
Time hr
Solar Flu
x, w/m
2
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18 20 22 24
Time,hr
Tamb
Tsol-air
Temp,ºc
*- indicates for both solid and liquid state
Solar insulation and weather data of
Coimbatore city, Tamilnadu, India during
June was used. Tsol-air was found out by
using the formula
Tsol= Tamb + (αqs /h0).
The graph between the Time vs. Tamb and
Time vs. Tsol-air was plotted as shown in the
Fig.3.
Figure 2. Solar Radiation Data for Coimbatore
during June 2006
Figure.3. Weather data for Coimbatore during
June 2006
6. TYPES OF ROOF TAKEN FOR
STUDY
Roof structures were modeled and solved
using thermal module of Ansys finite element
analysis software. Three types of roof were
considered for the case study as shown in the
Fig.4. Grid refinement was carried out and the
numbers of elements used were 5000. The roof
was maintained at uniform temperature of 25ºc to
start the solution for the transient thermal
analysis. The effect of this initial condition on the
end results are avoided by repeating the solution
for several days till the temperature distributions
at the end of two consecutive days are equal.
About 5days x 24 hours was found to be
sufficient for attaining the solution. The 5th day
results are presented and discussed.
Figure.4. Roof Structures
7. RESULTS AND DISCUSSION
The solar radiation data for Coimbatore during
June 2006 was recorded as shown in the fig 2.
The thickness of all the three roof structures are
different, so the distance is normalized
(Y*=Y/Ymax) with Y=0, Y*=0 referring to the
bottom of the roof and Y=Ymax and Y*=1.0
referring the top surface of the roof.
7.1 Temperature distribution across the roof
structure
At τ =0 hr to τ =6 hr, there is no solar radiation
on the building surface. But the heat accumulated
in the middle structure during the previous day,
travels on both the sides of the roof. The
temperatures at the top and bottom surfaces are
lower compared to the temperature inside the
roof. The average temperature for the concrete
structure is the highest among all the other types
of roofs as the thermal conductivity of RCC is
more compared to the WC and PCM. As the
thermal conductivity of RCC is higher, more heat.
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508
20
25
30
35
40
45
0
0.2
0.4
0.6
0.8
1
Y*
RCC
WC
PCM
Temp,ºc
20
4
6
8
10
0
0.
0.
0.
0.
1
Y*
RC
WC
PC
Temp,ºc
22
24
26
28
30
32
34
0
0.
0.
0.
0.
1
Y*
WC
20
RCC
PC
Temp,ºc
20
25
30
35
40
45
50
55
60
0
0.2
0.4
0.6
0.8
1
Y*
RCC
WC
PCM
Temp,ºc
2
2
3
3
4
4
0
0.
0.
0.
0.
1
Y
RC
C
W
PC
M
Temp,ºc
τ = 0 hr
τ = 6 hr
τ = 12 hr
τ = 18 hr
τ = 24 hr
will be stored during the previous day. The
thermal conductivity of Roof 3 is lowest
compared to Roof 2 and Roof 1
The curve for Roof 3 falls below the other
curves because PCM absorbs maximum heat
energy passing through the roof. It brings down
the temperature to the room temperature at the
place where it is located.
At τ =6 hr to τ =12 hr, as the solar radiation
falling on the surface increases, the heat transfer
characteristics varying from the previous time
period. As the thermal conductivity of the RCC is
highest, whatever the heat enters all the heat will
be transferred to the bottom so the curve is linear
and average. The curve for the Roof 2 is also
similar to the Roof 1 curve but slightly falls
below the Roof 1 curve at the bottom and peak
value at the top. The curve for the PCM reaches
the least value at the bottom and it reaches the
peak value in the top layer.
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20
25
30
35
40
45
0 2 4 6 8 10 12 14 16 18 20 22 24
Time, hr
RCC
WC
PCM
Tr
Temp,ºc
At τ =12 hr to τ =18 hr, the solar radiation
falling on the roof decreases but the heat that
has already entered travels inside the roof. The
mid plane temperature values are higher than
the τ =12 hr. As the heat flux during this τ =18
hr is very small value, so the convection at the
roof top dominates during this period.
Compared to the Roof 1, Roof 3 has reduced the
temperature at the bottom of the roof by 12º.
During τ =18 hr to τ =24 hr, there is no solar
radiation entering the roof. So the temperature
at the top and bottom of the roof is nearly at the
same temperature. For the Roof 3, temperature
reaches peak value at the middle and in the WC
region where PCM is located temperature falls
suddenly to room temperature as the PCM
absorbs all the heat passing through the roof.
And it reaches almost least value at the bottom
as the PCM installed region acts as thermal
energy storage.
7.2 Variation of roof top surface
temperature
Figure.5 Roof top surface temperature
Fig.5 shows the variation of top surface
temperature along with Tamb and Tsol-air. During
τ =0hr to τ =6hr and τ =18hr to τ =24hr the top
surface temperature reaches the low values. The
temperature for the WC roof attains the lowest
value and WC with PCM attains the highest
value, because even though in the absence of
solar radiation the PCM has stored heat energy
during the previous day releases the energy to
the top surface. During τ =6hr to τ =18hr the
solar radiation initially increases and drops
later, the top surface temperature for all the roofs
increases initially and drops later. The lease value
is observed for the RCC structure and highest
value for the WC with PCM structure. This is
because WC and WC with PCM structure offers
more resistance for the heat flow than the RCC
structure makes the top surface temperature to go
high for these two structures.
7.3 Variation of roof bottom surface
temperature
Figure. 6 Roof bottom surface temperature
The bottom surface temperature for three
different roof structures are plotted for different
time period during the day along with the room
temperature (Tr). The net heat entering in to the
room is mainly determined by the bottom surface
temperature. In the case of RCC roof, it has good
thermal conductivity, so the heat travels freely
into the room and the room temperature is
remarkably high value. The RCC with WC
structure offers some resistance so the bottom
temperature drops significantly. For the WC with
PCM structure, as the thermal conductivity of
PCM is very low it offers high resistance for heat
flow, so the bottom temperature is nearly
maintained constant.
7.4 Variation at the middle of the structure
In the RCC roof the middle surface at τ =0 hr to
τ =6 hr, there is no solar radiation on the building
surface. But the heat accumulated in the middle
structure travels on both the sides of the roof. The
temperatures at the top and bottom surfaces are
lower compared to the temperature inside the
roof. The average temperature for the concrete
structure is the highest among all the other types
of roofs as the thermal conductivity of RCC is
RCC
WC
0
10
20
30
40
50
60
70
80
90
100
0
2
4
6
8 10 12 14 16 18 20 22 24
PCM
Temp,ºc
Time, hr
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510
0
10
30
40
50
60
0
3
6
9 12 15 18
21
24
RCC
WC
PCM
20
Time, hr
Temp,ºc
2085.68
1107.9
601.43
0
500
1000
1500
2000
2500
RCC
WC
PCM
W/m
2 - d
ay
more compared to the WC, WC with PCM. As
the thermal conductivity of RCC is higher, more
heat will be stored during the previous day. The
thermal conductivity of WC with PCM is lowest
compared to WC and RCC. The curve for WC
with PCM falls below the other curves because
PCM absorbs maximum heat energy passing
through the roof. It brings down the temperature
to the room temperature, where the PCM is
locate face temperature goes on decreasing up
to τ =7hr. As the solar radiation increases later
the temperature starts increasing up to τ =0 hr to
τ =6 hr, there is no solar radiation on the
building surface. But the heat accumulated in
the middle structure travels on both the sides of
the roof. The temperatures at the top and bottom
surfaces are lower compared to the temperature
inside the roof. The average temperature for the
concrete structure is the highest among all the
other types of roofs as the thermal conductivity
of RCC is more compared to the WC, WC with
PCM. As the thermal conductivity of RCC is
higher, more heat will be stored during the
previous day. The thermal conductivity of WC
with PCM is lowest compared to WC and RCC.
The curve for WC with PCM falls below the
other curves because PCM absorbs maximum
heat energy passing through the roof.
Figure.7 Middle surface temperature
It brings down the temperature to the room
temperature, where the PCM is located. rts
increasing up to τ =16hr and then drops later.
For WC and PCM roof the surface temperature
goes on decreasing up to τ =9hr as these roofs
offer some resistance to heat flow compared to
the RCC roof. Up to τ =18hr, both the curves
are linearly increases and then drops later.
7.5 Heat flux entering into the room
From the Fig.8 it clearly states that PCM
installed roof is better than the WC roof and RCC
roof. If the roof is installed with PCM it can
reduce the heat entering the room about more than
two-third than the RCC laid roof.
Figure.8 – Heat Flux entering the room
When compared with the RCC, the WC and
PCM roof reduces the heat transfer by 46.88%
and 71.16%.As compared to WC roof with PCM
roof, reduction in net heat transfer was found to
be 45.71%.The reduction in heat transfer is
directly proportional to the corresponding
reduction in the electrical energy consumption for
to maintain the room at 25ºc.
8. CONCLUSIONS
Natural Cooling of building with phase change
material was studied. The heat entering in to the
room was maximum with RCC laid roof, because
the thermal conductivity of RCC is high value. So
almost all the heat entering the roof was
transferred to the room. When WC was laid along
with RCC and WC with PCM laid roof the heat
entering the room was reduced by 46.88% and
71.16%.As the PCM is having low thermal
conductivity, it offers the resistance for the heat
flow and heat transfer was reduced by 45.71
compared to the RCC with WC roof. With
various combinations of PCM, the test can be
repeated to find the best and effective material for
cooling application. The effects of thermo
physical properties of PCM, installation
methodology, location of PCM are the scope for
future work.
9. NOMENCLATURE
Tsol = Sol temperature
Tamb = Ambient temperature
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511
α = absorption coefficient
qs = heat flux in w/m2
h0 = room outside heat transfer coefficient
hi = roominside heat transfer coefficient
Ar = cross sectional area of the room in m2
Tr = room temperature in degree
10. ACKNOWLEDGEMENTS
The authors are grateful to the management of
BIT for providing the necessary facilities and
Prof Sakthivel for his encouragement during the
study.
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