Journal of Energy Storage 4 (2015) 74–88
Contents lists available at ScienceDirect
Journal of Energy Storage
journal homepage: www.elsevier.com/locate/est
Review
Review on phase change material based free cooling of buildings—The
way toward sustainability
Muthuvelan Thambiduraia , Karthik Panchabikesanb , Krishna Mohan Na ,
Velraj Ramalingamb,*
a
b
Department of Mechanical Engineering, Annamalai University, Chidambaram 608002, India
Institute for Energy Studies, Anna University, Chennai 600025, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 14 June 2015
Received in revised form 3 August 2015
Accepted 9 September 2015
The present world energy scenario signifies the importance of renewable energy utilization and paves the
pathway towards green and net zero energy building concepts for a sustainable future. In the recent
years, substantial energy is spent in building space heating/cooling applications to meet the human
comfort requirements. In order to reduce the unnecessary losses associated with the buildings, several
advancements toward energy efficient concepts are also being proposed and implemented in many
buildings. Free cooling is one such novel concept through which building cooling demands can be met
without compromising the indoor air quality. Free cooling concept stores the abundant atmospheric
night cool energy in phase change materials (PCM) and uses the stored energy during the day hours to
achieve the desired room comfort conditions. This review article aims to update and consolidate the
substantial work carried out in the recent years by various researchers on free cooling technology using
PCMs in latent heat thermal energy storage (LHTES) systems. In addition, future potential of free cooling
technologies, scope for further improvement, policies that needs be promoted by the government toward
its sustainability to ensure market penetration of free cooling technologies are also discussed in detail.
ã 2015 Elsevier Ltd. All rights reserved.
Keywords:
Latent heat thermal energy storage
Phase change materials
Free cooling
Sustainability
Green buildings
Contents
1.
2.
3.
4.
5.
6.
7.
8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concept of free cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Phase change materials for free cooling . . . . . . . . . . . . . . . . . . . .
Selection criteria of PCM for free cooling applications . . . . . . . .
PCM temperature range . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Subcooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
4.3.
Cooling degree days . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.
Geometry of the PCM container . . . . . . . . . . . . . . . . . . . .
Air flow rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.
Investigations on free cooling technology . . . . . . . . . . . . . . . . . .
Free cooling technology—applications . . . . . . . . . . . . . . . . . . . . .
Free cooling of buildings using PCM—a way forward . . . . . . . . . .
Assessment of free cooling potential . . . . . . . . . . . . . . . . .
7.1.
7.2.
Economics and CO2 emission analysis . . . . . . . . . . . . . . . .
Scope for future improvement in free cooling technology
7.3.
7.4.
Promotion policies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Corresponding author. Fax: +91 4422351991.
E-mail address: velrajr@gmail.com (V. Ramalingam).
http://dx.doi.org/10.1016/j.est.2015.09.003
2352-152X/ ã 2015 Elsevier Ltd. All rights reserved.
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M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
75
1. Introduction
2. Concept of free cooling
Residential and commercial buildings are among the few
sectors that possess large energy saving potential by means of
renewable energy utilization, green building concepts and
building energy management. Ancient buildings were constructed with heavy mass that reduced fluctuations in indoor
air temperature during the day and night. In recent years these
concepts are not being followed much by the architects/
construction engineers and in addition the buildings are
constructed with low thermal mass to reduce the cost of
construction. Hence modern buildings consume lots of energy
to meet the requirements of human comfort. Mechanical type air
conditioners/electric heaters are being used for space cooling/
heating applications which are not only energy intensive but also
responsible for indirect damage to the environment. The
increasing demand for energy along with worldwide environmental threat has drawn the attention of researchers to devise the
necessary steps for energy efficiency and sustainability in
buildings. In order to reduce the energy consumption and to
address the global environmental issues in buildings, more
importance is given toward the implementation of energy
efficient passive cooling technologies. Passive cooling technology
adopts the principle of supplying cool air to the buildings with
minimal electricity consumption by avoiding the energy intensive
mechanical type air conditioning systems.
The use of PCM as storage medium for both cooling and heating
applications appreciably reduces the energy demands of building
sectors during the peak hours. The concept of using PCMs for
building application has gained momentum only in the recent
years. Though free cooling potential shows promising characters
toward space cooling applications, it is not yet been commercialized and implemented in residential sectors. In order to create
awareness and to make it sustainable some initiatives need to be
taken by building technocrats, engineers and policy makers.
Reviews on phase change material based thermal storage for
energy efficiency in buildings have been carried out by various
groups of researchers in recent years [1–12]. Fig. 1 represents the
number of worldwide publications on PCM based energy storage
from the year 1995 to 2014, which indicates the increase in
development of PCM based storage in several applications. Fig. 2
shows the number of publications on free cooling technology by
various authors with a minimum of three publications to their
credit.
The main principle of free cooling is to either receive or release
an adequate amount of cool energy during phase transition at
constant temperature with low amplitude of temperature [13].
Free cooling technology requires a storage unit which stores the
thermal energy either by varying the internal energy of the storage
medium (sensible heat storage) or by varying the phase of storage
material (latent heat storage) or by both these processes. When the
PCM loses its cool energy, it gets discharged and to charge it again,
cool ambient air is allowed to pass through it during the night or
early morning hours. Fig. 3(a) and (b) shows the operation of free
cooling system during the day time and night time. The system
consists of a shell and tube structured PCM regenerative heat
exchanger in which the PCM is placed in the shell side and air is
circulated through the tube passages. The cool energy available in
the atmospheric air during the early morning hours is made to pass
through the regenerative heat exchanger. When the cool air passes
through the regenerative heat exchanger, PCM in the modules gets
charged and stores the cool energy. Air circulation is made by using
a fan during the night/early morning hours and dampers are used
to control the air flow rate. Hot air from the room is made to pass
through the PCM module through a small capacity fan and thus
PCM releases the stored cool energy to the room.
3. Phase change materials for free cooling
PCMs are commonly classified into three main categories based
on organic, inorganic and eutectic compounds. Based on the
operating temperatures, PCMs are also classified as low temperature PCM, medium temperature PCM and high temperature PCM.
The advantages and disadvantages of various types of PCM are
briefed in Table 1. Various researchers have summarized the list of
suitable PCMs for building applications [11,14–17]. Detailed
reviews of phase change materials for free cooling of buildings
have been done by various researchers [11,18,19]. Suitable phase
change materials used by researchers in recent years for free
cooling applications are given in Table 2 and the commercial PCMs
available are given separately in Table 3.
4. Selection criteria of PCM for free cooling applications
Selection of suitable PCM is very essential for the successful
implementation of free cooling concept. Performance of the free
Fig. 1. No. of publications on PCM based energy storage.
Source: www.scopus.com, as on March 2015.
76
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
Fig. 2. No. of publications on free cooling.
Source: www.scopus.com, as on March 2015.
Fig. 3. Free cooling concept.
Table 1
Classifications of PCM.
Classification
Characteristic
descriptions
Inorganic
- Typical crystalline solids of general formula
AB. nH2O
- Molten salt, metal or alloy, crystalline
hydrates
Organic
- Saturated hydro carbons (CnH2n+2)
- Acid/esters, high aliphatic hydrocarbon, or
salts, alcohols, aromatic hydrocarbons, aromatic ketone, lactam, freon, multi-carbonated category, polymers
Eutectic
- Eutectics are alloys of inorganics (mostly
hydrated salts) and/or organics
- They have a single melting temperature
Advantages
Higher energy storage density, higher thermal
conductivity, non-flammable, inexpensive
Physical and chemical stability, good thermal
behavior, adjustable transition zone
- Sharp melting temperature (could be used to
deliver the desired melting temperature
required)
- Volumetric thermal storage density slightly
above organic compounds
- No segregation and congruent phase-change
Disadvantages
Subcooling, phase segregation, corrosive,
incongruent melting point
Low thermal conductivity, low density, low
melting point, highly volatile, flammable,
volume change
- Limited data are available on their thermophysical properties
- Some fatty eutectics have a quite strong odor
and therefore they are not recommended for
use as PCM wallboard
Methods for
improvement
Mixed with nucleating and thickening agents,
thin layer
arranged horizontally, mechanical stir, shape
stabilized PCM
High thermal conductivity additives, fireretardant additives
–
77
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
Table 2
List of potential organic, inorganic and eutectic PCMs used for free cooling applications.
Compound
Melting point ( C)
Heat of fusion (kJ/kg)
Density
(kg/m3)
Reference
Organic phase change materials
Methyl stearate
Polyglycol E600
Paraffin C16–C18
Paraffin C18
Paraffin R20
Paraffin R26
Paraffin R27
n-Octadecane
Vinyl stearate
1-Dodecanol
Paraffin C13–C24
Propyl polmite
Dimethyl sulfoxide (DMSO)
45% Capric acid + 55% lauric acid
Glycerin
n-Heptadecane
Butyl stearate
Dimethyl sebacate
Octadecyl 3-mencaptopropylate
62.5% Trimethylolethane + 37% water
29
22
20–22
28
20-22
24–28
26–28
28
27–29
26
22–24
16–19
16.5
17–21
17.9
19
19
21
21
29.8
169
127.2
152
244
172
131
179
200
122
200
189
186
85.7
143
198.7
240
140
120–135
143
218
–
1126 (liquid, 25 C)
–
774 (liquid)
870 (750)
880 (760)
870 (750)
814 (774)
–
–
900 (760)
–
1009 (liquid)
–
–
760 (liquid)
760 (liquid)
–
–
1120 (liquid)
[20]
[26,21]
[3,26,21,22]
[3,26,21–23]
[20]
[20]
[20]
[21]
[3,21,22]
[3,21]
[3,21,22]
[1,15]
[15,21]
[20]
[23]
[23]
[3,26,21,22]
[3,21,22]
[22]
[11]
Inorganic phase change materials
45% Ca(NO3)26H2O + 55% Zn(NO3)26H2O
NaOH(3/2)H2O
LiNO32H2O
LiNO33H2O
FeBr36H2O
Mn(NO3)6H2O + MgCl26H2O
25
15
30
30
21
15–25
130
–
136
296
105
125.9
[22]
[21]
[23]
[3,26,21–23]
[23]
[20]
45–52% LiNO33H2O + 48–55% Zn(NO3)26H2O
55–65% LiNO33H2O + 35–45% Ni(NO3)2
50% CaCl2 + 50% MgCl2 + 6H2O
67% Ca(NO3)2 + 33% Mg(NO3)2
47% Ca(NO3)24H2O + 53% Mg(NO3)26H2O
CaCl26H2O
kF4H2O
4.3% NaCl + 0.4% KCl + 48% CaCl2 + 47.3% H2O
CaCl212H2O
17.2
24.4
25
30
30
29
18.5
27
29.8
220
230
95
136
136
190.8
231
188
174
1930 (liquid)
–
–
–
–
1738 (liquid,
20 C)
–
–
–
1670 (liquid)
–
–
–
1640 (1530)
–
Eutectic phase change materials
Octadecane + docosane
45/55 Capric + lauric acid
48% Butyl palmitate + 48% butyl stearate + 3% other
66.6% CaCl26H2O + 33.3% Mgcl26H2O
48% CaCl2 + 4.3% NaCl + 0.4% KCl + 47.3% H2O
47% Ca(NO3)24H2O + 53% Mg(NO3)26H2O
60% Na(CH3COO)3H2O + 40% CO(NH2)2
25.5–27.0
21
17
25
26.8
30
30
203.8
143
140
127
188
136
200.5
–
–
–
–
–
–
–
[27]
[1,26,23]
[20]
[24]
[28]
[28]
[27]
Fatty acids
61.5 mol% Capric acid + 38.5 mol% lauric acid
Capric acid + lauric acid
26.5% Myristic acid + 73.5% capric acid
34% Myristic acid + 66% capric acid
75.2% Capric acid + 24.8% palmitic acid
Lactic acid
86.6% Capric acid + 13.4% stearic acid
Emerest 2325
Emerest 2326
82% Capric acid + 18% lauric acid
50% Butyl stearate + 48% palmitate
19.1
21
21.4
24
22.1
26
26.8
20
20
19–24
20
132
143
152
147.7
153
184
160
134
139
–
–
–
[20]
[3]
[20]
[29]
[20]
[23]
[20]
[30]
[31]
[32]
[31,33]
cooling concept is influenced by parameters such as (a) PCM
thermo-physical properties, (b) local climatic conditions (c) mode
of heat transfer during melting and solidification, (d) materials and
configuration of heat transfer surface, (e) inlet flow rate and (f) the
thermo-physical properties of the heat transfer fluid. In LHTES
system it is possible to transfer the stored energy at a relatively
small temperature difference between the storage material and
heat transfer fluid. Charging/discharging cycles, cost of the
insulation, air flow rate, inlet air temperature, encapsulation
thickness and ratio of energy/volume in the encapsulation are the
–
–
–
–
–
–
–
–
[22]
[22]
[23]
[22]
[3,21]
[3,21,22]
[28,24,25]
[26,22]
[23]
main parameters that influences the performance of free cooling
based LHTES system [41]. In this section the role of phase change
temperature, cooling degree hours, geometry of the container and
the airflow rate in designing an effective PCM based free cooling
technology are reviewed in detail.
4.1. PCM temperature range
Phase change temperature range is one of the primary
influencing factors that have to be considered while choosing a
78
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
Table 3
List of commercial PCMs suitable for free cooling.
Product
Melting temperature ( C)
Heat of fusion (kJ/kg)
Density (kg/m3)
References
Paraffin
RT 18HC
RT 21
RT 21HC
RT 22HC
RT 24
RT 25
RT 25HC
RT 27
RT 28HC
18
21
21
22
24
26
25
28
28
250
155
190
200
150
148
230
179
245
880
880
880
760
880
880
880
880
880
[34]
[34]
[34]
[34]
[34]
[34]
[34]
[34]
[34]
Salt hydrates
S27
S25
S23
S21
Climsel C23
Climsel C24
SN 27
STL 27
E17
E19
E21
E28
E30
27
25
23
22
23
24
27
27
17
19
21
28
30
183
180
175
170
148
216
207
213
143
146
150
193
201
–
–
–
–
–
1480
–
–
1490
1480
1480
2100
1300
[34]
[34]
[34]
[34]
[35]
[35]
[36]
[37]
[38]
[38]
[38]
[38]
[38]
Organic mixture
OM 11
OM 21
OM 32
FS 21
6–16
16–26
28–37
16–26
260
140
235
125
–
–
–
–
[39]
[39]
[39]
[39]
Inorganic salts
HS 22
HS 24
HS 29
HS 34
17–27
19–29
29
29
185
185
190
150
–
–
–
–
[39]
[39]
[39]
[39]
Salt hydrates
S30
S27
S25
S23
S21
S19
S17
S15
30
27
25
23
21
19
17
15
190
183
180
175
170
160
160
160
–
1530
1530
1530
1530
1520
1525
1515
[40]
[40]
[40]
[40]
[40]
[40]
[40]
[40]
Organic mixture
A29
A28
A26
A25
A24
A23
A22
A16
A15
29
28
26
25
24
23
22
16
15
226
155
150
150
145
145
145
213
130
–
–
790
785
790
785
785
–
–
[40]
[40]
[40]
[40]
[40]
[40]
[40]
[40]
[40]
Salt hydrate
STL 27
27
213
–
[37]
Compound PCM
C 21
C 24
21
24
122
180
–
–
[35]
[35]
PCM. Free cooling system works better in places where the
temperature range between day and night is more than 15 C.
However, with optimum and careful design of heat transfer units,
free cooling of buildings can be implemented in locations where
diurnal temperature range prevails less than 15 C [11]. A
temperature difference of 3–4 C is required between the cooling
medium (night atmospheric air) and freezing temperature of the
(770)
(770)
(770)
(700)
(770)
(760)
(770)
(760)
(770)
PCM [42]. In hot and dry conditions, PCM melting temperature for
free cooling application depends upon the comfort or mean
temperature of the summer months [43]. The air coming out from
the PCM storage tank should be within the range of defined room
comfort levels [13,44]. Different authors have suggested different
criteria for selecting the appropriate melting temperature of the
PCM. Selection of phase change temperature of the PCM is one such
79
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
key parameter to ensure maximum charging of the PCM and must
be close to the designed room temperature [45]. PCMs with higher
melting temperature are most suitable for buildings located in
warmer climates so that the maximum cooling potential of the free
cooling system can be attained. Medved et al. [46] suggested the
following relation for finding the optimal melting temperature for
free cooling application,
T p ¼ T a þ 2K
where, Tp is the PCM peak melting temperature and Ta is the
average ambient temperature.
When the cooling demand is high, PCM phase change
temperature should be lower in order to maintain a specific
temperature level [47]. On the other hand, for very less cooling
demand the phase change temperature should be close to the
objective temperature level. In hot and dry climatic conditions,
PCM with melting temperature equal to comfort temperature can
increase the potential of free cooling system [48]. Turnpenny et al.
[49], proposed that more temperature difference (15 C or more) is
required for complete charging and discharging of the PCM in a
defined time period (7–10 h). For a temperature difference of 5 C
between the cold air and PCM temperature the heat transfer rate
was 40 W over a melting period of 19 h. Numerical simulations of
Arkar and Medved [50] showed that a PCM with a melting
temperature between 20 and 22 C is most suitable for free cooling
in continental climates. Experimental studies on the influences of
inlet air temperature, airflow and PCM storage unit for dry and hot
climate conditions were carried out by Waqas and Kumar [51].
Their results showed that, 33% charging time was required to
reduce the inlet air temperature from 22 C to 20 C and while
increasing inlet air temperature the time taken for complete
charging was high.
experiments on a vertical concentric tube LHTES to investigate the
subcooling trends of RT 21 PCM. Experiments were conducted in a
test room where the desired temperature was simulated using a
7 kW system. Subcooling characteristics were analyzed at different
temperatures (12 and 14 C) with different flow rates (3,4,5,and
6 m/s) and at different axial locations (212.5 mm, 162.5 mm,
112.5 mm and 62.5 mm), respectively. One of the conclusions
from their experimental analysis was, increase in driving
temperature gradient and higher air velocity had direct effect on
the subcooling of the PCM.
4.3. Cooling degree days
The degree days are estimated in order to find the heating/
cooling demand in buildings. Cooling degree day (CDD) is
calculated to estimate the energy required to cool the indoor
air to a comfortable temperature. Higher the CDD, higher the
cooling load or higher the energy required for maintaining the
comfort temperature. Optimization of free cooling system can be
performed by calculating the CDD. CDD are generally calculated
by considering mean daily average and the base temperature.
Base temperature is the temperature at which the cooling or
heating systems need not be operated to maintain the comfort
conditions inside a room. The calculation of degree days is a
theoretical approach; however the level of insulation and
building heat gains affect the demand of heating/cooling load.
Medved and Arkar [46] analyzed the free cooling potential for six
selected cities in Europe and they calculated cooling degree hours
(CDH) for selected cities for the months from June to August
(summer season). In addition they discussed the influences of
width of phase change temperature range and optimum melting
temperature of the PCM in the selection of LHTES. They
determined the CDH using the formula,
4.2. Subcooling
Subcooling is normally experienced while attempting to freeze
a material. During subcooling, material temperature drops well
below the melting point even before freezing initiates [52].
Subcooling or super cooling is one of the crucial factors that
influence the solidification characteristics of a PCM. Subcooling
property of a material makes it to remain in the liquid phase far
below its melting point and results in prolonged solidification
time. Though subcooling cannot be eliminated fully, it can be
reduced by adding additives and nucleating agents. Subcooling is
experienced more in inorganic PCM than organic PCMs. Conceptual analysis of subcooling characteristics of PCMs was undertaken
by various researchers [53–57]. Ryu et al. [58] explored ways to
prevent subcooling in inorganic hydrated salts. They stated that,
though inorganic PCMs possess high storage density and high
thermal conductivity when compared to organic PCMs, in order to
use them effectively further, preventive measures of subcooling
were needed. They also suggested that by adding various
nucleating agents, subcooling of the hydrated salts could be
prevented. Turnpenny et al. [49] in their study, discussed the
subcooling of Na2SO410H2O. They added 1.5% of borax as
nucleating agent to prevent the subcooling. Waqas and Kumar
[51] conducted an experiment to examine the influence of
subcooling on solidification time of SP29 PCM. They analyzed
the subcooling nature of the selected PCM for three different inlet
air temperatures, 20, 22, 24 C. Subcooling showed dominant
characteristics and prolonged for 2.3 h for high inlet air
temperature (24 C). Correspondingly for low air inlet temperature
subcooling was experienced for minimal duration (0.5 h). They also
stressed the fact that for higher air inlet temperatures, subcooling
should be taken into the account. Solomon et al. [59] carried out
CDH ¼
2X
208
ðT a
T o Þd
i¼1
where, d = 1 h, when Ta > To, and d = 0, when Ta < To.
Priya et al. [60] calculated the average annual CDD for
Tiruchirapalli (a district in South India) with eleven base temperatures (18–28 C) and estimated the cooling energy cost using the
computed CDD. In general, base temperature is taken as 18.33 C
(65 F). Using the base temperature as 18.33 C, CDD is equated as
(T-18), where T is the average temperature of the given day. The
formulae used by the authors for computing the daily CDD are
given in Table 4.
Where T = (Tmax + Tmin)/2 is the average temperature, Tbase is the
base temperature, Tmin is the minimum temperature, Tmax is the
maximum temperature for a given day.
Borah et al. [61] estimated the degree days for different climatic
zones of North East India using ASHRAE formula, UKMO equations
and Schoenau–Kehrig method. The CDD was calculated for five
various base temperatures (20 C, 22 C, 24 C, 26 C and 28 C). It
was inferred that, annual CDD for warm and humid climate
condition was much higher than other climatic zones. They
Table 4
Computation of daily CDD.
Condition
Formula used
Tmin > Tbase
Tmin 2 Tbase
T < Tbase
T max < Tbase
CDD = T – Tbase
CDD = [(Tmax – Tbase)/2] – [(Tbase – Tmin)/4]
CDD = (Tmax – Tbase)/4
CDD = 0
80
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
computed the CDD using the formula proposed by Mourshed [62].
"
#
24
X
ðT b T i Þþ
CDDd ¼
i¼1
24
where, Tb is the base temperature, Ti is the outdoor temperature at
the i-th hour of the day. Only the positive differences between the
Tb and Ti were considered.
4.4. Geometry of the PCM container
PCM encapsulation, geometry thickness and the type of
container material have significant effect on the surface area per
unit of volume storage area, pressure drop characteristics, heat
transfer rate, solidification and melting process of the PCM. In
general, PCM is specified as the core material and the container
which holds the PCM is considered as the shell material. Since the
availability of cool energy is limited to only a few hours (early
morning 02.00–06.00 h), shell material and the thickness of
container material should be selected in such a way that, it should
effectively transfer the cool energy to the PCM in short time span.
Based on the geometry, PCM container can be categorized as
cylindrical, tubular, spherical encapsulation and rectangular
encapsulation. Based on the size, the PCM encapsulation can be
classified as macro, micro and nano encapsulated PCM. Fig. 4
represents the different geometries of PCM encapsulation adopted
by various researchers.
Experimental and numerical investigations on PCM with
characteristic length of 2–5 mm for rapid heat discharge were
carried out by Wei et al. [63]. In their analysis higher heat transfer
rate was determined for spherical shape encapsulation compared
to rectangular, tubular and cylindrical shapes. The shell diameter
of 0.4 mm showed enhanced heat release over the 0.2 mm shell
diameter. Akgun et al. [64] analyzed the possibilities of heat
transfer enhancement in the heat storage geometry by tilting the
surface of the outer shell by an angle of 5 . Salunkhe and
Shembekar [65] reviewed the effects of PCM encapsulation on the
thermal performance of a system. They discussed the types of
PCM encapsulation, manufacturers of encapsulated PCMs,
encapsulation size, shell thickness, encapsulation geometry and
the effects of PCM encapsulation on solidification/melting
process of a PCM. In their analysis, conduction and natural
convection showed dominating characteristics during charging
and discharging cycles. Shell size, thermal conductivity of the
shell material and temperature of the HTF influenced the
solidification and melting characteristics of the PCM.
4.5. Air flow rate
Air flow rate influences both solidification and melting
characteristics of a PCM. From the free cooling experiment
conducted by Waqas and Kumar [51] it was inferred that, by
changing the air flow rate from 4 m3/h to 5 m3/h, solidification time
of a PCM was reduced up to 16%. In their analysis an airflow rate of
5 m3/h/kg of PCM and 1.3 m3/h/kg of PCM was maintained during
the charging and discharging process. They concluded that while
charging, if the air temperature is not lesser than the subcooling
temperature of the PCM, higher air flow rate is not beneficial.
Efficiency of free cooling system with two LHTES tanks integrated
to the ventilation system for heavy/low weight low energy
buildings was studied by Arkar et al. [43]. In their study, different
air flow rates were maintained for different operational modes. The
authors investigated the charging and discharging characteristics
of the selected PCM with respect to the step change of inlet air
temperature. They reported that during day time by decreasing the
air flow rate, extraction of cool energy from LHTES system is
possible via free cooling. Lazaro et al. [47] conducted experiments
on two PCM—air heat exchanger prototypes. Prototype 1 was filled
with inorganic PCM and prototype 2 was filled with organic PCM.
The authors carried out the experiments for different air flow rates
and observed the influence of air flow rate on the melting time and
cooling power of the PCM. When temperature range between the
PCM and inlet air is less, increase in inlet air flow beyond certain
value will have only a negligible effect on solidification characteristics of the PCM [51,59]. From their study, Wei et al. [63] inferred
that the variation in air flow rate was effective during charging/
discharging process. By increasing the air flow rate or by
decreasing the inlet air temperature, rapid discharge of thermal
energy was experienced.
Fig. 4. Various PCM encapsulation geometries.
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
It is inferred from literature, that air flow rate has its influences
on both charging and discharging process of PCM storage. Higher
inlet air flow rates reduce the time required for charging the PCM;
however, during discharging process, lower air flow rate is
recommended for a longer time rather than increased air flow
rate.
5. Investigations on free cooling technology
n this chapter, experimental investigation, simulation analysis
and PCM based heat exchanger units for free cooling applications
carried out by various researchers are reviewed in detail.
Experiment and simulation studies for a free cooling system
integrated to a mechanical ventilation system were done by Arkar
and Medved [50]. The experiments were conducted in four
different cases. In the first three cases, the potential of natural
ventilation system with different air exchange rate was analyzed
and in cases 4a and 4b potential of integrated system (mechanical
ventilation with LHTES unit) was analyzed. Due to solar and
internal temperature gains weak cooling potential was experienced in natural ventilation. They reported that in case of natural
ventilation, with five air exchanges per hour efficient cooling was
ensured; however with two air changes per hour the same cooling
effect could be achieved using the LHTES system. Zalba et al. [41]
tested the performance of PCMs (RT 25 and C22) in a flat plate
encapsulation for free cooling application. The major influencing
parameters such as ratio of energy/volume in encapsulates, load/
unload rate of the storage and cost of the installation were
determined. The major advantages of flat plate encapsulation as
stated by the authors were (i) the melting and freezing process of
the PCM on a plate surface was symmetric (ii) heat transfer in the
PCM could be controlled with the selected thickness of the
encapsulation and (iii) high area to volume ratio of storage was
obtained. Their statistical analysis showed that thickness of
encapsulation, air inlet temperature and air flow rate were the
main parameters influencing the solidification and melting
process of a PCM. Yanbing et al. [45] proposed the night
ventilation system with PCM packed bed storage (NVP) to cool
the space during day time and developed a mathematical model
for their proposed system. Lazaro et al. [47] studied the efficacy of
two real- scale prototypes air-PCM heat exchangers. The authors
followed ANSI/ASHRAE standards 94.1-2002 (method of testing
active latent heat storage devices based on thermal performance).
Precision thermopiles were used to measure inlet/outlet air
temperature and the air flow to obtain accuracy. Prototype
1 consisted of aluminium pouches and Prototype 2 had aluminium panels. PCM in aluminium pouches had the disadvantage of
leakages due to the thermal expansion in liquid phase. Free
cooling system for hot and dry climatic conditions using PCM
storage unit as a heat sink was studied by Waqas and Kumar [51].
The author’s major observations were PCM melting temperature
is equal to the comfort temperature of the hottest summer month
and the storage unit performance is maximized for all the months
during summer season and performance of the system is more
sensitive to the melting temperature than the airflow rate.
Experimental investigations on the performance of selected
PCM’s for free cooling/cold thermal energy storage concept were
conducted by Stritih and Butala [66]. They suggested that E19,
ClimSel C21, E21, RT 20, E23, ClimSel C24 are the suitable PCM’s
for free cooling concept. From their experimental results they
reported that, PCM (Rubitherm RT20) could cool the air to a
temperature below 24 C for more than 2.5 h when the air velocity
was 1m/s and the inlet air temperature was at 26 C. Turnpenny
et al. [67] conducted experiments on night time cooling system
using a heat pipe incorporated with PCM. In their experiment,
they analyzed parameters such as temperature, discharging
81
character of the PCM, cooling potential of the storage unit and
the effect of heat pipe tilt angles. The latent heat storage rate was
100 W h per unit for 2–3 h time period with the heat transfer rate
of 200 W. They also revealed that, the free cooling system reduced
the CO2 emissions by 430 t per year and could efficiently replace
the conventional air-condition units in 2000 offices around
United Kingdom. Raj and Velraj [68] carried out heat transfer
analysis for the fluid and PCM over a modular heat exchanger
concept with air spacers between each module of heat
exchangers. Their DSC analysis of the selected paraffin PCM
showed that a major phase change occurred in the temperature
range of 26 2 C. For single module and two air spacers,
transient/steady state CFD modelling was performed. Pressure
drop characteristics were determined using the steady state CFD
analysis and PCM solidification characteristics were analyzed
using the transient analysis. In their CFD analysis, PCM domain
was considered to be static, k–e turbulence model was used and
fully implicit method with PISO algorithm software as a solver
option was adopted for transient simulation. Isometric view of
single heat exchanger module and contours from CFD results are
shown in Fig. 5.
Takeda et al. [69] studied a ventilation system using direct heat
exchanger between PCM granules and air. They analyzed the
ventilation potential of the proposed system for eight cities in
Japan. Their results showed that the packed bed had high capacity
to stabilize the diurnal fluctuations of outdoor air temperature.
Arkar and Medved [70] adopted an appropriate packed bed
numerical model to analyze the non-uniformity of PCM porosity,
fluid velocity, and PCM temperature dependent thermal properties. The apparent heat capacity for different heating and cooling
cycles (5, 1 and 0.1 K/min) of RT 20 PCM was determined from the
DSC analysis and integrated into their numerical modelling.
Mosaffa et al. [71] undertook numerical investigations on
optimizing the free cooling system using LHTES with multiple
PCMs (Climsel 24 and KF. 4H2O). They calculated the energy
storage effectiveness and coefficient of performance (COP) of the
system. The charging and discharging process in flat multiple PCM
slabs were determined using effective heat capacity method. The
authors compared the energy storage effectiveness and other
energy based optimization methods to find out the appropriate
method of optimizing the free cooling operation using LHTES. One
of their major observations was, due to the low thermal
conductivity of liquid PCMs, time taken for the solidification
was higher than the discharging process.
Energy and exergy analysis of a multiple-PCM thermal storage
unit for free cooling applications were performed by Mosaffa et al.
[72]. Their system was designed in order to meet the requirements
of thermal comfort for the climate conditions of Tabriz, Iran. The
chosen Multi PCM's were CaCl26H2O and RT25. The authors used
COMSOL Multi physics for carrying out the analysis on thermal
performance of the system. Their results revealed that both inlet
air temperature and air flow rate increased the heat transfer rate,
outlet temperature and amount of heat absorbed by the PCMs. Chiu
et al. [73] carried out the techno-economic feasibility study with
multi objective optimization of active free cooling LHTES system
for Sweden in comparison to conventional air conditioning system.
They used finned pipe heat exchanger in which the HTF circulated
in the pipe and the PCM was filled in between the fins. The authors
calculated the cost of space cooling unit based on the Swedish
statistics, with the cost/(m2 year) considered at 230 s; for Stockholm, Tay et al. [74] determined the actual useful energy required
for a multi-storey commercial building with a total floor area of
8000 m2. The storage effectiveness of their PCM tank ranged from
0.68 to 0.75. Effect of PCM plate thickness, air flow rates, cooling
power, Stefan number and inlet air temperature on a free cooling
system was numerically investigated by Darzi et al. [75]. They
82
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
Fig. 5. Isometric view of a single heat exchanger module and simulation contours of Raj and Velraj [68].
analyzed the performance of the free cooling system in two cases,
(i) by changing the temperature gradient and (ii) by changing the
air flow rates. Fig. 6 shows the temperature profiles of the PCM
plates at different airflow rates. In their analysis, SP22A17 (from
Rubitherm) was used as PCM, because of its lower volume
expansion and suitable phase transition temperature. Their major
observations were (a) conduction dominated during the start and
the natural convection had the major impact in the melted zone of
the PCM, (b) PCM melted faster with higher Stefan number and
higher air flow rate, (c) PCM thickness had strong influence in the
performance of the PCM, (d) cooling power increased as airflow
rate increased and (e) reduced airflow rate was advisable to lower
the outlet temperature by increasing the resident time of the
flowing air.
6. Free cooling technology—applications
PCM based free cooling concept is most suitable for telecommunicating base station (TBS), data centers and particularly
for the buildings (commercial and residential) located in regions
where the temperature variations between the day and night is
very high. In this section, the technical feasibility along with the
energy saving potential achieved through free cooling technology
are discussed and reviewed.
Walsh et al. [42] investigated the application of PCMs in an
industrial evaporative cooling system. The night time cool energy
stored in the PCM is used during the day peak time to reduce the
water temperature. Through the free cooling application, 67%
reduction in peak time chiller operation was achieved. Jaber
and Ajib [76] proposed indirect evaporative and storage unit
(IESU) for the space cooling applications of residential buildings
located at Mediterranean climate. The authors investigated the
potential of ISEU in technical as well as in the economic point of
view. From technical point of view, initial cost of 5671 s is
required for the ISEU unit in order to cover the chosen cooling
load whereas from the economic point of view by optimizing the
dimensions of the room, initial cost of the proposed ISEU system
was reduced to 1195 s and the annual electricity consumption
including both the primary and secondary fans was reduced to
443 kWh.
Xiaoqin et al. [77] opted the free cooling technology in TBS to
achieve the energy savings and reduced the operating hours of
conventional air conditioning system. The authors developed a
full scale LHTES prototype system which was installed and tested
in TBSs located at five different parts of China. As a result of
adopting the free cooling concept, 67% of annual energy savings
was experienced in the Kunming city and 50% energy savings was
achieved in rest of the four cities. Eduard et al. [78] integrated the
free cooling concept in the data centres located at various
European cities and its energy saving potential was critically
analyzed. The researchers recommended the LHTES system only
in the atmospheric conditions with high humidity ratio and
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
83
Fig. 6. Temperature contours of Darzi et al. [75].
lesser ambient temperature (below 18 C). In the regions with
less humidity ratio, the cool ambient air is mixed with the part of
the room return air and it is directly used to maintain the desired
temperature in the data centre (direct air free cooling). The
authors achieved 51% (depending on locations) savings in cooling
electricity cost due to the integration of free cooling with the
conventional air chillers in data centres.
Peng et al. [79] investigated the energy saving potentials of the
air based free cooling technology with different enthalpy controls
incorporated in data centres located in 17 climatic zones. The
performance of their free cooling system varied based on the local
climatic conditions. The free cooling technology installed in the
cool climatic zones showed significant energy savings potential
whereas in the dry and humid zones, benefits of the same system is
substantially reduced due to the operation of high energy intensity
fans and the local climatic factors. In addition, the authors
observed that the energy savings potential of their free cooling
system got reduced by 2.8–8.5% for every 2 C temperature decline
in the indoor environment of the data center.
7. Free cooling of buildings using PCM—a way forward
The selection of PCM is based on the phase transition
temperature, thermal conductivity and subcooling nature of the
PCM and the other operational parameters like velocity/temperature of the inlet/outlet air and local geographical conditions which
have been explained in the previous chapters. In the second part of
the review, case studies on free cooling potential, scope for future
improvements in free cooling technology, economic analysis and
84
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
promotion policies to be initiated by government agencies to
commercialize and implement the free cooling concept in
buildings are discussed in detail.
7.1. Assessment of free cooling potential
Various literature pertaining to night ventilation, free cooling
technologies with and without thermal storage, case studies on
free cooling potential carried out in different regions of the world
are reviewed and presented in this section. Free cooling would be
best studied for regions where diurnal temperature range lies
between 12 C and 15 C. Desert and interior regions are highly
profited by the application of free cooling concept however in
coastal areas where diurnal temperature variation is less, it could
be less beneficial [51]. Indirect evaporative and storage unit (IESU)
consisting of a PCM heat exchanger was designed, modelled and
simulated by Jaber and Ajib [76] for a typical Jordanian residential
building named “Dar”. Experimental investigations on PCM
melting temperature, heat exchanger parameters, and air flow
rate were carried out. The authors calculated the heat absorbed/
released using the following equation,
Q 1 ¼ ap AðT ai
T pcm Þ; T pcm > T m ðLiquidÞ
Q t ¼ ml ðTransitionÞ
Q s ¼ ap AðT ai
T pcm Þ; T pcm < T m ðSolidÞ
where, Ql is the sensible heat in liquid state (kJ), Qt is the latent heat
stored during transition state (kJ), Qs is the sensible heat in solid
state (kJ), ap is the fictive convective heat transfer coefficient
(kJ m 2 K 1 s 1), m is the mass of PCM (kg), l is the latent heat of
fusion (kJ/kg), Tm,Tpcm, Ta are the melting temperature, PCM
temperature, air temperature ( C), respectively and ‘A’ is the heat
exchanger area in m2. Effectiveness of night ventilation technique
for residential buildings in Malaysia for hot and humid climate was
investigated by Kubota et al. [80]. Results from their experiment
showed that night ventilation displayed better performance for
building with terraces, compared with the other ventilation
strategies in terms of operative temperature. Parys et al. [81]
studied the feasibility of passive cooling of buildings solely by the
diurnal manual window operation, for the climatic conditions in
Belgium. PCM with higher temperature can be used for temperature moderation during summer season for load reduction [82].
Free cooling with low melting temperature PCM can be used
during winter. For places above 20 N latitude, the year-round
ambient temperature will not be above 30 C. Heating will be
required during winter. Here free cooling can be effected during
summer. Rajagopal et al. [83], studied the efficacy of free cooling
potential for Bangalore city in India, using the hourly weather data
for a period of one year, and adopting the CLTD method of cooling
load estimation. They reported that Bangalore city did not require
any mechanical cooling throughout the year. In Istanbul, energy
savings during the transition period April, May, September and
October was observed by Bulut and Aktacir [84], using hourly dry
bulb temperature measurements. During the hotter months June–
August the system could be made beneficial only if the air supply
temperature was high.
7.2. Economics and CO2 emission analysis
Zalba et al. [41] conducted an economic analysis of free cooling
technology using PCM for building applications. The authors
considered all the materials including the fan for the cost analysis
of their free cooling system. In Fig. 7, the cost distribution of PCM
based free cooling system by Zalba et al. [41] is shown.
Viability analysis between free cooling and conventional
refrigeration system was done by Waqas and Din [18]. They
inferred that the PCM storage system needed an additional
investment of 9% with a payback of 3–4 years with free cooling.
The electric power consumption of the free cooling system was
9.4 times lower than the conventional split type air-conditioning
unit. From the study by Bulut and Aktacir [84], it was understood
that for the air supply temperatures less than 24 C, cooling unit
installed in Istanbul could achieve 100% free cooling for 4671 h
which represented 54% of the year. Their cooling unit achieved the
partial free cooling for 3108 h, which was 35% of the year.
Therefore, total free cooling would be available for 89% of the year,
which represented considerable energy and cost savings. The
Fig. 7. Cost distribution of the PCM based storage system for free cooling applications.
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
85
Fig. 8. Equivalent cost savings and simple payback years of the free cooling system.
average part-load ratio was 0.5, if the compressor power was
500 kW and the average energy cost in Istanbul was 11 c/kWh. They
also estimated that the for air supply temperature less than 24 C,
operating cost savings would be 342.375$ per annum.
In tropical countries like India where 300 sunny days exists, the
cooling demand requirement is much high rather than the heating
load. In most of the cities in India during the day peak hours, the
operation of the air conditioning system (A/C system) is unavoidable
in order to meet the human thermal comfort. Hence the
implementation of the free cooling technology in buildings,
telecommunication base stations and data centres will lead to the
lesser operation of conventional air conditioned system and results
in electrical energy savings as well as the reduction of CO2 emissions.
In this section, a cost comparison of PCM based free cooling
technology with conventional air conditioning system (21 kW
cooling capacity), electrical energy savings achieved through free
cooling technology implementation and the estimation of simple
payback period are made. In Table 5, the electricity consumption and
operating cost per annum of a 21 kW A/C are shown.
Selection of suitable PCM for free cooling application is based on
the local climatic conditions. However, in this calculation, PCM with
phase change temperature of 20–25 C and the PCM average latent
heat storage capacity of 185 kJ/kg are considered. It is assumed that,
the operation of fans in free cooling system consumes 15% of total
electricity consumption of a conventional A/C system and accordingly the operating cost of the free cooling system is calculated.
Quantity of PCM required to produce 21 kW cooling, equivalent
electrical energy savings expected and simple payback period of a
free cooling system are described in Table 6. In Fig. 8, the total
investment required, equivalent cost savings expected by means of
free cooling system and simple payback period are shown. From
Fig. 8 it is also inferred that, better cost savings and quicker payback
period of the free cooling system can be achieved for high capacity air
conditioning system (>18 kW cooling system).
The annual CO2 emission comparison between the conventional A/C system and PCM based free cooling system is made in this
section. The CO2 emission factor (India) for producing 1 kWh
energy from coal fired power plant including the transmission and
Fig. 9. CO2 emission comparison of conventional A/C and free cooling system.
86
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
Table 5
Energy consumption of a 21 kW A/C system.
System parameters
Quantity with units
Thermal energy required to produce 21 kW cooling capacity
Coefficient of performance (COP) of the conventional A/C system
Power consumption of 21 kW cooling system
A/C operational hours considered
Average energy consumption per annum (250 days)
Average operating cost of the cooling system per annum
72,000 BTU/h (75,960 kJ/h)
3.514
7.8 kW (energy efficiency ratio [EER] = 2.7)
6 h (day peak time)
11,700 kWh
Rs. 76,050 (@ Rs. 6.5/kWh)
distribution losses is considered as 1.58 kg CO2/kWh [85]. Fig. 9
shows the CO2 emission analysis for the conventional A/C system
and the PCM based free cooling system.
7.3. Scope for future improvement in free cooling technology
The increasing energy demand and carbon emission are the
main driving forces that made the researchers to work on green
building technologies. Due to its advantages and facilities over
space cooling applications in buildings, free cooling technology
using PCM as storage medium has gained popularity among
researchers and HVAC engineers. Though extensive research work
on free cooling technology has been carried out over the past
decade, commercialization of free cooling concept continues to
remain the dream of architects and researchers. In this chapter,
further improvements and investigations required for the commercialization of free cooling concept are discussed in detail and
are summarized below.
Phase-change materials with high energy storage density have
an unacceptably low thermal conductivity and hence heat
transfer enhancement techniques are required for free cooling
applications as the available driving potential is very low during
charging and discharging.
The quantity of the phase change material required for buildings
would be enormous hence the cost effective PCM has to be
installed to optimize the resources.
Attempts should be taken to implement the hybrid cooling
technologies for building applications that operate with the
combination of free cooling, radiative cooling and evaporative
cooling concept, which could result in high efficiency and a
sustainable system.
Energy and exergy analysis carried out on PCM based free cooling
concept in literature are found to be less and insufficient. The
application of PCMs for free cooling in buildings is subjected to
local climate conditions. Feasibility analysis of free cooling
technology for different climatic conditions such as hot and dry,
warm and humid, moderate, cold and composite is required to
potentially utilize the effect of free cooling.
Appropriate design of the heat exchangers, mode of operation
and efficient control strategies are essential for efficient
utilization of free cooling technology.
Adopting free cooling system using PCM reduces the building
related CO2 emissions compared to the conventional air
conditioning system. Hence quantification of emission reduction
due to free cooling is essential.
7.4. Promotion policies
A consolidated mapping of free cooling potential for the entire
country will be very supportive for building architects/engineers
to proceed further in the free cooling technology. Hence
initiation by appropriate authorities is essential.
Innovative promotional schemes/incentive should be introduced
by the government through various agencies, toward the
implementation of free cooling concept in buildings.
Awareness should be created among the public about the free
cooling concept. Interested entrepreneurs should also be trained
and encouraged to commercialize this concept on a large scale.
If all the above promotional policies are adopted, it will translate
to a large energy saving potential in residential buildings, which
will be very helpful for fast developing countries like India to
meet the increasing energy demand and to achieve energy
sustainability.
Implementation of free cooling concept in buildings is also
essential to the global scenario to curtail the carbon emission
from the building sectors that consumes the major portion of
electricity being generated in any country.
8. Conclusion
In this review article, the concept of free cooling, PCMs suitable
for free cooling applications in buildings, commercial PCMs that
are available, effects of temperature and air flow rate on free
cooling, cooling degree days, subcooling of PCM, various experiments carried out and mapping of free cooling technology along
Table 6
Simple payback period calculation.
Description
Values with units
Latent heat storage capacity of the PCM
PCM quantity required to produce equivalent cooling
Cost of the PCM
Operating cost
Other cost including heat exchanger, insulation, instrumentation and storage tank cost
Total cost of the free cooling system
Electrical energy savings expected due to the operation of free cooling system during the day peak time
Total cost savings
Simple payback period
185 kJ/kg
2,464 kg
Rs. 320,263 (Rs. 130/kg of PCM)
Rs. 11,407
Rs.270,000
Rs. 590,263
11,700 kWh/annum
Rs. 64,643/annum (76,050–11,407)
9.1 years
M. Thambidurai et al. / Journal of Energy Storage 4 (2015) 74–88
with the promotional policies needed toward energy/environmental sustainability are summarized in detail. The major
conclusions from the present review are as follows,
Selection of PCM with the optimum melting temperature with
high heat storage capacity determines the effectiveness of the
free cooling system.
Free cooling concept is best suited for less humid and maximum
diurnal temperature range regions rather than warm and humid
climatic conditions. However, with careful design of heat
exchangers along with dehumidification of air, free cooling
concept can be effectively implemented even in warm and
humid areas.
By decreasing the inlet air temperature with optimal inlet air
velocity, solidification time can be reduced and it results in
complete charging of the PCM in lesser time.
Subcooling nature of the PCMs can be reduced by adding
nucleating agents, however it cannot be mitigated completely.
PCM, encapsulation material, air ducts and packaging are the
parameters that should be given more importance for a cost
effective free cooling technology.
Mapping of free cooling potential zones, construction of large
scale demonstration projects and promotion policies by the
government for free cooling technology are the essential steps to
be taken to make the technology commercially viable.
Commercializing and mass implementation of free cooling
technology in residential sectors will curtail air conditioner
(AC) running hours and corresponding greenhouse gas emissions.
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