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31 

 

 

 

Article 

Energy consumption reduction in a building by 

free cooling using phase change material (PCM) 
Aidin Shaghaghi1, Reza Eskandarpanah2, Siavash Gitifar3, Rahim Zahedi4, Hossein 

Pourrahmani5, Mansour Keshavarzzade6, Abolfazl Ahmadi1* 

1Department of Energy Systems Engineering, Iran University of Science and Technology, Tehran, Iran 
2Department of Energy, Islamic Azad University Science and Research Branch, Tehran, Iran 
3Faculty of Mechanical and Energy Engineering, Shahid Beheshti University, Tehran, Iran 
4Department of Renewable Energy and Environmental Engineering, University of Tehran, Tehran, Iran 
5Group of Energy Materials (GEM), École Polytechnique Fédérale de Lausanne (EPFL), 1951 Sion, Switzerland  
6Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa 

A R T I C L E   I N F O 
 

Article history: 
Received 04 July 2023  
Received in revised form 
06 August 2023 
Accepted 14 August 2023 
 
Keywords: 
Latent heat thermal energy storage, Phase change 
material, Free cooling, Numerical analysis 
 
*Corresponding author 
Email address:  
a_ahmadi@iust.ac.ir  

 
DOI: 10.55670/fpll.fuen.3.2.4 

A B S T R A C T 
 

It is significantly important to implement energy storage systems nowadays. 
Latent heat thermal energy storage (LHTES) systems contain numerous 
advantages as a result of their small temperature variation and higher energy 
storage densities during storage. The present paper deals with the cooling load 
of a room in Zanjan, Iran using Carrier software. Then, a free cooling system 
using commercial paraffin RT25 was numerically analyzed as phase change 
material (PCM) while investigating the effects of the flow rate of the storage 
tank and inlet air temperature overcharging and discharging procedures. Based 
on cold energy storage simulation, by airflow with the temperature of 20°C at 
night, the paraffin is solidified in 4 h. Stored cold energy of 1.4 kW in PCM 
releases energy through a free cooling system within 2.1 h of July afternoon in 
the room. 

 
1. Introduction  

The domestic and commercial sectors consume about 
40% of the world's total energy and produce a third of the 
greenhouse gases, a significant portion of which is spent on 
heating, cooling, and air conditioning. One of the effective 
ways to improve the use and protection of energy resources 
is the development of energy storage systems. Latent thermal 
energy storage using phase change materials has high 
efficiency and reliability and also, due to high energy storage 
capacity, has received much attention [1]. The basis for using 
phase change materials for free ventilation of buildings has 
two stages: first, at night, when the ambient temperature is 
lower than room temperature, the flow of cool ambient air 
passes through the energy storage unit, and the heat of the 
phase change material in the liquid state and the material 
begins to freeze at a constant temperature. Then, when the 
room temperature rises above the desired level during the 
day, the cool air stored in the phase change material is 
evacuated. As the hot air passes through the energy storage 
unit, the phase change material receives this heat and melts 
at a constant temperature, and finally, the desired cooled air 
enters the room. The efficiency of an open-air conditioning 

system largely depends on the climatic conditions of the 
environment. The properties of the phase change material, 
especially its melting temperature, are very important, and 
this temperature should be in the range of daily temperature 
changes. Airflow is very important for the successful 
operation of an open-air conditioning system, and if this flow 
is adequate, then heat transfer will be successful [2]. 
According to ASHRAE Standard 55, thermal comfort is a 
mental condition that expresses the sense of human 
satisfaction with environmental conditions. This standard has 
presented a proposed list of temperatures and airflows for 
different environmental conditions and buildings, which 
generally provides room comfort temperature in summer at 
23.5°C to 25.5°C Therefore, in free ventilation applications for 
buildings, phase change materials are preferred with a phase 
change temperature of 18°C to 30°C [3]. One of the most 
important studies on the free ventilation system was carried 
out by Chinnasamy et al. [4], which aimed to reduce the air 
conditioning load in buildings. Na2SO4.10H2O hydrated salt 
with a melting temperature of 21°C was used as a phase 
changer. The prototype of this system was installed and 
tested in an ordinary office, so results show that 270wh of 

 

 

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A. Shaghaghi et al. /Future Energy                                                                                                  May 2024| Volume 03 | Issue 02| Pages 31-36 

32 

 

thermal energy was stored during 8 hours of office work. If 
the pilot system replaces conventional cooling units in the UK, 
CO2 emissions will decrease by 430 tons per year. Takeda et 
al. [5] studied the storage system with a platform of phase 
change materials for air conditioning of the building in 
Japanese weather conditions. Room temperature for 
ventilation was stabilized at 26°C, and the experimental 
model consisted of a rectangular air conditioning duct with a 
granule phase change material (The phase change 
temperature was 22.5°C to 25°C). Using computer simulation, 
the potential of this system in reducing air conditioning load 
during summer was investigated for eight Japanese cities, and 
Kyoto having the highest efficiency with a reduction of the air 
conditioning load of about 62.8%. Yamaha and Misaki [6] 
considered an air distribution system with phase change 
materials inside the air duct, which was designed to cool the 
air and reduce the cost of electricity consumption in Japan. 
The phase change material was a combination of fatty acid 
and paraffin, which initially measured and recorded the 
properties of the compound. The charging process of the 
phase change material (cold storage) was done from 5 to 8 
a.m., and the discharge process was from 13 to 16 pm. For a 
typical office building in Nagoya city, 5.4 kg/m2 of the phase 
change material was able to keep the room temperature 
constant and desirable, and the suitable melting temperature 
of the material for successful system performance was 20°C. 
Streeti and Butala [7] considered a latent heat storage unit for 
free ventilation, which contained 3.6 kg of commercial RT20 
paraffin with a melting point of 22°C and a heat storage 
capacity of 172 kJ / kg. The results of numerical simulation 
and experimental work are very close to each other. The 
results showed that when the inlet air temperature is 26°C 
and the inlet air velocity is 1 m/s, the stored cold can keep the 
air below 24°C for 2.1 hours. 

In the present article, first, the cooling load of one of the 
rooms in Zanjan, with a cold and dry climate, was calculated 
in the hottest month of summer (July). Then, a numerical 
simulation of the free ventilation system with latent heat 
storage using RT25 phase change material of commercial 
paraffin type made by the German company Robiterm, was 
performed to investigate the potential of free ventilation to 
cool the room. 

2. Physical model 

In this paper, a flat plate heat exchanger [8] with layers 
containing phase change materials is used. The reason for 
choosing this type of heat exchanger is flexibility in adjusting 
the surface area, controlling the flow rate of passing air, and 
easy construction. The volume of the selected control for 
numerical simulation is shown in Figure 1, and this geometry 
was created using Gambit software; the dimensions can be 
seen in Figure 2. Due to the temperature gradient, a smaller 
mesh should be used in the area adjacent to the air with the 
phase change material. For this research, the number of cells 
was calculated at 15 thousand. 

3. Numerical modeling 

Fluent software uses the enthalpy-Porosity method to 
model the melting and freezing process [9]. In this method, 
the location of the joint surface separating the two phases is 
not explicitly specified; instead, there is a value called the 
Liquid Fraction, which represents a fraction of the cell volume 
that is liquid, and this value is calculated in each repetition 
based on the enthalpy equilibrium [10]. The Mushy Zone is 
the part where the Liquid Fraction varies between zero and 
one, and this region is modeled like a quasi-porous medium 

in which the amount of porosity changes from one to zero as 
the material freezes. When the material inside the cell is 
completely frozen, the porosity is zero. 

 
Figure 1. Selected control volume for numerical analysis 

 

 
Figure 2. The geometry of problem (mm) 

 
3.1 The governing equations 

The conservation equations of mass and momentum are 
derived from relationships (1) and (2) [11]: 

𝜕𝜌

𝜕𝑡
+ ∇. (ρv) = 0                              (1) 

𝜕

𝜕𝑡
(𝜌𝑣) + ∇. (𝜌𝑣𝑣) = ∇. [𝜇(∇𝑣 + ∇𝑣𝑇)] − ∇𝑃 + 𝜌𝑔 + 𝑆            (2) 

S is a momentum source that contains parts of the sensitive 
area porosity, surface tension at the joint surface of the two 
phases, and other external forces entering the surface. This 
value is calculated using Eq (3) below [12]: 

 

𝑆 =
𝐶(1−𝛽)2

𝛽3 𝑣             (3) 

Table 1 shows the properties of RT25. According to Eq (4), the 
enthalpy is written as the sum of the tangible enthalpy (h) and 
the latent heat (h), where the tangible enthalpy (h) is obtained 
from Eq (5). 

𝐻 = ℎ + ∆𝐻                                                                         (4) 

ℎ = ℎ𝑟𝑒𝑓 + ∫ 𝐶𝑝. 𝑑𝑇
𝑇

𝑇𝑟𝑒𝑓
                                                                     (5) 

Liquid fraction (β) is defined as Eq (6): 

𝛽 = {

0
1

𝑇−𝑇𝑠

𝑇𝑙−𝑇𝑠

                        

𝑇 ≤ 𝑇𝑠
𝑇 ≥ 𝑇𝑠

𝑇𝑠 ≤ 𝑇 ≤ 𝑇𝑙

                                  (6) 

The amount of latent heat H can be written in Eq (7): 

∆𝐻 = 𝛽𝐿                                                                                               (7) 

Finally, the energy equation is written as an Eq (8): 

𝜕

𝜕𝑡
(𝜌𝜌𝐻) + ∇. (𝜌𝑣𝐻) = ∇. (𝐾∇𝑇) + 𝑆𝑛                                          (8) 



A. Shaghaghi et al. /Future Energy                                                                                                  May 2024| Volume 03 | Issue 02| Pages 31-36 

33 

 

Table 1. Properties of RT25 [13] 

Property Value 
Melting and freezing 
temperatures 

24 (C) 

Latent heat 155 (kJ/kg) 
Heat capacity 2000 (J/kg.K) 
Density 770 (kg/m3) 
Thermal conductivity 0.2 (W/m.K) 

 

3.2 Boundary conditions 
In this paper, the boundary conditions are such that the 

inlet air enters the system at a certain speed and temperature, 
so the inlet is selected as the Velocity Inlet type. In the part 
where the air comes out, the boundary condition of the 
Outflow is selected, and the boundary condition of the layers 
between the air and the phase change material is wall type. 
Due to the symmetry in the problem and according to the 
selected control volume, the upper and lower walls are 
considered symmetrical. 

3.3 Initial conditions 
The velocity and temperature of the air entering the tank 

are two important variables to evaluate the performance of 
the thermal energy storage system in the melting and freezing 
processes. To consider different states for temperature and 
speed, it is best to make these variables dimensionless. 
Reynolds number is used to de-dimensionalize the velocity, 
which is calculated for the flow through the channel using Eq 
(9). For Reynolds numbers less than 2,300, the flow is calm, 
and for Reynolds numbers larger than 2,300, the flow is 
turbulent. In order to de-dimensionalize inlet air 
temperature  in the melting process, it is better to use the 
Stephen number, which is defined by Eq (10) [14]. 

𝑅𝑒 =
𝜌.𝑉.𝐷ℎ

𝜇
                                                                                            (9) 

Where ρ is density, V is the velocity of the input air, Dh is the 
hydraulic diameter of the channel, and µ is the dynamic 
viscosity of the air. 

𝑆𝑡𝑒 =
𝐶𝑝(𝑇𝑖𝑛−𝑇𝑚)

𝐿
                                                                 (10) 

Where Cp is the specific heat capacity, Tin is the temperature 
of the input air, Tm is the melting temperature of matter, and 
L is the latent heat of matter. 
The initial conditions during the melting and freezing process 
are briefly listed in Table 2. During the charging process, 
given that the rt25 phase change temperature is about 24°C, 
it is assumed that the initial temperature of the material in the 
tank is 3 degrees above the freezing temperature of 27°C. 

Table 2. Initial conditions 

Process Velocity 
(m/s) 

Reynolds 
number 

Temperature 
(C) 

Stephen 
number 

Charge 7 10811 20 - 

charge 9.3 14446 20 - 

Discharge 3 4660 30 0.075 

discharge 4 6213 31 0.088 

 
According to Figure 3, the dry temperature of Zanjan is 20°C 
during the night to the morning. Also, due to low airflow, 
using a ventilation fan the air speed entering the tank 
increases. From 4 p.m. onwards, when the ambient 
temperature is the highest, the common ventilation of the 
building has been extinguished, and free ventilation is used 
for cooling. The air temperature entering the tank is 30°C and 

31°C in the melting process with an average temperature. 
Also, using a ventilation fan, the air velocity entering the tank 
reaches 3 m/s and 4 m/s. 

 

Figure 3. The temperature of Zanjan City on the hottest day 

of July 

3.4 Solution conditions 
Fluent software has two different solution methods, one 

based on pressure and the other one based on density in the 
simulation of the melting and freezing process; only a 
pressure-based solver can be used. The flow is transient, and 
the standard k-e method is used to model the turbulence. 
Piezo algorithm is selected for the pressure-velocity 
relationship, and also Presto method is selected for pressure 
discretization, and the second-order upstream design method 
is used for energy discretization. The Under-relaxation 
factors for pressure, density, momentum, liquid fraction, and 
energy are 0.7, 1, 0.3, 0.9, and 1, respectively. Convergence 
criteria were selected values of 10-4 for mass and momentum 
conservation and 10-8 for energy conservation. 

4. Calculation of room cooling load 

Basically, the correct estimation of the cooling load 
depends on the detailed examination of the load components 
in the ventilated environment, like complete plans of the 
building and the general design of the space. In this article, the 
building shown in Figure 4 is a residential unit with an area of 
120 square meters as a south ground floor.  

 
Figure 4. Plan of the building 



A. Shaghaghi et al. /Future Energy                                                                                                  May 2024| Volume 03 | Issue 02| Pages 31-36 

34 

 

The area of the building is asphalt, the average reflection 
coefficient of the ground surface is 0.2, and also the thermal 
conductivity of the soil in the area is 1.35 W/m.K. First, in 
order to check the amount of cooling load required to design 
the ventilation system, the cooling load of one of the rooms 
selected here, bedroom A, is calculated. The door of the room 
is made of wood with dimensions of 0.9*2.5 m2, the window 
is made of aluminum with dimensions of 1.8 *1.2 m2, and the 
lighting equipment consists of a fluorescent lamp and an 
incandescent lamp. Also, the number of people using a room 
is considered. By using the carrier software for July, the 
amount of cooling load during one day and night is calculated 
and shown in Figure 5. Also, the amount of aeration required 
for the room is about 420 m3/h. 

Figure 5. The cooling load of sleeping room A 

 
5. Results and discussion 

5.1 Validation of numerical simulation 
To validate numerical simulation, the results are first 

compared with the experimental work done in reference [8] 
and shown in (Figure 6). The average error rate between 
numerical and experimental results is about 4%, which is 
valid numerical results. 

5.2 Cooling storage process 
Figure 7 shows the change in temperature of the phase 

change material and the output air of the system over time. 
With the entry of cool air into the system due to heat 
absorption from the material, first, the temperature of the 
material decreases to freezing temperature, then phase 
change begins, and the temperature decreases with a gentle 
slope. Finally, after complete freezing of the material, the 
temperature decreases again with a steep slope. At a constant 
temperature, the higher the velocity of the air entering the 
system, the faster the material freezes. When the air arrives 
at 20°C and the tank speed of 7m/s, the complete freezing of 
the material lasts for 4 hours. By increasing the inlet speed to 
9.3m/s, the total freezing time of the material reaches 3.5 
hours. 

5.3 The process of discharging cooling energy 
According to the profile of ambient air temperature 

during the day for Zanjan, the cooling energy discharge 
process has been studied in the following four different 
conditions. In the first and second cases, the Reynolds number 
is 6213, and the Stephen number changes from 0.075 to 
0.088. In the third and fourth cases, the Reynolds number is 
4660, and the Stephen number changes from 0.075 to 0.088. 

Figure 6. Validation of numerical analysis during charging 
process 

 
Figure 7. The temperature of the PCM and outlet air during 
the charging process 

 
Figure 8 shows the temperature change of the phase 

change material over time. When hot air enters the tank, the 
frozen phase change material absorbs heat from the hot air 
and reaches its melting temperature after 0.5 hours. Then the 
phase change begins, and the temperature rises with a gentle 
slope, and finally, after the material has completely melted, 
the temperature rises again with a steep slope. The liquid 
fraction during the melting process is shown in Figure 9. In a 
constant Reynolds number with a 17% increase in Stephen's 
number, the melting time decreased by 0.5 hours, and in a 
constant Stephen number with a 25% reduction, the melting 
time increased by 1 hour. 

One of the most important results of the paper is the 
temperature of the output air for ventilation of the room, 
which is shown in Figure 10. As time goes on and the material 
melts more and more, the outlet air temperature increases. 
Tamaskani and Esfahankalateh [15] have conducted a study 
with the aim of determining the range of thermal comfort for 
the city of Zanjan. The results show that the comfort 
temperature inside the room for July is about 23°C to 28.5°C. 
According to the calculations, the output air temperature of 
the storage tank for ventilation of the room up to 26.5°C is 
considered the permissible temperature. The results are 
summarized in Table 3, so in the lowest Reynolds number and 



A. Shaghaghi et al. /Future Energy                                                                                                  May 2024| Volume 03 | Issue 02| Pages 31-36 

35 

 

the lowest Stephen number, the best situation occurs, and 
there is comfortable air ventilation for 2.1 hours. 

 
Figure 8. Temperature variation of the PCM during the 
discharging process 

 

 
Figure 9. Variation of liquid fraction during the discharging 
process 

 

 
Figure 10. Temperature variation of the outlet air during the 
discharging process 

 
 

Another important result is the amount of cooling air 
created by free ventilation. During the melting process, the 
cooling load is constantly decreasing, and the higher the 
Reynolds number or Stephen number, the higher the cooling 
load. In Figure 11, a comparison is made between the amount 
of cooling air required by the room and the cooling air created 
by the free ventilation from 16:00 onwards. According to this 
diagram, free ventilation has provided the required cooling 
air for up to 2.1 hours, an average of 1.4 kW of cooling air has 
been injected into the room. 

Table 3. Result of discharging process 

Condition  Outlet air 
temperature 
(C) 

Aeration 
(m3/h) 

Duration 
of 
ventilation 
(h) 

Melting 
rate (%) 

First 25.5 – 26.5 570 1.5 56 
Second  25.9 – 26.5 570 1 45 
Third 25.1 – 26.5 420 2.1 74 
fourth 25.4 – 26.5 420 1.5 55 

 

 

 
Figure 11. Comparison of cooling load during the discharging 
process 

 
6. Conclusion 

In this paper, the effects of temperature and velocity of 
the air entering the free ventilation system with cold storage 
were investigated by a numerical simulation, and the 
following results were obtained: 
• In the charging process, the complete freezing of the phase 

change material takes 4 h to complete when the air enters 
the heat exchanger with a temperature of 20 ° C and a speed 
of 7 m/s. 

• In the discharge process, in a constant Reynolds number 
with a 17% increase in Stephen's number, the melting time 
decreased by 0.5 hours, and in a constant Stephen number 
with a 25% reduction, the melting time increased by one 
hour. 

• In the discharge process, in the lowest Reynolds number, 
which is 4660, and the lowest Stephen number, which is 
0.075, the best ventilation occurs. In this case, the melting 
process takes 5 hours, and 2.1 hours of optimal air is 
available for room ventilation. The amount of aeration, in 
this case, is 420 m3/h, which is sufficient. Also, during this 
period, an average of 1.4 kW of cooling air was injected into 
the room. 



A. Shaghaghi et al. /Future Energy                                                                                                  May 2024| Volume 03 | Issue 02| Pages 31-36 

36 

 

Ethical issue 
The authors are aware of and comply with best practices in 
publication ethics, specifically concerning authorship 
(avoidance of guest authorship), dual submission, 
manipulation of figures, competing interests, and compliance 
with policies on research ethics. The authors adhere to 
publication requirements that the submitted work is original 
and has not been published elsewhere in any language. 

Data availability statement 
Datasets analyzed during the current study are available and 

can be given following a reasonable request from the 

corresponding author. 

Conflict of interest 

The authors declare no potential conflict of interest. 

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