PCM integrated to external building walls: An optimization study on maximum activation of latent heat

Journal Pre-proofs PCM integrated to external building walls: An optimization study on maximum activation of latent heat Müslüm Arıcı, Feyza Bilgin, Sandro Nižetić, Hasan Karabay PII: DOI: Reference:

S1359-4311(19)33615-4 https://doi.org/10.1016/j.applthermaleng.2019.114560 ATE 114560

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Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

25 May 2019 24 September 2019 16 October 2019

Please cite this article as: M. Arıcı, F. Bilgin, S. Nižetić, H. Karabay, PCM integrated to external building walls: An optimization study on maximum activation of latent heat, Applied Thermal Engineering (2019), doi: https:// doi.org/10.1016/j.applthermaleng.2019.114560

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PCM integrated to external building walls: An optimization study on maximum activation of latent heat Müslüm Arıcı1*, Feyza Bilgin1, Sandro Nižetić2, Hasan Karabay1

1Department

of Mechanical Engineering, Faculty of Engineering, Kocaeli University, Umuttepe Campus, Kocaeli 41380, Turkey

2LTEF-

Laboratory for Thermodynamics and Energy Efficiency, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Rudjera Boskovica 32, 21000 Split, Croatia

*Corresponding author; tel.: +90 262 303 34 52; e-mail: [email protected] 1

Highlights: 

Optimization study based on the maximum activation of latent heat fusion



Optimization of PCM location, melting temperature (Tm) and thickness (L) for Turkey



Investigation of energy saving, decrement factor and time lag



Optimum Tm and L vary 6-34oC and 1-20 mm depending on climatic conditions



Up to 13.3 h increment in time lag and 18% of energy saving can be obtained.

2

Abstract Integration of the phase change materials (PCM) into the external building walls is an efficient method for reduction of energy consumption and regulation of energy demands due to increasing thermal inertia of the walls. This study aims to reveal the contribution of latent heat to the thermal performance of the wall and to determine the location, thickness and melting temperature of PCM for the maximum exploitation of latent heat for different climatic conditions. A comparative study is carried out for the wall coupled with PCM and the wall with Phase Stabilized PCM (PSM) to reveal the improvement provided by the latent heat. The influence of location, fusion temperature and layer thickness of PCM on energy saving, decrement factor and time lag was examined. The annually optimized PCM fusion temperature and layer thickness which utilizes the latent heat at maximum level considering both heating and cooling loads are determined for three cities of Turkey. The computed results show that the monthly optimized PCM melting temperature and PCM layer thickness vary from 6 to 34oC, from 1 to 20 mm depending on climatic conditions. It was concluded that an optimization study should be conducted in order to prevent PCM behaves like PSM. Keywords: buildings; decrement factor; energy savings; latent heat; PCM; time lag

3

1. Introduction In recent years, the applications of thermal energy storage in buildings have increased together with the increasing interest in renewable energy technologies. Thermal energy can be stored in three forms; sensible heat, latent heat and chemical energy [1]. The latent heat storage by phase change materials (PCMs) is more preferred among other energy storage methods because it can be absorbed (or released) the great amount of energy at almost isothermal temperature or in a very narrow temperature range [2]. The desired properties from PCM are high thermal storage capacity, suitable melting temperature (Tm), chemical stability, low cost, small volume change during solidification and availability without flammable, toxic, distortion and crystallization problem [3]. PCMs are often used in passive cooling of the photovoltaic panels [4-7], ice-bank applications [8], hot water tanks [9], agricultural industry [10], thermal protection of electronic devices [11,12] and in buildings components such as external wall [13-19], roof [20-22], floor [23,24], and glazing units [25-27]. Nowadays, the integration of PCM into building wall has become a common method for damping indoor temperature variation, regulating energy demand from the grid and consequently increasing energy saving. For instance, Athienitis et al. [28] carried out an experimental analysis of PCM in a test-room and indicated that the peak room temperature can be reduced by about 4oC during the day and heating load can be decreased at night. Similarly, the concrete wall containing two PCM layers (Tm=16oC) was numerically investigated in [29] which reported that the heating load decreased about 25% compared to the wall without PCM. In paper [30] it was indicated that the maximum indoor temperature reduced by 1oC by integrating microencapsulated PCM in a concrete wall of a building in Lleida, Spain. In the same study was shown that peak temperature shifted about 2 h due to the increase in the thermal inertia of the wall. Elnajjar [31] numerically studied the effect of an embedded PCM on the energy saving of a building wall in the United Arab Emirates. It was reported that the heat flux decreased by 30% by using bricks filled with PCM (Tm=47oC) compared to bricks without PCM. Regarding the reduction of cooling load of the building in spring and summer times, the research outcomes of studies [32,33] demonstrated that the peak air conditioner load can be decreased by using PCM, and the overheating of building rooms can be prevented thanks to the stored heat. A similar conclusion based on the experimental and numerical results was drawn by Figueiredo et al. [34], which indicated that the overheating of the indoor temperature can be prevented by the use of PCM in the university building in Aveiro, Portugal. Also, the authors suggested that the ventilation rate of air conditioner which is linked to a geothermal system is necessary to provide the solidification process of PCM. It was also emphasized that optimization analysis of PCM melting temperature is important for effective usage of the latent heat. Furthermore, a significant amount of work has been done by many researchers in order to evaluate the impact of climatic conditions on the energy savings. For example, Guarino et al. [35] experimentally and numerically analyzed the south-facing building wall including PCM for cold climatic conditions in Montreal, Canada and showed that the use of PCM can reduce the heating and cooling loads by 17% and 20%, respectively. Ye et al. [36] also conducted a comprehensive work (both experimentally and numerically) to study the effect of shapestabilized PCM on energy saving in buildings based on two newly defined parameters (energy saving equivalent and energy saving index). They carried out an experimental study in two different test rooms in Hefei, China and conducted simulations for three typical cities in China; Beijing (cold), Shanghai (hot summer and cold winter) and Guangzhou (hot summer and warm 4

winter). It was stated that thermal comfort can be improved by reducing the indoor temperature fluctuation by integrating PCM. Moreover, they reported that the performance of PCM in building wall varies depending on climatic conditions and seasons, and the insulation layer has a better performance than PCM during the year. The research team in [37] considered Harbin and Haikou cities of China, apart from those in [36], and concluded that the PCM has a significant effect on thermal performance in some months, however also minor impact in the other months of the year (thermal performance analysis should be performed throughout the year). Mazzeo et al. [38] notified that the PCM increased the energy saving of the building wall in some months of the year for two different cities (Turin and Cosenza) of Italy. To determine the impact of climatic conditions on the energy performance of the building wall containing PCM, a large-scale study has been performed in [39] for twenty-five metropolitan cities in the world. The authors emphasized that the heating and cooling energy consumptions reduced for the cities with arid and warm temperate climatic conditions. They also noted that the improvement of energy saving introduced by PCM is limited for tropical and snow climatic conditions. As stated previously, the optimization analysis of PCM melting temperature is necessary to use the latent heat effectively. A numerical study was performed by Huang et al. [40] for two different PCM materials for winter (Tm=28oC) and summer (Tm=43oC) months which showed that the properly selected PCM layer (Tm=28oC, L=20 mm) enhances the thermal comfort requirements. An analytical approach was developed in [41] to detect the optimum PCM melting temperature in a typical passive solar room in Beijing, China. In the same study it was reported that the suitable PCM melting temperature depends on the average indoor temperature and solar radiation. Park et al. [42] experimentally and numerically investigated the optimization of PCM melting temperature in order to conserve the energy of the building for four different cities (Gangneung, Gwangju, Incheon and Ulsan) of South Korea. They suggested that PCMs with the melting temperatures of 21oC and 24oC can achieve the highest energy savings for heating and cooling, respectively. A comprehensive study was conducted by Saffari et al. [43] which investigated the influence of PCM melting temperature on the energy performance of buildings for fifty-seven different cities from all over the world (corresponding to nineteen climatic conditions according to Köppen-Geiger classification). They found that the optimum PCM melting temperature of 20oC and 26oC contribute to the reduction of heating and cooling loads, respectively. However, an optimization work for the climatic conditions of Spain [44] concluded that there is not a certain PCM melting temperature which minimizes the thermal load of building wall and suggested that PCM melting temperature varies from 5oC to 35oC, depending on the considered parameters. Even though there are some studies in the literature regarding the optimum location of the PCM layer in the building wall, there is no a universal conclusion. For instance, Diaconu and Cruceru [45] performed an annual simulation in Algeria and suggested that PCM should be located close to the outer surface for summer months and the inner surface for the winter months. On the other hand, two different studies for Minneapolis, Louisville and Miami (USA) [46] and Bayern (Germany) [47] reported that placing PCM in the central of the wall performs better than PCM located at exterior or interior. On the contrary, Jin et al. [48] suggested place PCM near the exterior surface when PCM has high melting temperature and high layer thickness, and place near the interior surface when the room temperature is high. In light of the above information, it is clear that an extensive optimization analysis examining different parameters is essential to achieve the maximum energy saving. Zhou et al. [13] 5

investigated the effect of thermal properties of PCM including melting temperature, latent heat, thermal conductivity and surface heat transfer coefficients on both exterior and interior building wall. Authors stated that the optimum PCM melting temperature equals the average room temperature for interior building wall while it depends on both air temperature and room temperature for exterior building wall. Also, they reported that high latent heat has a significant effect on the daily energy storage if PCM completes melting-solidification process during a day. Moreover, they indicated that the thermal conductivity of PCM has no strong influence but the surface heat transfer coefficients have an important effect on thermal performance of building walls. Recently, another research group [49] carried out an optimization analysis in order to determine energy saving of PCM enhanced office building in Mediterranean climatic conditions in Palermo and Turin, Italy. They investigated several parameters such as PCM positions in the wall, PCM melting temperatures, PCM layer thicknesses, thermal conductivity of PCM, U-value of the wall, the wall configurations, and the window types. It was found that placing PCM close to internal surface provides higher energy saving and optimum. PCM melting temperature and latent heat are the most two significant parameters to obtain the highest energy savings. It was also noted that economic and building structure analyses are essential to determine the U-value of the wall and the optimum PCM layer thickness. Wang et al. [50] studied the impacts of PCM melting temperature (31oC, 35oC, 42oC, 47oC, 50oC and 55oC), PCM layer thickness (5-20 mm with 5 mm intervals) and position (exterior, middle, interior) on the energy saving of the south-facing wall in Shanghai, China. It was indicated that the reduction of heat flux achieves about 35% by the use of best combination of studied parameters (Tm=42oC, L=20 mm and close to outside surface). Decrement factor and time lag are two parameters which have been suggested in the existing scientific literature for the evaluation of the heat storage capacities of the building walls. As demonstrated in the study of Asan [51], the type and thickness of the building material have a considerable effect on the decrement factor and time lag. The storage and release processes of latent heat at the PCM melting temperature reduces the fluctuations of exterior and interior surface temperatures significantly. For example, Zhou et al. [52,53] studied a shape-stabilized PCM wallboard to analyze the effect of PCM melting temperature, PCM layer thickness, latent heat, thermal conductivity, inner surface heat transfer coefficient (on the decrement factor) and time lag. Authors notified that PCM melting temperature is the most significant parameter while PCM layer thickness and latent heat have less effect on the decrement factor and time lag. Furthermore, authors reported that thermal conductivity of PCM and inner surface heat transfer coefficient have a significant effect on the decrement factor while they have less influence on the time lag. By using the decrement factor and time lag, the authors investigated the efficiency of PCM-impregnated gypsum boards on the thermal performance of buildings to provide the highest energy saving in [54]. They concluded that the indoor peak temperature can be reduced. Namely, the period in which the interior room temperature stays within the comfort temperature can be increased where the amount of required energy can be cut down by utilization of a PCM with a melting temperature near the comfort temperature. By being different from others, Mazzeo et al. [55] defined the decrement factor and time lag considering the variation of heat flux on the external and internal surfaces. They also investigated the melting and solidification dynamics of five different PCMs by using various parameters related to the latent heat storage efficiency in order to evaluate the effective thermal characterization of the PCM layer in the air-conditioned building walls in two different cities (Turin and Cosenza) of Italy. They found that the phase change rate of 35% in the PCM layer is enough to achieve a high dynamic thermal performance. The literature survey presented above indicates that the usage of PCM in the building walls has an important effect on the decrease of the indoor temperature fluctuations, maintaining the 6

comfort temperature which decreases heating or cooling load, thus increases energy savings. However, to the best knowledge of authors, there is not any optimization study of PCM layer thickness considering the amount of activated latent heat of fusion. As it is evident that any addition of PCM layer to the wall improves thermal inertia of the wall, the analysis was performed for Phase Stabilized PCM (PSM), also in which no phase change occurs during the period of analysis so that effective use of latent heat fusion can be identified and evaluated. Besides, this study explores the optimum PCM layer thickness with optimum PCM location and PCM melting temperature for the first time for the climatic conditions of Turkey. The main objective of this work is focused on investigation of the thermal performance of PCM coupled with the external wall for the climatic conditions of Diyarbakır, Konya and Erzurum, Turkey. The effects of PCM position, melting temperature (ranging between -10oC to 40oC with 1oC intervals) and thickness (varying from 1 to 20 mm with 1 mm increments) on the energy saving, decrement factor and time lag were numerically investigated. The optimum PCM location, PCM melting temperature and thickness were obtained with respect to the annual heating and cooling loads for the considered geographical locations.

2. Methodology In this study, the external building wall without PCM (base wall) consists of four layers; interior plaster (20 mm), concrete (250 mm), insulation (20 mm) and exterior plaster (8 mm). Two different scenarios of integration of PCM to the base wall were considered: i) locating PCM between the insulation and exterior plaster, (Case 1, Fig.1a) and ii) placing PCM between the interior plaster and concrete (Case 2, Fig.1b). PCM layer thickness, L is varied between 1 and 20 mm for both two different locations of PCM in the base wall.

Fig. 1 The schematic sketch of the building walls including PCM a) Case 1, b) Case 2

In the study, PCM of the RT-category is used as it is available for a wide range of applications of which melting temperature can be adjusted between -10oC to 90oC [56]. Thermal properties of components of the external building wall are listed in Table 1. As seen in Table 1, the thermophysical properties of wall components do not vary with temperature except the density of PCM which is phase-dependent.

7

Table 1. Thermal properties of the external building wall materials [44,56] k (W/mK)

 (kg/m3)

Cp (J/kgK)

Tm (C)

LH (kJ/kg)

Plaster

0.57

1,100

1,000

-

-

Insulation

0.038

32

840

-

-

Concrete

1.3

1,900

1,000

-

-

PCM

0.2

880(solid)/760(liquid)

2,000

-10 to 40 (1C intervals)

250

Materials

Ezan et al. [57] notified that the difference between the models including and excluding convection is about 0.3% for aspect ratio (H/L) of 15. Since the aspect ratio is very high (H/L varies from 140 to 2,800) in the present work, the convective currents in the molten PCM are assumed to be negligible. The governing equation for one-dimensional, transient heat conduction for both solid and liquid phase of PCM and for other wall components can be expressed by Eq. (1)      t x

(1)

The melting and solidification process of PCM is handled by modifying the equation as following [58]    L f    H C p t t x

(2)

where f represents liquid fraction in the considered cell. The liquid fraction is set to zero when the PCM node temperature is lower than the melting temperature and to unity when the PCM node temperature is larger than the melting temperature. When the liquid fraction is between 0 and 1, both solid phase and liquid phase coexist in the cell. The cell which experiences this phenomenon is called the mushy zone where PCM node temperature is equal to PCM melting temperature. In other words, there is no temperature increase or decrease in the cell during the phase change process which is assumed to occur in isothermal temperature in this study. The boundary conditions on the interior and exterior surfaces of the external wall are given by Eq. (3) and Eq. (4), respectively. k

k

T x

in

T x

ex

 hin Troom  Tin 

(3)

 hex Tex  Tair    Fsky  Tex  Tsky    Fsky     Tex  Tair     Fground Tex  Tair     G I (4)

where T represents temperature and hin (7.69 W/m2K) stands for the internal heat transfer coefficient. αG is the absorptivity of external surface (0.9), ε (0.85) is the emissivity factor for external surface, σ is the Stefan-Boltzmann constant (5.67 10-8 W/m2K4). Fsky represents the view factor between the exterior surface of the external wall and the sky dome and Fground stands for the view factor between the exterior surface of the wall and the ground surface [59]. Moreover, β is used to divide the longwave heat exchange of the sky dome between sky and air radiation. It is assumed that the ground surface temperature equals the air temperature in this 8

paper. As the comfort room temperature varies along the year, different room temperatures are considered for each season. The comfort temperature is assumed to be 20oC in January, February, March, November and December, 22oC in April, May, September and October and 24oC in June, July and August. The external heat transfer coefficient (hex) is estimated by Eq. (5) [44]    w( t ) hex ( t )     .  . w( t )

if

w( t )   m / s

if

w( t )   m / s

(5)

where w is the wind velocity (see in Fig.2b). The sky temperature (Tsky) can be predicted by the clear-sky approximation which is proposed by Swinbank (Eq. (6)) [60] Tsky ( t )   . Tair ( t )

 .

(6)

Computations are performed for three different cities which are located in different climate conditions of Turkey: Diyarbakır (37.9094oN, 41.2133oE), Konya (37.8687oN, 32.4713oE) and Erzurum (39.9058oN, 41.2544oE). The cities are classified as Csa (very hot summer and warm winter), Bsk (hot summer and cold winter) and Dsb (warm summer and severe cold winter) by the Köppen-Geiger climate classification system, respectively [61]. These climate classifications constitute 65.1%, 9.5% and 10.3% of climatic regions of Turkey, respectively. The environmental outdoor temperature (Tair) and wind velocity (w) are obtained from [62]. The solar radiation data were taken from PVGIS-CMSAF database for a stationary vertical wall [63]. The annual variations of climatic data are shown in Fig. 2. The variations of environmental climatic conditions are assumed to be same during the month according to average days of month proposed by [64].

Fig. 2 The climate data a) air temperature, b) wind velocity and c) solar radiation [62,63] An explicit finite difference based code written in Fortran programming language is used to discretize the governing equations given above and obtain the temperature field. Grid size (x) and time step (t) are taken to be 1 mm and 0.5 s, respectively after a few numerical trials. The 9

validity of present numerical method was accomplished in two steps. At first, the numerical result was compared with the analytical solution of one-dimensional time-dependent heat conduction in a plane wall (l=0.2 m), which is subjected to convective heat transfer on the outer and inner surfaces. In the problem employed for the validation, the initial wall temperature is 20oC, the air temperature is 60oC and Biot number (Bi=hl/k) is 3. The comparison of numerical results and analytical solutions is given in Fig. 3a for different Fourier numbers (Fo=t/x2). Then, the code was verified also with the model of melting process in a semi-infinite PCM slab, of which the analytical result is presented by Solomon [65] (Fig.3b). As seen in Fig.3, a good match is observed among the results.

Fig. 3 The comparison between numerical results and analytical results The outputs of analysis are the temperature and the liquid fraction of PCM which are updated with time for each node. Energy savings, decrement factor and time lag were calculated based on the output data. In order to determine heating and cooling loads, the instantaneous heat loss or heat gain is obtained considering the heat transfer rate between the interior surface of the wall and indoor air (Eq. (7)). While the heating load was considered in the first four months and last two months of the year, the cooling load was considered in the other months. The total heat loss or heat gain during a month is calculated by Eq. (8). h T T  t )   in  room in  Q(  hin Ti n  Troom 

for heating

(7)

for cooling

 P    Q  d    Q( t )  t      t 

(8)

where P and d respectively represent a daily period (86,400 s) and the number of days in a month (30 days). Energy saving was calculated by Eq. (9) that accounts the difference between the energy consumption of the wall without PCM (base wall) and the wall with PCM. It is obvious that any additional layer of PCM increases the thermal resistance of the wall even if the latent heat is not activated. Therefore, energy saving considering Phase Stabilized PCM (PSM) was also calculated by Eq. (10). E PCM  Qbase

wall

 Q PCM

(9)

E PSM  Qbase

wall

 Q PSM

(10)

Energy saving taking the base wall as reference does not show any extremum points as it increases with increasing PCM layer. In order to determine the optimum conditions (PCM layer 10

thickness and melting point) an objective function is defined in Eq. (11), which compares the energy consumption for the wall with PCM and the wall with PSM and which maximizes the utilization of latent heat for the energy saving.

Eopt  max QPSM  L   QPCM  L,Tm 

(11)

Optimization study was carried out by altering the layer thickness from 1 mm to 20 mm and PCM melting temperature ranging from -10oC to 40oC. The values obtained by Eq. (11) were identified as the optimum value in the case when they resulted in the largest energy saving with respect to PSM Decrement factor and time lag are important parameters to evaluate the heat storage performance of the external building walls. Decrement factor of the maximum excursion of the heat flux (Eq. (12)) and time lag of the maximum and the minimum peak of the heat flux ((Eq. (13) and (Eq. (14), respectively) were calculated as following [55]: DF 

 in max  in min  ex max  ex min

TL max  t TL min  t

in

in

max

min

 t  t

ex

ex

(12) (13)

max

(14)

min

3. Results and Discussion In the present work, energy saving from an external building wall incorporating PCM for climatic conditions of three cities of Turkey (namely Diyarbakır, Konya and Erzurum) was studied considering PCM location, PCM melting temperature (ranging from -10oC to 40oC with 1oC increments), PCM layer thickness (varying from 1 to 20 mm). The analysis is performed for the external wall without PCM (base wall) and the wall with Phase Stabilized PCM (PSM). The influence of considered parameters on the energy saving, decrement factor and time lag together with the monthly and yearly optimized PCM melting temperatures are obtained and discussed below. Increasing the PCM layer thickness is expected to improve thermal resistance of the external wall due to increased thermal resistance of the wall and activated latent heat. In order to distinguish the contribution of latent heat, calculations were carried out for the PSM (the material with the same thermal properties of PCM apart from being stabilized, that is no phase change) also and obtained results are plotted in Fig.4. It is evident in Fig.4 that the energy saving for PSM (shown in red) increases linearly with the increase in layer thickness. With the activation of latent heat, the amount of energy saving further increases (blue line). The enhancement in the energy saving brought by activated latent heat is shown in the figure also (green line), which indicates the difference between energy saving of PCM and PSM. As can be seen from the figure, the benefit of latent heat increases with the increase of PCM layer thickness until L=12 mm and then decreases. It can be deduced from the results shown in Fig.4, that there is an optimum PCM layer thickness in which utilization of latent heat fusion of PCM can be maximized, and that there is a need of optimization of PCM layer thickness in the wall also. In this optimization study, therefore, the optimum PCM melting temperature and thickness are determined by the largest energy saving compared to the building wall incorporating Phase Stabilized PCM (not the base wall). 11

Fig. 4. The effect of latent heat activation on energy saving for different PCM layer thicknesses (Case 1, Diyarbakır, January, Tm=11oC) The computations are performed for different locations of PCM in the external wall (Case 1 and Case 2). The optimum PCM melting temperature and optimum PCM layer thickness are identified by the highest energy saving, when compared with the wall containing PSM, are shown in Fig. 5. From the Fig. 5, one can read the optimum PCM melting temperatures (Fig.5a, c and e) and optimum PCM layer thickness (Fig.5b, d and f) for the each location of PCM for the studied cities. For example, it is seen from Fig. 5a that the optimum PCM melting temperature in Diyarbakır varies between 11oC and 28oC for Case 1 (PCM is placed near the exterior surface) and from 20oC to 26oC for Case 2 (PCM is located close to the interior) depending on month of year. It was observed that while the optimum PCM melting temperature is mainly determined by the external environment for Case 1, it is determined by the indoor temperature for Case 2. It is interesting to note that there is no optimum PCM in February for Konya, and in May and June for Erzurum which implies that either latent heat is not activated at all (i.e. PCM is equivalent to PSM) or activation of latent heat defeats the purpose (i.e. it causes an increase in the energy consumption). A similar observation can be made for the optimum PCM layer thickness which also varies depending on the location of PCM and month (see Fig.5 (b, d and f)). The monthly optimum PCM layer thicknesses ranges from 1 to 20 mm depending on the climatic conditions for three cities and PCM location in the wall, but the annual optimum PCM layer thickness is 20 mm for the optimum PCM locations. The annual maximum energy saving in Erzurum is provided with the combination of Tm=16oC and L=20 mm when the PCM is placed near the exterior surface of the wall (Case 1). By locating the PCM close to the interior surface (Case 2), the annual largest energy savings are obtained with the combination of Tm=23oC and L=20 mm in Diyarbakır and Tm=25oC and L=20 mm in Konya. Although the results obtained in this study for the given climatic conditions show that the larger PCM thickness performs better, it is suggested to carry out yearly optimization study on the PCM layer thickness for different environmental conditions.

12

Fig. 5 The optimum PCM melting temperatures (a, c and e) and thicknesses (b, d and f) for Case 1 and Case 2 (Diyarbakır, Konya and Erzurum) The energy savings which correspond to the optimized PCM parameters are shown in Fig.6. As seen in Fig.6 (a, b and c) neither of Case 1 (placing PCM between the insulation and exterior plaster) or Case 2 (locating PCM between the interior plaster and concrete) provides energy saving for all the months of year. Generally speaking, Case 1 results in higher energy saving in winter months and Case 2 in summer months. Considering the annual energy saving in Diyarbakır and Konya (Fig.6d), it is obvious that locating PCM close to the interior surface of the wall provides much higher energy saving due to utilizing latent heat of PCM effectively. Conversely, the annual largest energy saving in Erzurum is provided by placing PCM near the exterior surface of the wall. A similar conclusion was drawn in [66] which emphasizes that the optimum location of PCM depends on the season. Diaconu and Cruceru [45] suggested placing the PCM close to the interior surface in winter and the exterior surface in summer months for Béchar, Algeria, which falls in the same Köppen-Geiger climate classification. Cascone et al. [49] also concluded that placing PCM close to the interior surface provided higher energy saving for Palermo and Turin cities in Italy. It is noted that the further analyses in this work are performed for Case 1 for Erzurum and for Case 2 for Diyarbakır and Konya which provided higher energy savings. 13

Fig. 6 The monthly (a, b and c) and annual (d) energy savings for Case 1 and Case 2 compared with the base wall (Diyarbakır, Konya and Erzurum) In order to demonstrate the activation level of latent heat on the phase change process and the indoor surface temperature, consequently on the energy saving, the results shown in Fig.7 are produced for different PCM melting temperatures. The small fluctuations in the PCM temperature seen in Fig.7a indicate that the phase change occurs. It was observed that melting and solidification occur during the day for Tm=0oC and Tm=8oC while there is no phase change for Tm=40oC. This process can be more clearly seen in the variation of liquid fraction (Fig.7b) which varies between 0 and 1 for Tm=8oC while it remains to be constant at zero (that is it behaves like Phase Stabilized PCM) during the day for Tm=40oC. For Tm=0oC, the PCM begins to melt again during the day before it completely solidifies, thus the stored latent heat is not fully utilized. This situation can be observed in the variation of the liquid fraction, which ranges from 0.13 to 1. Therefore, the amount of energy saving of Tm=8oC is higher than that one at Tm=0oC, which shows that the latent heat of PCM could be used more effectively in Tm=8oC. The start and end times of phase change affect the variation of indoor surface temperature. It can be seen from Fig.7c that Tm=8oC provides better thermal comfort as the indoor surface temperature approaches to indoor room temperature. Considering the base wall, Fig.7d shows the energy savings of PSM and PCM for different fusion temperatures. Moreover, the values given in bold on the orange columns indicate the energy saving owing to using the latent heat (QPSM-QPCM). Consequently, the largest energy saving is achieved by Tm=8oC (4397 kJ/m2month) which is followed by Tm=0oC (3447 kJ/m2month). The contribution of latent heat in the energy saving is 753 kJ/m2month and 1703 kJ/m2month for Tm=0oC and 8oC, respectively.

14

Fig. 7 A detailed analysis on the variation of temperature and liquid fraction, and energy saving (Case 1, Erzurum, March, L=15 mm) The variations of annual energy saving for various PCM melting temperatures and PCM layer thicknesses are plotted in Fig.8. The energy saving shown in the figure is calculated for the wall including PSM (QPSM-QPCM). As seen in Fig. 8a, the optimum PCM melting temperature varies while the PCM layer thickness ranges (Tm=8oC, 11oC and 13oC for L=5 mm, 10 mm and 15 mm, respectively.). Energy saving increases with increasing PCM melting temperature, it reaches a maximum (Tm=16oC for L=20 mm) and then starts to decrease, which indicates that an optimum PCM melting temperature exists. Although the behaviors of curves are similar for different PCM layer thickness, the annual energy savings are different due to different amount of the activated latent heat. The highest energy saving is attained for Tm=16oC as the optimum melting temperature is affected by the average optimum melting temperature of months during the year. As it can be seen in Fig. 8b, an optimum PCM layer thickness depends on the melting temperature considerably. The energy saving due to latent heat is attained at L=20 mm for Tm=16oC (10582 kJ/m2year), 6oC (6009 kJ/m2year) and 26oC (5185 kJ/m2year). In Tm=36oC, the annual energy saving due to the activated latent heat reaches a maximum (3650 kJ/m²year for L=16 mm) and then starts to decrease.

15

Fig. 8 The variation of annual energy saving for different a) PCM melting temperatures and b) PCM layer thicknesses compared to the wall with PSM (Case 1, Erzurum) In order to show the effect of climatic zone on the optimum PCM melting temperature and PCM layer thickness, the optimized parameters are presented for Diyarbakır (Case 2), Konya (Case 2) and Erzurum (Case 1) cities in Fig. 9. The annual optimized values are also shown in the figure for the considered cities. As can be noticed in Fig. 9, the climatic zone has a big effect on the optimized parameters. Careful evaluation of the results shows that the optimum PCM melting temperature is near the average of the mean outdoor environment and indoor comfort temperatures when PCM is located close to the exterior surface. For example, the optimum PCM melting temperature for Erzurum in March is 8oC which is the average of mean outdoor (-4oC) and indoor temperature (20oC). When PCM is placed near the interior surface, the optimum PCM melting temperature is close to the indoor comfort temperature in Diyarbakır and Konya, except for summer months. It is worth to note that the monthly optimum PCM melting temperature is in a narrow range for Diyarbakır (20oC and 26oC) and Konya (22oC and 25oC), whereas it varies 6oC and 34oC in Erzurum similar to the values (5oC to 35oC) reported in Ref. [44]. The reason for the wide range in Erzurum is that the location of PCM is near the exterior surface, thus the optimum PCM melting temperature is determined by the summer and winter outdoor environmental conditions. It is also seen from Fig. 9a that, there is no optimum PCM melting temperatures in January, March and December for Diyarbakır, and in January, February, March and December for Konya, since the melting temperature at which melting/solidification cycling occurs, keeps the indoor surface temperature below the comfort temperature. Previously mentioned circumstances are leading to increase in the heating load, no phase change cycling is established and at higher melting temperature due to the fact that the amplitude of variation in the outdoor environment is not large enough. An negative effect similar to the one mentioned above is observed in May, June and September in Erzurum as phase change occurs at high PCM melting temperature which keeps the internal surface temperature above the comfort temperature, thus increases the cooling load. As seen in Fig. 9b, the monthly optimum PCM layer thickness ranges from 1 mm to 20 mm depending on the season. Although, the annual optimum PCM layer thickness is the largest value (20 mm) for all the selected cities in this study, it may be a lower thickness for different environmental conditions, which suggests that in order to make application of PCM in the building facades economically more viable, an optimization study regarding the activation of latent heat of PCM is needed.

16

Fig. 9 a) The optimum PCM melting temperatures and b) PCM layer thicknesses for Diyarbakır (Case 2), Konya (Case 2) and Erzurum (Case 1) Taking the base wall as a reference, the monthly and annual energy savings of PSM and PCM for Diyarbakır (Case 2), Konya (Case 2) and Erzurum (Case 1) cities are shown in Fig. 10. Besides, referring to the mentioned figure, the values are given on each column represents the energy saving due to activation of latent heat (QPSM-QPCM). Careful examination of Fig.10a and Fig.10b shows that the amount of energy saving potential of cooling load in Diyarbakır and Konya is much higher compared to the heating load. Contrarily, the monthly energy saving potential of heating load in Erzurum is greater than the cooling load (Fig.10c). It is worth to emphasize that the highest energy saving with the addition of PCM may not be considered as the most efficient use of latent heat. For instance, in Erzurum (Fig.10c), the energy saving with the inclusion of PCM in the wall is 4582 kJ/m2month in January while it is 3194 kJ/m2month in November. However, the highest energy saving due to the activated latent heat of PCM is much higher in November (2648 kJ/m2month) compared to that in January (1702 kJ/m2month). Fig. 10(a, b and c) indicates that the highest energy savings provided by utilization of latent heat are 9262 kJ/m2month, 9071 kJ/m2month and 3318 kJ/m2month in Diyarbakır (in September), Konya (in July) and Erzurum (in February), respectively. By integration of PCM in the wall (in Fig. 10d), the highest annual energy saving is attained in Diyarbakır (42871 kJ/m2year), which is followed by Erzurum (24432 kJ/m2year) and Konya (22224 kJ/m2year). The highest contribution of latent heat in the energy saving is achieved in Diyarbakır (18906 kJ/m2year). Although the locations of PCM in the wall (Case 2) and the optimum PCM thicknesses (L=20 mm) for Diyarbakır and Konya are the same, the energy saving potential of PCM is higher in Diyarbakır due to higher cooling energy demand.

17

Fig. 10 The monthly (a, b and c) and d) annual energy saving for Diyarbakır (Case 2), Konya (Case 2) and Erzurum (Case 1) compared to the base wall

The decrement factors for the base wall and the wall with PCM are compared in Fig. 11 for three cities. Besides, the percentage reduction in the decrement factor compared to the base wall is shown in the same figure. As it is evident in Fig. 11, by integrating PCM to the external building wall, the decrement factor was reduced considerably, implying that the interior surface temperature approaches to the thermal comfort level thus the energy consumption decreases. As can be seen in Fig. 11a and Fig. 11b, the reduction in the decrement factor is approximately 35% in months except for May (93%), September (95%) and October (99%) for Diyarbakır, and April (73%), July (97%), November (67%) and August (84%) for Konya. The reduction in the decrement factor of Erzurum (in Fig. 12c) varies between 15% (in July and August) and 94% (in June).

18

Fig. 11 Comparison of decrement factors of the maximum excursion of the heat flux between the base wall and the wall with PCM Fig. 12 compares the time lag of the base wall and the wall including PCM which are calculated based on the heat flux. Fig.12 (a, c and e) show the maximum and Fig.12 (b, d and f) show the minimum time lags of the heat flux. Additionally, the enhancement in the time lag compared to the base wall is given on the bars in the same figure. Based on the results presented in Fig. 12 the time of peak heat flux is delayed when PCM is integrated into the wall. The delays in the maximum peak of the heat flux are up to 4.4 h in Diyarbakır (in April), 5.8 h in Erzurum (in April) and 10.3 h in Konya (in August) while the delays in the minimum peak of the heat flux are 4.3 h in Diyarbakır (in October), 7.2 h in Erzurum (in November) and 13.3 h in Konya (in July). It is interesting to note that the time lag for the wall containing PCM decreases in some situations (e.g. in October in Erzurum) due to inappropriate melting temperatures.

19

Fig. 12 Comparison of time lag of a) the maximum and b) the minimum peak of the heat fluxes between the base wall and the wall containing PCM

4. Conclusions In this work, the thermal performance of the external building wall containing PCM is numerically investigated considering heating and cooling loads. The main goal of the study was to identify the contribution of latent heat of PCM layer on the thermal mass of the wall and to optimize the PCM layer thickness and melting temperature for potential improvement in buildings energy performance. Adhering to this goal, in the study, the impacts of PCM location (near the indoor environment or outdoor environment), PCM melting temperature (Tm varies from -10oC to 40oC with 1oC increments) and PCM layer thickness (L ranges from 1 to 20 mm) are examined on the energy saving, decrement factor and time lag. The calculations are performed for the external building wall without PCM (base wall), the wall with Phase Stabilized PCM (PSM) and the wall with PCM. Computations were carried out for three cities of (Diyarbakır, Konya and Erzurum) Turkey by using the hourly variation of air temperature, wind velocity and solar radiation. The obtained results in this paper are highlighted below.

20

 



  

Placing PCM between the insulation and exterior plaster provides the maximum energy saving in heating conditions. Under cooling conditions, the maximum energy saving is obtained by placing PCM between the interior plaster and concrete, Considering both heating and cooling loads (annual energy consumption), locating PCM between the interior plaster and concrete performs better for Diyarbakır and Konya. On the contrary, the annual energy saving of Erzurum is higher when PCM is placed between the insulation and exterior plaster, The monthly optimum PCM layer thickness varies from 1 to 20 mm depending on the seasons. The range of monthly optimum PCM melting temperature depends on the location of PCM (near the interior or exterior surface). The largest range is observed in Erzurum, which varies from 6oC to 34oC, While the annual optimum PCM layer thicknesses are the same for the studied cities, the annual optimum PCM melting temperatures are different, which are 20oC, 25oC and 16oC for Diyarbakır, Konya and Erzurum, respectively, Compared to the base wall, the largest annual energy saving by incorporating PCM to the external wall is achieved in Diyarbakır corresponding to 18% of energy reduction, The decrement factor can be reduced significantly and time lag can be increased up to 10.3 h by activating the latent heat of PCM effectively.

It was concluded that optimization of PCM thickness and PCM melting temperature based on the maximum exploitation of latent heat of PCM is of very significant from economic aspect of building passive energy systems.

Nomenclature Bi

: Biot number (=hl/k)

Cp

: Specific heat capacity (kJ/kgK)

d

: The number of days in a month (30 days)

DF

: Decrement factor

E

: Energy saving (kJ/m2month)

f

: Liquid fraction

F

: The view factor

Fo

: Fourier number (=t/x2)

h

: Heat transfer coefficient (W/m2K)

H

: Height of wall (mm)

I

: Solar radiation (W/m2)

k

: Thermal conductivity (W/mK)

l

: Half thickness of plane wall (m)

L

: PCM layer thickness (mm)

LH

: Latent heat (kJ/kg) 21

P

: A daily period (86400 s)

Q

: The total heat loss or heat gain (J/m2month)

Q

: The instantaneous heat loss or heat gain (W/m2)

t

: Time (s, h)

T

: Temperature (oC, K)

TL

: Time lag (h)

w

: Wind velocity (m2/s)

Greek symbols α

: Thermal diffusivity (m2/s)

αG

: Absorptivity

β

: A factor for radiative heat exchange on the exterior surface of the wall

t

: Time step (s)

x

: Grid size (mm)

ε

: Emissivity factors

ρ

: Density (kg/m3)

σ

: Stefan-Boltzmann constant (5.67x10-8 W/m2K4)

Φ

: Heat flux (W/m²)

Subscripts air

: Air

base wall

: External building wall without PCM

ex

: External surface

ground

: Ground surface

i

: Node number

in

: Internal surface

m

: Melting

max

: Maximum

min

: Minimum

PCM

: Phase change material

PSM

: Phase Stabilized PCM

room

: Room

sky

: Sky

22

References [1]

Jeon, J., Lee, J.H., Seo, J., Jeong, S.G., Kim, S., Application of PCM thermal energy storage system to reduce building energy consumption. Journal of Thermal Analysis and Calorimetry 2013;111(1):279-288. doi:10.1007/s10973-012-2291-9.

[2]

Zalba, B., Marin, J.B., Cabeza, L.F., Mehling, H., Review on thermal energy storage with phase change materials, heat transfer analysis and applications. Applied Thermal Engineering 2003;23(3):251-283. doi:10.1016/S1359-4311(02)00192-8.

[3]

Schröder, J., Gawron, K., Latent heat storage. International Journal of Energy Research 1981;5(2):103-109. doi:10.1002/er.4440050202.

[4]

Brano, V.L., Ciulla, G., Piacentino, A., Cardona, F., Finite difference thermal model of a latent heat storage system coupled with a photovoltaic device: Description and experimental validation. Renewable Energy 2014;68:181-193. doi:10.1016/j.renene.2014.01.043.

[5]

Smith, C.J., Forster, P.M., Crook, R., Global analysis of photovoltaic energy output enhanced by phase change material cooling. Applied Energy 2014;126:21-28. doi:10.1016/j.apenergy.2014.03.083.

[6]

Arıcı, M., Bilgin, F., Nižetić, S., Papadopoulos, A.M., Phase change material based cooling of photovoltaic panel: A simplified numerical model for the optimization of the phase change material layer and general economic evaluation. Journal of Cleaner Production 2018;189:738-745. doi:10.1016/j.jclepro.2018.04.057.

[7]

Nižetić, S., Arıcı, M., Bilgin, F., Grubišić-Čabo, F., Investigation of pork fat as potential novel phase change material for passive cooling applications in photovoltaics. Journal of Cleaner Production 2018;170:1006-1016. doi:10.1016/j.jclepro.2017.09.164.

[8]

Ismail, K.A.R., Jesus, A.B., Parametric study of solidification of PCM around a cylinder for ice-bank applications. International Journal of Refrigeration 2001;24(8):809-822. doi:10.1016/S0140-7007(00)00059-1.

[9]

Mehling, H., Cabeza, L.F., Hippeli, S., Hiebler, S., PCM-module to improve hot water heat stores with stratification. Renewable Energy 2003;28(5):699-711. doi:10.1016/S0960-1481(02)00108-8.

[10]

Kürklü, A., Energy storage applications in greenhouses by means of phase change materials (PCMs): A review. Renewable Energy 1998;13(1):89-103. doi:10.1016/S0960-1481(97)83337-X.

[11]

Gharbi, S., Harmand, S., Jabrallah, S.B., Experimental comparison between different configurations of PCM based heat sinks for cooling electronic components. Applied Thermal Engineering 2015;87:454-462. doi:10.1016/j.applthermaleng.2015.05.024.

[12]

Ali, H.M., Ashraf, M.J., Giovannelli, A., Irfan, M., Irshad, T.B., Hamid, H.M., Hassan, F., Arshad, A., Thermal management of electronics: An experimental analysis of triangular, rectangular and circular pin-fin heat sinks for various PCMs. International Journal of Heat and Mass Transfer 2018;123:272-284. doi:10.1016/j.ijheatmasstransfer.2018.02.044.

[13]

Zhou, D., Shire, G.S.F., Tian, Y., Parametric analysis of influencing factors in Phase Change Material Wallboard (PCMW). Applied Energy 2014;119:33-42. doi:10.1016/j.apenergy.2013.12.059. 23

[14]

Sun, X., Zhang, Q., Medina, M.A, Lee, K.O., Energy and economic analysis of a building enclosure outfitted with a phase change material board (PCMB). Energy Conversion and Management 2014;83:73-78. doi:10.1016/j.enconman.2014.03.035.

[15]

Akeiber, H., Nejat, P., Majid, M.Z.A., Wahid, M.A., Jomehzadeh, F., Famileh, I.Z., Calautit, J.K., Hughes, B.R., Zaki, S.A., A review on phase change material (PCM) for sustainable passive cooling in building envelopes. Renewable and Sustainable Energy Reviews 2016;60:1470-1497. doi:10.1016/j.rser.2016.03.036.

[16]

Mi, X., Liu, R., Cui, H., Memon, S.A., Xing, F., Lo, Y., Energy and economic analysis of building integrated with PCM in different cities of China. Applied Energy 2016;175:324-336. doi:10.1016/j.apenergy.2016.05.032.

[17]

Bilgin, F., Arıcı, M., Effect of phase change materials on time lag, decrement factor and heat-saving. Acta Physica Polonica Series A 2017;132(3-Ⅱ):1102-1105. doi: 10.12693/APhysPolA.132.1102.

[18]

Mazzeo, D., Oliveti, G., Gracia, A.D., Coma, J., Solé, A., Cabeza, L.F., Experimental validation of the exact analytical solution to the steady periodic heat transfer problem in a PCM layer. Energy 2017;140(1):1131-1147. doi:10.1016/j.energy.2017.08.045.

[19]

Mazzeo, D., Oliveti, G., Thermal field and heat storage in a cyclic phase change process caused by several moving melting and solidification interfaces in the layer. International Journal of Thermal Sciences 2018;129:462-488. doi: 10.1016/j.ijthermalsci.2017.12.026.

[20]

Koschenz, M., Lehmann, B., Development of a thermally activated ceiling panel with PCM for application in lightweight and retrofitted buildings. Energy and Buildings 2004;36(6):567-578. doi:10.1016/j.enbuild.2004.01.029.

[21]

Li, D., Zheng, Y., Liu, C., Wu, G., Numerical analysis on thermal performance of roof contained PCM of a single residential building. Energy Conversion and Management 2015;100:147-156. doi:10.1016/j.enconman.2015.05.014.

[22]

Li, D., Wu, Y., Zhang, G., Arıcı, M., Liu, C., Wang, F., Influence of glazed roof containing phase change material on indoor thermal environment and energy consumption. Applied Energy 2018;222:343-350. doi:10.1016/j.apenergy.2018.04.015.

[23]

Arnault, A., Mathieu-Potvin, F., Gosselin, L., Internal surfaces including phase change materials for passive optimal shift of solar heat gain. International Journal of Thermal Sciences 2010;49(11):2148-2156. doi:10.1016/j.ijthermalsci.2010.06.021.

[24]

Ansuini, R., Larghetti, R., Giretti, A., Lemma, M., Radiant floors integrated with PCM for indoor temperature control. Energy and Buildings 2011;43(11):3019-3026. doi:10.1016/j.enbuild.2011.07.018.

[25]

Li, D., Wu, Y., Liu, C., Zhang, G., Arıcı, M., Energy investigation of glazed windows containing Nano-PCM in different seasons. Energy Conversion and Management 2018;172:119-128. doi:10.1016/j.enconman.2018.07.015.

[26]

Li, D., Wu, Y., Liu, C., Zhang, G., Arıcı, M., Numerical investigation of thermal and optical performance of window units filled with nanoparticle enhanced PCM. International Journal of Heat and Mass Transfer 2018;125:1321-1332. doi:10.1016/j.ijheatmasstransfer.2018.04.152.

[27]

Liu, C., Zhang, G., Arıcı, M., Bian, J., Li, D., Thermal performance of non-ventilated 24

multilayer glazing facades filled with phase change material. Solar Energy 2019;177:464-470. doi:10.1016/j.solener.2018.11.044. [28]

Athienitis, A.K., Liu, C., Hawes, D., Banu, D., Feldman, D., Investigation of the thermal performance of a passive solar test-room with wall latent heat storage. Building and Environment 1997;32(5):405-410. doi:10.1016/S0360-1323(97)00009-7.

[29]

Faraji, M., Numerical computation of solar heat storage in phase change material/concrete wall. International Journal of Energy and Environment 2014;5(3):353360.

[30]

Cabeza, L.F., Castellón, C., Nogués, M., Medrano, M., Leppers, R., Zubillaga, O., Use of microencapsulated PCM in concrete walls for energy savings. Energy and Buildings 2007;39(2):113-119. doi:10.1016/j.enbuild.2006.03.030.

[31]

Elnajjar, E., Using PCM embedded in building material for thermal management: Performance assessment study. Energy and Buildings 2017;151:28-34. doi:10.1016/j.enbuild.2017.06.010.

[32]

Halford, C.K., Boehm, R.F., Modeling of phase change material peak load shifting. Energy and Buildings 2007;39(3):298-305. doi:10.1016/j.enbuild.2006.07.005.

[33]

Kuznik, F., Virgone, J., Noel, J., Optimization of a phase change material wallboard for building use. Applied Thermal Engineering 2008;28(11-12):1291-1298. doi:10.1016/j.applthermaleng.2007.10.012.

[34]

Figueiredo, A., Vicente, R., Lapa, J., Cardoso, C., Rodrigues, F., Kämpf, J., Indoor thermal comfort assessment using different constructive solutions incorporating PCM. Applied Energy 2017;208:1208-1221. doi:10.1016/j.apenergy.2017.09.032.

[35]

Guarino, F., Athienitis, A., Cellura, M., Bastien, D., PCM thermal storage design in buildings : Experimental studies and applications to solaria in cold climates. Applied Energy 2017;185(1):95-106. doi:10.1016/j.apenergy.2016.10.046.

[36]

Ye, H., Long, L., Zhang, H., Zou, R., The performance evaluation of shape-stabilized phase change materials in building applications using energy saving index. Applied Energy 2014;113:1118-1126. doi: 10.1016/j.apenergy.2013.08.067.

[37]

Xie, J., Wang, W., Liu, J., Pan, S., Thermal performance analysis of PCM wallboards for building application based on numerical simulation. Solar Energy 2018;162:533-540. doi:10.1016/j.solener.2018.01.069.

[38]

Mazzeo, D., Oliveti, G., Arcuri, N., A Method for Thermal Dimensioning and for Energy Behavior Evaluation of a Building Envelope PCM Layer by Using the Characteristic Days. Energies 2017;10(5):659. doi:10.3390/en10050659.

[39]

Marin, P., Saffari, M., Gracia, A.D., Zhu, X., Farid, M.M., Cabeza, L.F., Ushak, S., Energy savings due to the use of PCM for relocatable lightweight buildings passive heating and cooling in different weather conditions. Energy and Buildings 2016;129:274-283. doi:10.1016/j.enbuild.2016.08.007.

[40]

Huang, M.J., Eames, P.C., Hewitt, N.J., The application of a validated numerical model to predict the energy conservation potential of using phase change materials in the fabric of a building. Solar Energy Materials and Solar Cells 2006;90(13):1951-1960. doi:10.1016/j.solmat.2006.02.002. 25

[41]

Xiao, W., Wang, X., Zhang, Y., Analytical optimization of interior PCM for energy storage in a lightweight passive solar room. Applied Energy 2009;86(10):2013-2018. doi:10.1016/j.apenergy.2008.12.011.

[42]

Park, J.H., Lee, J., Wi, S., Jeon, J., Chang, S.J., Chang, J.D., Kim, S., Optimization of phase change materials to improve energy performance within thermal comfort range in the South Korean climate. Energy and Buildings 2019;185:12-25. doi:10.1016/j.enbuild.2018.12.013.

[43]

Saffari, M., Gracia, A.D., Fernández, C., Cabeza, L.F., Simulation-based optimization of PCM melting temperature to improve the energy performance in buildings. Applied Energy 2017;202:420-434. doi:10.1016/j.apenergy.2017.05.107.

[44]

Izquierdo-Barrientos, M.A., Belmonte, J.F., Rodríguez-Sánchez, D., Molina, A.E., Almendros-Ibáñez, J.A., A numerical study of external building walls containing phase change materials (PCM). Applied Thermal Engineering 2012;47:73-85. doi:10.1016/j.applthermaleng.2012.02.038.

[45]

Diaconu, B.M., Cruceru, M., Novel concept of composite phase change material wall system for year-round thermal energy savings. Energy and Buildings 2010;42(10):17591772. doi:10.1016/j.enbuild.2010.05.012.

[46]

Zwanzig, S.D., Lian, Y., Brehob, E.G., Numerical simulation of phase change material composite wallboard in a multi-layered building envelope. Energy Conversion and Management 2013;69:27-40. doi:10.1016/j.enconman.2013.02.003.

[47]

Fateh, A., Klinker, F., Brütting, M., Weinläder, H., Devia, F., Numerical and experimental investigation of an insulation layer with phase change materials ( PCMs ). Energy and Buildings 2017;153:231-240. doi:10.1016/j.enbuild.2017.08.007.

[48]

Jin, X., Medina, M.A., Zhang, X., Numerical analysis for the optimal location of a thin PCM layer in frame walls. Applied Thermal Engineering 2016;103:1057-1063. doi:10.1016/j.applthermaleng.2016.04.056.

[49]

Cascone, Y., Capozzoli, A., Perino, M., Optimisation analysis of PCM-enhanced opaque building envelope components for the energy retrofitting of office buildings in Mediterranean climates. Applied Energy 2018;211:929-953. doi:10.1016/j.apenergy.2017.11.081.

[50]

Wang, Q., Wu, R., Wu, Y., Zhao, C.Y., Parametric analysis of using PCM walls for heating loads reduction. Energy and Buildings 2018;172:328-336. doi:10.1016/j.enbuild.2018.05.012.

[51]

Asan, H., Numerical computation of time lags and decrement factors for different building materials. Building and Environment 2006;41(5):615-620. doi:10.1016/j.buildenv.2005.02.020.

[52]

Zhou, G., Yang, Y., Wang, X., Cheng, J., Thermal characteristics of shape-stabilized phase change material wallboard with periodical outside temperature waves. Applied Energy 2010;87(8):2666-2672. doi:10.1016/j.apenergy.2010.02.001.

[53]

Zhou, G., Yang, Y., Xu, H., Performance of shape-stabilized phase change material wallboard with periodical outside heat flux waves. Applied Energy 2011;88(6):21132121. doi:10.1016/j.apenergy.2011.01.016.

[54]

Sharifi, N.P., Shaikh, A.A.N., Sakulich, A.R., Application of phase change materials in 26

gypsum boards to meet building energy conservation goals. Energy and Buildings 2017;138:455-467. doi:10.1016/j.enbuild.2016.12.046. [55]

Mazzeo, D., Oliveti, G., Arcuri, N., Definition of a new set of parameters for the dynamic thermal characterization of PCM layers in the presence of one or more liquid-solid interfaces. Energy and Buildings 2017;141:379-396. doi:10.1016/j.enbuild.2017.02.027.

[56]

Web source a: https://www.rubitherm.eu/en/index.php/productcategory/organischepcm-rt (accessed 24 October 2018).

[57]

Ezan, M.A., Yüksel, C., Alptekin, E., Yılancı, A., Importance of natural convection on numerical modelling of the building integrated PVP/PCM systems. Solar Energy 2018;159:616-627. doi:10.1016/j.solener.2017.11.022.

[58]

Zivkovic, B., Fujii, I., An analysis of isothermal phase change of phase change material within rectangular and cylindrical containers. Solar Energy 2001;70(1):51-61. doi:10.1016/S0038-092X(00)00112-2.

[59]

Walton, G.N., Thermal Analysis Research Program Reference Manual, NBSSIR 832655, National Bureau of Standards, 1983.

[60]

Swinbank, W.C., Long-wave radiation from clear skies. Quarterly Journal of the Royal Meteorological Society 1963;89(381):339-348. doi:10.1002/qj.49708938105.

[61]

Peel, M.C., Finlayson, B.L., McMahon, T.A., Updated world map of the Köppen-Geiger climate classification. Hydrology and Earth System Sciences 2007;11(5):1633-1644. doi:10.5194/hess-11-1633-2007.

[62]

The Turkish State Meteorological Affairs General Directorate (DMI).

[63]

Web source b: http://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#HR (accessed 24 October 2018).

[64]

American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE), Handbook of Fundamentals. 1985, Atlanta, Georgia.

[65]

Solomon, A.D., An easily computable solution to a two-phase Stefan problem. Solar Energy 1979;23(6):525-528. doi:10.1016/0038-092X(79)90077-X.

[66]

Jin, X., Shi, D., Medina, M.A., Shi, X., Zhou, X., Zhang, X., Optimal location of PCM layer in building walls under Nanjing (China) weather conditions. Journal of Thermal Analysis and Calorimetry 2017;129(3):1767-1778. doi:10.1007/s10973-017-6307-3.

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Conflicts of Interest Statement Manuscript Title: “PCM integrated to external building walls: An optimization study on maximum activation of latent heat”

The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Author names: Müslüm Arıcı, Feyza Bilgin, Sandro Nižetić, Hasan Karabay

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