Abstract
Abstract: Phase change materials (PCM) in the construction industry became attractive because of several interesting attributes, such as thermo-physical parameters, open air atmospheric condition usage, cost and the duty structure requirement. Thermal performance optimization of PCMs in terms of proficient storage of a large amount of heat or cold in a finite volume remains a challenging task. Implementation of PCMs in buildings to achieve thermal comfort for a specific climatic condition in Iraq is our main focus. From this standpoint, the present paper reports the experimental and numerical results on the lowering of heat flux inside a residential building using PCM, which is composed of oil (40%) and wax (60%). This PCM (paraffin), being plentiful and cost-effective, is extracted locally from waste petroleum products in Iraq. Experiments are performed with two rooms of identical internal dimensions in the presence and absence of PCM. A two-dimensional numerical transient heat transfer model is developed and solved using the finite difference method. A relatively simple geometry is chosen to initially verify the numerical solution procedure by incorporating in the computer program two-dimensional elliptic flows. It is demonstrated that the heat flux inside the room containing PCM is remarkably lower than the one devoid of PCM.
Introduction
Over the years, it has been established that thermal energy storage systems (TESSs) with phase change materials (PCMs) can efficiently reduce the excessive usage of fossil fuels and subsequent global warming [1, 2] . Thermal energy storage (TES) is found to play a vital role in a broad range of industrial and residential applications. The use of TESSs with PCMs in buildings enhances the human comfort by reducing the internal air temperature fluctuation. Consequently, the indoor air temperature remains near the required temperature over an extended time period [3] . Sensible heat TES, which stores heat in fluid or solid form, latent heat TES, which uses latent heat during the phase change process, and thermoelectric devices are the common examples of different types of TES technologies. TESSs can rapidly discharge or store huge amounts of heat with solar energy and heat as alternating sources [4] . Recent research reveals the advancements of extensive building architecture and management based on TES with PCMs. TES is attractive due to its absolute suitability to reduce the gap between energy demand and supply [5] . room temperatures are observed to vary~27˘3˝C during the experiment. Despite many dedicated efforts, a comprehensive understanding of the PCM-mediated reduction in heat flux inside the building is far from being achieved.
In this paper, experimental and numerical investigations were performed using PCMs in building architecture to determine their impact on lowering the heat flux inside the building. Two identical rooms, one without and the other with PCM, were considered for experiments. Numerical simulation was done based on the transient heat transfer model. The heat flux inside the room is determined to assess the thermal performance of such PCMs. Results are discussed, analyzed, compared and validated.
Numerical Scheme
The boundary condition on the inner surface of the aluminum frame follows natural convection. Most of the previous researchers considered the bottom wall as insulated, because the temperature difference between the room and the wall was very small. The heat transfer coefficient (h) inside the room is calculated (FORTRAN programming). Figures 1 and 2 display the schematics for the numerical model formulation, which assumes the following:
One dimensional heat conduction in the composite wall is considered, and the end impacts are not taken into account.
The thermal conductivity of the aluminum frame and the roof top slab are constant irrespective of temperature variation.
The PCM is uniform and isotropic. (iv)
The convection impact in the molten PCM is not considered.
The interfacial resistances are negligible.
The value of C p for the PCM panel is considered as follows:
T ă T m´∆ T, C p " C ps (1) T ą T m`∆ T, C p " C pl (2)
where C p is the specific heat capacity, h sl is the enthalpy change of solid-liquid, ∆T is half of the temperature range over which the phase change occurs and T m is the phase transition temperature. (vii)
The latent heat being highly sensitive to the phase transition process of the PCM is modeled over a range of temperatures, where C p is considered to be uniform during the phase conversion. Although, in reality, C p varies with temperature.
Energies 2016, 9, 30 3 efforts, a comprehensive understanding of the PCM-mediated reduction in heat flux inside the building is far from being achieved. In this paper, experimental and numerical investigations were performed using PCMs in building architecture to determine their impact on lowering the heat flux inside the building. Two identical rooms, one without and the other with PCM, were considered for experiments. Numerical simulation was done based on the transient heat transfer model. The heat flux inside the room is determined to assess the thermal performance of such PCMs. Results are discussed, analyzed, compared and validated.
The boundary condition on the inner surface of the aluminum frame follows natural convection. Most of the previous researchers considered the bottom wall as insulated, because the temperature difference between the room and the wall was very small. The heat transfer coefficient (h) inside the room is calculated (FORTRAN programming). Figures 1 and 2 display the schematics for the numerical model formulation, which assumes the following: 
where is the specific heat capacity, hsl is the enthalpy change of solid-liquid, ΔT is half of the temperature range over which the phase change occurs and Tm is the phase transition temperature. (vii) The latent heat being highly sensitive to the phase transition process of the PCM is modeled over a range of temperatures, where is considered to be uniform during the phase conversion. Although, in reality, varies with temperature. Following these assumptions, the governing equation and the boundary conditions are written as:
where m = 1 for the roof top slab, m = 2 for the PCM panel and m = 3 for the bottom aluminum frame. The same equation holds for all three material regions and takes different values of k, ρ and .
In the outer walls (x = 0), where the floor is exposed to solar radiation, the boundary condition is expressed as:
The radiation effect is considered only during the sunshine hours. The boundary condition (x = L) in the bottom layer of the aluminum frame is:
The instantaneous continuity of heat flux and temperature at the interfaces x = and is preserved.
The equation for the top volume cell is written as:
The equation for the volume cells located in between the top and bottom volume cells yields:
The above-mentioned discretized equations are applicable to volume cells for 2-4, 7 and for 10-12 of the roof top slab, PCM panel and aluminum frame, respectively, with: = 1, = 2, 3, 4; = 2, = 7; and = 3, =10, 11, 12. The equation for the interface volume cell 5 is written as: Following these assumptions, the governing equation and the boundary conditions are written as:
where m = 1 for the roof top slab, m = 2 for the PCM panel and m = 3 for the bottom aluminum frame. The same equation holds for all three material regions and takes different values of k, ρ and C p .
In the outer walls (x = 0), where the floor is exposed to solar radiation, the boundary condition is expressed as:
The radiation effect is considered only during the sunshine hours. The boundary condition (x = L) in the bottom layer of the aluminum frame is:
The instantaneous continuity of heat flux and temperature at the interfaces x = L 1 and L 2 is preserved.
The equation for the top volume cell is written as:
The equation for the volume cells located in between the top and bottom volume cells yields:
The above-mentioned discretized equations are applicable to volume cells for 2-4, 7 and for 10-12 of the roof top slab, PCM panel and aluminum frame, respectively, with: m = 1, i = 2, 3, 4; m = 2, i = 7; and m = 3, i =10, 11, 12. The equation for the interface volume cell 5 is written as:
where ∆x 1 and ∆x 2 are the cell thickness of the roof top slab and PCM panel, respectively. Similarly, the equation can be written for volume cell 6. A similar process is extended to control Volumes 8 and 9, which involve cell thicknesses ∆x 2 and ∆x 3 corresponding to the PCM panel and the bottom aluminum frame, respectively.
The equation for the bottom volume cell 13 is given by:
" f rh i p´2qs`p1´fq
Development of the Numerical Model

One-Phase Solution
For either completely solid or liquid PCM layers, the temperature distribution on the n-th layer of the wall follows the one-dimensional diffusion equation given by:
with the boundary condition:
T pL, tq " T l ptq
The numerical solution for a single material node yields:
The finite difference equation for the inner interface is written as:
The finite difference equation for the outer surface is written as:
Depending on whether the PCM is completely solid or completely liquid upon entering the one-phase subroutine, this solution is run until the maximum nodal temperature within the PCM layer increases above the melting point or until the minimum temperature within the material drops below Energies 2016, 9, 30 6 of 17 the freezing point. The temperature distribution at this particular time instant becomes the initial condition for the two-phase subroutine.
Two-Phase Solution
The boundary points between the solid and liquid are held constant at the melting temperature of the material when the PCM layer consists of more than one phase and is expressed as:
The movement of the solid-liquid boundary is given by:
where xsl`and xsl´denote the limits approaching the solid-liquid interface from the right and left, respectively. An explicit numerical solution for a single material two-phase node is given by:
where λ is the volume fraction of the i-th node, which is melted at the j-th time step. For the two PCM-insulation interface points, the finite difference equation is modified to consider that the node is half insulation and half PCM. For these two nodes, λ = 1/2 = λ max indicates the fully-melted state. For the inner interface node, the two-phase finite difference equation takes the form:
For the outer interface node, the two-phase finite difference equation yields:
Two-phase subroutine prepares a kind of switching between Equations (13)- (15) and Equations (15)- (18), based on the nodal state. Figure 3 shows a typical temperature for exiting the one-phase solid solution. In this case, the material is completely solid (l = 0) for all nodes with the outermost PCM node at some temperature slightly above T m .
Energies 2016, 9, 30 An explicit numerical solution for a single material two-phase node is given by:
where λ is the volume fraction of the i-th node, which is melted at the j-th time step. For the two PCM-insulation interface points, the finite difference equation is modified to consider that the node is half insulation and half PCM. For these two nodes, λ = 1/2 = λmax indicates the fully-melted state. For the inner interface node, the two-phase finite difference equation takes the form:
For the outer interface node, the two-phase finite difference equation yields:
Two-phase subroutine prepares a kind of switching between Equations (13)- (15) and Equations (15)- (18), based on the nodal state. Figure 3 shows a typical temperature for exiting the one-phase solid solution. In this case, the material is completely solid (l = 0) for all nodes with the outermost PCM node at some temperature slightly above Tm. The temperature of this node is shifted back to the PCM melting temperature and kept constant. Equation (18) is used to determine the fraction of the melted node. This is continued until l returns to zero, meaning the complete re-freezing of node or λ = λmax, implying the complete melting of nodes. In such an instance, the temperature is again allowed to alter according to Equation (12) . For the nodes that are below , the temperatures are governed by the one-phase equations until they exceed . At this time, the node is in two phases, and the method of calculation is toggled to Equations (15)- (18). The temperature of this node is shifted back to the PCM melting temperature and kept constant. Equation (18) is used to determine the fraction of the melted node. This is continued until l returns to zero, meaning the complete re-freezing of node or λ = λ max , implying the complete melting of nodes. In such an instance, the temperature is again allowed to alter according to Equation (12) . For the nodes that are below T m , the temperatures are governed by the one-phase equations until they exceed T m . At this time, the node is in two phases, and the method of calculation is toggled to Equations (15)-(18).
Experimental Scheme
Experiments are set up using two full-scale rooms. The test room with PCM is constructed to determine its effect on the roof and wall for thermal management of a residential building. The thermal performance of this organic eutectic PCM having a melting temperature within 40-44˝C is computed. The following criteria are satisfied for precise measurements:
The PCM heat transfer is one-dimensional. (2) The heat flow from uncontrolled outside influences is negligible compared to the applied heat. (3) The rate of the temperature rise and fall of the PCM is comparable to reality.
The general properties of the PCM are listed in Table 1 . 
Full-Scale Test Rooms
Two identical full-scale outdoor test rooms with dimension (3 mˆ2.5 mˆ2 m) as shown in Figure 4 are built at the University Technology site of Iraq. The thermal performance (internal thermal gains) of these two test rooms (PCM incorporated wall and roof) is analyzed to determine their impact under the Iraqi climate. The wall and roof of one room is covered with PCM and the other without any PCM. The basic structures (windows and supports) of both rooms are identical and made of a metallic insulated door positioned on the north wall. The thermal behavior of these rooms is compared to obtain the performance and effect of PCM. The detailed structure of these rooms is as follows:
The REF is a test room without PCM. The walls (without insulation) are built using a commune brick system, cemented plaster and gypsum board. Conversely, the roof contains two layers, where the bottom slab (thickness 12 cm) is made from concrete and the top slab (thickness 10 cm) is made using a brick mixture plus mortar ( Figure 4) . (2) Room 1: The structure is the same as the REF together with the incorporation of a PCM layer in the southern, eastern and western walls, as well as in the roof. The PCM (paraffin) layer is 2.5 cm thick and contained in aluminum panels ( Figure 4 ).
Energies 2016, 9, 30 8 of 17
7
(1) The REF is a test room without PCM. The walls (without insulation) are built using a commune brick system, cemented plaster and gypsum board. Conversely, the roof contains two layers, where the bottom slab (thickness 12 cm) is made from concrete and the top slab (thickness 10 cm) is made using a brick mixture plus mortar ( Figure 4 ). (2) Room 1: The structure is the same as the REF together with the incorporation of a PCM layer in the southern, eastern and western walls, as well as in the roof. The PCM (paraffin) layer is 2.5 cm thick and contained in aluminum panels (Figure 4 ). Figure 5 illustrates the average variation of ambient temperature and the temperature inside the test rooms with and without PCM at a 1.5 m height in the month of August. As shown in the figure, the temperature of the room without PCM is found to increase at τ = 12 h, reaches the maximum Figure 5 illustrates the average variation of ambient temperature and the temperature inside the test rooms with and without PCM at a 1.5 m height in the month of August. As shown in the figure, the temperature of the room without PCM is found to increase at τ = 12 h, reaches the maximum (35˝C) between 16 h and 17 h and then decreases. This is due to the fact that initially (up to 12 h), the heat is absorbed by the room walls and roof exposed to sun and then slowly released, which caused a greenhouse-like effect of an inner increase of the temperature, as expected. Needless to say, the sun sign (solar irradiance) reaches the maximum at 12 h and remains there for a few hours before gradually dropping in later in the afternoon (beyond 17 h). On the other hand, the implementation of the PCM in the building structures reduced the room temperature peak load by 5˝C for the same time period (between 16 h and 17 h). This lowering in the peak temperature inside the room and the creation of thermal comfort is attributed to the effect of the heat storage capacity of the PCM on the temperature variation. Actually, during the first few hours of the day (up to 12 h), paraffin absorbed the latent heat from the room environment and then underwent phase transformation, thereby reducing the peak temperature. However, with the decrease of the sun sign (beyond 17 h), the latent heat is slowly released into the atmosphere. The temperature fluctuation in this case is much weaker than the one without PCM incorporated in the building. It is evident that the greenhouse effect was much lowered in the presence of PCM. This verifies the environmental friendliness of PCM when used in building management and architecture.
Results and Discussion

Energies 2016, 9, 30 (35 °C) between 16 h and 17 h and then decreases. This is due to the fact that initially (up to 12 h), the heat is absorbed by the room walls and roof exposed to sun and then slowly released, which caused a greenhouse-like effect of an inner increase of the temperature, as expected. Needless to say, the sun sign (solar irradiance) reaches the maximum at 12 h and remains there for a few hours before gradually dropping in later in the afternoon (beyond 17 h). On the other hand, the implementation of the PCM in the building structures reduced the room temperature peak load by 5 °C for the same time period (between 16 h and 17 h). This lowering in the peak temperature inside the room and the creation of thermal comfort is attributed to the effect of the heat storage capacity of the PCM on the temperature variation. Actually, during the first few hours of the day (up to 12 h), paraffin absorbed the latent heat from the room environment and then underwent phase transformation, thereby reducing the peak temperature. However, with the decrease of the sun sign (beyond 17 h), the latent heat is slowly released into the atmosphere. The temperature fluctuation in this case is much weaker than the one without PCM incorporated in the building. It is evident that the greenhouse effect was much lowered in the presence of PCM. This verifies the environmental friendliness of PCM when used in building management and architecture. Figure 6 depicts the average variation of ambient temperature and the temperature inside the test rooms with and without PCM at a 1.5 m height in the month of January. The temperature of the room without PCM started to increase at 10 h and reached its peak load at 20 h. The temperature of the roof top reaches the maximum between 12 h and 16 h before dropping. The higher value of the roof top temperature compared to the ambient and inside ones clearly indicates the role of both radiation and convection throughout the day, as long as the sun signs; while the temperature inside the room with PCM in the winter month of January is stable all day under ambient temperature. This stability is due to the effect of using the PCM, which acts as a heat storage capacitor and diminishes the rapid temperature fluctuation via phase transformation. During phase conversion in paraffin, a huge amount of latent heat exchange occurs at a constant temperature. This elevated absorption of heat by the PCM materials is indeed responsible for the reduction of the heat flux inside the room and the maintenance of thermal comfort. Specifically, the use of PCM in the building walls and roof increased their thermal resistance and thereby reduced the overall heat transfer through the walls with much lowered heat load compared to the building without PCM.
The temperature variation for the PCM and non-PCM walls is theoretically examined. The east, south and west walls are used to calculate the heat flux through the external and internal walls' surface and the amount of heat storage in the walls. In Figures 7-9 the external heat flux, heat storage and internal heat flux on the east wall of both types of rooms have been compared. Figure 9 (heat flux through the external wall) clearly reveals that the heat flux for the PCM incorporated room is slightly higher than the one without PCM on the walls. This observation is attributed to the low thermal conductivity of the liquid PCM, which reduced the heat transmission to the room and thereby increased the wall top surface temperature. The maximum value of heat flux through the PCM including and excluding the external wall at 9 h is found to be 312 W/m 2 and 162 W/m 2 , respectively. Conversely, the heat flux (at 21.5 h) through the internal east wall of the room with and without PCM is discerned to be 26 W/m 2 and 44 W/m 2 , respectively. Furthermore, heat storage is observed to be maximum at 12 h with values of 240 W/m 2 and 135 W/m 2 for the PCM integrated and non-integrated east walls, respectively.
Energies 2016, 9, 30 conductivity of the liquid PCM, which reduced the heat transmission to the room and thereby increased the wall top surface temperature. The maximum value of heat flux through the PCM including and excluding the external wall at 9 h is found to be 312 W/m 2 and 162 W/m 2 , respectively. Conversely, the heat flux (at 21.5 h) through the internal east wall of the room with and without PCM is discerned to be 26 W/m 2 and 44 W/m 2 , respectively. Furthermore, heat storage is observed to be maximum at 12 h with values of 240 W/m 2 and 135 W/m 2 for the PCM integrated and non-integrated east walls, respectively. The variation of heat flux and storage for the south and west walls for the rooms with and without the incorporation of PCM is demonstrated in Figures 10-15 . These variations in heat and storage profiles are similar to those of the east wall.
The value of thermal flow and heat storage through a selected wall is computed using finite volume method (FVM), where the thermal flow (through the wall for both the internal and external surface) is calculated. A fixed value of the internal and external heat transfer coefficient is used, which is 7.73 W/m 2 •°C and 23.3 W/m 2 •°C for the internal and external wall surface, respectively. The amount of stored heat in the walls containing PCMs is found to be larger than the traditional (REF) walls. This elevated storage in PCM walls is ascribed to the high heat capacity and heat-retaining The value of thermal flow and heat storage through a selected wall is computed using finite volume method (FVM), where the thermal flow (through the wall for both the internal and external surface) is calculated. A fixed value of the internal and external heat transfer coefficient is used, which is 7.73 W/m 2¨˝C and 23.3 W/m 2¨˝C for the internal and external wall surface, respectively. The amount of stored heat in the walls containing PCMs is found to be larger than the traditional (REF) walls. This elevated storage in PCM walls is ascribed to the high heat capacity and heat-retaining susceptibility without connecting any storage space to the air conditioner. The observed higher thermal storage for the western walls is related to the longest exposure to the solar radiation. The It is evident that the ceiling (concrete) temperature of the room containing PCM maintained the temperature constant (27 °C) throughout the day as compared to that of the conventional room. This demonstrates that the environment insignificantly affects the inner surface of the ceiling, because all of the heat energy is absorbed by the PCM installed in the roof. Conversely, a considerable temperature fluctuation is observed in the ceiling of the REF (without PCM), because the outside environment immediately influenced its ceiling. Furthermore, the experimental results for PCM2 revealed a small reduction in ceiling temperature during the day time and slight augmentation It is evident that the ceiling (concrete) temperature of the room containing PCM maintained the temperature constant (27 °C) throughout the day as compared to that of the conventional room. This demonstrates that the environment insignificantly affects the inner surface of the ceiling, because all of the heat energy is absorbed by the PCM installed in the roof. Conversely, a considerable temperature fluctuation is observed in the ceiling of the REF (without PCM), because the outside environment immediately influenced its ceiling. Furthermore, the experimental results for PCM2 It is evident that the ceiling (concrete) temperature of the room containing PCM maintained the temperature constant (27˝C) throughout the day as compared to that of the conventional room. This demonstrates that the environment insignificantly affects the inner surface of the ceiling, because all of the heat energy is absorbed by the PCM installed in the roof. Conversely, a considerable temperature fluctuation is observed in the ceiling of the REF (without PCM), because the outside environment immediately influenced its ceiling. Furthermore, the experimental results for PCM2 revealed a small reduction in ceiling temperature during the day time and slight augmentation during the night time. This diminished temperature fluctuation of PCM2 arose from the large heat storage capacity of the PCM. The occurrence of the observed temperature differences between the simulation and experimental results is ascribed to the following reasons:
The room ceiling is influenced by interior condition, where an actual temperature variation occurred. (2) The effective thermal conductivity of the PCM in the experiment is higher due to the presence of uniformly-distributed high conductivity heat exchanger material in the PCM panel.
The actual phase change may not occur during the phase change temperature as prescribed in the theory. Figure 22 shows the simulated heat flux entering the room. It is clear that the PCM incorporated roof is better than the one without PCM. The implementation of PCM in the building structure remarkably reduced (more than two-thirds) the heat entry compared to the room without PCM integration. Moreover, the presence of PCM reduced the heat transfer by 46.71% which is directly proportional to reduction in the electricity consumption to maintain the room at 25˝C. Hence, the incorporation of PCM in the building architecture of Iraq is recommended because of thermal comfort, cost-effectiveness and environmental friendliness. 
Conclusions
Thermal management of locally-extracted PCM (paraffin with 40% oil and 60% wax) incorporated building structures in the specific climatic condition of Iraq is reported. Experiments and numerical investigations are made to examine the heat flux reduction inside a residential building using such PCMs. These inexpensive PCMs are obtained from waste petroleum products in Iraq. Two rooms of identical internal dimensions are built, one with PCM and the other without PCM in the roof and walls. The thermal performances of these full-scale test rooms are evaluated. The experimental results are complemented using two-dimensional numerical transient heat transfer (laminar and turbulent flows) model simulation, where the finite difference method is used to solve the discretized equations. The computations are conducted to obtain the solution of heat transfer in 
Thermal management of locally-extracted PCM (paraffin with 40% oil and 60% wax) incorporated building structures in the specific climatic condition of Iraq is reported. Experiments and numerical investigations are made to examine the heat flux reduction inside a residential building using such PCMs. These inexpensive PCMs are obtained from waste petroleum products in Iraq. Two rooms of identical internal dimensions are built, one with PCM and the other without PCM in the roof and walls. The thermal performances of these full-scale test rooms are evaluated. The experimental 
Conflicts of Interest:
The authors declare no conflict of interest. 
Nomenclatures


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