2] . Measurements of both surface temperatures and heat flux related to a wall in steady state conditions were thus determined. Also, one must to consider that, notwithstanding the extreme accuracy of the measurement in the laboratory, the thermal resistance of the wall determined in-situ might present a significant deviation from the resistance determined from the prototype. This deviation is due not only to the inevitable differences among supplies of the same material made in different time periods, but also to the inevitable differences in the conditions of wall-building  .
In our procedure, we chose the period from 8th to 10th July in this study, because it shows some moderation in Casablanca weather fluctuations, compared to other periods of summer.
2.4. Weather Data
The cavities are located in Casablanca FSAC (33˚32'N latitude, 7˚39'W longitude and altitude 57 m). The weather data used in this study are those relating to 8th to10th July 2014, measured using our weather station installed on the roof of the FSAC. These data were measured with a time step of one hour and concerning: Outdoor temperature, humidity, global solar radiation, wind speed and wind direction. Note that for this period, external temperature fluctuations are not very important and do not exceed 6˚C as shown in Figure 2. Global heat radiation is presented in Figure 3. It is practically identical for the three days of measurement. It maximum value is around 850 W/m2. Against, the wind speed has varied considerably between the nights of 8th to 9th July, Figure 4. These fluctuations will act on the outer heat exchange coefficient and thus the thermal resistance of the walls.
The equation used to calculate the thermal resistance for different walls is as follows:
Figure 2. Outdoor temperature, 8th to 10 July 2014.
Figure 3. Solar radiation (W/m2), 8th to 10 July 2014.
Figure 4. Wind Speed variation, 8th to 10 July 2014.
n: number of the wall layers.
However, in large spaces such as wall cavities, heat can still be lost across the air layer (inside the wall) by convection and radiation.
Table 1 summarizes the calculation of the thermal resistance of the various walls taking into account their composition.
The thermal conductivity of air is about 0.026 W/m∙K, the value 0.09 W/m∙K, considered in this work (Table 1, found in TRNSYS16 database) reflects the weak natural convection that may exist in the layer air.
The heat flux exchanged is calculated relating to the overall strength taking into account the internal natural convection and the external free convection:
Table 1. Characteristics of the different layers of the walls.
The thermal convective heat transfer coefficient describing heat exchanged along the external and internal faces of the building walls, can be calculated using approximated correlations from previous work   , respectively:
4. Results and Discussion
In order to minimize the heat gain in the summer period, we placed the PCM on the roof of the test cell. To highlight the contribution of PCM for insulation against the solar radiation and storage, we performed a systematic comparison of the internal and external temperatures of the walls and the room temperature with and without PCM. The preliminary results presented in this paper are obtained for the period from 8th to 10th July 2014, for which we consider that the boundary conditions are fairly stable and uniform in order to use the steady state method in our evaluation process of thermal resistance.
Figure 5 and Figure 6 illustrate the temperature variations of west and south walls of the two cavities, respectively. These walls are exposed to solar radiation more than the other plates. In case of the west wall of the sample with PCM (Figure 5), the temperature of the inside surface is lower than the inner temperature of the same wall of that without PCM, during the day. The situation is reversed during the night. It turns that the integration of PCM on the roof can reduce the temperature fluctuations of 2.53˚C during our test period. Figure 6 shows that the external temperature fluctuated from 20.70˚C to 31.56˚C and the internal temperature is varying between a minimum of 25.67˚C and a maximum of 29.01˚C for the cavity with PCM while in the case without PCM, it varies from 25.7˚C to 30.04˚C, during these three days. The incorporate PCM in the ceiling allows a reduction of the temperature for different walls of the cavity (different orientations).
Moreover, we can observe a remarkable difference between the surface temperatures of west wall and that of the south wall, in both cases (with and without PCM), it is due to the significant insolation received by the south faces.
The variation of the indoor temperature of the two cells (with and without PCM) is shown in Figure 7. It is found that the indoor temperature of the cubicle with PCM is reduced by 1.5˚C compared to the one without PCM. This can be explained by the good thermal inertia presented by the phase change material (PCM). This difference between indoor temperatures is directly affecting the comfort sensation, according the well-estab- lished comfort theories. In Table 2 we summarized the thermal time shifts for the cell walls.
The reduction of heat inputs through the ceiling (by introduction of the PCM) does not only affect the
Figure 5. Evolution of the external and internal temperatures of the west face of the two cavities.
Figure 6. Evolution of the external and internal temperatures of the south face of the two cavities.
temperature of the walls but also the indoor temperature of the cell. The latter is the result of heat exchange between the cavity and the outside through the walls (usually it is calculated by weighting temperatures of different surfaces). The latter (walls) are at different temperatures, for given time (depending on their orientation). The differences that present the wall temperatures for both cavities are not synchronized (when the difference related to the southern walls is low, that of the East walls are important and so on). Knowing that the indoor temperatures are closely related to those of the walls at a specific time, it is normal that the gap between them will not be comparable to those of walls.
Figure 7. Evolution of the indoor temperatures of the cavities.
Table 2. Time shifts between the temperatures of the two cells.
We present the heat transfer between the two cavities and the outside in terms of heat flux densities through the various walls. In this paper, only those relating to the ceilings, the west and the south walls will be exposed and discussed.
Figure 8 shows the heat flux evolution of the two ceilings with and without PCM. First, we note that, there is a significant gap between the two heat flux densities during this period. The insulation effect of PCM is clearly visible during the day as can be seen on the maximum curve (φmax, with_PCM = 58.35 W/m2, φmax, without_PCM = 63.80 W/m2). The Aluminium layer reflects a significant portion of solar radiation in our setup. It will reduce, significantly, the solar flux absorbed by the PCM, knowing its reflectivity against the radiation which is about 0.9. In this work, we have not considered this aspect because our objective was focused on the study of the thermal efficiency of the local, but we evaluated the effect of the entire PCM panel. In further configurations, we will cover the panels by a very few reflector materials to highlight the only effect of PCM. The effect of the thermal inertia is very clear; a delay of three hours is observed (Table 2). This leads to a significant reduction in energy consumption during the discharge period (night). The cells do not cool in the same way. In fact, the heat flux density on the cell with PCM remains greater than or equal to zero while that of the reference cell drops to −10 W/m2. This is due to the refund of the amount of heat previously stored (in the form of latent heat) in the PCM.
Based on the analysis of representative figures of the heat flux exchanged through the vertical walls, we distinguish the emergence of two phenomena during the day. As an example, we present Figure 9 and Figure 10 showing heat fluxes related to the West and South walls of the two cavities. Generally, it is noted that the heat flux values range from maximum (positive) and minimum (negative) for a period of 24 hours. From 6 h 00 to 13 h 00 both cells yield the same heat flux density while for the rest of the day, the exchanged heat flux are different
Figure 8. Evolution of the heat flux densities of ceilings with and without PCM.
Figure 9. Evolution of the heat flux densities of west wall for the two cavities.
from one cell to the other one. Through the west wall, the maximum value in the cavity without PCM is 2.98 W/m2, while in the cell with PCM; it reaches 2.62 W/m2. This remark also applies to the south wall. We can note also that the time shift is approximately 6 hours for the two walls. So it is an important thermal inertia introduced by the PCM.
During the first period, the similarity of the heat flux is due to internal wall temperatures, which are substantially equal. The difference observed in the second period is due to the time shift between the internal and external temperatures of the two cells (see Figure 2 and Figure 3).
Figure 10. Evolution of the heat flux densities of south wall for the two cavities.
The heat flux through the ceiling is almost 10 times greater than those for vertical faces. The difference of the heat fluxes of the two ceilings arrives to 11.2 W/m2. This suggests the importance of the ceiling insulation against solar radiation in Morocco.
We conducted an experimental study of the thermal performance of two cavities like living quarters built, in-situ within the Faculty of Science Ain Chock, Casablanca. One of them is equipped with a PCM on the roof, during summer of 2014.
The results show that the integration of PCM on the roof of the building reduces the indoor temperatures of the cells by 1.5˚C and those of the internal walls of 2.53˚C. We also note that the amplitude of thermal oscillations of south and west walls decreases.
The study of heat transfer, through the vertical walls and ceilings of the two cells, led to conclude that the PCM was a good insulation and storage agent on the one hand, and greatly increased the thermal inertia of the cavity with PCM, on the other hand (due to the return of the heat stored during the day).
According to the same study, it is imperatively important to insulate the ceilings of residential premises against solar radiation: the gap between the two streams of the ceiling with and without PCM reaches 11.2 W/m2. Note that solar radiation constitutes the major part of the overall gain in Morocco. Therefore, there will be a significant reduction of energy used to cool the local. Furthermore, there is a significant time shift presented by the temperatures and the heat flux densities due to PCM incorporated in the ceiling.
Authors acknowledge the financial support provided by IRESEN “Institut de Recherche en Energie Solaire et Energies Nouvelles” Morocco.
Cite this paper
Amina Mourid,Mustapha El Alami,Mostafa Najam, (2016) Passive Study of Energy Efficiency of a Building with PCM on the Roof during Summer in Casablanca. Journal of Power and Energy Engineering,04,26-37. doi: 10.4236/jpee.2016.48003
e: Thickness, mm.
he: Outdoor thermal convective heat transfer coefficient (W/m2∙K).
hi: Inside thermal convective heat transfer coefficient (W/m2∙K).
PCM: Phase Change Material.
r: Thermal resistance of a wall layer (m2/W∙K).
Rt: Global thermal resistance of wall (m2/W∙K).
Δθ: Temperature gap (ϴwi-ϴwe) (˚C).
Δts: Thermal Time shift.
f: Heat flux, W/m2.
λ: Thermal conductivity (W/m∙K).
ϴwi: Internal temperature of the wall (˚C).
ϴwe: External temperature of the wall (˚C).
we: external face of wall.
wi: internal face of wall.
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