^{1}

^{1}

^{2}

^{3}

PCMs (Phase Change Materials) can be integrated into building envelopes to decrease the building energy consumption, refine the indoor thermal comfort, shift and reduce the peak electricity load due to its relatively large latent heat. In this study, influence of the PCM layer location on the multilayer wall thermal performance is numerically researched in four walls under the climate conditions of Chengdu, China. The results only shows when the phase change of PCM occurs; its latent thermal storage performance can be played and have the significant influence on wall thermal performance. Due to phase change of PCM occurs, the fluctuation amplitudes of inner surface temperature and heat flow are reduced obviously; the temperature peak value is delayed in the phase-change occurred periods. In addition, the PCM layer can reduce inner surface heat flow, especially in summer and transition season, which is in the phase-change occurred periods. The average annual heat flow can be reduced by 8.5% - 11.8%. And when the PCM layer is closer to the wall internal side, the influence of the PCM layer location on the multilayer wall thermal performance is more significantly.

With the development of society, demand in thermal comfort of buildings is rising increasingly; the energy consumption is correspondingly increasing. [

Mandilaras, et al. [

However, most of studies have ignored the influence of phase-change temperature arrange of PCM. As is known, only when the phase change of PCM occurs can the latent thermal storage performance be played, so the reasonable phase- change temperature arrange of PCM is the most important factor. Based on the above analysis, this paper builds four wall models, of which one is a reference subject and other three are the walls integrated with the PCM layer in the different location, and according to the previous studies [

To research the optimal location of the PCM layer in the multilayer wall, four walls are built as shown in

Convection thermal boundary conditions are adopted in inner and outer sides. In outer side, the air temperature and the solar radiation intensity in Chengdu

Materials | Density (kg/m^{3}) | Heat capacity (J/(kg・K)) | Heat conductivity coefficient (W/(m・K)) |
---|---|---|---|

Plaster layer | 1860 | 840 | 0.87 |

Sintered brick layer | 1700 | 1051.6 | 0.63 |

PCM layer | 1300 | 1785 | 0.45 (liquid), 0.7 (solid) |

city are as shown in

Considering the actual conditions of air-conditioning and heating, if the indoor air temperature from Equation (1) is greater than 25˚C in summer, indoor air temperature is set as 25˚C. Meanwhile, if the indoor air temperature from Equation (1) is less than 20˚C in winter, indoor air temperature is set as 20˚C. According to these,

Considering the actual conditions of air-conditioning and heating, if the indoor air temperature from Equation (1) is greater than 25˚C in summer, indoor air temperature is set as 25˚C. Meanwhile, if the indoor air temperature from Equation (1) is less than 20˚C in winter, indoor air temperature is set as 20˚C. According to these,

Only when the phase change of PCM occurs, the latent thermal storage performance can be played. And thus, the reasonable selection on the PCM phase-

change temperature arrange directly affects the PCM phase-change thermal storage performance effect. Therefore, this study refers to the calculation methods of the phase-change temperature range proposed by Meng, et al. [

where_{ }are solidus and liquidus temperatures ˚C; ^{2}・K); h_{in} and h_{out} are inside and outside convective heat transfer coefficient, (W/(m∙K)); ^{2}∙K); ^{3};

Meanwhile, this study only consider the phase-change happening in summer and transition seasons. And PCM latent heat is 178.5 kJ/kg in this study. According to Equations (2)-(3), the theoretical values of PCM phase-change range under the different location of the PCM layer are shown in

Actually, the wall heat transfer is three-dimensional, but as the wall heat transfer occurs between inner and outer surfaces and thereby, there is only heat transfer along the wall thickness. Therefore, a three-dimensional problem can be approximately simplified to one-dimensional heat transfer along the wall thickness directions.

Cases | The location of the PCM layer | PCM phase-change range | |
---|---|---|---|

Solidus temperature (T_{S}_{-TV}) | Liquidus temperature (T_{L}_{-TV}) | ||

1 | Internal side (L = 0 mm) | 18.90 | 27.30 |

2 | Middle (L = 110 mm) | 15.99 | 30.52 |

3 | External side (L = 220 mm) | 13.06 | 35.00 |

sol-air temperature includes the effect of outdoor air temperature combined with solar radiation. Air temperature at the wall inner surface represents indoor air temperature, which may be equal to the set temperature of air-conditioning.

Under the outer thermal environment variation with time, the heat transfer of the multilayer wall integrated with the PCM layer is the transient heat conduction with both melting and solidification of PCM. If a one-dimensional coordinate system is established with the coordinate origin at the thickness direction

where T denotes the material temperature, ˚C; t is time, s; In the non-PCM layer, H can be shown as following:

where

In the PCM layer, H can be shown as following:

where

The convective heat transfer boundary conditions are adopted on the outer and inner surfaces (

On the outer surface (

On the inner surface (

where ^{2}.

The equations of the heat transfer model have been solved using the finite volume method in this simulation region. The finite volume formulation utilized in this algorithm ensures the energy conservation of wall heat transfer. A fully implicit scheme is applied for discretizing the time derivatives and a second-order central difference scheme is used for the diffusion terms. The corresponding algebraic equations are solved by the tri-diagonal matrix algorithm (TDMA). The convergence of the computations is declared at each time instant, when the following criterion is satisfied:

where n is the internal iteration number.

To verify both accuracy and reliability of the unsteady calculation procedure on the enthalpy-porosity model, the research is done to numerically simulate the dynamic thermal response of the multilayer wall integrated with the PCM layer, which is researched experimentally by Kuznik and Virgone [

In order to study the influence of the PCM layer location on the wall thermal performance, indoor air temperature is designed as in ^{2}・K) and 8.7 W/ (m^{2}・K) respectively [

temperature is less than the solidus temperature, and thereby, PCM is presented as the solid state. Therefore, in summer and transition season of

of the PCM layer, the fluctuation amplitude is reduced for inner surface heat flow and the peak heat flow value is delayed with the different degrees, which shows PCM can shift and reduce the peak air-conditioning load. However, due to the fact that PCM has not changed its phase in winter, the PCM layer location has not the influence on inner surface heat flow. On the other hand, in summer and transition season, when PCM can change its phase with outdoor thermal environment, the reduced fluctuation amplitude with the PCM layer located in the internal side is obviously larger than that with the PCM layer located in the middle and the external side. Namely, when the PCM layer is closer to the wall internal side, the reduced fluctuation amplitude of inner surface heat flow is larger.

Due to the fact that PCM can change its phase with outdoor thermal environment, their reduced percentage of the average heat flow values is larger than that in winter. On the other hand, when the PCM layer is closer to the wall internal side, the reduced percentage of inner surface heat flow is larger, and thereby, PCM has the higher energy saving efficiency.

In this study, the influence of the PCM layer location on the multilayer wall thermal performance is numerically researched under the climates conditions of Chengdu, China. However, influence of temperature and heat flow has been analyzed and the following conclusions can be drawn from the results obtained:

1) For three kinds of PCM layer, phase change is all occurred in summer and transition season. It has no effect in winter.

2) The application of PCM layer can reduce wall inner surface temperature and heat flow fluctuation, and the closer to inner surface, the more obviously this phenomenon is; which means the closer to wall inner surface, the better the effect of improving indoor comfort and wall thermal performance.

3) For three kinds of walls of different PCM layer location, wall 1 is the biggest energy saving wall, wall 2 is second and wall 3 is the last during summer and transition season, which means the best location of PCM layer is most close to inner surface when meeting other demand of wall structure.

4) Even though the walls are simplified ideal walls, the rule of temperature and heat flow is applicable for a real wall. So when making walls integrated PCM, PCM layer should laid close to wall inner surface as much as possible.

This research was supported and funded by MEXT, Japan (NO.142264)

Gao, Y., Gao, W.J., Meng, X. and Long, E. (2017) Influence of the PCM Layer Location on the Multilayer Wall Thermal Performance. Open Journal of Energy Efficiency, 6, 1-13. https://doi.org/10.4236/ojee.2017.61001