Advances in Materials Science and Engineering

Volume 2016 (2016), Article ID 6359414, 15 pages

http://dx.doi.org/10.1155/2016/6359414

## Experimental Research on the Thermal Performance of Composite PCM Hollow Block Walls and Validation of Phase Transition Heat Transfer Models

^{1}School of Energy and Power Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, Jiangsu 212013, China^{2}College of Urban Construction, Nanjing Tech University, Nanjing, China

Received 1 September 2015; Revised 27 December 2015; Accepted 29 December 2015

Academic Editor: Fernando Lusquiños

Copyright © 2016 Yuan Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A type of concrete hollow block with typical structure and a common phase change material (PCM) were adopted. The PCM was filled into the hollow blocks by which the multiform composite PCM hollow blocks were made. The temperature-changing hot chamber method was used to test the thermal performance of block walls. The enthalpy method and the effective heat capacity method were used to calculate the heat transfer process. The results of the two methods can both reach the reasonable agreement with the experimental data. The unsteady-state thermal performance of the PCM hollow block walls is markedly higher than that of the wall without PCM. Furthermore, if the temperature of the PCM in the wall does not exceed its phase transition temperature range, the PCM wall can reach high thermal performance.

#### 1. Introduction

As the building energy consumption increases, the energy-saving technology in buildings has become one of the hot fields in the world; meanwhile, people’s demand on the indoor thermal comfort also increases continuously. One of the traditional ways to solve this problem is to introduce thermal insulation materials into building envelope to enhance the thermal performance of wall. Traditional thermal insulation materials have low thermal conductivities which can effectively reduce the heat flow through the building envelope; however, they are mostly organic thermal insulation materials with low specific heat capacity. This can probably result in the indoor temperature fluctuation with large amplitude and frequency, which reduces the indoor thermal comfort degree, gives rise to the frequent open and close of air-conditioning device, and decreases the opportunity of device operating under the optimum condition, which increases the energy consumption of device. In addition, the fire safety hidden trouble is accompanied, and the thermal insulation material does not have a lifetime as long as the building itself (the general service life of the organic thermal insulation material is 10 to 20 years). Therefore, people start to find new and better methods to save energy.

It is one of the potential ways to use phase change material (PCM) into building envelope. The proper use of the latent heat of phase transition effect can substantially increase the thermal capacity of building envelope. By doing this, the heat flow into building envelope in daytime is conserved; when the outdoor air’s temperature falls to a low value in night time, the PCM releases heat and then makes the conserved heat back to the outside. In this way, the heat entering into the interior of the room throughout a day will be remarkably reduced; furthermore, the peak of heat entering building envelope in daytime will enter into the interior of the room at dusk or night with a lower peak value and larger time lag because of the decrement and delay effects. At this time, the indoor cooling load is reduced; also, the electric charge is in the valley time period, by which the indoor thermal comfort degree is improved and the money is saved.

At present, a lot of researchers studied the thermal performance of the building envelope outfitted with PCM layer. Aelenei used a new structure of wall with photovoltaic board and PCM board and calculated and tested the application effect under the climate of winter. The results indicated that the electrical energy was saved by 10% and the efficiency of the whole system reached 20% by using this type of wall [1]. Ahmed et al. introduced the PCM (paraffin) into the wall of the truck box which was used for the refrigeration of things. The test results demonstrated that the peak heat flow rate entering the truck box was averagely reduced by 29.1% when all the walls were outfitted with PCM; if one wall of the walls was outfitted with PCM, the heat flow rate can be decreased for 11.3%–43.8%; the heat flow rate that entered the truck box every day can be reduced for 16.3% [2]. Alqallaf and Alawadhi analyzed the thermal performance of a concrete roof deck outfitted with PCM. The test results indicated that the heat gain became lower with the increasing amount of PCM; furthermore, the heat gain was influenced by the phase transition temperature of the PCM [3]. Bontemps et al. applied PCM into the wall of an experimental box with twin-box structure. The experimental box was placed under the environment of the local typical outdoor climate. The test results demonstrated that the temperature fluctuation inside the test box with PCM was significantly lower than that of the box without PCM [4]. Borreguero et al. tested the thermal performance of a gypsum board outfitted with PCM. The result showed that the thermal storage capacity of the gypsum board was strengthened by the increasing amount of PCM; meanwhile, the temperature fluctuation inside the wall was reduced; in addition, containing the same thermal performance, by increasing 5% (mass fraction) amount of PCM inside the board, the thickness of the board can be decreased for 8.5% [5]. Cabeza et al. built a concrete room with actual size. The south and west walls and the roof were outfitted with 5% PCM, and the room was exposed to the natural environment. The results demonstrated that the thermal inertia of the wall with PCM was remarkably higher than the wall without PCM, and the interior surface temperature of the PCM wall was lower (reducing for 0 to 2°C); the peak heat was shifted for about two hours by the PCM wall [6]. Castell et al. built the house using two types of traditional perforated bricks with and without PCM. The houses were tested under the typical climate of Spain. The results showed that the PCM made the indoor peak temperature reduce for about 0.9°C and 0.73°C, and the energy consumption reduced for 15% and 17% [7].

It can be seen from the above research that the building envelope outfitted with PCM can reach better thermal performance. At present, the study of the phase transition heat transfer model is mostly around the one-dimensional models [4, 5, 8]. However, it is easy to leak and be damaged and occupies the thickness of wall, if the PCM exists in building envelope with the form of one layer. The hollow brick wall and hollow block wall are widely used in some parts of world for energy conservation now. If the PCM is properly put into the holes of the hollow bricks and hollow blocks, the construction procedure will be simplified and the negative influences will be avoided with maintaining high thermal performance.

On the field of multidimensional heat transfer, Antoniadis et al., using Comsol Multiphysics software [9] and two-dimensional finite element method, analyzed the impacts of the hole rows, hole thickness, and the staggered or unstaggered arrangement of hole rows on the thermal performance of hollow brick, taking the effective thermal conductivity as the evaluation index [10]. Arendt, using two-dimensional energy equation, studied the impacts of hollow ratio on the steady-state and unsteady-state thermal parameters of hollow bricks, with the evaluation indexes of time lag, decrement factor, and equivalent thermal conductivity [11]. Li et al. studied the heat transfer process of the hollow bricks with different structures using the Tri-diagonal Matrix (TDMA) method and Alternating Direction Implicit (ADI) method [12, 13]. The computational efficiency and accuracy of the methods are both superior to or equivalent to the Gaussian elimination method [14], which are appropriate for the solution of multidimensional heat transfer problems of the nonhomogeneous materials such as hollow blocks [15, 16]. Mackerle et al. analyzed the thermal performance of a concrete roof outfitted with PCM using three-dimensional heat conduction model and finite element method. In the mathematical model, the specific heat of PCM within the phase transition temperature range was equivalent to a certain value. The discrete equations were solved by preconditional generalized minimum residual (PGMR) [17] solver. This concrete roof had some cylindrical holes inside, which were filled with PCM (A39). The numerical calculation and experimental test were performed on the roofs with and without PCM, respectively. The results demonstrated that the maximum errors between the calculation results and the experimental data were 7.25% and 7.4%, and there was reasonable agreement between calculation results obtained by the PCM mathematical model and the experimental data [3]. Carbonari et al. developed a two-dimensional phase transition heat transfer model. He used the model and finite element method calculated the heat transfer process of a type of wall board with PCM layer. After the comparison between the calculation results and the experimental data, it was found that a good agreement is achieved [18]. It can be known that, at present, the study on the multidimensional heat transfer process of the constant-property material wall is relatively sufficient but that of the composite PCM wall is insufficient.

In consideration of the reasons above, a type of concrete hollow blocks in typical structure and a commonly used phase change material (PCM) were adopted and the multiform composite PCM hollow blocks were produced in this paper. The PCM was put into different positions of hollow blocks. The temperature-changing hot chamber method was used to test the thermal performance of the hollow block walls with or without PCM under different temperature conditions; meanwhile, the two-dimensional enthalpy model and effective heat capacity model were used to calculate the phase transition heat transfer process under the conditions of experiments. The results of the models were validated by the experimental data, and the thermal performances of the hollow block walls with and without PCM and the walls with the PCM at different positions were compared. The difference on the thermal performance of each wall was analyzed, which provides the data reference and guide for the engineering application of composite PCM hollow block wall.

#### 2. Mathematical Models

##### 2.1. Assumptions

To simplify the heat transfer model, the following assumptions were made: all the materials were considered to be thermally homogeneous and isotropic; because the heat transfer process was mainly heat conduction, the thermal conductivity of the liquid PCM was considered to be not changed; because the specific volume ratio of the solid state to that of the liquid state was less than 1.15, the volume change of the PCM during the phase change process was ignored; the supercooling effect during the freezing process and the natural convection effect were ignored (the used PCM in the experiment and calculation was the composite PCM which was the mixture of the PCM and gypsum in a certain ratio; the natural convection effect was effectively restrained); the equivalent thermal conductivity was applied for the air cavities inside the blocks [19]; because the net heat flow was 0 on the vertical direction of wall, the heat transfer in the computational domain was simplified to be two-dimensional heat transfer, and the total heat flow was on the direction of thickness (the boundaries of other directions were adiabatic, where the second type thermal boundary condition was applied).

For point , there are some explanations which are worthy to state. According to the formula, the Grashof and Prandtl numbers can be calculated as and . After the calculations, for the side holes with air in the experimental hollow blocks, the average is calculated as 0.0167, and the 0.3998 for the middle holes with air. The average for the side holes with air and the middle holes with air is around 0.699. The average for the side holes with air and the middle holes with air is relatively small, which demonstrates that the buoyancy force is small, relative to the viscous force. Meanwhile, the average is relatively large, which indicates the obvious influence of viscidity during the heat transfer process. Therefore, it can be seen that the convection effect in the holes with air is unapparent and the main heat transfer approach is heat conduction. For the holes with PCM, the used PCM in the experiment and calculation was the composite PCM which was the mixture of the PCM and gypsum in a certain ratio. Then the liquid PCM (if it has melted) exists in the microvoid of the solid gypsum. Because of these, the natural convection effect was effectively restrained, the value of the for the holes with PCM becomes very small, and the is the same as the status of the holes with air. With the situations above, we assumed the natural convection to be negligible and we considered that it will not produce large errors in the calculations.

For point , it is worthy to explain that, according to the instruction book of the device, the heat loss of the chambers and the wall frames is less than 5%. Furthermore, the thermal conductivity of the insulation material inside the frames of chambers (extruded polystyrene) is low enough (0.029 W/m·K) and the thickness of the frames is large enough (larger than 0.15 m). Therefore, we consider that the heat losses through the chambers and the wall frames have little impact on the accuracy of the experimental data.

##### 2.2. Governing Equation

In this research, the traditional two-dimensional heat transfer equation was used to solve the problem of the hollow block wall without PCM [20], and the two-dimensional enthalpy model and effective heat capacity model were adopted for the hollow block wall with PCM.

###### 2.2.1. Two-Dimensional Enthalpy Model

One haswhere , , and were the density, thermal conductivity, and enthalpy of PCM, respectively. The thickness and width directions were represented by and , and the temperature at any point in time was . The enthalpy of PCM was calculated bywhere and were the specific heat of the PCM in its solid and liquid states, respectively, was the mean temperature of the phase transition temperature range. The phase change radius represented the half range of the phase change temperature range and was the latent heat during phase change. The temperature range of phase transition was from to . When or when , the value of was equal to the value of kJ·kg^{−1}·K^{−1}; when , the value of was equal to , where was the equivalent specific heat of the PCM during phase change.

###### 2.2.2. Two-Dimensional Effective Heat Capacity Model

One haswhere and represented the density and the thermal conductivity of the PCM, respectively. was the effective heat capacity of the PCM, which was calculated using (4). The parameters and were the thickness and width directions, and was the temperature of the specimen at a point in time ; and were the heat flow rate across the PCM and the mass of the PCM. The heating or cooling rate of the PCM was given by .

##### 2.3. Initial and Boundary Conditions

Because the parameters were tested under the periodical thermal boundary conditions in the experiments, four temperature periods were performed in each experiment to eliminate the impact of the initial condition on the testing results. The initial temperature was shown as follows: where was the temperature at point at time . was the initial temperature.

The third type thermal boundary condition was applied when (exterior surface on the thickness direction) and (interior surface on the thickness direction): where was the total number of the meshes on the direction; point and were the boundary meshes when and ; , , and were the points right to, above, and under point ; , , and were the points left to, above, and under point ; , , , and were the convective heat transfer coefficients and boundary air temperatures when was equal to 1 and , respectively. Each item of the two equations took the positive value when heat flowed into the point (or ) and negative value when heat flowed out of the point (or ).

The boundaries on the direction of (width direction, when and , was the total number of the meshes on the direction) were adiabatic (the second type thermal boundary condition). The equation is where was the heat flow function in which the independent variable was time , .

##### 2.4. Numerical Method

The point iteration method was used for the two-dimensional numerical calculation in this work. The governing equations and boundary conditions were discretized by Finite Difference Method (FDM) [21, 22]. The sizes of the mesh and time steps were 0.005 m and 2 s, which guarantees that the results were independent of the used mesh and the time step sizes. Central differences were applied in space step while a fully implicit finite difference scheme and forward differences were applied in time step. The Gauss-Seidel scheme was employed in iteration. The convergence criterion is shown as in (8). The calculation program was developed in MATLAB environment [23]:where and were the temperatures of the point at the time of and , respectively.

#### 3. Experiments and Models Validation of Composite PCM Hollow Block Walls

##### 3.1. PCM and Composite PCM Hollow Blocks

Figure 1 is the schematic diagram of the four blocks used in experiments (the shaded part in the figures is PCM), they are the hollow block without PCM (Block 1), the block with a hole row of PCM which is near the left surface (Block 2, the left side of the block wall was disposed as the outdoor side, and the right side was disposed as the indoor side), the block with a hole row of PCM which is in the middle of the block (Block 3), and the block with a hole row of PCM which is near the right surface (Block 4), respectively. With filling PCM into the different rows of blocks, and taking the block without PCM as a reference, the thermal performance of composite PCM hollow block walls was analyzed. The PCM used in the experiments is Capric Acid. The thermal-physical parameters are shown in Table 1 [24, 25].