EXPERIMENTAL INVESTIGATION OF AGGLOMERATE MARBLES CONTAINING PHASE CHANGE MATERIALS. I. Mandilaras, M. Founti

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1 EXPERIMENTAL INVESTIGATION OF AGGLOMERATE MARBLES CONTAINING PHASE CHANGE MATERIALS I. Mandilaras, M. Founti National Technical University of Athens School of Mechanical Engineering 9, Heroon Polytechniou, Polytechnioupoli Zografou Athens 15780, Greece ABSTRACT The work investigates the thermal response of a new composite building material based on natural stone wastes and Phase Change Materials (PCM). In order to be able to measure the transient thermal response of the new products under various temperature conditions, a specialized device has been developed at NTUA and implemented for the study of the nonlinear heat-transfer characteristics associated with phase-change of the PCM. Thermal performance analysis and evaluation for three different agglomerate marble samples with different percentages of PCM in their mass, have been undertaken. Comparative analysis of the results shows the ability of the new materials to stabilize their temperature near the melting range of the PCM, and potentially reduce heat losses of a building. 1. INTRODUCTION Thermal analysis of conventional building materials requires determination of specific heat and thermal conductivity. These two thermal properties along with the density of the material can fully characterize and predict its performance. Incorporation of a Phase Change Material (PCM) in a building material necessitates a different approach in thermal characterization. Additional properties have to be measured such as melting/solidification range of the PCM and enthalpy change during phase transition. Furthermore, comparative analysis including measurements of specific heat capacity and heat-flux/energy is required [1,2]. Current methods for thermal conductivity measurements can be applied for the measurement of materials containing PCMs as long as the temperature of the sample during the measuring procedure remains outside of the phase change region. However, specific heat capacity cannot be measured with conventional transient methods. The only applicable method is Differential Scanning Calorimetry, which requires expensive equipment and a very small quantity (in the order of mg) of the material that in many cases is not representative of the original sample [3]. For the needs of inexpensive and reliable measurements a prototype experimental set up was specifically developed for the thermal characterization of building materials containing PCM. The application of the new experimental apparatus for the investigation of an agglomerate marble containing PCM provides the focus for this paper.

2 2. EXPERIMENTAL SET-UP An experimental setup (Fig. 1,2) has been developed for the measurement of heat capacity, thermal mass and thermal energy conservation of building materials containing PCMs. The device applies variable thermal loads at the two sides of a flat-surface material sample, while measuring its thermal response. Two thermo-regulated aluminum plates that support and hold the sample are used to impose thermal loads. Thermoelectric elements with a control unit are capable of applying any constant temperature or temperature profile on the sample facing sides of the aluminum plates. Thermal response and heat flux on the two surfaces of the sample are continuously recorded via a data acquisition system. A thin insulating layer of Neopren between the sample and the device simulates air-wall convection heat transfer. An insulation of 5 mm with thermal conductivity of k = W/m k corresponds to convection heat transfer coefficient h = 13 W/m 2 k which is a typical value for wall-air heat transfer cases. In addition, this flexible layer assures perfect contact between samples with rough surface and the device and allows the sensors to be placed on the two surfaces of the sample. Flexible interface Aluminium Plates Water out Sample Water in Figure 1. Sample holder The test rig is pictured in figs. 1-2 and can be divided into four main components: The frame that holds the plates and the sample (Fig. 1) Thermoelectric heating/cooling devices The temperature control unit The data acquisition system

3 Control Unit Thermoelectric device 1 PC DAQ Sample Holder Thermoelectric device 2 Figure 2. Schematic representation of the experimental set up The aluminium plates are rectangular in shape (200x200 mm 2 ) and they can move along a horizontal slide allowing specimen insertion. The dimensions were chosen to ensure that there exists a central section suitable for one-dimensional heat transfer. To achieve the uniform onedimensional temperature field required for the calculations, the lateral surfaces of the specimen and the device are covered with insulating material of 60 mm thickness and thermal conductivity of approximately k=0.03w/mk. The temperature control of the surfaces is achieved with the recirculation of water into four independent channels in each plate.. Heating/cooling of the water is performed by two 500 W thermoelectric type devices. The devices are capable of extended use at temperatures from 5 o C to 75 o C. Up to 10 L/min of water are supplied to the plates via two 220 V centrifugal pumps. The control unit is capable of stabilizing the temperature of the plates within the range of ±0.05 o C. It comprises of two temperature controllers and two power supply units 500 W each. It is connected to the PC with RS232 interface. For the data acquisition (Fig. 2) an Agilent DAQ/Switch unit is used. The unit collects the signals from 12 sensors attached on the plates and the sample to be measured. Temperatures are measured with NTC thermistors. The probes are very small with 0.46mm diameter, to ensure fast time response of 200 ms and accuracy of ±0.2 o C at 25 o C. Heat flux is measured with two thin (0.18mm) heat flux sensors with sensitivity 2.06 μv/w/m 2. A PC running LabView software collects and records all the data from the control and the DAQ unit. Main sources of systematic error in the system are associated with: Sensors accuracy DAQ unit accuracy Control Unit accuracy Deviation from the one-dimensional heat transfer characteristics through the sample Maximum systematic errors are expected in the measurement of the heat capacity, and for the specific configuration they have been estimated to be of the order of ± 10% of the measured value.

4 3. AGGLOMERATE MARBLE The material examined is a new type of agglomerate marble in the form of tile. It consists of wastes of the Greek semi-white dolomitic marble Thassos Crystallina and PCM in microencapsulated form. Chips and powder from the marble are mixed with the PCM and bounded with polyesteric resin. The samples have been developed by GeoAnalysis S.A [4]. The PCM used is the BASF Micronal 5001 DS, in powder form (micro-encapsulated paraffin). The melting temperature of the material is 26 o C and the specific heat capacity is approximately 110 kj/kg. The value of the density varies from 250 to 350 kg/m 3. The product has the form of white microcapsules with average size less than 150μm. It is composed of a mixture of paraffin encapsulated in highly cross-linked polymethyl methacrylate microcapsules. The paraffin wax absorbs or releases heat as it changes from solid to liquid or vice versa. The release and absorption of heat in a narrow temperature range tends to stabilize the temperature of the material near the phase change region. Three agglomerate marble mixes were prepared (Fig. 3). The reference mix does not contain any PCM in its mass. In the other two mixes, 10% and 20% of the marble mass was substituted by PCM. The composition of the mixes is shown in table 1. Thermal conductivity (measured by hot wire transient method at 30 o C) and density of the three samples is presented in table 2. (a) (b) (c) Figure 3. Agglomerate marble samples with (a) 0%, (b) 10% and (c) per marble mass per marble mass Marble Chips 4.0 kg 4.0 kg 3.5kg Marble powder 1.0 kg 0.5 kg 0.5kg PCM kg 1.0 kg Resin 1.0 kg 1.5 kg 2.5 kg Table 1. Mixture composition of the samples per marble mass per marble mass Thermal Conductivity [W/mK] Density [kg/m 3 ] Table 2. Thermal conductivity and density of the samples

5 4. THERMAL ANALYSIS The first part of the thermal analysis includes measurements of specific heat capacity and thermal mass of the samples. The second part is an experimental attempt to estimate and compare the performance of the materials in terms of energy saving assuming that the material constitutes a wall of a room with constant inner temperature. 4.1 Specific heat capacity/thermal mass For the specific heat capacity measurements three samples of the three different mixes were prepared at the appropriate dimensions, 200mm x 200mm x 15.5mm. The samples were introduced in the sample holder of the device at a temperature of 10 C and were heated up to 36 C. The temperature of the two plates during the heating process was set 4 o C higher that the sample temperature. This corresponds to a constant heating rate of approximately 61 W/m 2. The temperature of the samples and the heat flux from the device to the samples were recorded. Temperature and heat flux measurements allow the calculation of the heat capacity and thermal mass of the samples (Figures 4 and 5). Assuming one-dimensional heat transfer and according to the lump capacitance method (Bi<0.1) we have: c p = Aq dt m dt (1) M th = mcp (6) where Cp is the heat capacity of the sample, M th the thermal mass, A the heat exchange area of the sample, q the heat flux per square meter, m the mass of the sample, T the temperature of the sample and, t the time. Figures 4 and 5 present the measured specific heat capacity and thermal mass for the three samples versus temperature. In both Figures, the effect of increasing the percentage of PCM in the mixture is apparent in the melting temperature range of the PCM (25 C 30 C). Comparison of figures 4 and 5 indicates that, as expected, increasing the amount of PCM in the mixture increases significantly the maximum value of specific heat capacity (up to 4 times for the content). However, the thermal mass does not exhibit the same trend. The mixture has more thermal mass than the 10% mixture inside the melting range of the PCM but less outside of this region. This is the consequence of the decreasing density of the sample with the increasing PCM content. As a result, there is a limitation on the PCM percentage in the material depending mostly on the temperature range that the material is going to be used. For the temperature range from 16 o C to 31 o C the mean thermal mass for 10% and 20% p/m PCM content has the same value. The use of higher PCM percentage in this range will reduce the mean thermal mass of the sample. Moreover, the use of a mixture in a wider temperature range will have a negative effect regarding the mean thermal mass in comparison to the 10% mixture.

6 5000 Specific Heat Capacity [J/kgK] Temperature [ o C] Figure 4. Specific heat capacity 9000 Thermal mass [J/K] Temperature [ o C] Figure 5. Thermal mass 4.2 Energy saving aspects Potential energy savings with the agglomerate marbles can be estimated by measuring heat losses through the samples when they are assumed to be part of a wall. In this case the device is used to simulate indoor and outdoor temperatures and appropriate temperature profiles are imposed on the two sides of the sample. The outdoor temperature is assumed to have a sinusoidal variation from 21 C to 31 C for 48 hours (resembling temperature variations in a South European country), while the indoor temperature is set stable at a level of 23.5 C. Temperatures and heat

7 fluxes on both surfaces of the same samples as the ones used for the specific heat capacity measurements are recorded. Integration of the measured heat flux (Figure 6) on the inner side of the sample provides a measure of the total heat losses towards the indoor environment. The heat flux measurements of Figure 6 demonstrate an up to 18% variation in the measured maximum and minimum peak values for the sample with content. The calculated energy (Figure 7) corresponds to the energy required by an air-conditioning system to maintain the indoor temperature constant at 26 o C. Savings up to 20% can be expected as a result of the inclusion of 20% p/m PCM in the mix Heat Flux [W/m 2 ] Time [h] Figure 6. Heat flux on the side of the sample corresponding to the indoor wall surface 900 Energy [Wh/m2] Time [h] Figure 7. Energy required for maintaining indoor temperature stable at 26 o C

8 The measured total energy consumption under the above conditions in 48 hours for the reference, 10% and 20% mixtures are 885 Wh/m 2, 774 Wh/m 2 and 705 Wh/m 2 respectively. As it can be seen the PCM mixes improve significantly the thermal performance of agglomerate marble in terms of energy saving. This is not only due to the increased thermal mass but also due to the improvement in thermal insulation (conductivity reduction). 5. CONCLUSIONS The thermal performance of a new composite building material based on natural stone wastes and PCM was presented in this paper. For the needs of the experimental investigation, a simple to use apparatus for measuring thermal properties of samples containing PCMs was designed and constructed. Three samples with different percentages of PCM were investigated. Experiments showed a significant increase of the specific heat capacity of the samples containing PCM in the temperature region within the phase change. The thermal mass of the sample was also increased inside the melting region but decreased outside of the region as a result of the decrease in the density of samples containing PCM. This fact reveals a limitation in the increase of the PCM in the mass of the mixture, which depends on the temperature range at which the material is used. Finally, the energy saving potential with this new material was examined. Experiments showed the capability of the material to reduce energy requirements of a building by reducing heat losses from the building envelope. 6. ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the European Commission though the I-Stone project, FP6-NMP-IP and the MESSIB project, FP7-NMP2-LA REFERENCES 1. M. Founti, I. Mandilaras, K. Laskaridis, M. Patronis, M.D. Romero-Sánchez, A.M. López-Buendía, Multi- Functional Building Products Based On Natural Stone And PCMs With Stabilised Thermal And Dynamic Load, 7th IIR Conference on Phase Change Materials and Slurries for Refrigeration and Air Conditioning, Paris, France, M.D. Romero-Sánchez, M. Founti, C. Guillem-López, A.M. López-Buendía, Thermal Energy Storage in Natural Stone Treated with PCMs, 11th International Conference on Thermal Energy Storage-EffStock, H. Mehling, H.-P. Ebert, P. Shossig, Development of standards for materials testing and quality control of PCM, 7th IIR Conference on Phase Change Materials and Slurries for Refrigeration and Air Conditioning, Paris, France, GeoAnalysis S.A, http: //