THERMAL ENERGY STORAGE IN NATURAL STONE TREATED WIT H PCMs Romero-Sánchez, M.D.* (1), Founti, M. (2), Guillem-López, C. (1), López-Buendía, A.M.(1) 1 AIDICO. Asociación De Investigación de Las Industrias de la Construcción. Camí de Castella, 4. 03660 Novelda. Alicante. Spain. 2 National Technical University of Athens, School of Mechanical Engineering, Lab. of Heterogeneous Mixtures and Combustion Systems, Heroon Polytechniou 9, 15780 Zografou, Greece *Contact person: md.romero@aidico.es. Tel.34 965608302

ABSTRACT The objective is to improve thermal properties of natural stone by means of latent heat storage by using phase change materials (PCMS). As a consequence, PCM treated natural stone could be used as construction material able to store thermal energy. In this way, energy consumption in buildings (cooling/heating systems) is decreased and also negative effects and damages for the environment are reduced. In this study, the selection of PCMs has been done depending on their chemical nature and melting temperature. PCMs with a melting point close to the human comfort temperature have been used (aprox. 24ºC). Different experimental techniques have been used for the characterization of the used PCMs and for the characterization of the natural stone after treatment with the PCMs. The results have shown that these new products based on natural stone have the ability to store energy. Temperature fluctuations in the place of use are reduced as well as the temperature peaks maximum-minimum between day-night are reduced. 1. INTRODUCTION It is well known that the building sector is responsible for more than 40% of the total EU energy consumption. Improving energy efficiency in the building sector can drastically contribute to reduce CO2 emissions and EU energy dependence. Significant efforts are currently made to promote the introduction of new technologies around improved materials (such as insulation, glazing, energy storage through phase change materials…) that can find direct application in “energy efficient buildings” such as dwellings, offices, hospitals, schools, factories, airports, rail stations, public buildings, for new and retrofitting buildings and retrofitting Cultural Heritage. The addition of PCMs in construction materials such as concrete, gypsum or plasterboard panels is promoted, as an energy storage solution to reduce energy demands in buildings. In this study, the use of natural stone treated with PCMs is proposed as a system for thermal energy storage. The viability of natural stone as a system for thermal energy storage is considered of great importance due to the high number of applications: walls, floors, indoors, outdoors, etc. The use of PCMs as thermal storage system for buildings has been of interest since the first applications in the 1940s. PCMs store latent heat as the ambient temperature rises up to their melting point (PCM changes from solid to liquid state). As the temperature cools down, the PCM returns to the solid phase and the latent heat is released. This absorption and release of heat takes place at a constant temperature, which is ideal to smooth temperature fluctuations. Some references have been found about the use of PCMs to improve thermal properties of construction materials, such as concrete or gypsum. Hawes [Hawes, 1990] has studied the

thermal performance of PCMs in different types of concrete blocks. Thermal storage in concrete containing PCMs was increased more than 200%. Salyer et al [Salyer, 1995] have developed different methods of PCM incorporation to building blocks: by imbibing the PCM into porous materials, PCM absorption into silica or incorporation of PCMs to polymeric carriers. Natural stones are commonly used for external (façades) and internal (floors, walls etc) building applications. The use of PCMs for the treatment of natural stone in order to improve its thermal properties is proposed for several reasons: 1) Energy savings in heating/cooling systems; 2) Enhancement of thermal comfort inside the building (reduction of temperature differences between day and night and different rooms inside the building); 3) Storage of the heat from outdoors; 4) Avoid excessive heat from outdoors; 4) Protection of the natural stone in extreme climatic variations. The objective is to use PCMs incorporated to natural stone as an environmentally friendly system in buildings as a way to improve thermal properties and to achieve heat storage when temperatures are high and release this energy when temperatures decrease, thus reducing the use of heating/air conditioning systems, increasing comfort of the people in the room, contributing to energy savings. Natural stone is a building material, which can effectively contribute to energy savings. 2. MATERIALS AND EXPERIMENTAL TECHNIQUES 2.1. MATERIALS Bateig azul has been selected in order to study the thermal storage properties of natural stone materials after PCM treatment. This material is extracted in Novelda-Alicante-Spain. It is blue and composed of calcite and quartz, with medium porosity size. The Micronal DS 5000X (provided by BASF) has been selected as the phase change material. It is a water-based solution with following characteristics: viscosity equal to 30-100 mPa·s, solid content of 1-43% and meeting temperature ca. 26ºC. Bateig azul has been immersed in this solution to be impregnated with the PCM. 2.2. EXPERIMENTAL TECHNIQUES

Different experimental techniques have been used for the characterization of Bateig azul without and with PCM treatment. -Scanning Electron Microscopy (SEM). The presence of PCM in the pores of Bateig azul has been observed, distribution and shape. Hitachi S-3000 N. -Porosimetry. Porosity of Bateig azul before and after PCM treatment has been evaluated (Hg porosimeter, Micromeritcs Autopore IV). -Differential Scanning Calorimetry (DSC). Thermal behaviour of PCMs, melting temperature, entalphy, decomposition, etc can be evaluated with DSC (Stare SW 8.10, Mettler Toledo). -Thermal conductivity and heat capacity. These properties have been evaluated in pieces of Bateig azul with and without PCMs (CT METRE de SA TELEPH). 3. RESULTS AND DISCUSSION The impregnation of Bateig azul has been carried out by immersion of the pieces into a solution containing the PCMs. Pieces have been weighted before and after the treatment in order to calculate the amount of PCM incorporated into the bulk of the natural stone. It was

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found that the weight of the sample increased by 0.5% when immersed in the PCM solution, which corresponds to 0.011 g PCM/cm3 Bateig azul. DSC was used to characterize thermal properties of Bateig azul with PCM (Figure 1, 20 to 60ºC at 5ºC/min). Considering the 0.5% PCMs introduced into Bateig azul, the entalphy for the melting process is 39.60 J/g PCM. Figure 1. DSC measurements of Bateig azul+PCM. Data obtained from Hg porosimetry show that the porosity of Bateig azul decreases from 14.6 to 13.9 % when treated with the PCMs, confirming the presence of PCMs into the bulk of Bateig azul. Moreover, the pore size has been decreased because of the presence of PCMs. Figure 2 shows the accumulated intrusion of Hg (ml/g) in Bateig azul and Bateig azul with PCM versus pressure. For all the range, the intrusion is lower for Bateig azul with PCM, indicating again the presence of PCMs in the sample. Figure 2. Hg porosimetry. Accumulated intrusion versus pressure for Bateig azul and Bateig azul+PCM.

Melting point of PCM

Cross sections of Bateig azul and Bateig azul with PCMs have been observed with SEM. Figure 3 includes micrographs of Bateig azul showing the distribution and porous structure. Micrographs of Bateig azul with PCMs are included in Figure 4. It can be observed that the PCMs are in the bulk of the stone, as previously obtained with Hg porosimetry. Figure 3. SEM micrographs of Bateig azul. Figure 4. SEM micrographs of Bateig azul with PCMs. Thermal conductivity and volumetric thermal capacity of Bateig azul and Bateig azul with PCMs have been measured in order to evaluate the effect of PCMs in the thermal storage properties of the natural stone. Data are included in Table 2. Table 2. Thermal properties of Bateig azul and Bateig azul with PCMs.

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5.3 The data show a 6.7% increase in the thermal conductivity of Bateig azul with PCMs compared to Bateig azul without PCMs. However, the influence of porosity on the thermal conductivity has to be considered, because a higher amount of pores means lower thermal conductivity, due to the lower thermal conductivity of air. The introduction of PCMs decreases the porosity of Bateig azul, thus an increase in thermal conductivity can be produced for this reason. Nevertheless, the volumetric thermal capacity has been increased (5.3%) with the introduction of PCMs into the bulk of Bateig azul. This indicates that the sample with PCMs requires a higher amount of energy to modify its temperature, because the PCM absorbs part of the energy. With these results, it has been checked that thermal

properties of natural stone can be modified with the introduction of PCMs. The degree of modification can be optimized by changing the type and amount of PCM. To evaluate these modifications on thermal properties of Bateig azul when PCMs are introduced, an experiment has been designed to analyze the response of Bateig azul without and with PCMs towards heating/cooling processes. The experiment consists of the analysis of the propagation of thermal waves between different points of Bateig azul. Figure 5 shows the experimental set-up. It consists of a cylinder (12 cm length, 5 cm diameter). 3 holes have been opened with 3cm spacing (3, 6 and 9 cm from cylinder edge), where thermocouples have been placed. Another hole has been opened at one of the vertical faces of the cylinder to place a heating source. Figure 5. Experiment designed to evaluate the heating/cooling behaviour in Bateig azul with and without PCMs. The Bateig azul cylinder is heated at a constant temperature via the heating element inserted in one of the vertical faces of the sample. The heating source is switched off and the cylinder is left to cool down. Temperatures are measured in the positions of the 3 thermocouples. Figure 6 shows the results for Bateig azul and Bateig azul with PCMs. As expected, the temperature decreases, from thermocouple 1 to 3, as the distance from the heating source is increased. It is interesting to note that the sample cylinder with the PCMs maintains the high temperature ca. 45 min longer than the cylinder without PCMs. This indicates that the energy stored during the heating process is released at a lower rate than in the case without the PCMs. Moreover, the decrease to lower temperatures is smoother for the PCM-cylinder. This effect is re-produced for the 3 thermocouples, independently of the maximum temperature obtained. Figure 6. Heating/cooling behaviour of Bateig azul with and without PCMs.

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In addition, a second experiment has also been carried using the same experimental set up as above. In this case, heat pulses have been produced by the heating source for 15 min. After the pulses, the samples have been left to cool (Figure 7). Although, the form of supplied heat is different in the two cases (continuous versus sinusoidal variation of heat supply) the recorded behaviour is similar. Maximum temperatures of Bateig azul with PCMs (measured at the locations of the 3 thermocouples) are reduced with respect to the Bateig azul without PCMs. Moreover, a delay in the time required to get maximum temperatures is obtained.

Figure 7. Heating/cooling behaviour by heat pulses in Bateig azul with and without PCMs.

As a further step to demonstrate the effectiveness of PCM treated natural stones, a number of pilot stations made out of concrete and Bateig azul (with and without PCMs) as natural stone for the façade have been built and placed outdoors (Figure 8).

Figure 8. Pilot station made of concrete and Bateig azul as natural stone for the façade.

The objective is to analyze the effect of the PCM treated stones in different parts of the pilot station: south wall, east wall, ceiling, temperature in the air gap, in contact with the concrete or the natural stone, etc. In all cases, sensors have been placed in the control and the “PCM” stations in the same positions. Variations of temperature in different points of the pilot

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stations have been monitored every 10 min for several day-night cycles. Figure 9 shows the recorded temperature variation for 1 cycle.

Figure 9. Temperatures in the pilot stations of Bateig azul with and without PCMs as façade materials.

Several differences between the temperatures in the pilot station and the station with PCM treated Bateig Azul as façade can be observed. The maximum recorded temperature in the PCM treated station – although it occurs at the same time point as in the reference station - it is 1.3ºC lower than for the station without PCMs, demonstrating at pilot scale the energy storage potentials of PCMs. The cooling process is smoother for the pilot station with PCMs (the temperatures occurring in the reference station are reached with an hour’s delay in the station with the PCM treated stones), indicating that the stored energy is being released. Minimum temperature in the PCM treated station is 0.6ºC higher compared to the station without PCMs. These temperature measurements at the pilot stations confirm the effectiveness of PCM treated stones as a system for energy storage. At temperatures higher than the melting point of the PCM (26ºC in this experiment), the PMCs store energy. When ambient temperature decreases (cooling process), the PCMs release energy at a controlled rate maintaining higher temperatures in comparison to the station without PCMs. In a real building this process would imply a reduction in the temperature fluctuations between day-night, reducing the energy demands of the heating/cooling systems and increasing human comfort indoors.

4. CONCLUSIONS

The thermal storage properties of natural stone treated with PCMs have been confirmed with the use of several experimental techniques. As a consequence of the PCM treatment, two main effects are observed: Reduction on the peak temperatures between day-night and delay in the time point to reach the minimum or maximum temperatures in a closed space.

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As a result of the experimental observations, a reduction in energy consumption can be anticipated and an increase in human comfort, due to a reduction of temperature variations during day and night.

ACKNOWLEDGEMENTS: Financial support of the E.C., 6th Framework Prog., Integrated Project I-STONE, Contract No. NMP2-Ct-2005-515762 and the IMDPHA/2008/18 project. REFERENCES -Abhat, A., 1983. Low temperature latent heat thermal energy storage: heat storage materials, Solar energy, 30, pp. 313-332. -Farid, M., Kong, W.J., October 2001, Proceedings of the I MECH E Part A Journal of Power and Energy, Vol 215, No.5, pp.601-609. -Halawa, E., Bruno, F., Saman, W., 2005. Numerical analysis of a PCM thermal storage system with varying wall temperature. Energy Conversion and Management, 46, pp. 2592-2604. -Hawes, D.W., Banu, D., Feldman, D., 1990. Latent heat storage in concrete, Solar energy Mater, 21, 61-80. -Khudhair, A.M., Farid, M.M., 2004. A review on energy conservation in buildings applications with thermal storage by latent heat using phase changing materials. Energy Conversion and Management, 45, pp. 263-275. -Lee, T, Hawes, D.W., Banu, D., Feldman, D., 2000, Control aspects of latent heat storage and recovery in concrete, Solar Energy Materials & Solar Cells, 62, pp.217-237 -Salyer, I.O., Sircar A.K., Kumar A., 1995. Advanced phase change materials technology: evaluation in lightweight solite hollow-core building blocks. In: Proceedings of the 30th Intersociety Energy Conversion Engineering Conference, Orlando, FL, USA, pp.217-224. -Zalba, B., Marín, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering, 23, pp. 251-283. -Zhang, D., Li, Z., Zhou, J., Wu, K., 2004. Development of thermal energy storage concrete, Cement and Concrete Research, 34, pp. 927-934.