Improving the thermal conductivity of fiber-reinforced concrete panels for exterior facades with phase change materials

Abstract

New technologies and modern developments in the production industry, and building construction have reduced the time and cost of construction. However, most of these developments have caused lightweight materials and structures with low thermal mass. Thermal mass lack can fail to reduce the dynamic thermal load by combining with dynamic thermal stimulation and cause an increase in thermal conductivity, energy consumption for space conditioning, and large temperature fluctuations throughout the day. The building facade is the most important part of a building related to temperature stability and energy consumption. Which, like an envelope, covers the building and protects it from severe climatic conditions. In different climates, facades are designed with unique and different specifications, to provide comfortable thermal conditions for residents. To provide the thermal comfort of the interior, heat losses must be minimized which occurs mostly due to the temperature difference between indoors and outdoors. According to the currently mentioned aspect, fossil fuel consumption and anthropomorphic environmental effects rising and resulting in the wastage of energy and carbon dioxide production. In this case, finding alternative energy sources or developing storage methods becomes important. Since the most energy loss occurs from the facades, according to the aspects currently mentioned, the current project's development aims to provide comfortable conditions for the interior areas by adding phase change materials (PCM) to the exterior panels on the facade. This study aims to reduce the heat transitions between the indoor and outdoor environments in existing or newly constructed buildings and to provide the thermal comfort of the indoor space and consume less energy. For this reason, searching around the materials or systems to be applied on the exterior facade, was targeted to reducing or delaying the heat transfer between the interior and exterior areas. To reduce thermal conductivity, the temperature difference between the inner and outer surfaces of the materials and elements should be reduced and balanced. PCMs can stabilize and reduce heat transfer due to their special thermal and storage properties. Adding these materials to prefabricated facade panels which are frequently used as Polypropylene-Fiber Reinforced Concrete (PPFRC) panels, causes the building shell to act as a heat balancer and prevent indoor areas heat losses. The ability of building elements in storing thermal energy has a sufficient role in properly using solar energy. Due to their ability to store latent heat, phase change materials (PCMs) are a group of functional materials with high energy storage densities over a constrained temperature range. (Cabeza et al., 2011). PCMs added to building facades contribute to reducing indoor temperature fluctuations, reducing heating and cooling loads, and lowering energy consumption by making the system have high thermal capacity. Several projects discussed in the literature review section on adding PCM materials contribute to the overall energy performance of the building by causing an increase in the thermal storage capacity of the elements. From a thermodynamic point of view, a change in the entropy of a phase change material (PCM) results in the absorption or release of thermal energy, commonly referred to as latent heat, which depends on PCMs unit mass. By adding thermal energy and starting the melting process, molecules' bones are broken. Current phase change materials are mixtures of liquid and solid molecules. The melting phase begins by gaining kinetic energy and heating the particles of the solid phase to break the forces that keep them together in the solid structure. Eventually, the molecules rearrange themselves and cause an entropy change, this phase is an endothermic process (Safaralipour and Karagüler, 2023). Most typically, PCMs used in building envelope applications must undergo a complete phase transition within 24 hours to be fully effective. This is why the temperature at which the PCM is installed must fluctuate (perhaps daily) within the functional temperature range of the PCM. Ideally, all the heat of transition must be available at the melting and freezing temperature points. However, this happens with paraffin-based PCM. Therefore, this range of temperature should be as little as possible for designing the best PCM systems. The fact of temperature hysteresis is one of the difficulties between the melting and solidification of PCM. In this study, the ability to use phase change materials in facade cladding to improve the insulating properties of PPFRC panels was investigated. With the latent heat storage feature of the phase-change material and the delayed action it will create in heat transfer, it is expected to reduce the heat losses that occur due to the temperature difference between the indoor and outdoor areas. According to this characteristic of the phase change material, the phase change material begins to melt if the outside environment's temperature exceeds the melting temperature point of the phase change material used in the facade, and starts to store the heat with the latent heat storage system. It stores this heat in itself until the outdoor temperature drops below the melting point, preventing or delaying its transfer to the indoor space. Conversely, if the temperature of the external environment falls below the melting point of the phase-change material, then it releases the stored heat, causing the difference between the internal and external temperatures to decrease. Due to this feature of the phase change material, it always acts as an insulating barrier in the system. To assess the produced composite's thermal conductivity coefficients, one reference sample without PCM, and five samples with different PCM ratios were prepared. The proportion of PCM added is prepared as 10%, 20%, 30%, 40%, and 50% of the total mass volume. For the coding system of the prepared samples, SV (Sample by Volume) was used as the title and the current PCM ratio in the sample was shown as a number in front of it. In this study, the latent heat storage properties of phase change materials added to PPFRC concrete mortars were investigated, and the thermal conductivity values of the composites obtained at the desired temperature were reduced. As mentioned earlier, the calculation and evaluation process were done by comparative methods due to the experimental setup. By the following calculation method, the thermal conductivity of the prepared sample was determined and compared with the reference sample (sample without PCM). Therefore, a barrier should be created on the exterior of the building to reduce the heating and cooling energy used in harsh climates (dry, hot, cold) and reduce heat loss. The most basic feature of this barrier is that it consists of insulating or heat-balancing materials. Phase change materials that can be used as heat stabilizers have a temperature range according to the needs of different climates and can be used as heat regulators. The use of phase change materials with melting points close to the indoor comfort temperature is a common way to regulate indoor temperature. According to the area's climate and annual average temperature records, the best melt point for phase change material could be selected. Since each region has a different climate, the average annual temperature of that region should be taken into account to obtain a more efficient system. In addition, to extend the working life of the phase change material and to get the most efficiency from the system, at least one phase change should occur every day, and for this reason, the melting temperature of the phase change material should be close to the annual average temperature of the region. The current climate change and energy consumption crisis in the world have led As a result, the heat storage and heat transfer delay action of the phase change material starts at the melting temperature point. In the applied test system, since two different temperatures are controlled, one side is assumed to be indoor and the other side is assumed to be outdoor. Thereupon, the phase change material causes the indoor environment to be less affected by the temperature fluctuation of the external environment, due to both heat storage and heat transfer retardation. To obtain an efficient system from phase change materials, they must be selected from the right group and have the right melting point. Additionally, phase change should occur continuously and at least once a day in the system to maintain its efficiency for a long time. According to the data obtained, by rising the outdoor temperature above the phase change materials melting point, PCM starts to melt and store the excess energy as latent heat and prevents the temperature increase of the material. In addition, as the temperature increased, the thermal conductivity coefficient decreased more. Afterward, with the decrease in the outdoor temperature, the phase-change material solidifies and the stored heat is released to the outside environment and causing the temperature difference between indoors and outdoors to decrease again. Based on the information obtained from the experiments, the expected efficiency in decreasing the thermal conductivity coefficient was realized using phase change material in PPFRC mortar. The increase in efficiency was proportionate to the use of PCM at a higher rate in the main mortar. However, as to the quantity of phase change material used, as the use of PCM increases, the density and compressive strength of the composite material decrease. Therefore, the PCM ratio should be determined by considering the physical properties and thermal conductivity value expected from the composite material.