Thermo-optical reaction changes of a PCM filled glass system

Content from this work may be used under the terms of theCreativeCommonsAttribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 8th International Building Physics Conference (IBPC 2021) Journal of Physics: Conference Series 2069 (2021) 012195 IOP Publishing doi:10.1088/1742-6596/2069/1/012195 1 Thermo-optical reaction changes of a PCM filled glass system T Cabanová1, M Kuruc1, J Čurpek1,2, D Urbán1 and M Čekon 1,2 1 Department of Materials Engineering and Physics, Slovak University of Technology 2 Department of Building Structures, Brno University of Technology terezia.cabanov[email protected], [email protected] Abstract. This paper analyzes thermo-optical reactions of the PCM-based glass element which has the capability to store thermal energy together with a variable transparency level through the energy storage process corresponding to phase change. Optical properties are determined by the level of phase transition at given boundary conditions over time. Special uncommon thermooptical changes occur during its internal phase transition processes, from liquid to solid phase and vice versa (latent heat of fusion) within a given narrow range of temperature interval. PCM acts as random and diffusive media with relevant scattering effects in solid phase, however in liquid state are highly transparent with direct transmission and no relevant scattering effect. These internal physical changes were detailly identified by experimental test procedures based on optical properties measurements performed using a spectrophotometry, and parallelly with the stabilization of each temperature set provided by environmental chamber. As result of that, relevant differences in the PCM spectral feature can be identified for its different states (solid/liquid) in which transmittance spectra are unstable during rapid phase change process. This provides a substantial base line for the optimization of a PCM glazing system in terms of various degree of freedom for different building types and climate zones. 1. Introduction Energy efficient building envelope technologies are currently being investigated regarding the integration of various types of responsive materials. In adaptive façade principles, transparent and/or translucent elements serve as a functional and also aesthetic part of the building envelopes. Compared to opaque building envelopes, transparent facades have a low ability to store thermal energy [1]. Hence, phase change materials (PCMs) integrated into the glazing systems represent a potential approach [2]. The key advantage is that when accumulating a large amount of thermal energy, a smaller amount of material is used, compared to conventional walls [3]. In order to achieve the required properties of a transparent façade with PCM, it is necessary to take into account a number of factors, such as climatic zone and adjust the melting / solidification temperature range, PCM layer thickness, glazing structure, building use, etc. [4]. Here, the transmittance parameters represent a very specific phenomenon that respond to the temperature variation. Gowreesunker et al. [5] investigated that during sudden changes in temperature or phase, the spectral transmittance of RT27 paraffin based double glazed unit is unstable. In addition, the appearance of the PCM varies depending on the phase state in which the material is present. Liu et al. [6] demonstrated that the transmittance of the PCM-glazed unit, especially in the solid state, is lower compared with air-filled, due to the significant diffusion effects. However, considering to its transparency, PCM in the liquid state achieved a higher transmittance comparable to a conventional air-filled glazed unit. Using spectroscopic methods, Li et al. [7] has proposed a new inverse method for 8th International Building Physics Conference (IBPC 2021) Journal of Physics: Conference Series 2069 (2021) 012195 IOP Publishing doi:10.1088/1742-6596/2069/1/012195 2 the analysis of the optical properties of liquid paraffin integrated into a double-glazed unit based on the transmittance spectrum. The transmittance decreased with increasing paraffin thickness. Regarding the absorption properties, it has been concluded that the larger the layer of liquid paraffin, the greater the absorbency. Heim et al. [8] analyzed effects of the transmittance change in complex phase transition phase of PCM over time, under different solar radiation conditions. The solid state showed a high diffusion effect with very low transmittance, while in the liquid state it represented high transmittance. 2. Research scope, method and obtained results The phase change process of PCM during its integration into glass components represents a complex process based on significant changes in terms of their thermal and optical characteristics in accordance to Figure 1. Accordingly, it is a dynamic system with variable light transmittance and heat transfer. These aspects depend on the intensity of the incident solar radiation and the overall principle of implementing PCM in the glass element. Figure 1. Physical phenomena of action-reaction processes in PCM-GB (from solid to liquid). In this work, the paraffin based PCM is applied to clear glass block (GB) of a 190 x 190 x 80 mm. This represents a potential variant to PCM incorporation compared to conventional transparent systems. The optical properties of clear GB and that of filled with paraffin RT27 Rubitherm (PCM-GB) were measured in Figure 2. The PCM-GB sample was conditioned in an environmental chamber to reach its full liquid state. The transmittance was measured in the spectrum from 360-880 nm with using a FLAME-XR spectrometer and a light source HL-2000-HP with a nominal power of 20W. Figure 2. PCM-GB (liquid state) attached to the apparatus and clear GB reference (REF). Figure 3. Thermo-optical changes in 2 hours duration, first 90 minutes 1-4, next 20 minutes 5-20. 8th International Building Physics Conference (IBPC 2021) Journal of Physics: Conference Series 2069 (2021) 012195 IOP Publishing doi:10.1088/1742-6596/2069/1/012195 3 Experimental measurement of the transmittance parameters was performed at an initial material temperature of 39.7 °C (measurement 1) up to a decrease to 27.4 ° C (17-20). In Figure 3, results show that the empty sample REF has slightly lower values (around 0.6) than the first four measurements of the PCM-GB (1-4) in a full liquid state (up to 0.7 in maximum peak). When the temperature of the paraffin reached 28.2 °C (4), the transmittance decreased rapidly, though the sample was still highly transparent. After this stage, according to Figure 4, the mushy zone interface (crystallization) occurs inside, as a result of which the effect of attenuation of light transmission to almost zero was observed within only 20 minutes. This phenomenon is caused by the effect of a narrow beam of light, which already showed scattering at the indication of the slight crystallization in the PCM layer. Accordingly, the transmittance values obtained during the change of liquid state do not show relevant optical changes due to used radiation source. However, in the liquid state values indicates high transparency. Figure 4. Gradation of the paraffin layer recrystallizing along with the temperature change. 3. Conclusion and research remarks Based on the results demonstrated, the behavior of paraffin in the liquid phase confirmed the expected assumptions, where the transmittance of simulated radiation dominated in comparison with air-filled concept. Though in the solid phase, the PCM is almost translucent (most of the light is diffuse and scattering occurs), measurements were significantly sensitive to these phenomena. On the contrary, in the liquid phase, as it was highly transparent to the light used, measurements provide a reliable data with regard the direct light transmittance. As for the slurry of the material, the scattering effect prevailed and the rapid onset of crystallization in the paraffin layer generated minimal and even up to zero transmittance values, despite the visible transparency was still evident. Therefore, this method (light source used) provides a relevant data for transparent samples only, however when effect of diffusivity and scattering is employed, diffuse light source based on artificial sun (solar simulator) need to be incorporated in measurement procedure. Accordingly, interreflections and refraction of light on semi layers characterized by different physical states will be identified in complex. Acknowledgement This research was supported by the project VEGA 1/0680/20 and by the project GA 20-00630S. 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