Design of a cool color glaze for solar reflective tile application
Introduction
Climate change is one of the most discussed problems of this century. One of the most efficient ways to mitigate it is through passive cooling techniques, such as increasing the surface albedo [1], [2], [3], [4], [5], [6]. Cool roof is one of the most suitable solutions to increase a surface׳s albedo through an improvement of solar reflectance and thermal emittance if compared with analog building materials. Urban heat island mitigation will limit summer overheating, also reducing smog and improving energy saving, grid stability and public health [1], [7].
Moreover, solar reflective materials are appreciated, thanks to their ability to reduce air conditioning costs, improve comfort inside buildings and reduce structural stress and material degradation [1], [7], [8], [9], [10], [11], [12].
Cool roof technology developed from naturally white products and white products designed on purpose with enhanced solar reflectance. These products, even though very efficient, can be hardly framed in traditional building framework where, roof tiles can be seen from the streets and warm colors such as brick, brown and beige are used. A solution to this problem can be found using cool colors [12], [13], [14]building materials are characterized both by high solar reflectance, high thermal emissivity and also by different colors. These materials are, in fact, usually characterized by spectrally selective spectral reflectivity, keeping in the visible portion of the solar spectrum, a response similar to the one of a colored product, but increasing deeply the performance in the near infra-red range of the solar spectrum.
Irradiance spectra represent different conditions at which a sample receives energy from the sun. It ranges from 300 to 2500 nm and it is divided into three portions: 300–400 nm UV radiation, 400–700 nm visible radiation and 700–2500 nm near infrared variation. Several irradiance spectra are available depending on the irradiance condition. One of the better spectrum to represent these conditions is AM1GH spectrum, which considers direct, diffused and reflected radiation on a horizontal sample considering Air Mass 1 (Sun at its Zenith position) [15], [16]. According to this spectrum 7% of the total irradiance amount is referred to the UV radiation, 45% to the visible and 49% to the near infra red radiaton. It is important to highlight how, as reported by percentages, infra-red is the most influent fraction, focusing the attention for solar reflective materials on this range.
Among all solar reflective materials, ceramic based ones represent an interesting solution thanks to their durability against ageing and weathering, their naturally high thermal emissivity (ε=0.90) and their peculiar behavior in the near infra-red fraction of the solar spectrum [17], [18], [19]. A previous study regards the development of solar reflective clay roof tiles [20], while in 2013 Ferrari et al. [19] developed a new solar reflective engobe, which reaches 0.90 of solar reflectance through the application of a 200 μm layer of a zirconium silicate engobe on a stoneware ceramic tile. In 2007, Levinson et al. [14] established an effective structure to create a nonwhite solar reflective surface suggesting two models. Among these, the three-layer model, made by a substrate, a solar reflective basecoat and a spectrally selective topcoat, is strongly comparable to the traditional structure of a glazed ceramic tile. This study is, then, aimed to create a ceramic-based solar reflective nonwhite surface considering a s tile, as a substrate, the engobe as a solar reflective basecoat formulated in [19] and, as topcoat, ceramic glazes with different finishing and different colors.
The formulated glazes tried to analyze the wide range of products available considering a transparent glaze, a white gloss glaze, a white matt glaze and a glaze made by recycled glass. Among the colors, eight different pigments were selected to represent the wide range of colors characterizing steep slope roof in the Mediterranean area. The relation between solar reflectance, microstructure and color was, then, investigated.
Section snippets
Sample preparation
Four different glaze formulations were prepared and each formulation was characterized by the presence of different frits, which develop different surface finishing. In particular were produced a transparent glaze, a recycled glass based glaze (Table 1), a white gloss glaze and a white matt glaze (Table 2).
The frit in the T glaze was made by 68% SiO2, 13% CaO, 4.7% Al2O3, 1.7% MgO, 2.9% alkali and 9.7% others; the frits in R glaze were made by 56.8% SiO2, 7.6% Na2O, 5.3% B2O3, 5.2% BaO, 4.2% K2
XRD analysis
The mineralogical analysis was, first of all, carried out on samples without pigments in order to establish which phases were eventually formed during the heating treatment.
In Fig. 1 the comparison between two glaze layers (with different thicknesses) applied on engobed ceramic tiles is shown; both transparent and matt glaze are reported in order to show that there is no influence in phase formation with 200 and 400 µm of glaze.
According to what reported in Fig. 1, the samples coated with 400 µm
Conclusions
Cool roof can be identified as one of the most efficient solutions to mitigate urban heat island effect. Traditionally these products are characterized by light colors which limit their application in urban areas. New colored products with high solar reflectance can be an interesting response to increase the use of solar reflective materials. In particular, ceramic-based cool colors can combine both the advantages brought by cool colors and higher durability, if compared with polymeric based
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