Cool Surface Strategies with an Emphasis on the Materials Dimension: A Review

  Citation: Mourou, C.; Zamorano, M.; Ruiz, D.P.; Martín-Morales, M. Cool Surface Strategies with an Emphasis on the Materials Dimension: A Review. Appl. Sci. 2022,12, 1893. https://doi.org/10.3390/ app12041893 Academic Editor: Elza Bontempi Received: 7 January 2022 Accepted: 5 February 2022 Published: 11 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Review Cool Surface Strategies with an Emphasis on the Materials Dimension: A Review Chaimae Mourou 1, Montserrat Zamorano 2, Diego P. Ruiz 3and María Martín-Morales 1,* 1Department of Building Construction, School of Building Engineering, University of Granada, 18071 Granada, Spain; [email protected] 2 Department of Civil Engineering, School of Civil Engineering, University of Granada, 18071 Granada, Spain; [email protected] 3Department of Applied Physics, Faculty of Sciences, University of Granada, 18071 Granada, Spain; [email protected] *Correspondence: [email protected] Abstract: The need to tackle the urban heat island effect demands the implementation of cool surfaces as a mitigation strategy. This study comprehensively reviews the evolution of this research field from a materials perspective. It provides a bibliometric analysis of the relevant literature using the SciMAT software processing of bibliographic records from 1995 to 2020, for the evolution of cool surfaces. The results obtained show an increased interest in the field from 2011 to 2020, particularly for roof applications, and present the scientific evolution of reflective materials. According to the materials dimension adopted by the development of the research field, the study is refined from a bibliometric analysis of 982 selected records for the analysis of five themes: (i) Pigments; (ii) Phase change materials; (iii) Retroreflective materials; (iv) Ceramic materials; and (v) Glass. These materials present promising results in terms of their solar reflectance performances in the mitigation of the urban heat island phenomenon. At the end of this review, recommendations for future studies are provided for the creation of economic and environmentally friendly materials based on waste glass recycling. This study represents a valuable contribution that provides a scientific background with regard to cool surfaces from a materials perspective for future investigations. Keywords: cool surface; cool material; cool roof; urban heat island 1. Introduction Urban areas represent 2% of the Earth’s surface, yet they consume 75% of the world’s energy resources [ 1 ]. A portion of this energy is dissipated as anthropogenic heat, which increases the ambient temperatures in urban areas. This phenomenon was labelled as the “urban heat island (UHI) effect” by meteorologists more than a century ago, and it is the result of the increase in the ambient temperatures and the amount of solar heat trapped in urban areas, as well as of the increase in greenhouse gas emissions [ 2 – 4 ]. Consequently, in addition to climate change, associated environmental challenges and public health problems have occurred [ 4 ]; for instance, the occurrence of urban smog and the increase in cooling energy consumption [ 2 ], as well as a decrease in indoor and outdoor thermal comfort and quality [ 3 , 5 ]. It can therefore be stated that the UHI effect is an environmental problem that requires theoretical and practical studies to mitigate its impact [6]. In order to palliate the UHI effect, a growing number of studies and investigations have been conducted to develop mitigation strategies that can be implemented in urban spaces and buildings. Several solutions have been developed, including urban geometry reshaping [ 1 ], designing green and cool roofs [ 7 , 8 ], using permeable, porous, water-retentive, and cool pavements [ 9 ], incorporating green spaces into the urban landscape, and utilizing water and wind for cooling effects [ 4 , 10 , 11 ]. These solutions could yield a median reduction in the air temperature of between 1.8 and 2.1 K [ 12 ]. Furthermore, the combination of Appl. Sci. 2022,12, 1893. https://doi.org/10.3390/app12041893 https://www.mdpi.com/journal/applsci Appl. Sci. 2022,12, 1893 2 of 24 different measures could be more effective [ 13 ], and the choice of the optimal strategy depends on the regional atmospheric and geographic specifications of each individual urban environment [8,14]. Among the cited UHI mitigation strategies, cool surfaces have emerged as a viable solution [ 15 , 16 ]. The term “cool surfaces” basically refers to surfaces with reflective materials and coatings that reflect the solar energy radiation that hits building envelopes and urban areas [ 17 ], including roofs, facades, and pavements (Figure 1). Cool surfaces are able to reduce the thermal infrared radiation outflow in the atmosphere, as well as the temperature and the solar heat gain [ 18 , 19 ]. In fact, it has been proven that the implementation of cool surfaces to replace dark and highly absorptive materials during routine maintenance increases the albedo over time [ 2 ]. These materials come in a huge variety and include natural materials, artificial cool coatings, and nonwhite high-albedo materials [ 20 , 21 ]. In addition to the fabrication process and conditions, the thickness, particle size, and the substrate and binder materials are all key parameters that could affect the optical and thermal properties of cool materials, such as the albedo, permeability, conductivity, total solar reflectance, and emissivity [21,22]. Figure 1. Schematic representation of the cool surface effect on a UHI through the ambient temperature. The thermal performance of a material is mainly evaluated by the albedo (solar radiation) and emissivity (longwave radiation) [ 21 ]. The solar reflectance potential of cool materials initially relied on their whiteness, which promoted the use of white paints and light-colored aggregates [ 23 ]. Following that, the research field advanced toward enhancing the near infrared (NIR) reflectance of cool materials, as it represents almost 52% of the electromagnetic spectrum of light (i.e., from 700 to 2500 nm) [ 24 ]. Thus, novel methodologies and techniques have been considered to cover a wide range of solar reflectances in order to enhance the performance of cool materials. The cool surface strategy has started an extensive series of studies concerning pigments that has created an industry of geoengineering and chemical solutions for the development of cool materials to enhance the solar reflectance performances. The integration of pigments in cool materials has recently been discussed in detail within both the research and industrial contexts [ 25 ]. With regard to the aesthetic requirements of a design, selective pigments have been developed to maintain the optical color desired on top of the material while achieving important NIR reflectance results [ 26 ]. The pigments range from organic, to complex inorganic color mixed-metal oxides [ 27 , 28 ]. These substances have demonstrated high solar reflectances of up to 95%, compared to TiO2[29–33]. In addition to cool pigments, other solutions for cool surfaces have emerged to improve urban climate conditions and energy consumption, such as retroreflective (RR) and phase change materials (PCMs). Independently of the incidence direction, retroreflectivity refers to the capacity of a surface to reflect an incoming light beam to a surface back towards its source [ 34 , 35 ]. RR materials have been demonstrated to be effective in several studies in terms of the solar radiation reflectance beyond urban canyons and canopies [ 34 – 36 ]. PCMs have the ability to change their physical characteristics as a consequence of heat release or absorption [ 37 ]. In recent decades, a variety of PCMs have been investigated as dynamic Appl. Sci. 2022,12, 1893 3 of 24 components in structures [ 38 ]. The implementation of PCMs in the matrix of roof finishing materials decreases the flux of the roof heat gain by 54%, compared to the cool roof [ 16 ], and it helps to compensate for the effect of the thermal stress generated by the latter [ 39 ]. Moreover, PCMs can be used to regulate the indoor thermal comfort and reduce the heating penalty during the summer and winter, respectively, more so than do cool paints [40–43]. Sustainable adaptation has been a parallel concern in the development of cool material solutions. On this basis, the use of recycling materials to save energy and natural resources, and to enhance the solar reflectance, presents a promising eco-friendly strategy [ 14 ]. For instance, full body porcelain aggregate from waste tiles has been used as a cool pavement coating that exhibits important thermal performance values compared to asphalt pavement, with an NIR solar reflectance of 6.4 ◦ C, and a surface temperature reduction of up to 6.4 ◦ C during the peak periods [ 44 ]. In addition, the use of recycled glass cullet in the fabrication of a sustainable asphalt roof shingle improved the solar reflectance [45]. Considering the evolution of materials applied in cool surfaces, the current objective of this study is to perform a bibliometric analysis to review the scientific development of this solution in the mitigation of the UHI effect. Section 2presents the research methodology employed. Section 3describes the collection process for the materials and the bibliometric evaluation, provides a descriptive analysis of the findings and analyzes the most important contributions of the studies identified previously. Finally, Section 4provides the most important conclusions of this study, which contribute to the existing body of knowledge by highlighting the trends in the research field of cool surfaces for building envelopes and urban areas, as well as by recommending research areas for future studies. 2. Methodology To achieve the objective of this study, a dual bibliometric study based on science mapping and performance analysis was conducted first using the science mapping analysis software, SciMAT, to obtain the necessary patterns and bibliometric measures [ 46 , 47 ]. This bibliometric study contains publications from 1995 to 2020, and it was conducted in 2021. Hence, recent findings and limitations are included in the literature review section in order to enrich the discussion, which is necessary because of the constant evolution of the field. Science mapping visualizes, analyzes, and models a broad range of scientific and technological activities, and it follows a general workflow of data retrieval, data preprocessing, network extraction, network normalization, mapping, analysis, visualization, and finally, the interpretation of the results [ 46 ]. Furthermore, the performance analysis as a complementary methodology uses different bibliometric measures and indicators to complement the visualization results and to help identify the impacts and productivities of the themes in the research field. With consideration to the results obtained, a number of publications were selected in order to conduct a review of the evolution of cool surfaces. 2.1. Sample Definition and Steps for the Data Collection This review addresses two concepts: the UHI effect, and the different types of cool surfaces applied. To achieve this, an extensive search was carried out that employed the keywords linked to both concepts, as well as the satellite materials directly connected to the research field. Therefore, the first stage of the data collection was performed using the field, “Title/Abstract/Keyword”, through the following keywords and search strings (Figure 2): “Heat island” OR “Reflect*” AND “Cool surface*”, “Cool facade*”, and “Cool roof*” AND “Cool pavement*”, within the Web of Science Core Collection and Scopus databases. The asterisk is used at the end of keywords to broaden the research. As a result, 982 publications were found, 347 of which were excluded after the deduplicating and cleaning of the raw data. After reading the abstracts, another 121 publications were excluded because they were not aligned with the purpose of this research. Finally, the bibliometric study was performed with 514 publications. Appl. Sci. 2022,12, 1893 4 of 24 Figure 2. Data collection flowchart. The next step was dedicated to the restriction of the data and the refinement of the sample on the basis of a conceptual approach. According to the SciMAT visualization results, it was possible to follow the internal links in the cluster networks of the concepts with the highest potentials for reviewing, which returned 63 records. This process of data collection is described in the flowchart above (Figure 2). The analysis of these publications was the basis of the literature review of the evolution of cool surfaces through the materials applied to mitigate the UHI effect. 2.2. Systematic Literature Review The analysis of the 514 documents selected for the systematic literature review resulted in the following data: total numbers and years of publications; and authors with the highest contributions to the field based on the number of publications, sources, and journals. Furthermore, strategic diagrams, thematic networks, overlay graphs, and evolution maps were used to demonstrate the relationships between each theme of the strategic diagrams, the keywords, and their interconnections. These were also used to identify the research motor themes, the highly developed and isolated themes, the emerging or declining themes, and the basic and transversal themes, and to then trace the evolution of these themes along the studied period [ 48 – 50 ]. The sample of academic publications processed through this science mapping analysis was dependent on the specific input conditions, such as the unit of analysis and the keywords. 3. Results and Discussion The aim of this section is to assess and analyze the collected material that was released during the period between 1995 and 2020 using quantitative and qualitative methods. A descriptive analysis was conducted through: (i) An analysis of the evolution of the numbers of documents; and (ii) The main sources of the publications and a review of the most prolific authors. Science mapping and visualization were then conducted to obtain an assessment of the evolution of the cool-material-application domain. Finally, a literature revision of Appl. Sci. 2022,12, 1893 5 of 24 the materials applied for cool surfaces was developed, which led to five analytical sections employed for the evaluation of the materials: (i) Pigments; (ii) RR materials; (ii) PCMs; (iv) Ceramic materials; and (v) Glass. 3.1. Descriptive Analysis 3.1.1. Evolution of Number of Documents Since the first article identified in these databases was published in 1995, the time horizon used in this study was from 1995 to 2020. In order to analyze the trends and patterns in the publications, three periods were identified (1995–2001, 2002–2010, and 2011–2020) on the basis of the main turning points and milestones in the evolution of cool surfaces. The results of the contributions for the three periods identified are summarized below. The first period (1995–2001): In this period, the contribution of the academic literature to the topic was very poor, with only five publications found. However, the period was marked by promoting cool roofs through building codes. In fact, starting in 1999, several energy-building standards adopted cool roof credits or requirements, such as ASHRAE 90.1, ASHRAE 90.2, the International Energy Conservation Code, and California’s Title 24. The second period (2002–2010): This period is characterized by the rising concern with regard to the topic, which coincided with the third assessment report of the Intergovernmental Panel on Climate Change, which highlights the increase in greenhouse emissions and the global average surface temperature in the 20th century by 0.6 ◦ C. The number of publications increased considerably (to 58) during this period (Figure 3). Figure 3. Number of documents per year. The third period (2011–2020): The cool surface strategy was a rapidly growing research topic during this period. In fact, this period is by far the most prolific in terms of publications, with 87.74% of the records published during this time (Figure 3). This period coincides with the foundation of the European Cool Roofs Council, which seeks to develop knowledge and research regarding cool roof technology and promotes the use and implementation of this technology in Europe. Considering the significant evolution of the number of academic documents in this field, an increasing interest in this topic has been explicitly detected (Figure 3). Appl. Sci. 2022,12, 1893 6 of 24 3.1.2. Main Source Publications The nine most prolific journals account for 40% of the total records of the sample principally considered. It is possible to conclude that the most common aspect of the journals considered is related to energy efficiency and buildings, as well as to the science and technology of solar energy applications. In fact, Energy and Buildings has the dominant share, with 16.53% of the published records, followed by Solar Energy (6.19%), and Solar Energy Materials and Solar Cells (4.12%). Concerning the authorship of the publications, Akbari, H., who is affiliated with the United States and Canada, has made the most contributions to the field, with 36 publications, followed by Pisello, A.L. (33), and Levinson, R. (28). Most of the publications are affiliated with Italian universities (97 papers), followed by the United States and Canada, with 74 and 36 publications, respectively. During the last decade, with the increases in the UHI effect and climate change, it became necessary to draw attention to the development of geoengineering-based solutions, such as cool surfaces. This strategy has been implemented for roof applications, building facades, and pavements. Nevertheless, the academic research placed an emphasis on cool roofs, with a much higher number of articles published on this application than for any of the others (Figure 4). Thus, the number of publications regarding cool roofs in the third period reached 251 documents, while, in the case of pavements and facades, only 58 and 22 papers were identified, respectively. Figure 4. Evolution of cool material application domain: roofs, pavements, and walls/facades. 3.2. Science Mapping and Visualization Figure 5plots the overlapping map representing the three periods of research and the evolution of the keywords. Figure 6presents the thematic evolution map of the research field, based on the h-index (Figure 6a) and on the numbers of published documents for the three cited periods (Figure 6b). Figure 5shows that the number of keywords grew substantially in the second period, with 52 units, and in the third period, with 16 more. Thus, it is possible to confirm that cool surfaces represent a growing research field. In fact, the first period was the least developed in terms of published documents, with most of them focused on the use of lighter-colored materials to provide high solar energy reflectance and to decrease the solar heat entrapped in urban areas [ 2 , 23 ]. This period was characterized by the “cooling” theme (Figure 6b), as the increases in the temperatures in cities urged the use of high-albedo surfaces instead of darker materials. Appl. Sci. 2022,12, 1893 7 of 24 Figure 5. Overlapping map of the sample. Figure 6. (Thematic evolution map according to the h-index ( a ) and the number of published documents (b). Later, several studies showed that the whiteness of materials enhances the surface albedo and increases the solar reflectance in the visible spectrum range, which reduces the cooling load of buildings [ 51 ]. For instance, in a study exploring alternative methods for creating high-albedo concrete for pavement applications, Boriboonsomsin and Reza [ 52 ] found that replacing cement with whiter constituents (70% slag) achieves an albedo of 0.582, which is 71% higher than the conventional mix. To encourage the implementation of white reflective materials in buildings and urban areas, as well as to facilitate their integration into the construction sector, standards and product labelling were adopted and promoted on the basis of the spectral reflection examination of these materials [ 2 , 53 , 54 ]. In this sense, some efforts have been taken to incorporate cool roofs as an effective sustainable strategy in the revised ASHRAE building standards, S90.1 [ 55 ]. This approach was developing in the second period and it coincides with the integration of building regulations to enhance energy performances, such as the first version of the Energy Performance of Buildings Directive, 2002/91/EC, as well as its subsequent update (Directive 2010/31/EU). In this second period, the research field started to receive more interest, which is highlighted by the inclusion of 52 new keywords (Figure 5) and the following six emerging themes (Figure 6): “buildings”; “solar-energy”; “solar-radiation”; “urban-area”; “pigments”; and “standards-codes”. The h-index impact in Figure 6a emphasizes buildings, solar energy, and solar radiation, and the number of published documents is approximately the same for each theme (Figure 6b). These studies began because of the rising interest in cool materials with solar radiation reflectance properties not only in the visible range, but also in the NIR spectrum, to reduce energy consumption and enhance thermal comfort. Appl. Sci. 2022,12, 1893 8 of 24 Finally, the third period increases the number of keywords to 99 (Figure 5), and it shows a link between the research field and the creation of balanced solutions in the development of coatings, membranes, and materials designed to save energy in buildings (Figure 6b). This represents the most prolific period in terms of published documents. In this period, the conceptual evolution of the themes was developed into more specific areas directed toward creating solutions and developing materials, such as: “roofs”; “urbanarea”; “buildings”; “coatings”; “building-materials”; “albedo”; “membranes”; and “coolmaterials”. Researchers started to explore more alternatives that conformed to the energy efficiency and aesthetic requirements, both indoors and outdoors, and there was increasing interest in materials with the appropriate thermal emissivity/absorption spectrum. For this purpose, the careful selection of nanoparticles and pigments was developed to optimize the thermal and optical performances of materials, such as radiative cooling painting. Several works were led in this sense to develop high-reflective paints known as “cool paints”, such as smart coatings with high NIR reflectance to reduce the solar heat gain and the energy consumption [ 56 – 59 ]. In addition, the coatings were developed in order to achieve the color and efficient radiative cooling requirements in a simple, low-cost, and scalable way [60]. Since the third period (2011–2020) was the most prolific for this field of research, it is analyzed in detail below. To achieve this, Figure 7was created, which shows a strategic diagram of a two-dimensional space that was built by plotting the themes according to their centralities and their density rank values. This includes four quadrants, each containing a specific theme: (i) Motor themes, in the upper-right quadrant; (ii) Basic and transversal themes, in the lower-right quadrant; (iii) Highly developed and isolated themes, in the upper-left quadrant; and (iv) Emerging or declining themes, in the lower-left quadrant [ 61 ]. Figure 7. Strategic diagram of the third period, the volume of the spheres is proportional to the number of documents published ( a ), to the h-index ( b ) and to the number of citations ( c ) in the third period associated with each theme. According to Figure 7a, almost 70% of the published records represent the roofs, urbanarea, and coatings themes, and they have the highest impacts (Figure 7b). It can be seen that roofs and urban-area are the most cited themes, with 7079 and 3497 citations, respectively, followed at a distance by coatings, with 1442 citations (Figure 7c). In the case of the roofs theme, its origins show an association with the concepts, building, solar-radiation, solar energy, and urban areas (Figure 6), and it represents the most applied domain of cool materials, as is shown in Figure 4. It is noteworthy that the urban-area motor theme is associated with all of the specific themes of the third period as an origin theme. The most detailed analysis of the cluster network allowed for the obtainment of information regarding the materials applied for cool surface purposes. Thus, the coatings theme showed a strong relation with the development of pigments (Figure 6a), especially the ones performing in the NIR spectrum as reflective materials, as can be seen in its cluster network (Figure 8d). In terms of the isolated membranes theme (Figure 8c), it mainly concerns the latent strategy of the thermal energy storage resumed in the PCMs. Finally, the two emerging themes of building-materials and cool-materials (Figure 8a,d, respectively) discuss ceramic Appl. Sci. 2022,12, 1893 9 of 24 materials and glass, and retroreflective materials (RR materials), respectively. The most relevant contributions to these topics are analyzed in the next section for the review of materials applied for cool surfaces. Figure 8. Cluster networks of: ( a ) building-materials; ( b ) cool-materials; ( c ) membranes; and ( d ) coatings. 3.3. Literature Revision of Materials Applied for Cool Surfaces The use of cool materials for building envelopes and pavements helps to increase the solar reflectance index, which decreases the energy use for the cooling demand and enhances the indoor and outdoor thermal comfort. The fabrication of such materials to mitigate the UHI implies the creation of an equilibrium between the technical use of these materials and their environmental impact in order to secure a sustainable production and service loop. In this section, the most applied materials are analyzed: (i) Pigments; (ii) RR materials; (ii) PCMs; (iv) Ceramic materials; and (v) Glass. The implementation of more than one strategy to create a cool surface could be complementary. These materials were the subject of 63 publications, from which ~25% evaluate ceramic materials, 22% are dedicated to pigments, and 19, 19, and 14% evaluate PCMs, glass, and RR materials, respectively. This sample was selected by means of clustering during the third period. However, six publications were extracted from the previous periods and added to the analysis because of the impacts and contributions of the authors, such as Akbari, H, who has made the most contributions in terms of the records published related to the field of cool surfaces. The literature reviewing of these materials is included below. 3.3.1. Pigments According to the publications presented in Table 1, the pigments were evaluated through the 14 most relevant publications that are related to the theme of energy use in buildings. Most of the publications are affiliated with research entities based in India and China. Appl. Sci. 2022,12, 1893 16 of 24 Table 4. Cont. Title of Publ. Authors Journal Year of Publ. Ref. Country of Affiliation New strategy to mitigate urban heat island effect: Energy saving by combining high albedo and low thermal diffusivity in glass ceramic materials. Enríquez, E., Fuertes, V., Cabrera, M. J., Seores, J., Muñoz, D., and Fernández, J. F. Solar Energy 2017 [100] Spain White sintered glass-ceramic tiles with improved thermal insulation properties for building applications. Marangoni, M., Nait-Ali, B., Smith, D. S., Binhussain, M., Colombo, P., and Bernardo, E. Journal of the European Ceramic Society 2017 [101] Italy, France, Saudi Arabia, United States A composite cool colored tile for sloped roofs with high “equivalent” solar reflectance. Ferrari, Chiara, Libbra, A., Cernuschi, F. M., De Maria, L., Marchionna, S., Barozzi, M., . . . Muscio, A. Energy and Buildings 2016 [102] Italy Optical properties of traditional clay tiles for ventilated roofs and implication on roof thermal performance. Di Giuseppe, E., Sabbatini, S., Cozzolino, N., Stipa, P., and D’Orazio, M. Journal of Building Physics 2019 [103] Italy Study on the cool roof effect of Japanese traditional tiled roof: Numerical analysis of solar reflectance of unevenness tiled surface and heat budget of typical tiled roof system. Takebayashi, H., Moriyama, M., and Sugihara, T. Energy and Buildings 2012 [104] Japan Development of clay tile coatings for steep-sloped cool roofs. Pisello, A. L., Cotana, F., Nicolini, A., and Brinchi, L. Energies 2013 [105] Italy Thermal-energy analysis of roof cool clay tiles for application in historic buildings and cities. Pisello, A. L. Sustainable Cities and Society 2015 [106] Italy Study on roof tile’s colors in Malaysia for development of new anti-warming roof tiles with higher Solar Reflectance Index (SRI). Yacouby, A. M. A., Khamidi, M. F., Nuruddin, M. F., Farhan, S. A., and Razali, A. E. National Postgraduate Conference—Energy and Sustainability: Exploring the Innovative Minds 2011 [107] Malaysia The publications showed the development of innovative solutions for a better solar reflectance index of ceramic materials, such as tiles, glazes, and engobes. The use of ceramic tiles is considered an effective component of the cool roof strategy, thanks to its durability and its solar properties, especially if it is glazed [ 95 – 97 ]. The substrate material was tested with the application of different ceramic coatings through several studies, and the developed nonwhite coatings enhanced the solar reflective performance and showed interesting results in terms of energy saving [ 98 , 99 ]. Moreover, the wollastonite–hardystonite glass–ceramic porous tiles showed high reflectances of solar radiation, coupled with low thermal conductivity in an arid environment [ 100 ], which highlights the complementary function with regard to the thermal insulation properties [ 101 , 102 ]. In the same sense, an improvement in the thermal performance of a residential building was found during the summer and the winter, and 75% of the solar radiation reflectance in the NIR spectrum was registered, i.e., 10% more with respect to traditional tiles, without altering the visible appearance [ 64 ]. The latter property is highly considered for historical buildings that are required to maintain their original aesthetic appearance. These types of buildings often exist in the center of urban areas, which are strongly affected by the UHI. Their retrofitting Appl. Sci. 2022,12, 1893 17 of 24 using innovative cool clay tile coatings increased the solar reflectance by 20% while maintaining the original color intact. This kind of retrofitting enhances the thermal responses of buildings and the urban climate in general [103–106]. Glazes present a good complementary component for cool tiles as glass–ceramic materials; they are fabricated through a controlled crystallization process for a desired microstructure. The incorporation of cool pigments in the composition of glazes yields the total reflective performance of the product up to 82.8% [ 66 ]. The results of an experimental study show that a tile coated with glass ceramic material induces 20% energy savings, compared to TiO 2 -based paints, and that it could be used for roofs and pavements [ 107 ]. The application of engobes enhances the solar reflectance as well, by up to 0.90 [ 95 ]. As an intermediate layer between the substrate and the glaze, it provides a high degree of adhesion while taking into account the convenience of all the coefficients of thermal expansion. The application of developed glazes and engobes enhances the solar reflective performance of tiles [ 108 ]. In recent studies, the development of cool ceramic coatings was strongly discussed in terms of the low-cost routes, the use of secondary materials, and the self-cleaning abilities [ 109 ]. In turn, research has been focused on enhancing the compositions of glass ceramic frits, opacifiers, and pigments to reach the optimal potential for NIR reflectance. The development of coatings has been oriented toward the use of dynamic coatings, such as the passive ones: photochromic and thermochromic coatings [ 110 ]. It is clear that the research field is an intersection of multiple complementary techniques and materials, which include pigments, glazes, and tiles [ 73 , 111 ], and this creates a wide range of opportunities to attain the energy-saving potential of cool materials in future studies. 3.3.5. Glass According to the publications presented in Table 5, the use of glass is discussed through the 12 most relevant publications that are related to the themes of buildings and the cleaner production of materials. The publications are affiliated with research entities based in different countries and that are not concentrated in a specific one. Table 5. Most important publications about glass. Title of Publ. Authors Journal Year of Publ. Ref. Country of Affiliation Optic-energy and visual comfort analysis of retro-reflective building plasters. Castellani, Gambelli, Nicolini, Rossi Building and Environment 2020 [83] Italy Waste glass in civil engineering applications—A review. Kazmi, D., Williams, D. J. and Serati, M. International Journal of Applied Ceramic Technology 2020 [112] Australia Reuse of waste glass in building brick production. Demir, I. Waste Management and Research 2009 [113] Turkey Utilization of waste glass to enhance physical-mechanical properties of fired clay brick. Phonphuak, N., Kanyakam, S., and Chindaprasirt, P. Journal of Cleaner Production 2016 [114] Thailand Properties of Fired Clay Bricks Mixed with Waste Glass. Abdeen, H., and Shihada, S. Journal of Scientific Research and Reports 2017 [115] Palestine The role of glass waste in the production of ceramic-based products and other applications: A review. Silva, R. V., de Brito, J., Lye, C. Q., and Dhir, R. K. Journal of Cleaner Production 2017 [116] Portugal, United Kingdom Effect of waste glass on properties of burnt clay bricks. Hameed, A., Haider, U., Qazi, A. U., and Abbas, S. Pakistan Journal of Engineering and Applied Sciences 2018 [117] Canada Thermal performance evaluation of eco-friendly bricks incorporating waste glass sludge. Kazmi, S. M. S., Munir, M. J., Wu, Y. F., Hanif, A., and Patnaikuni, I. Journal of Cleaner Production 2018 [118] Australia, Pakistan, Hong Gong Appl. Sci. 2022,12, 1893 18 of 24 Table 5. Cont. Title of Publ. Authors Journal Year of Publ. Ref. Country of Affiliation Glass recycling in the production of low-temperature stoneware tiles. Lassinantti Gualtieri, M., Mugoni, C., Guandalini, S., Cattini, A., Mazzini, D., Alboni, C., and Siligardi, C. Journal of Cleaner Production 2018 [119] Italy Effect of glass powder on the technological properties and microstructure of clay mixture for porcelain stoneware tiles manufacture. Njindam, O. R., Njoya, D., Mache, J. R., Mouafon, M., Messan, A., and Njopwouo, D. Construction and Building Materials 2018 [120] Burkina Fasu, Cameroon Incorporating hollow glass microsphere to cool asphalt pavement: Preliminary evaluation of asphalt mastic. Du Yinfei, Dai Mingxin, Deng Haibin, Deng Deyi, Cheng Peifeng, Ma Cong Construction and Building Materials 2020 [121] China Cool White Polymer Coatings based on Glass Bubbles for Buildings. Nie, YoungjaeYoo, Hasitha Hewakuruppu, Sullivan, Krishna, Jaeho Lee Scientific Reports 2020 [122]South Korea, United States Recently, numerous solutions for cool materials have been developed through the academic research in terms of sustainability, which have paved the way for substituting raw materials and using secondary ones. Waste glass was introduced as an alternative as a solution to the increased need for efficient waste management strategies. Using glass as a secondary material in the fabrication of ceramic materials enhances the mechanical behavior and the sintering action because of its amorphous structure [ 112 ]. Different percentages of waste glass have been tested in different studies for the fabrication of clay bricks, and, in general, the mechanical and thermal behaviors of these materials were affected by the percentage of the substitution, the size of the waste glass particles, and the chemical composition of each type [ 113 – 118 ]. For the fabrication of tiles, 41 wt.% of waste glass demonstrated good flexural strength and abrasion resistance when using the boron-rich waste glass as a sintering promoter [ 119 ]. However, Njindam et al. [ 120 ] demonstrated that the addition of high amounts of glass (>30 wt.%) into ceramic bodies is undesirable because of its negative effect on the physical properties. The integration of glass into the matrix of cool materials was mostly in its finest structure; in general, good results were obtained when using small-sized particles of glass. For instance, hollow glass microspheres were integrated into an asphalt mixture, which resulted in a 40% decrease in the thermal conductivity, and a 60% increase in the infrared reflectance [ 121 , 122 ]. In the same sense, it was demonstrated that glass spheres incorporated in RR materials showed good results in terms of the energy reflected beyond the canyon for the building envelopes, in addition to the road-traffic-marking efficiency [83]. According to the different studies, each material manifests specific thermal, physical, and mechanical properties, depending on different conditions, such as the chemical composition and the particle size of the waste glass. The evaluation of the solar reflectance of ceramic materials, such as tiles and bricks, for building envelopes incorporating waste glass has been little discussed in the academic research. Nevertheless, waste glass cullet was successfully mixed with conventional materials to fabricate sustainable asphalt roof shingles, which have a solar reflectance greater than 25%, and better solar reflectance properties [ 45 ]. Recent research has shown that the use of waste glass tile coatings should be considered for a global solar reflectance analysis [ 123 ]. Future studies are needed to evaluate the influence of using waste glass on Appl. Sci. 2022,12, 1893 19 of 24 the solar radiation reflectance in ceramic materials, especially in the NIR solar spectrum, where 52% of the solar energy is concentrated. 4. Conclusions On the basis of a bibliometric analysis using SciMAT software, a sample of 982 academic records in the field of cool surfaces was processed for three periods, from 1995 to 2020. The most prolific period, between 2011 and 2020, with 87.74% of the total records, was analyzed specifically. It was shown that cool surfaces are a developing research field that aims to create sustainable cool materials and coatings, with efficient solar reflectivity, that conform to the retrofitting requirements, and that are adaptable to the specificities of each domain of application: pavements, facades, and particularly roofs. The research field has placed emphasis on the materials perspective, as most materials trending in this topic are pigments, RR materials, PCMs, ceramic materials, and glass. The heat energy mostly falls within the NIR wavelength region; thus, several recent studies have been developing solar radiation reflective materials. However, the efficiency and the manufacturing of these materials depend not only on the good thermal and optical performances, but also on the environmental impact during the production and the service life. The results of the analysis of the key publications are summarized below: • Developing pigments with high NIR radiation reflectance is a growing industrial domain, which provides up to 95% of the solar reflectivity performance. However, the acquisition, the production process, and the methods of synthesis of these nanoparticles may inconveniently affect the environment; • Retroreflective materials, such as backscattering materials, present good performances by reflecting the incident solar radiation beyond the urban canyon and easing the heat trapped in the urban canopy with respect to the diffusive materials. However, their retroreflective behavior occurs mainly for low angles of incidence, therefore limiting their performance at large angles of incidence. More studies should discuss alternative solutions for the angular selective behavior; • PCMs represent a good solution as cool-surface dynamic switch materials, and their implementation in the reflective materials matrix enhances the solar reflectivity performance. However, some PCM-based materials may contain toxic metals, and the encapsulation methods should be further addressed in future studies for better potential uses; • Ceramic materials are complementary effective materials for cool materials and coatings that enhance the total solar reflectance performance, with a broad range of reflective glazes and engobes; • Most of the studies discuss the integration of glass in the fabrication of ceramic materials as fine particles. The use of glass in these materials, with the optimal dosage and particle size, enhances their physical, mechanical, and thermal properties; • All the cool materials discussed in this review have a common aim, and that is to conform to energy efficiency while using sustainable materials and methods, these materials could be used as complementary strategies for an energy saving system. • The use of secondary materials as substitutes for the manufacturing of cool materials have paved the way toward more sustainable solutions. The incorporation of glass generated from waste in the manufacturing of cool materials is poorly discussed in the academic literature; thus, more research on the solar reflectance performance of waste glass particles as raw material should be further investigated. This study presents a scientific contribution to the cool surfaces research field in terms of the materials and strategies it presents to counter the urban heat island effect. Appl. Sci. 2022,12, 1893 20 of 24 Author Contributions: Conceptualization, experiments, writing, formal analysis, and the postprocessing, C.M. and M.M.-M.; conceptualization, writing, project administration, funding acquisition, and supervision, M.Z. and D.P.R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the State Research Agency (AEI) of Spain and the European Regional Development Funds (ERDF), under project: PID2019-108761RB-I00. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available upon request. Acknowledgments: The research group TEP-968 technologies for the circular economy of the University of Granada, Spain. Conflicts of Interest: The authors declare no conflict of interest. References 1. Gago, E.J.; Roldan, J.; Pacheco-Torres, R.; Ordonez, J. 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