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Review

The Review of Radiative Cooling Technology Applied to Building Roof—A Bibliometric Analysis

by
Linlin Guo
1,2,*,
Zhuqing Liang
2,
Wenhao Li
2,
Can Yang
2 and
Endong Wang
3
1
Henan Key Laboratory of Grain and Oil Storage Facility & Safety, Henan University of Technology, Zhengzhou 450001, China
2
School of Architecture, Henan University of Technology, Zhengzhou 450001, China
3
Sustainable Construction Management Program, State University of New York (SUNY-ESF), Syracuse, NY 13210, USA
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(16), 6936; https://doi.org/10.3390/su16166936
Submission received: 15 July 2024 / Revised: 7 August 2024 / Accepted: 10 August 2024 / Published: 13 August 2024
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Abstract

In the continuous growth trend of global energy demand, the energy consumption of building cooling occupies a significant proportion. The utilization of alternative or partially alternative energy-input cooling methods in buildings, for example, the application of radiative cooling technology to building roofs, can effectively achieve better cooling performance. This has a positive impact on reducing energy consumption in the building field and slowing down global warming. This paper uses bibliometric analysis methods to systematically review the application of radiative cooling technology on building roofs. The development trajectory, hotspot issues, cutting-edge trends, and future research prospects in the research field over the past 20 years are analyzed and summarized. This study provides insights for the scaled application of radiative cooling technology in buildings and references for the application of radiative cooling technology in the field of architecture to reduce energy consumption, improve energy efficiency, achieve energy conservation, carbon reduction, and sustainable development.

1. Introduction

At present, the whole human society is faced with two biggest challenges: global warming and energy crisis [1]. In order to solve the problem of rising temperatures, energy-input cooling methods are increasingly used but emit more greenhouse gases, which aggravates the global warming issue [2]. The key to curbing the vicious circle between rising temperatures and increasing energy demand is to enhance energy efficiency and reduce energy consumption [3,4]. The building industry is an important player in growing global energy demand. According to the International Energy Agency, global building-related energy consumption accounts for about 46.7% of total societal energy consumption. Especially, building operation accounts for about 36% of total societal energy consumption, and about 65% of building operation energy is used for cooling [5]. Radiative cooling, a novel cooling approach that employs alternative or partially alternative energy-input cooling technology in buildings [6,7], has garnered significant attention, as it can save energy by 20–60%, demonstrating a positive influence on reducing energy demand and mitigating global warming [8,9].
Radiative cooling technology applies the basic principle that high-temperature objects can always cool themselves by thermal radiation. Radiative cooling achieves temperature reduction through spectral regulation. In the “atmospheric transparency window” of 8~13 μm mid-infrared band, by enhancing the emissivity of an object surface in the band range, the surface objects are directly facing the cold source space close to zero to realize radiative heat transfer, so as to achieve the cooling effect of surface objects [7,10]. The roof of the building is the upper layer of the building covered by the envelope, large heat transfer, and the roof of the building has a high sky field of view, less surrounding shielding, and is more conducive to radiation heat transfer [11,12]. Therefore, when applying radiative cooling technology in buildings, the building roof is the best deployment position to achieve good cooling performance [13,14]. Since the form of the building roof is affected by climatic environment, building type, regional culture, material, structure, and other factors [15,16,17,18], the application of radiative cooling technology to the building roof needs systematic research such as physical experimentation, numerical simulation, and onsite case application.
In the late 1970s, the research of radiative cooling technology began to rise. It was not until the early 21st century that researchers gradually began to study the application of radiative cooling technology on building roofs with the progressive developments of radiative cooling materials and technologies [11]. The application of radiative cooling technology on building roofs can be a roof constructive apparatus, a new type of radiative cooling material on the outer surface of the roof, or a combination of a roof the constructive apparatus and the new type of radiative cooling material on the outer surface of the roof [19,20,21,22,23,24,25,26,27,28,29,30]. In the past two decades, research papers on the application of radiative cooling technology on building roofs mainly focused on the demonstrations of individual building cases [19,20,31,32]. Some review papers related to radiative cooling have been published but mainly focused on the basic principles, material prospects, and application prospects [33,34,35]. However, these publications do not provide a systematic guide for applying radiative cooling technology to building roofs.
This systematic review makes a comprehensive examination of the application of radiative cooling technology in building roofs by using the bibliometric analysis method. It analyzes the research progress involving the design strategy, technical principle, numerical simulation, cooling effect, and energy-saving efficiency of radiative cooling roofs. It also summarizes the current hot issues and research trends to provide a reference for the subsequent large-scale and generalized application of radiative cooling technology in building roofs and offer support for promoting the sustainable development of the architecture field.

2. Methodology

2.1. Data Collection

We conducted a systematic literature review by comprehensively gathering relevant literature from the Web of Science Database, Frontiers Database, MDPI Database, ScienceDirect Database, Scopus Database, and Springer Link Database. Overlaps and redundancies were eliminated by using theliterature management tool. These databases are widely utilized by researchers globally for accessing academic literature, providing a common information basis for understanding and exploring radiative cooling technology and its applications in this study. In order to more accurately select the literature required for this study, a method of integrating two sets of keywords based on the research topic was selected for paper retrieval and divided into the following two groups:
(A) Retrieval words including the professional terms related to radiative cooling technology, namely “Radiative cooling”, “Radiative sky cooling”, “Daytime radiative cooling”, “Daytime cooling”, “Passive sky cooling”, “Atmospheric window”, “Spectral selectivity”, “Radiation control”, and “Radiant cooling”.
(B) Retrieval words including the professional terms related to building roof cooling, namely “Building roof”, “Cool roof”, “Passive roof cooling”, “Building roof cooling”, and “Building cooling”.
For effective searching, each keyword from Group A was combined with each keyword from Group B, and a paper was considered relevant if it contained at least one keyword from both groups in the title, abstract, or keywords. The examination window ranges between 1 January 2000 and 31 December 2023. In the execution of the above searching process, the following two criteria were used to select the final literature:
1. Only papers written in English and published in peer-reviewed academic journals or conference proceedings were retained.
2. Literature on the application of radiative cooling technology in other related fields apart from the building field (such as photovoltaic cell cooling, fabric cooling, and thermal management) was excluded.
The retrieved literature was summarized and organized by using the “EndNote” tool. Firstly, based on the title and abstract, eliminate the studies that are not relevant to this study. Then, read the full text of the included literature, and two researchers independently conduct the literature assessment and screening. When there is inconsistency in the screening of the literature, the two sides discuss and negotiate whether to include the literature or let a third researcher make the decision. Finally, through literature tracking, check the references of the included literature to find the missed literature. A total of 175 papers were ultimately selected for in-depth analysis in this study.

2.2. Data Analysis

Bibliometric analysis is a method for quantitative analysis of existing literature. Bibliometric analysis uses visual nodes and lines to show the relationships among keywords, authors, and institutions and can analyze and present the research process, research hotspots, and research trends of a certain topic. In this paper, we mainly analyze the following aspects, including publication volume and publication trend, keyword co-occurrence network, and document co-citation network, using bibliometric analysis on the obtained literature. The development path, the hot issue, and frontier trends of radiative cooling technology applied to building roofs in the past two decades are analyzed and discussed.

3. Results

3.1. Bibliometric Analysis of Research Progress

The research on radiative cooling technology emerged even earlier than the 1970s, and the initial research was to solve the problem of low efficiency of solar cells at high temperatures [36]. Later, researchers found that radiative cooling technology can be used to solve the problem of high temperatures on building surfaces caused by global warming, and the application of radiative cooling technology to building cooling can reduce the energy consumption of building refrigeration [21,37,38,39]. The research on the application of radiative cooling technology to building roofs has gradually attracted attention in the past decade. In 2005, a research paper was published on the use of cold roofs (solar-reflective roofs) to cool buildings and save energy [11]. The paper monitored the effect of increasing the solar reflectivity of roofs on cooling and saving energy in retail store building, school building, and cold storage building. The results showed that installing cool roofs could reduce the daily peak roof surface temperature of each building and decrease air conditioning energy consumption to varying degrees. This paper did not explicitly mention radiative cooling, but the principle of a cool roof is essentially a prototype for radiative cooling on a building roof. In 2006, the first paper on the application of radiative cooling technology to building roofs was published [6]. In the paper, a prototype roof component exploited radiative cooling, utilizing water as the fluid medium, and its cooling performance was investigated. The research showed that the water-based radiative cooling roof component can help to cool down the building and save energy, but its cooling performance is affected by the operation of the component itself, weather conditions, and the surrounding environment of the building.
Through bibliometric analysis, we conducted a study on the quantity of literature regarding the utilization of radiative cooling technology in building roofs and its temporal distribution. A total of 175 SCI papers from 2005 to 2023 were subjected to statistical analysis, leading to the formation of an annual publication trend chart and a price curve (wherein a coefficient closer to 1 indicates faster growth in the number of published articles within the research field). According to Figure 1, the trend coefficient of the publication literature is 0.9039, indicating that there has been a rapid increase in the number of high-level academic papers on the application of radiative cooling technology to building roofs over time. Radiative refrigeration technology uses the high permeability of the 8–13 μm band “atmospheric window” to exchange the heat of the ground object with the low temperature of outer space, so that the ground object can obtain a lower temperature [40]. Radiative cooling is not a new concept. The formation of frost and dew is a natural result of radiative cooling. The Persians used the principle of radiative cooling to make ice at night as early as the 1st century BCE [41]. Radiative cooling technology can be divided into nocturnal radiative cooling and daytime radiative cooling [42]. Nocturnal radiative cooling primarily relies on the 8–13 μm atmospheric window to emit heat to outer space, while also exchanging heat with the surrounding environment, making it relatively easy to achieve cooling below ambient temperature [43,44,45,46]. During the day, objects on the surface absorb part of the sunlight, and they also exchange heat with the surrounding environment. Daytime radiative cooling requires a high emissivity in the atmospheric window at 8–13 μm, as well as a sufficient high solar reflectance, in order to ensure that the heat dissipation is greater than the heat absorption and achieve the cooling effect [47,48,49,50,51,52,53]. The demand for building cooling is much higher during the day than at night [54], and the advancements in daytime radiative cooling technology and materials are the key factors that have led to a rapid growth in research literature on the application of radiative cooling technology to building roofs [7,10,36].
Based on the statistical analysis of the sample literature, the research on the application of radiative cooling technology to building roofs in the past 20 years can be roughly divided into three stages (Table 1). The research was in its infancy from 2005 to 2013, focusing mainly on the application of nighttime radiative cooling on building roofs, as well as research on increasing the solar reflectance of building roof surfaces for building cooling and energy saving. Later, in 2014, Raman and Shanhui Fan from Stanford University in the United States contributed a publication in Nature, describing a photonic crystal consisting of seven alternating layers that achieved spectral selective emission in the atmospheric window and achieved a radiative cooling effect below the ambient temperature during the day [7]. In 2017, Zhai et al. from the University of Colorado in the United States reported in Science a hybrid metasurface radiative cooling film that achieved a high solar spectral reflectance and high atmospheric window emission rate compared to previous studies [10]. In 2018, Mandal et al. from Columbia University in the United States published a study on the phase-inversion-based technique P(VdF-HFP)HP coatings with a wide emission spectrum in Science, which opened a new chapter in high-performance radiative cooling [36]. Therefore, from 2014 to 2018, research on the application of radiative cooling technology to building roofs entered a transitional exploration period, focusing on the cooling and energy-saving effects of different types of radiative cooling materials and components applied to building roofs. The research in this stage from 2019 to the present is in an explosion period, with 80% of the total sample literature being published in the past 5 years. During this stage, the demand for reducing energy consumption and carbon emissions in the building field was more urgent, and the research and production of daylight radiative cooling technology and materials were turning more mature. The role of using radiative cooling technology on building roofs in achieving building cooling, energy saving, and reducing energy demand has been widely recognized and accepted. In summary, research on the application of radiative cooling technology to building roofs has been rapidly progressing due to two aspects, including the urgent need for energy conservation in buildings and carbon reduction and the progress in the development of radiative cooling technology and materials.
In order to better understand the current research status of radiative cooling technology applied to building roofs, we used bibliometric analysis of the countries and institutions that published papers, resulting in a network diagram of cooperative relationships between countries and institutions with 190 nodes, 528 edge connections, and a network density of 0.0294 (Figure 2). In the cooperation network diagram, the nodes represent countries or research institutions, and the size of the nodes reflects the volume of their literature output. The edge connections line between the nodes reflects the cooperative relationships between different countries or research institutions, and the thickness of the edge connections line reflects the degree of closeness of the cooperative relationships between the countries or research institutions. As shown in Figure 2, China, the United States, the EU member states (plus Switzerland, the United Kingdom, Norway, and Serbia), Australia, and India have the highest number of documents published; the top three institutions in terms of document publication are Southeast University of China, the Hong Kong Polytechnic University of China, and the University of Colorado, USA. It is evident that China, the United States, and the EU member states (plus Switzerland, the United Kingdom, Norway, and Serbia) have made significant contributions to research in this field. In recent years, China, the United States, and India have ranked among the top three countries in global energy consumption. The energy supply in the European Union is tense. The current research status of radiative cooling technology being applied to building roofs clearly reflects that significant scientific attempts are being made to meet the urgent need to reduce energy consumption.

3.2. Research Hotspot Based on Keywords Co-Occurrence and Clustering Analysis

The co-occurrence of keywords refers to the phenomenon that frequently appearing keywords within the same research field are found together. By utilizing bibliometric analysis of keyword co-occurrence in the literature sample, a network diagram depicting keyword co-occurrence was generated, consisting of 286 nodes, 1441 links, and a network density of 0.0354 (Figure 3). This reflects the distribution and intensity of research hotspots in radiation cooling technology applied to building roofs over the past two decades. The size of the nodes and circles represents the frequency and centrality of the keywords, while the links between nodes indicate their co-occurrence relationship in Figure 3. This suggests that the research on radiation cooling technology applications to building roofs has experienced significant growth in recent years, with strong connections between keywords. The top 10 most frequently occurring keywords were subjected to statistical analysis (Table 2), which revealed that the predominant research topics in the field are “radiative cooling”, “performance”, “cooling roof”, “energy saving”, and “system”. The central keywords reflect the status and influence of the corresponding research content in the field. The keyword “energy saving” has the highest centrality, indicating a focus on energy-saving aspects in research by applying radiative cooling technology to building roofs. Additionally, keywords with centrality greater than 0.1 include “performance”, “cooling roof”, “building”, “coating”, “system”, and “optical property”, highlighting their core roles in the development of this research field.
Keyword clustering involves grouping keywords with similar themes within a specific research field. Bibliometric analysis offers two indices for network-based clustering analysis: the clustering module number (Q-value) and the clustering average silhouette value (S-value). A Q-value greater than 0.3 indicates a clear module structure in the network, while an S-value greater than 0.7 signifies high reliability of the clustering. With a clustering Q-value of 0.4675 and an S-value of 0.76338, it is evident that the clustering falls within the credible interval and is of high quality (Figure 4). The sample of 175 documents is clustered into nine categories: passive radiative cooling (#0), atmospheric window (#1), thermal comfort (#2), energy savings (#3), solar reflectance (#4), radiative sky cooling (#5), thermal performance (#6), building design (#7), and roof albedo (#8).
By analyzing the clustering results and integrating the relevant literature, the nine clusters reveal three research hotspots in the field of building roof radiative cooling technology.
Research Hotspot 1 focuses on the application of radiative cooling technology to building roofs in order to save energy. This research area is primarily centered around four clusters in the literature: passive radiative cooling (#0), energy conservation (#3), radiative sky cooling (#5), and roof albedo (#8). The primary recipients of incident solar radiation are the extensive roof surfaces of buildings [55], and the heat absorbed by these roofs accounts for 5–10% of the total energy consumption for indoor cooling [17,56]. Mitigating the absorption of heat from roof surfaces is an effective strategy for reducing building energy consumption [57]. Radiative cooling technology can be utilized to increase the albedo of building roofs, thereby decreasing the amount of solar radiation absorbed by the roof and reducing heat transfer through the roof structure [58,59,60,61,62]. This approach improves indoor thermal conditions, lowers building cooling requirements, and ultimately leads to energy savings [63,64]. Baniassadi et al. [60] conducted energy simulations on residential and commercial buildings and calculated the energy savings for cooling in radiative cooling roofs in eight cities in the United States. Radiative cooling roofs reduced the total energy consumption by 4–19% in commercial buildings and 28% in residential buildings. Wang et al. [31] studied the results of using photonic radiative coolers integrated with air conditioning systems (heating, ventilation, and air conditioning (HVAC) integrating the use of the photonic radiative cooler) in office buildings in Miami, Las Vegas, Los Angeles, California, and Chicago, and found that the system could produce significant energy-saving benefits in all five locations. Kolokotsa et al. [65] showed that radiative cooling roofs could save 17% of the annual total cooling energy for municipal buildings in Greece. Ma et al. [66] showed that radiative cooling metamaterial roofs produced significant energy-saving benefits in Beijing, Harbin, Kunming, Nanjing, and Guangzhou in China. Therefore, quantitatively studying the energy-saving benefits of radiative cooling technology applied to building roofs is crucial for accelerating energy-saving and carbon reduction in the building sector.
Research Hotspot 2 focuses on the cooling effect of radiation cooling technology applied to building roofs. This hotspot is the main focus of literature in the three clusters of atmospheric window (#1), thermal performance (#6), and solar reflectance (#4). The technology of radiative cooling exploits the high transparency of the “atmospheric window” in the 8–13 μm waveband [40], enabling ground objects to emit heat to the low-temperature outer space, thus facilitating self-cooling. In dry and clear weather conditions, the “atmospheric window” is nearly transparent to infrared radiation [13,67]. The two key parameters determining the cooling effectiveness of a radiative cooling roof are solar reflectance (the extent to which solar radiation is reflected) and infrared emittance (the extent to which heat radiation is emitted by the object in the infrared region) [52,68,69]. Wijesuriya et al.’s research [70] showed that in residential buildings in Texas, when the solar reflectance is 1, the infrared emissivity can increase from 0.5 to 0.9, and the cooling load savings can increase from 316 kWh to 346 kWh. However, when the infrared emissivity is 0.9, the solar reflectance increases from 0 to 0.4, and the cooling load savings increase from 218 kWh to 346 kWh. Solar reflectance has a greater impact on cooling than infrared emissivity. Chen et al.’s research [55] found that if the long-wave infrared emissivity is fixed at 0.9, the roof temperature decreases significantly as the solar radiation reflectance is increased from 0.2 to 0.95 in increments of 0.1. The maximum daily temperature difference between the roof surfaces with solar reflectance of 0.2 and 0.95 can reach 13.8 °C. Anand et al.’s research [38,47] also showed that cooling below ambient temperature can only be achieved when the solar reflectance reaches a very high value (>95%). Meanwhile, as clear skies are a fundamental requirement for achieving optimal radiative cooling effects [63], the performance of radiative cooling roofs varies significantly across different climate conditions. Clouds can impact radiative cooling by amplifying long-wave radiation in the atmosphere [62,63,71,72]. High water vapor content in the atmosphere can reduce the transmittance of the “atmospheric window”, resulting in poor radiative cooling effects during humid weather conditions [73,74,75,76]. Han et al. [77] demonstrated that an increase in cloud cover from 5% to 78% led to a 63% decrease in the cooling effect on surface temperature due to radiative processes. Gao et al.’s study [78] demonstrated that porous polymer radiative cooling materials can achieve a temperature reduction of 6.9 °C at a relative humidity of 38%, compared to only a 3.9 °C temperature drop at a relative humidity of 68%. Bijarniya et al.’s investigation [51,69] revealed that in humid climate conditions such as Mumbai and Calcutta in Indian coastal cities, radiative cooling performance is suboptimal. Conversely, in dry climate cities like New Delhi and Varanasi, radiative cooling performance is favorable. Radiative cooling technology has potential for year-round outdoor operation when applied to building roofs. However, it is crucial to consider parameters such as solar reflectance and infrared emittance of the radiative cooling surface as well as weather conditions (clear skies, clouds, haze, light rain), which are pivotal factors influencing its effectiveness. Sensitivity analysis should be conducted to comprehensively consider these factors during system design and implementation to maximize the cooling effect.
Research Hotspot 3 focuses on the applicability of radiative cooling technology to various building types and roof forms. This hotspot is the main focus of literature in the two clusters of thermal comfort (#2) and building design (#7). Currently, radiation cooling technologies and materials utilized in building roofs encompass radiation cooling devices, reflective radiation cooling films, radiation cooling rolls, and radiation cooling coatings [22,23,24,25,26,27]. These can be comprehensively applied to the surface and structure of building roofs to achieve the objective of building cooling [28,29,30]. Wang et al.’s experimental research on reflective radiation cooling film application on warehouse roofs revealed a decrease in roof temperature compared to ambient temperature, along with a noticeable improvement in thermal stratification within the warehouse environment [32]. Yuan et al.’s study demonstrated that industrial building galvanized steel roofs covered with reflective radiation cooling films exhibited surface temperatures of 25–30 °C lower at the south side and 20–25 °C lower at the north side compared to uncovered galvanized steel roofs, thus achieving significant roof-cooling effects [53]. The study by Fang et al. demonstrated that the application of radiative cooling film on wooden shingle roofs, a commonly used roofing material in the United States, led to a significant reduction in roof surface temperature [61]. Baniassadi et al. [18] conducted energy consumption simulations for typical commercial and residential buildings with radiative cooling coating materials applied on their roofs. The total energy consumption of the commercial building and the residential building was reduced by 4–19% and 28%, respectively, and the surface temperature of the roof of both types of buildings remained below the ambient air temperature throughout the day. Pisello and Cotana’s annual analysis of clay tile cool roofs on residential buildings in Italy revealed that during summer, the cool roof could lower the highest indoor attic temperature by 4.7 °C, and by 1.2 °C during winter [19]. Some of these studies take the cooling amplitude of the outer surface temperature of the building roof as the evaluation index, some take the degree of energy consumption reduction as the evaluation index, and some take the cooling amplitude of the indoor air temperature of the building as the evaluation index. Which specific evaluation index is chosen in the research mainly depends on different types of buildings, different forms of building roofs, the thermophysical properties of their roof materials, and the cooling demand of using radiative cooling technology. The ultimate goal of the research is to provide reliable data support for the applicability of radiative cooling technology applied to building roofs.

3.3. Research Trends Based on Co-Citation and Burstiness Analysis

The fluctuation in the number of literature references should reflect the changing degree of academic attention paid to the research field. Literature co-citation analysis plays an important role in bibliometric analysis. Through co-citation analysis, we can identify the trend of research over time and the important literature in the development of a discipline, providing direction for future research. Using bibliometric analysis for a co-citation analysis of 175 sampled articles resulted in the creation of a co-cited literature network map (Figure 5). In this map, the size of each node represents the number of citations for the corresponding article, and the connections between nodes indicate co-citation relationships. The network depicted in Figure 5 illustrates the distribution of frequently cited literature, and Table 3 lists the top 10 articles with the highest citation counts. The two most highly cited articles are “Research on High-Performance Radiation Cooling Paint” published in Science in 2018 and “Research on Radiation Cooling Metamaterial Film” published in Science in 2017. Combining the research of these sample articles, it is found that radiation cooling paint and radiation cooling thin film are currently the main trend in applying radiation cooling technology to building roofs, with radiation cooling paint being more dominant due to its convenience in construction.
The burst detection analysis in bibliometric analysis is utilized to identify abnormal activity between nodes over time, indicating heightened attention towards a potential topic. Word burst analysis is employed to characterize short-term keyword shifts, serving as indicators of active research fields. By examining the intensity and timing of keyword bursts, it becomes possible to partially explore emerging trends in research fields. Through conducting keyword mutation analysis on 175 sampled articles using bibliometric analysis (Figure 6), the research trends related to the application of radiative cooling technology on building roofs can be discerned. The data in Figure 6 indicate that prior to 2013, the keyword “model” stood out from other keywords with the strongest citation bursts, indicating the frequent early-adoption of model-based research in the field. From 2014 to 2018, “clear sky”, “selective emitter”, and “thermal radiation” demonstrated significant mutation strength, signifying rapid innovation in key radiation cooling technologies during that period. Since 2019, the keywords “cooling performance”, “performance evaluation”, and “strategy” have presented the strongest citation bursts, suggesting a heightened focus on continuously achieving higher cooling performance and optimizing various application strategies based on real-world scenarios in research pertaining to the practical application of radiation cooling technology on building roofs.
Through co-citation analysis and keyword emergence analysis, as well as the examination of exemplary literature, the research on the application of radiative cooling technology on building roofs demonstrates three distinct trends:
Research Trend 1: The accuracy of numerical simulations for heat transfer in spectral selective radiation cooling materials used to assess the roof cooling performance of radiative cooling technology is being improved. With the advancement of computer technology, numerical simulation has facilitated the study of thermal environments in buildings and energy conservation. In the numerical simulation of radiative cooling systems, the use of precise and suitable thermal balance models is essential for assessing their cooling performance [84]. Given that radiative cooling relies on high emissivity of materials within the “atmospheric window” wavelength range and high reflectivity in the solar spectrum, integrating the thermal properties of spectrum selective radiative cooling materials into building thermal balance simulations becomes a critical step for accurately evaluating cooling performance [85]. Currently, there are primarily two methods for evaluating the cooling performance of radiative cooling through numerical simulations: one is to compare the building’s energy savings before and after implementing radiative cooling technology, and the other is to compare the enhancement of the building’s thermal environment before and after applying radiative cooling technology. The former method mainly utilizes Energy Plus, a building energy consumption simulation software, to assess the cooling performance of the radiative cooling system [20,52,68,69,83,86,87]. However, in most energy consumption simulation software tools, such as Energy Plus, the material model only provides a constant emissivity. If the spectral selectivity of the radiative cooling material is not considered, it may lead to certain deviations in simulation accuracy [88,89]. Yu et al. [89] used Python code to calculate the longwave radiation heat flux, considering spectral selective materials and obtaining the surface temperature function of the radiative cooling material as the inputs into Energy Plus for calculation. The longwave radiation heat flux and material surface temperature calculations were iterated between Energy Plus and the Python code to ensure that they satisfied the energy balance between Energy Plus and the Python code, resulting in a high degree of simulation accuracy. The latter primarily utilizes Fluent, COMSOL, and TRNSYS software for research [21,58,59,66,85,90,91,92]. Feng et al. [85] proposed a novel approach by integrating TRNSYS and MATLAB software. TRNSYS is used for energy and thermal balance calculations. A thermal model is developed in MATLAB to calculate the surface temperature of the roof based on the spectral selective material thermal balance. The surface temperature of radiative cooling material is calculated. The obtained roof surface temperature is used as the roof boundary condition in the TRNSYS simulation to more accurately evaluate the cooling performance of the radiation cooling system. Chen et al. [90] proposed a new super-cool roof model to enhance the consistency of the thermal mass effects of the roof transfer model (RTTV) in numerical simulation with the characteristics of radiative cooling materials. This new model fully incorporates the spectral selectivity of daytime radiative cooling materials for evaluating roof cooling performance and correlates local precipitable water vapor (PWV) with atmospheric transmittance to more accurately assess the cooling performance of spectral selective radiative cooling materials by considering the response of atmospheric meteorological parameters to climate change. The increasing precision of numerical simulation will provide more favorable supporting data for the effective application of radiation cooling technology in building roofs.
Research Trend 2: The demand for radiative cooling technology application on building roofs is affected by geographical position, seasonal, and climate conditions, and a dynamic radiative cooling strategy is better suited for adaptability. In view of the differences in geographical location, season, and climate, the amount of solar radiation heat obtained by surface objects shows significant variations [49]. The energy consumption that can be saved by using radiative cooling technology on building roofs is directly related to the amount of solar radiation heat received by the roofs. In plain terms, more energy consumption can be saved by adopting radiative cooling technology in regions with hotter climates and during hotter periods [54,59,71,93,94]. In terms of the annual energy savings achieved by using radiative cooling technology on roofs, radiative cooling roofs show significant energy savings in hot summer but may increase heating loads during the heating and transition seasons, partially offsetting the cooling load savings achieved during the cooling season [93,94,95,96]. The study by Feng et al. [85] revealed that in Darwin, Australia, the indoor thermal comfort primarily requires significant cooling. The implementation of radiative cooling technology on building roofs can lead to significant energy-saving effects. However, in Sydney, Australia, to meet the indoor thermal comfort requirement, both cooling and heating are needed. It was found that the reduction in cooling loads in summer due to the application of radiative cooling technology on the building roof cannot fully offset the increase in winter heating loads. In order to minimize the increase in heating load during winter, it is essential to implement dynamic radiative cooling strategies [97]. Researchers have initiated the exploration of novel technologies for dynamically regulating heat transfer in radiative cooling systems [95]. Yazdani et al. [96] developed a dynamic cool roof that can alter the surface reflectance rate responding to seasonal changes. Under various climate conditions, this dynamic radiation cooling roof reduced annual energy consumption for residential buildings by 11.8–66.7 kwh/m2 (22.4–35.4%), making it an optimal choice for maximizing year-round energy savings. Self-adaptive radiative cooling systems with adjustable optical properties can modify their optical properties based on ambient temperature, enabling cooling in summer and preventing unnecessary cooling in winter, thereby reducing heating losses [98,99]. Tang et al. [98] conducted research on a temperature-adaptive radiation cooling coating capable of adjusting its solar reflectance rate based on seasonal variations. During summer, the overall solar reflectance rate of the coating reached 91.25%, while in winter, it decreased to 72.71%. The modulation of solar reflectance between seasons led to a significant reduction in energy loss for heating during winter. Further research on dynamic radiation cooling strategies will provide a scientific basis and technical support for the feasibility, standardization, and generalization of radiation cooling technology in building roof applications.
Research Trend 3: The aging of radiative cooling materials, combined with the impacts of outdoor dust pollution and precipitation, has led to a decrease in the performance of radiative cooling. This poses significant challenges for the application of radiative cooling technology on building roofs and remains one major challenge in this research field [100,101,102,103]. In relation to the impact of aging on radiative cooling materials, Mastrapostoli et al. [100] conducted an analysis in two school buildings on the aging characteristics of radiative cooling roofs that had been exposed to outdoor conditions for a period of four years in Greece. The study revealed that the solar reflectance of the radiative cooling roofs decreased by almost 25% after four years of exposure, and the surface temperature of the roof increased by approximately 7–12 °C. Lei et al. [101] utilized temperature cycling, ultraviolet aging, and natural aging experiments to assess the durability of radiative cooling coatings and found that the cooling performance significantly declined during natural aging. Shi et al. [64] conducted outdoor exposure experiments on 12 available high-reflectance coatings to study the aging effects of radiative cooling roofs in hot climate conditions. This study found that with varying weather conditions, the aging effect can reduce the energy savings of a radiative cooling roof to different levels, changing from 26.3% to 10.5%. Regarding the impact of dust pollution and rain on radiative cooling materials, Lei et al. [101] found that the cooling range of radiative cooling was 2.37 °C higher in dusty weather outdoors than under clear sky environments. Radiative cooling coatings can achieve full-day cooling on rainy days, but their cooling capacity will not immediately recover after rainfall. The relevant research showed that the accumulation of dust on the surface of radiative cooling materials, as well as the dirt and grime caused by the mixture of dust and rain, can cause a significant decline in the performance of radiative cooling [17,100,101,103]. Strategies such as self-cleaning, strong water cleaning, or rain cleaning have received widespread attention. To solve the problem of the declining cooling performance of radiative cooling coatings due to dust, rain, and dirt, the proposed self-cleaning high-reflective materials can be promising [104]. Superhydrophobic self-cleaning materials can avoid the contamination of radiative cooling coatings from dust or moisture in outdoor environments [27,105,106,107,108,109,110]. However, the comprehensive improvement of the anti-aging, anti-pollution, and waterproof performance of radiative cooling materials remains a challenge and needs to be further studied for the regular application of radiative cooling technology on building roofs.

4. Discussion and Conclusions

Roof space is responsible for a significant amount of indoor thermal energy loss and solar radiant heat gain, particularly in low-rise buildings where heat transfer plays a major role. In practical applications of radiative cooling technology for building roofs, single-story or low-rise buildings can achieve more effective cooling and energy-saving results within the 8–13 μm waveband “atmospheric window” range of radiative heat transfer. This is due to their large roof area, unobstructed views, and horizontal radiation heat transfer surfaces [8,22,57,111,112,113]. However, when applying radiative cooling technology to the roofs of multi-story or high-rise buildings, the cooling capacity is limited. This is because the cooling load demand of the building increases exponentially with the addition of floors, and the radiative cooling surface area of the roof is small compared to the overall external surface area of the buildings. As a result, it becomes challenging to achieve significant cooling and energy-saving effects for the entire building [114].
From the principles, theories, and experimental research of radiative cooling, it is evident that the cooling performance achievable by using radiative cooling technology on the exterior walls of buildings is limited [115]. When radiative cooling technology is applied to the exterior walls of buildings, the vertical direction of radiative heat transfer surfaces and surrounding obstructions causes the radiative heat transfer to bounce back and forth, and there is not a good range for radiative heat transfer in the 8–13 μm “atmospheric window” band. Therefore, the cooling effects are negatively impacted. Additionally, during the day, the surrounding environment temperature is high, and a limited “cold source” is available for radiative heat transfer on the exterior walls of buildings [115]. At night, although good radiative heat transfer can be achieved to obtain more cooling, the amount of cooling obtained at night is relatively small, and the building’s cooling demand is mostly dense during the daytime. The part of the cooling obtained at night cannot play a significant role in saving building energy consumption. However, if the excess cooling generated by radiation cooling technology at night can be stored and released for building cooling during the day, it can help maximize the utilization efficiency of radiation cooling. Some studies have integrated radiative cooling technology with other technologies and applied them to building walls. For example, incorporating radiation cooling technology, phase change materials, and micro-channel heat pipes into a “radiative cooling-phase change materials (RC-PCM) wall” [116,117] can store the excessive cooling generated by radiation cooling technology at night through PCMs and release it for building cooling during the day, enabling autonomous adjustment of cooling storage and extraction. This provides new ideas and methods for the application of radiative cooling technology on the roofs and exterior walls of high-rise buildings.
With the advancement of daytime radiative cooling materials and technologies, research on the application of radiative cooling technology in building roofs has experienced rapid growth due to the urgent need for energy conservation and carbon reduction in the building sector. This will provide more solutions for the development of the low-carbon, green, and sustainable construction field. At present, the research focus of radiative cooling technology in the application of building roofs mainly concentrates on the scientific assessment of the energy-saving and cooling effect of radiative cooling technology on building roofs and the evaluation of its applicability and feasibility in various buildings and roof systems. Based on the analysis of the hotspots and trends in this research category, future research and practice are still expected to be further enhanced in the following three aspects:
(1). During the numerical simulation process, comprehensively considering the thermal influence generated by the spectrally selective radiative cooling materials can scientifically and accurately evaluate the energy-saving effect and cooling performance of radiative cooling technology on building roofs. At present, it is necessary to adopt the means of integrating numerical simulation and experimental research to enrich the diversity of engineering case studies of different types of buildings and different roof forms so as to provide more abundant data support for the large-scale application of radiative cooling technology in building roofs.
(2). Comprehensively consider the effects of geographical location, climatic conditions, weather elements, and seasonal demands on the radiative cooling performance of building roofs. Deeply explore the adaptability and energy-saving effectiveness of dynamic radiative cooling strategies for building roofs. Classify according to building types, geographical factors, climatic conditions, etc., and explore the dynamic adaptation strategies and specific measures of applying radiative cooling technology to building roofs and external building walls. Provide more practical and effective solutions for reducing energy consumption in the building sector through radiative cooling technology.
(3). In the exploration domain of design strategies and technological progress, give full play to the advantages of interdisciplinary cross-research. Further, solve the problems of aging and performance attenuation of radiative cooling materials caused by outdoor dust pollution and rainwater. Integrate radiative cooling technology with other technologies to maximize the performance of radiative cooling, providing scientific and technological support and research reference for the full realization of the goal of energy conservation and carbon reduction in the construction field. This will facilitate the sustainable development of buildings.

Author Contributions

Conceptualization, L.G. and E.W.; methodology, L.G. and Z.L.; software, W.L. and Z.L.; formal analysis, Z.L.; investigation, W.L. and C.Y.; resources, Z.L.; data curation, L.G.; writing—original draft preparation, L.G. and Z.L.; writing—review and editing, L.G. and E.W.; funding acquisition, L.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by: Henan Key Laboratory of Grain and Oil Storage Facility & Safety, HAUT, Zhengzhou, 450001, China [grant number 2021KF-B04]. Henan Key specialized research and development breakthrough- science and technology project, Zhengzhou, 450001, China [grant number 222102110272]. Doctoral Scientific Research Start-up Foundation from Henan University of Technology [grant number 2020BS058].

Data Availability Statement

The data applied in this study are available on request from the first and the corresponding author.

Conflicts of Interest

We declare that we do not have any commercial or associative interests that represent a conflict of interest in connection with the work submitted.

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Figure 1. Number of publications per year.
Figure 1. Number of publications per year.
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Figure 2. Collaborative networks between countries and institutions.
Figure 2. Collaborative networks between countries and institutions.
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Figure 3. Keywords co-occurrence network.
Figure 3. Keywords co-occurrence network.
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Figure 4. Keyword clustering and mapping.
Figure 4. Keyword clustering and mapping.
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Figure 5. Document co-citation network.
Figure 5. Document co-citation network.
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Figure 6. Analysis of burstiness based on keywords, ranked by the beginning year of burst.
Figure 6. Analysis of burstiness based on keywords, ranked by the beginning year of burst.
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Table 1. Research stage staging.
Table 1. Research stage staging.
PhasePeriodResearch Content and Findings
Phase I2005–2013Using nighttime radiative cooling to collect cold sources for daytime heat exchange on building roofs; utilizing the principle of increasing the solar reflectance of building roofs to set up a cool roof, reducing the amount of heat absorbed by the building roof during the day, thus achieving the goal of cooling the building.
Phase II2014–2018Studying the actual case applications of different types of radiation cooling components and radiation cooling materials on building roofs, and monitoring and observing the cooling effects.
Phase III2019 to the presentBy experimental measurements and numerical simulations, the potential for energy savings and cooling effects of radiative cooling technology applied to building roofs were studied, accumulating a large amount of research data for the regular and mass application of radiative cooling technology in building roofs.
Table 2. Top 10 frequency keywords and centrality.
Table 2. Top 10 frequency keywords and centrality.
No.KeywordsFrequencyCentralityYear
1Radiative cooling690.082006
2Performance590.172008
3Cooling roof450.162005
4Energy saving380.232012
5System350.142006
6Building240.162016
7Coating200.162008
8Optical property180.112014
9Surface170.062016
10Solar reflectance140.062008
Table 3. Top 10 frequent co-cited documents.
Table 3. Top 10 frequent co-cited documents.
NO.AuthorTitleYearFreqSourceRefs.
1Mandal, J
Fu, YK
Overvig, AC		Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling201889Science[36]
2Zhai, Y
Ma, YG
David, SN		Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling201777Science[10]
3Zhao, DL
Aili, A
Zhai, Y		Subambient Cooling of Water: Toward Real-World Applications of Daytime Radiative Cooling201947Joule[79]
4Li, T
Zhai, Y
He, SM		A radiative cooling structural material201946Science[80]
5Kou, JL
Jurado, Z
Chen, Z		Daytime radiative cooling using near-black infrared emitters201743Acs Photonics[81]
6Zhao, B
Hu, MK
Ao, XZ		Radiative cooling: A review of fundamentals, materials, applications, and prospects201941Applied Energy[33]
7Bao, H
Yan, C
Wang, B		Double-layer nanoparticle-based coatings for efficient terrestrial radiative cooling201738Solar Energy Materials and Solar Cells[82]
8Goldstein, EA
Raman, AP
Fan, S		Sub-ambient non-evaporative fluid cooling with the sky201736Nature Energy[83]
9Mandal, J
Yang, Y
Yu, NF		Paints as a Scalable and Effective Radiative Cooling Technology for Buildings202032Joule[23]
10Zhang, K
Zhao, DL
Yin, XB		Energy saving and economic analysis of a new hybrid radiative cooling system for single-family houses in the USA 201831Applied Energy[8]
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Guo, L.; Liang, Z.; Li, W.; Yang, C.; Wang, E. The Review of Radiative Cooling Technology Applied to Building Roof—A Bibliometric Analysis. Sustainability 2024, 16, 6936. https://doi.org/10.3390/su16166936

AMA Style

Guo L, Liang Z, Li W, Yang C, Wang E. The Review of Radiative Cooling Technology Applied to Building Roof—A Bibliometric Analysis. Sustainability. 2024; 16(16):6936. https://doi.org/10.3390/su16166936

Chicago/Turabian Style

Guo, Linlin, Zhuqing Liang, Wenhao Li, Can Yang, and Endong Wang. 2024. "The Review of Radiative Cooling Technology Applied to Building Roof—A Bibliometric Analysis" Sustainability 16, no. 16: 6936. https://doi.org/10.3390/su16166936

APA Style

Guo, L., Liang, Z., Li, W., Yang, C., & Wang, E. (2024). The Review of Radiative Cooling Technology Applied to Building Roof—A Bibliometric Analysis. Sustainability, 16(16), 6936. https://doi.org/10.3390/su16166936

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