CiteSpace-Based Visualization Analysis on the Trombe Wall in Solar Buildings

Abstract

The Trombe Wall is the main solar collector component in solar buildings, and it has attracted much attention due to its ability to maximize solar energy utilization and reduce buildings’ energy consumption. Numerous research studies have been conducted. Bibliometrics and CiteSpace visual analysis software are used in this paper to summarize and find that the research process for the Trombe Wall can be divided into three periods: the preliminary development period, the ice-breaking period, and the comprehensive development period. Then, we visually analyze information, such as countries, institutions, authors, journals, references, and keywords, from 537 selected articles in the Web of Science (WOS) database during the period 1991–2023. The results show that current research in this area primarily focuses on “thermal comfort”, “BLC”, “PCM-Trombe Wall”, “PV-Trombe Wall”, and “BIPV system”. On this basis, potential development trends in this field and some future research issues that need to be addressed are proposed. Furthermore, the study identifies potential development trends in this area. By providing a comprehensive understanding of the current research status, research frontiers, research hotspots, and research trends in this area, this study provides valuable theoretical guidance for subsequent research.

1. Introduction

With the rapid growth in the global population and the rapid development of the economy, energy crises and environmental problems are major problems faced by countries all over the world, forcing people to actively seek the development and utilization of renewable clean energy. Buildings are the main consumers of energy and also contribute to environmental pollution [1,2,3]; solar energy, as a renewable and clean energy, possesses the advantages of huge reserves, wide distribution, low cost, being pollution-free, and so on. Consequently, solar buildings have garnered attention from countries worldwide [4,5].

The research on utilizing solar radiation to collect and store heat on walls can be traced back to 1956. Professor Felix Trombe, director of the French Odeillo Solar Energy Research Institute, first proposed the heat collection and storage wall model. This wall model involves covering all the remaining walls of the south-facing exterior wall (excluding windows) with glass covers, leaving a certain gap. This gap allows the south wall to function as a heat collection and storage wall. At the same time, the absorption characteristics of the glass to the sunlight are utilized, enabling the south wall to maximize solar energy utilization. The absorbed heat is then transferred to the indoor environment through radiation, convection, and conduction, improving the overall indoor thermal environment.

In 1976, Professor Felix Trombe first proposed a passive solar house with this thermal storage wall and tested its internal air thermal cycle performance. The results indicated that when the indoor air temperature is maintained at about 20 °C in winter, the heat provided by solar energy can account for 60~70% of the total heat supply [6]. Since the thermal storage wall of this structure was first proposed by Professor Felix Trombe and his collaborators and successfully verified experimentally, this thermal storage wall is also called the Trombe Wall.

The Trombe Wall is one of the most effective systems among passive heating systems [7,8], and it can reduce building energy consumption by up to 30% in the process of building operation [9]. The application of the Trombe Wall has greatly expanded the area of heat collection in solar buildings, making its thermal efficiency a breakthrough [10]. This outstanding energy-saving advantage has attracted many scholars to research it. In addition to the research on the performance of the Trombe Wall itself, a variety of derivative types of the Trombe Wall have emerged. At present, the common types mainly include the PCM (Phase Change Material) Trombe Wall [11,12,13], the Trombe Wall with ventilation louvers [14,15,16], the composite Trombe Wall [17,18,19], the PV (photovoltaic) Trombe Wall [20,21,22], and the water-based Trombe Wall [23,24], etc.

By sorting through the literature, it can be found that many scholars at home and abroad have carried out relevant summary research on the field of the Trombe Wall in solar buildings. Omer K. Ahmed et al. [25] presented different designs of a PV–Trombe Wall system besides its thermal and electrical applications. The review covered in detail the influence of design and operational parameters. Furthermore, a comparison between the PV–Trombe Wall system and the classical Trombe Wall as well as the applicability of this novel system were revealed. Adil A. M. Omara et al. [8] presented a comprehensive review of the different advantages of integrating PCMs with Trombe Walls. In addition, the effectiveness of PCMs in providing protection from overheating and improving the efficiency of the energy management process and energy saving of Trombe Walls is demonstrated. Kostikov Sergei et al. [7] conducted systematic reviews on the progress of the Trombe Wall and discussed the main subspecies of the Trombe Wall. A qualitative assessment of the possibility for use in cold climatic conditions was given for each subspecies to revise the current potential of the Trombe Wall for cold climates. Jerzy Szyszka [26] showed the evolution of the classic Trombe Wall during the period 1967–2022 and indicated the impact of climate conditions on the decision-making process involved in the selection of the Trombe Wall design with respect to the optimization of energy effects.

A literature review is considered to be an effective way to deeply understand a field of research [27]. By systematically combing through the existing research, we can figure out the current research situation and development trend of the field, thus providing a direction for future research [27]. The primary purpose of this review is to describe in detail the results of existing studies. However, most of the existing relevant summary research was based on a specific perspective, lacking systematic sorting and induction. In addition, researchers mostly summarized the research in a narrative way, lacking visual and intuitive expression. CiteSpace.6.1.R6 software, a visual citation analysis software developed under the background of scientometrics and data visualization, is one of the most popular bifurcation mapping tools [28]. This paper uses the scientific method of big data statistics to analyze the visual knowledge map of the relevant articles in the WOS core collection database from 1991 to 2003 by using CiteSpace visualization software, collates the relevant research articles in the field of the Trombe Wall in an objective and scientific way, displays its historical evolution; analyzes its research status, research hotspots, and research frontiers; and explores its future development trend. This article is intended to provide theoretical and practical guidance for later research.

2. Research Method

2.1. Data Collection

The research documents were selected from the Web of Science core collection, which includes three university science citation indexes (SCIE, SSCI, A&HCI), two international conference proceedings citation indexes (CPCIs) (Social Sciences and Humanities and Natural Sciences), a book citation index (social science humanities and natural science), and two chemical indexes. The WOS core database has great influence and authority worldwide, containing more than one billion searchable citations across more than 250 disciplines.

Table 1. Data Sample Source.

Data Base	Web of Science Core Collection
Search method	TS (topic search) = (trombe wall)
Time range	1 January 1991 to 28 February 2023
Article type	Thesis; conference proceedings; review papers; published online.
Subject classification	Engineering; energy fuels; construction building technology; thermodynamics; physics; environmental science ecology et al.
Search quantity	537

shows the source information about the data samples studied in this article. The advanced search was carried out on the data resources in the WOS core collection database. The search topic word TS = (trombe wall). The articles were firstly searched with a longer time span, and it was found there were few publications before 1991; therefore, the search time span was set to 1991–2023. A total of 552 records were retrieved. In order to further refine the results, the literature related to the relevant fields and interdisciplinary fields of this study was screened through types of literature and research directions. A total of 537 relevant research documents were finally retrieved as the research objects of this paper.

2.2. Research Method

Bibliometrics is a discipline that uses mathematical, statistical, and philological methods to evaluate and predict the current situation and the development trend of scientific and technological research with the help of various characteristics and quantities of literature. It often uses quantitative methods and combines relevant professional knowledge to summarize and analyze the research hotspots and development trends in a certain field. CiteSpace is a bibliometric visual analysis software that was jointly developed by Professor Chen Chaomei from Drexel University and the WISE Laboratory of the Dalian University of Technology. The software presents the development history, frontier fields, hot topics, and other contents of research topics via a visual atlas, revealing the dynamic development law of related research fields, which is conducive to researchers more comprehensively grasping the relevant information on the research topic, so as to more accurately analyze and prospect the research status, research hotspots, and research trends.

In this paper, bibliometrics analysis and the CiteSpace.6.1.R6 software are used to sort and analyze 537 relevant research documents retrieved on the Trombe Wall. By using a cluster analysis of research countries and research authors, these documents main research contents and their interrelationships in the research topics can be determined. Through bibliometrics analysis of co-cited references, the references cited with high frequency can be identified, which reflects the research hotspots and provides systematic information on high-quality references for researchers to refer to. A cluster analysis of keywords and an analysis of their frequency and centrality [29] help to summarize the research status, frontier, and hotspots regarding the Trombe Wall. The co-occurrence network of keywords and the keywords with the strongest citation burst, combined with research interests, help to identify the characteristics of the research trends and to explore research trends. Overall, the use of CiteSpace software provides a valuable tool for analyzing and summarizing the research on the Trombe Wall.

Two quantified indicators in CiteSpace are mentioned in the following process: betweenness centrality and burst strength. Betweenness centrality is an indicator of the importance of nodes in a network, which is the ratio of the shortest paths between two nodes to the sum of all shortest paths [30]. If the betweenness centrality exceeds 0.1, the node is called a critical node. In general, the higher the betweenness centrality, the closer the connection between the node and other nodes; meanwhile, the occurrence of a high betweenness centrality is likely to be accompanied by a transformative discovery [31], while burst strength could reveal a sudden change in a citation over a period of time, and the nodes with a high burst strength may be the turning points and milestones in the development of literature themes.

3. Results and Discussion

3.1. Situational Analysis of Research Enthusiasm

Statistical analysis was carried out on the 537 related documents studied in this paper according to the annual literature publication volume, and the annual trends chart of the number of related documents about the Trombe Wall is shown in Figure 1. According to Figure 1, the research process regarding the Trombe Wall is divided into three periods:

• The first period was the “preliminary development period”, spanning from 1991 to 2006. During this period, the Trombe Wall faced several challenges, including the limitations of social economic, technological levels, and public awareness, among other factors. As a result, only a few experts have dedicated their efforts to researching the Trombe Wall during this stage, resulting in a modest annual output of fewer than 10 scholarly documents.
• The second period was the “ice-breaking period”, which spanned from 2007 to 2018, and was marked by the global economic crisis and a pressing energy problem, which heightened the public’s awareness of the importance of renewable and clean energy such as solar energy. In addition to the in-depth implementation of the sustainable development strategy during this period, the number of relevant research documents was rapidly increasing, making it a period with a rapid development of relevant research.
• The third period is the “comprehensive development period”, spanning from 2019 to the present. The 2018 World Energy Development Report emphasized the pivotal role of solar and wind energy in the global energy transition. In addition, breakthroughs in the computer fields were made in 2018, leading to a significant increase in the amount of relevant research in 2019 and showing a steady upward trend ever since.

In general, according to the annual trends of publications related to the Trombe Wall, it can be predicted that the amount of research in this field will continue to increase steadily until it is relatively mature, and then it will enter a stable development period. At present, the energy crisis is still a major problem faced by countries all over the world, so research in these related fields still has great potential for development and broad research prospects.

3.2. Countries of Publications and Their Geographical Distribution Characteristics

3.2.1. Analysis of the Countries Where the Documents Are Published

Through the national-network-analysis function of CiteSpace, the countries of the publishers of 537 relevant documents are clustered, and the knowledge mapping for the national distribution network is shown in Figure 2. It can be concluded that (1) almost all countries have formed cooperative relations with at least one other country and (2) in the related research fields, there is a noticeable level of collaboration between the cooperation between the United States, France, Italy, Norway, and other Western developed countries.

According to statistics, scholars from 64 countries have published articles related to the Trombe Wall. The top 10 research countries for this research, by the number of published articles, are listed in

Table 2. The number of publications by scholars from different countries of the Trombe Wall and degree of centrality.

NO.	Country	Publications	NO.	Country	Centrality
1	China	206	1	Peoples R China	0.47
2	France	32	2	France	0.40
3	Iran	28	3	Spain	0.28
4	Australia	23	4	Saudi Arabia	0.22
5	Spain	23	5	Japan	0.17
6	Saudi Arabia	22	6	England	0.16
7	Turkey	19	7	Malaysia	0.12
8	Portugal	19	8	Canada	0.11
9	India	18	9	Tunisia	0.11
10	Poland	18	10	Iran	0.10

. It can be found that (1) China published the largest number of papers, with 206 papers, accounting for 38.36% of the total, followed by France, with 32 papers published, accounting for 5.96% of the total, and in turn followed by Iran, with 28 papers published, accounting for 5.21% of the total; (2) most papers were published in Asia-Pacific region, including China, Australia, and other Asian countries, and the Western countries, where the highest number of papers were published in France and Spain; (3) the top ten countries with a centrality ranking are listed in Table 2, all of which have a centrality exceeding 0.1, indicating that these countries have established sufficient authoritative positions in this research field. Among them, China, France, and Japan show strong centrality at 0.47, 0.40, and 0.28, respectively, indicating that their academic research is closely combined with that of other countries; in a comprehensive comparison of the published papers, Iran ranked in the top three places, but it had a weaker centrality and did not form the same degree of close academic network relationship aligned with other countries.

3.2.2. Geographical Distribution Characteristics

Adapting to regional characteristics is fundamental to the design and application of the Trombe Wall in an overall building engineering system. Partitioning the area where the solar building is located according to the distribution of solar energy resources will help construction engineers to understand the local characteristics more clearly and carry out more breakthrough research on the Trombe Wall, thus the proposal for a feasible optimization design strategy according to local conditions.

Due to different natural geographical conditions, differences in the distribution of solar energy resources, and limitations on economic and technological capacities, the research on the Trombe Wall exhibits discernible differences across different countries and regions.

• Distribution Characteristics of Solar Energy Resources

According to the international classification of regions with solar thermal utilization, global solar energy resources can be divided into four areas, as shown in Figure 3: areas with extremely rich solar resources, areas with relatively rich solar resources, areas with available solar resources, and areas with deficient solar resources. Among them, the regions with the best intensity of solar radiation and duration of sunshine include North Africa, the Middle East, the southwestern United States and Mexico, southern Europe, Australia, South Africa, the east and west coasts of South America, and western China [32].

• Comparative Analysis of National and Regional Studies on Different Solar Energy Resource Partitions.

Based on a statistical analysis of data from 537 related papers, the related research results of the developed and developing countries on the Trombe Wall are summarized and sorted by different partitions of solar energy resources, as shown in Figure 4. The following results can be found.

• In developed countries with solar resources in extremely rich areas, including the United States and Australia, the number of related papers accounted for 6.71% of the total, while in developing countries, such as China, Iran, Turkey, India, and others, the number of related papers accounted for 72.33% of the total; the total number accounted for 79.04%.
• In developed countries with relatively rich solar energy resources, such as Canada, Spain, Portugal, Italy, and other countries, there are more related studies: the number of related papers accounted for 17.19% of the total, while in developing countries, such as Turkey, Serbia, and others, the number of related papers accounted for 9.68% of the total, for a total of 26.87%.
• Among the developed countries with areas that have available solar energy resources, such as France, Japan, Germany, Poland, and others, the number of related papers accounted for 14.23% of the total, while the developing countries in this region had less research on related fields, and the number of related papers accounted for only 1.38% of the total, for a total of 15.61%.
• Among the developed countries with deficient solar energy resources, such as the United Kingdom, Norway, Denmark, and other countries, the United Kingdom and Denmark were the main research forces in related fields in this region, and the number of related papers accounted for 5.33%, while the relevant research in developing countries in areas with insufficient solar energy resources is relatively scarce.

Through the above analysis, it can be concluded that (1) there is a notable geographical alignment between the geographical distribution of regions with research about the Trombe Wall and solar radiation resources; (2) developing countries demonstrate the highest proportion of research papers in areas with extremely rich or relatively rich solar resources, indicating that the Trombe Wall exhibits immense potential for development in economically underdeveloped regions; and (3) developed countries such as the United States and Canada have started employing advanced computer simulation programs and computer optimization algorithms for their research, but there is still a technological research gap between developing countries and developed countries.

3.3. Analysis of the Relevant Research Institutions

Based on an analysis of the research institutions to which the 537 research papers belong, with a threshold set to 3, Figure 5 represents the knowledge mapping for the main institutions researching the Trombe Wall. Figure 5 shows that the number of research institutions is large and scattered, but in recent years, some systems of cooperation have been formed between research institutions, and academic exchanges and cooperation between research institutions have not been blocked by geographical distance.

Table 3. Number of publications by relevant research institutions of the Trombe Wall.

NO.	Institutions	Country	Publications	Proportion
1	University of Science Technology of China Cas	China	53	9.87%
2	Nanjing Technology University	China	15	2.79%
3	Hong Kong Polytechnic University	China	14	2.61%
4	Hefei University of Technology	China	14	2.61%
5	Royal Melbourne Institute of Technology	America	14	2.61%
6	Hunan University	China	12	2.23%
7	Chongqing University	China	11	2.05%
8	City University of Hong Kong	China	10	1.86%
9	Dalian University of Technology	China	10	1.86%
10	Anhui Prov Key Lab Human Safety	China	9	1.68%
11	Northern Technology University	Iraq	8	1.49%
12	Harbin Institute of Technology	China	8	1.49%
13	Beijing University of Civil Engineering Architecture	China	7	1.30%
14	Chengdu University	China	7	1.30%
15	King Abdulaziz University	Saudi Arabia	7	1.30%
16	Rzeszow University of Technology	Poland	7	1.30%

lists the top 16 research institutions in terms of the number of articles published on the Trombe Wall. It can be found that (1) the University of Science and Technology of China is the leading research institution on the Trombe Wall, with 53 papers published, and the number of related papers accounted for 9.87% of the total, far ahead of China’s Nanjing Technology University, which ranks second with 15 articles, and (2) among the top 16 research institutions in terms of the number of publications, 12 are from China, indicating that China has taken a leading role in international cooperation related to the Trombe Wall and has conducted relatively systematic research. It can also be found from Table 3 that most of the research institutions are colleges and universities, while the number of enterprise researchers in related fields is relatively small. This suggests that the current academic circles represented by colleges and universities have conducted a relatively in-depth exploration of the Trombe Wall, and the research on related companies in the industry needs to be further promoted.

3.4. Analysis of Article Authors

Analyzing and studying the current situation of the articles’ authors allows us to summarize the maturity of the author group in a specific field and focus on the core research scholars in these fields. According to the inference of Price’s Law [33], the formula for calculating the minimum number of papers of core authors is

m ≈ 0.749 × √n_max

In the formula, √n_max is the maximum number of papers published by all authors, and m is the minimum number of papers published by core authors.

According to WOS data statistics, there are 200 researchers in this related fields, and the maximum number of papers issued is 39. Taking √n_max = 39, the minimum integer m can be calculated as 5. Therefore, the number of core authors in the related research fields should be greater than or equal to 5. Set the threshold value to 5, and

Table 4. List of core authors’ information in relevant research fields of the Trombe Wall (1991–2023).

NO.	Authors	Affiliation	Country	Publications
1	Ji, Jie	University of Science & Technology of China, CAS	China	39
2	Yu, Bendong	Nanjing Technology University	China	22
3	He, Wei	Hefei University of Technology	China	19
4	Hu, Zhongting	Zhejiang University of Technology	China	11
5	Shi, Long	Royal Melbourne Institute of Technology (RMIT)	American	9
6	Li, Niansi	Nanjing Technology University	China	8
7	Wang, Chuyao	University of Science & Technology of China	China	8
8	Zhou, Yuekuan	Hong Kong Polytechnic University	China	8
9	Wu, Shuang-Ying	Chongqing University	China	8
10	Xiao, Lan	Chongqing University	China	8
11	Ahmed, Omer K	Northern Technical University Iraq	Iraq	7
12	Xu, Lijie	Qilu University of Technology	China	7
13	Zhang, Guomin	Xiamen University	China	7
14	Boaventura-Cunha, Jose	INESC TEC	Portugal	6
15	Hong, Xiaoqiang	University of Science & Technology of China	China	6
16	Habib, Khairul	University Technology Petrona	Malaysia	6
17	Chen, Hongbing	University of Science & Technology Beijing	China	6
18	Lassue, Stephane	University Artois	French	6
19	Dehmani, Leila	Centre de Recherche et des Technologies de l’Energie de Borj Cedria, Hammam Lif	Tunisia	5
20	Luo, Kun	Central South University of Forestry & Technology	China	5
21	Zalewski, Laurent	University Artois	French	5
22	Irshad, Kashif	King Fahd University of Petroleum & Minerals	Saudi Arabia	5
23	Zhang, Guoqiang	North China Electric Power University	China	5
24	Bruno, Roberto	University of Calabria	Italy	5

shows the number of institutions and contributed papers of core authors from 1991 to 2023. It can be seen from Table 4 that from 1991 to 2023, 24 authors belonged to the core authors in the relevant research fields of the Trombe Wall. Ji, Jie from the University of Science & Technology of China published 39 papers, ranking first place, followed by Yu, Bendong from the Nanjing Technology University and He, Wei from the Hefei University of Technology, who published 22 papers and 19 papers, respectively. More information is found in Table 4.

Through the author network analysis function of CiteSpace, the authors of 537 related papers were clustered, and Figure 6 represents the author-network knowledge mapping for the research fields related to the Trombe Wall. The mapping highlights that most authors collaborate in research teams, yet these teams tend to be dispersed throughout the network. However, a small group of authors displays a preference for independent research.

3.5. Analysis of Journals and High-Frequency Co-Cited References

3.5.1. Analysis of Published Journals

Journals are a platform for displaying research fields and achievements.

Table 5. Ranking of publications by journals on the Trombe Wall.

NO.	Journals	Publications	Proportion
1	ENERGY AND BUILDINGS	62	11.55%
2	ENERGY	42	8.38%
3	SOLAR ENERGY	32	5.96%
4	RENEWABLE ENERGY	27	5.03%
5	ENERGIES	22	4.10%
6	APPLIED ENERGY	20	3.72%
7	JOURNAL OF BUILDING ENGINEERING	20	3.72%
8	ENERGY CONVERSION AND MANAGEMENT	17	3.17%
9	BUILDING AND ENVIRONMENT	16	2.98%
10	APPLIED THERMAL ENGINEERING	13	2.42%

shows the main journals and the number of publications of research documents related to the Trombe Wall from 1991–2023. It can be found that ENERGY AND ARCHITECTURE is the journal with the largest number of publications, with 62 articles, accounting for 11.55% of the total; ENERGY ranked second, with 42 articles, accounting for 8.38% of the total; and SOLAR ENERGY ranked third, with 32 articles, accounting for 5.96% of the total.

3.5.2. Analysis of High-Frequency Co-Cited References

The citation frequency of co-cited references can be used not only as one of the important indicators to evaluate the authors’ research ability and contribution to the research topics but also as an effective tool to evaluate the academic value and quality of research papers.

Table 6. Co-cited references on the Trombe Wall cited more than 100 times.

NO.	Title	Journals	Time	DOI	Authors	Citation Frequency
1	PCM thermal storage in buildings: A state of the art	RENEWABLE & SUSTAINABLE ENERGY REVIEWS	2007	10.1016/j.rser.2005. 10.002	Tyagi, Vineet Veer	726
2	Passive building energy savings: A review of building envelope components	RENEWABLE & SUSTAINABLE ENERGY REVIEWS	2011	10.1016/j.rser.2011. 07.014	Sadineni, Suresh B	709
3	A parametric study of Trombe Walls for passive cooling of buildings	ENERGY AND BUILDINGS	1998	10.1016/S0378-7788(97)00024-8	Gan, GH	218
4	Trombe Walls: A review of opportunities and challenges in research and development	RENEWABLE & SUSTAINABLE ENERGY REVIEWS	2012	10.1016/j.rser.2012. 06.032	Saadatian, Omidreza	201
5	A review of performance of zero energy buildings and energy efficiency solutions	JOURNAL OF BUILDING ENGINEERING	2019	10.1016/j.jobe.2019. 100772	Belussi, Lorenzo	189
6	A mathematical model of a solar chimney	RENEWABLE ENERGY	2003	10.1016/S0960-1481(02)00057-5	Ong, KS	154
7	Application of passive Wall systems for improving the energy efficiency in buildings: A comprehensive review	RENEWABLE & SUSTAINABLE ENERGY REVIEWS	2016	10.1016/j.rser.2016. 04.010	Omrany, Hossein	146
8	A review of research and developments of building-integrated photovoltaic/thermal (BIPV/T) systems	RENEWABLE & SUSTAINABLE ENERGY REVIEWS	2016	10.1016/j.rser.2016. 07.011	Yang, Tingting	141
9	A review on the application of Trombe Wall system in buildings	RENEWABLE & SUSTAINABLE ENERGY REVIEWS	2017	10.1016/j.rser.2016. 12.003	Hu, Zhongting	134
10	Experimental study of small-scale solar Wall integrating phase change material	SOLAR ENERGY	2012	10.1016/j.solener. 2011.09.026	Zalewski, Laurent	121
11	Optimum design of Trombe Wall system in mediterranean region	SOLAR ENERGY	2011	10.1016/j.solener. 2011.04.025	Jaber, Samar	115
12	Modeling of a novel Trombe Wall with PV cells	BUILDING AND ENVIRONMENT	2007	10.1016/j.buildenv. 2006.01.005	Jie, Ji	114
13	Performance of PV-Trombe Wall in winter correlated with south facade design	APPLIED ENERGY	2011	10.1016/j.apenergy. 2010.06.002	Sun, Wei	113
14	Numerical study on thermal behavior of classical or composite Trombe solar Walls	ENERGY AND BUILDINGS	2007	10.1016/j.enbuild. 2006.11.003	Shen, Jibao	109

shows the information summary of 14 co-cited references with a citation frequency of more than 100 times, which can guide researchers to obtain the most valuable information in the fields of the Trombe Wall in the future. The statistical results reflect the following situations:

• High-frequency co-cited references can be regarded as high-quality and high-level literature that has had an important academic impact on the relevant research fields of the Trombe Wall.
• Table 6 shows that “PCM thermal storage in buildings: A state of art” [34], which was published in 2007, was co-cited at the top of the list, with a frequency of 726.
• The top 4 frequently co-cited documents from the table were chosen for further analysis. Tyagi et al. [34] discussed various possible methods for heating and cooling in buildings and presented the thermal performance of various types of systems such as PCM-Trombe Wall, PCM wallboards, air-based heating systems, etc. All systems have good potential for heating and cooling in buildings through phase change materials and are also very beneficial to reduce the energy demand of the buildings; Sadineni et al. [35] made an exhaustive technical review of the building envelope components and respective improvements from an energy-efficiency perspective. They discussed different types of energy-efficient walls such as Trombe Walls, ventilated walls, and glazed walls. Various types of thermal insulation materials are enumerated, along with selection criteria of these materials. The effects of thermal mass and phase change materials on building cooling/heating loads and peak loads were discussed; Gan et al. [36] mainly studied the issue of summer cooling of buildings using Trombe Walls. Ventilation rates resulting from natural cooling were predicted using the CFD (computational fluid dynamics) technique. The renormalization group (RNG) k-epsilon turbulence model was used for the prediction of buoyant air flow and the flow rate in enclosures with Trombe Wall geometries. The effects of the distance between the wall and glazing, wall height, glazing type, and wall insulation were also investigated. The research methods and conclusions have laid a theoretical foundation for subsequent research; Saadatian et al. [37] discussed the characteristics of Trombe Walls, including Trombe Wall configurations and Trombe Wall technology. The advantages and disadvantages of this sustainable architectural technology have been highlighted, and future research questions have been identified.

3.6. Analysis of Research Frontiers and Hotspots

The keywords of the paper may highly reflect the scheme and thoughts of the paper, and it is feasible to determine the research frontiers, research hotspots, and directions of development in the fields of discipline according to the frequencies of the keywords [38]. Centrality is a key indicator in analyzing the importance of keywords. If the centrality of a node exceeds 0.1, it means that the node is a central node and has a great influence on relevant research fields. The term “trombe wall” was filtered out because it was used as the main search term for the paper. On the basis of screening and filtering, the keywords’ co-occurrence network is generated by identifying keywords of documents in the database and creating the keywords’ co-occurrence matrix with CiteSpace, as shown in Figure 7.

Table 7. Statistics of high-frequency keywords with a frequency of more than 40.

NO.	Keywords	Frequency	NO.	Keywords	Centrality
1	system	131	1	heat transfer	0.18
2	performance	129	2	flow	0.14
3	building	100	3	solar chimney	0.14
4	thermal performance	89	4	natural convection	0.13
5	design	82	5	energy efficiency	0.12
6	simulation	75	6	phase change material	0.11
7	phase change material	72	7	building	0.11
8	heat transfer	62	8	air flow	0.10
9	energy	62	9	chimney	0.09
10	solar energy	48	10	solar energy	0.09
11	solar chimney	47	11	design	0.09
12	natural ventilation	46	12	system	0.09
13	natural convection	44	13	performance	0.08
14	energy performance	44	14	model	0.07
15	behavior	40	15	energy performance	0.07

represents the statistical analysis of high-frequency keywords with a frequency of more than 40 and the keywords’ centrality in 537 relevant research documents during the period of 1991–2023. High-frequency keywords with a frequency of more than 40 include system, performance, building, thermal performance, design, simulation, phase change material, heat transfer, energy, solar energy, solar chimney, natural ventilation, natural convection, energy performance, and behavior, and keywords with a centrality of more than 0.1 include heat transfer, flow, solar chimney, natural convection, energy efficiency, phase change material, building, and air flow, indicating that these keywords are key nodes in the development process of this related research.

The clustering of keywords and the automatic labeling of clusters are important functions of CiteSpace that help to identify the main research topics related to the Trombe Wall. Figure 8 shows the clustering results.

Table 8. The cluster data statistics of references.

Cluster ID	Cluster Label (LLR)	Size	Silhouette	Mean Year	Ranked Terms and Their Frequency
#0	Solar chimney	63	0.869	2007	Building (100); solar chimney (47); natural convection (44); flow (37); ventilation (22); numerical simulation (21); chimney (15); channel (10); air (10); water (8); solar collector (7); passive heating (6); Trombe Wall channel (3); composite Trombe Wall (1); computational fluid dynamics technique (1); heat loss coefficient (1); etc.
#1	Direct gain	54	0.813	1999	Heat (10); passive solar system (5); passive solar heating (5); plate (5); passive solar (4); direct gain (4); attached sunspace (3); classical Trombe Wall (2); temperature distribution (2); water Wall (2); thermal network (1); composite Wall-collector system (1); computer simulation (1); etc.
#2	Phase change Material	53	0.769	2012	System (131); simulation (75); phase change material (72); PCM (32); collector (26); energy storage (14); convection (5); ventilated Trombe Wall (5); solar radiation (5); PV-Trombe Wall (2); heat transfer analysis (1); collector-storage Wall (1); etc.
#3	Building facade	47	0.714	2015	solar energy (48); ventilation blind (30); air flow (21); PV-Trombe Wall (19); double skin façade (17); performance evaluation (10); optimizing energy (5); convective heat transfer (4); energy building consumption (4); etc.
#4	BIPV	46	0.72	2017	energy performance (44); model (29); efficiency (17); heating system (5); energy analysis (4); experimental validation (4); BIPV Trombe Wall (3); radiation (3); BIPVT system (2); etc.
#5	Comparison	44	0.755	2015	thermal energy storage (22); energy consumption (14); climate (13); comfort (9); solar (7); architecture (5); strategy (5); heat storage (4); coefficient (2); house (2); simulated and experimental result (2); etc.
#6	Photocatalytic	39	0.814	2019	Storage (30); Trombe Wall system (18); Performance analysis (16); formaldehyde (11); PV-Trombe Wall (8); degradation (7); space heating (7); catalytic oxidation (5); challenge (4); indoor thermal (4); validation (4); etc.
#7	Thermal performance	36	0.826	2013	thermal performance (89); Energy (62); optimization (27); numerical analysis (11); passive solar house (9); cooling performance (8); thermal behavior (5); passive heating system (4); air gap (2); experimental work (2); buildings passive heating (1); etc.
#8	Energy efficiency	34	0.875	2007	energy efficiency (26); thermal comfort (23); energy saving (15); roof (4); change material pcm (3); ventilated façade (3); bioclimatic design (2); optic-variable Wall(ovw) (2); air temperature (1); etc.
#9	Heat transfer	34	0.747	2011	Design (82); Heat transfer (62); solar Wall (32); building envelope (8); solar heating (6); building energy saving (4); composite (3); composite Wall (3); classical Wall (2); passive technique (2); calculation methodology (1); etc.
#10	Natural Ventilation	30	0.847	2015	Performance (129); natural ventilation (46); renewable energy (17); room (12); empirical model (8); Wall (7); BIPV-Trombe Wall (3); computational fluid dynamics (3)
#11	Behavior	15	0.866	2015	Behavior (40); summer (5); building performance (3); thermal insulation (2); cumulative energy demand (1); building dynamic simulation (1); energy gain (1); etc.
#12	Building energy efficiency	6	0.991	2012	building energy efficiency (5); passive space heating (2); water heating (2); dual-function solar collector (1); additional sunspace (1); building-integrated dual-function solar collector (1)
#13	Performance enhancement	5	0.996	2020	array (3); performance enhancement (2); thermal measurement (2); high Rayleigh number (2); experimental heat transfer (2)

presents the cluster data statistics of references, and the Silhouette (S) in the table indicates that these main research fields have reasonable data clustering. The keywords were clustered into 14 categories, labeled from 0 to 13. The cluster names, in order, are Solar chimney, Direct gain, Phase change Material, Building facade, BIPV (Building Integrated PV), Comparison, Photocatalytic, Thermal performance, Energy efficiency, Heat transfer, Natural Ventilation, Behavior, Building energy efficiency, and Performance enhancement. Each cluster is composed of multiple closely related keywords. The smaller the cluster number, the more keywords it contains. Table 8 lists the main keywords contained in each cluster and their frequency.

It can be found in Figure 8 that the classification of node keywords involves research materials, contents, forms, etc. The present study reveals that the research areas pertaining to the Trombe Wall exhibit a dispersed pattern without being restricted to any particular domain. There are two values to pay attention to in this network: Q (modularity, clustering module value) and S (silhouette, cluster average contour value). These two values represent the quality of the clustering grid. It is generally believed that when the Q > 0.3, it means that the clustering structure is significant; when the S > 0.5, it means that the clustering is reasonable; and when S > 0.7, it means the clustering has high credibility [39]. The values obtained after clustering in this paper are as follows: Q = 0.52 > 0.3, which means that the clustering structure of this paper is significant, and S = 0.82 > 0.7, which means that the clustering of this paper has high credibility.

3.7. Analysis of Research Interests and Research Trends

3.7.1. Analysis of Research Interests

According to the subject classification in the WOS database spanning the past 32 years, research on the Trombe Wall has involved more than 50 subjects.

Table 9. The number of publications on the Trombe Wall by research interests.

NO.	Research Interests	Publications	Proportion
1	energy and fuel science	362	67.41%
2	building construction technology	159	29.61%
3	civil engineering	125	23.28%
4	thermodynamics	123	22.91%
5	green sustainability science	79	14.71%
6	materials science	50	9.31%
7	engineering machinery	48	8.94%
8	environmental science	30	5.59%
9	engineering chemistry	27	5.03%
10	applied physics	25	4.66%

shows the main disciplines involved in the 537 related documents, including energy and fuel science, building construction technology, civil engineering, thermodynamics, green sustainability science, materials science, engineering machinery, environmental science, engineering chemistry, and applied physics. Energy and fuel science, construction technology, civil engineering, and thermodynamics occupy absolutely dominant positions, including 362, 159, 125, and 123 publications, respectively; among them, energy and fuel science is the main subject direction, including 362 publications, accounting for 68.16% of the total.

In recent years, among the disciplines of publications, the proportion of publications on building construction technology, green sustainability science, and environmental science is on the rise, which shows that the research on the Trombe Wall has gradually shifted from focusing on the optimization design of structural monomers to considering the overall performance of the building and paying attention to the analysis of integration factors, including emphasizing its practical application in the construction engineering system, life cycle carbon emissions, indoor thermal comfort, indoor air quality, etc. At the same time, scholars have realized that the Trombe Wall is an important means to achieving green and sustainable buildings, which shows that the Trombe Wall plays an important role in realizing the sustainable development of clean energy, particularly solar energy.

3.7.2. Analysis of Research Trends

To fully represent the development process and the presentation of the knowledge cluster of the Trombe Wall, the cluster visual timeline analysis of keywords is shown in Figure 9, and the analysis of the top 40 keywords with the strongest citation bursts is shown in Figure 10. By combining the three periods of the research process of the Trombe Wall in 3.1, we summarize the characteristics of its research development at each stage, and analyze its research trends.

• The “preliminary development period” was from 1991 to 2006. Specifically, as shown in Figure 9 and Figure 10 the keywords cited at this stage include solar heating, passive solar systems, direct income, passive solar houses, natural convection, the cavity of the Trombe Wall, etc. The relevant research on the Trombe Wall in the early days mostly focused on theoretical analysis and preliminary comparative analysis and exploration, and the number of related documents during this period was relatively limited. It can be seen in Figure 10 that the high-frequency highlighted keywords appearing at this stage had a relatively long duration, indicating that, although the research vitality is relatively low at this stage, it laid the foundation for later research and development.

Bansal et al. [32] established a steady-state thermal network mathematics model based on the ventilation principle of the Trombe Wall and a solar chimney. They studied the changes in the airflow rate under different combinations of incident solar radiation and ambient temperature and verified that the theoretical model could effectively predict the performance of these systems. He et al. [40] studied the unsteady heat transfer process of the Trombe Wall through the unsteady-heat-transfer theory and computer simulation calculation. They calculated the value of the reaction performance index that can adapt to different wall types, further improving the calculation method of the unsteady heat transfer;

2.

The “ice-breaking period” was from 2007 to 2018. It can be observed from Figure 10 that a large number of keywords began to emerge at this stage, and most of the clusters were also established at this stage. Keywords such as solar heating, passive solar house, ad natural convection still had a strong citation outburst at this stage, and most of the research in this period focused on the thermal performance of the components of the Trombe Wall itself, such as performance, heat transfer, heat storage, and thermal efficiency, which had a burst at this stage. With the continuous advancement of technology, keywords such as simulation, parameters, and optimization have begun to appear; in addition, researchers have continuously researched and explored a variety of derivative types based on the classic Trombe Wall, such as the modified Trombe Wall, the composite Trombe Wall, and the Trombe Wall with ventilation louvers, and in the later stage of this stage, there was the PCM-Trombe Wall, photovoltaic Trombe Wall, and other types. The research areas mainly focus on energy and fuels, materials science, green sustainability science, and other fields.

Shen et al. [14] compared and analyzed the thermal efficiency of the composite Trombe Wall and the classic Trombe Wall through FDM (finite difference method) and computer simulation software. They concluded that the composite Trombe Wall has better performance than the classic Trombe Wall, which can solve the problem of energy storage at night. Hong, He et al. [41,42] analyzed the influence of the position and structural parameters of the louver curtain on the thermal performance of the Collector-Storage Wall with ventilation louvers by using a three-dimensional CFD (computational fluid dynamics) model. They established a dynamic numerical model to study the thermal behavior of the Collector-Storage Wall with ventilation louvers and its contribution to indoor thermal comfort. Yvonne et al. [43] proposed a light translucent Trombe Wall model composed of PCM and insulating gel. Through a CFD numerical simulation, it was concluded that the combination of a 4 cm PCM plate +1 cm aerogel plate had the most significant energy-saving effect.

3.

The “comprehensive development period” is from 2019 to the present. It can be observed from Figure 9 and Figure 10 that keywords such as the PCM-Trombe Wall and the PV-Trombe Wall, which appeared at the late end of the previous stage, produced a citation burst in this stage, resulting in the appearance of related keywords such as BIPVT system, photocatalytic oxidation, and cooling performance. The research at this stage is more focused on improving the heat collection and storage performance of the components and strengthening the natural convection heat transfer. Therefore, some new types of the Trombe Wall, such as the porous Trombe Wall and water wall have begun to rise and gradually become a research hotspot. The research on its specific performance is another important future research direction regarding the Trombe Wall. Keywords such as performance evaluation and the full life cycle have begun to emerge, which are closely related to the proposal of the BLC (Building Life Cycle). As a result, more and more scholars have adopted LCA (Life Cycle Assessment) and begun to comprehensively consider the design, construction, management, components, and other links and pay attention to the life cycle carbon emissions of solar buildings integrated with the Trombe Wall in the operation process; keywords such as thermal behavior and thermal comfort produced a citation outburst, indicating that relevant research has gradually started to focus on people-oriented, comprehensive consideration of the indoor thermal environment of buildings and focus on factor integration.

4.

Ozdenefe et al. [44], based on the existing building conditions, studied the optimal size ratio of the Trombe Wall through a comprehensive evaluation of the consumption of the Trombe Wall, the LCC (life cycle cost), and indoor thermal comfort; Lin et al. [45], based on the life cycle cost of buildings and indoor thermal comfort, conducted an optimal design study on a building with an integrated PV-Trombe Wall and PCM-Trombe Wall through computer simulation and the establishment of an ANN (Artificial Neural Network) model. It can be seen that, based on LCA, improving the indoor thermal environment of buildings, balancing building energy consumption and user comfort, and other related issues will be one of the key points in the research and development on the Trombe Wall in the future.

Overall, the research methodology regarding the Trombe Wall has shifted from phenomenon–problem–strategy to problem–law mechanism–optimized regulation, and the research methods have gradually shifted from qualitative research and quantitative research to a combination of qualitative and quantitative research. The main research areas have focused on energy and fuel science, engineering civil, environmental science, green sustainability science, and other fields. According to Figure 9 and Figure 10, combined with the above analysis, the performance, optimization, PCM-Trombe Wall, PV-Trombe Wall, LCC, thermal comfort, and other directions in the Trombe-Wall-related fields will be the hot topics for further research in the future.

However, there are still existing deficiencies in the research and development regarding the Trombe Wall that require further investigation and resolution.

It is worth studying the heat-collection efficiency and thermal comfort associated with the Trombe Wall under different climates. Dhahri, Maher et al. [46] carried out a numerical evaluation of the thermal behavior of the Trombe Wall installed in a room (TW-R) and used ANSYS CFX software to simulate the thermal efficiency of the Trombe Wall design during winter and summer peaks in semiarid climates in Tunisia. The results show that the normal Trombe Wall cannot ensure a satisfactory comfort level even in summer conditions.

In terms of the application technology, efficiency, and durability of the Trombe Wall, Karaseva L et al. [47] carried out research on the composite Trombe Wall. In severe climatic conditions, the temperature difference can be quite significant. As a result, moisture can form in a massive wall. Further, the massive wall generates condensation on the surface due to moisture. In addition, depending on climatic conditions, the wall’s thickness can vary greatly. Therefore, the useful space of the room is reduced, and the construction becomes more expensive. Also, the amount of sunlight in the daytime is reduced to illuminate the room, since the Trombe composite wall often does not have window openings. During the operating process, it may be difficult to clean the space between the glazing and walls from dust and dirt. As the research results [48] show, to specify the project carefully and resolve the main problematic issues, the composite Trombe Wall is considered a quite effective solution in severe climatic conditions. However, with the continuous changes in the technology and types of the Trombe Wall, these problems need to be repeatedly verified and solved to ensure that the use of the Trombe Wall is not affected in order to be more widespread and widely applied.

In terms of the economic cost of the Trombe Wall, such as the PCM-Trombe Wall, at present, there are a vast number of PCMs that have different characteristics. Many studies cite various PCMs and their thermal characteristics [49,50,51]. However, the authors provide very little information about the economic component, the durability, and the accumulating efficiency assessment concerning the durability. The use of PCM can significantly increase the Trombe Wall’s construction cost. As a result, when designing the wall, it is imperative to conduct a comparative technical–economical analysis between PCM and other possible options. For example, the cost of PCMs and their durability is several times greater than the explicit costs for the standard building envelope with enhanced thermal insulation. As a result, it is possible to reduce the emissions of harmful substances into the atmosphere [7]. The most problematic part of active solar installations such as photovoltaic systems is the batteries’ high cost. As a result, the primary input and operating costs of a photovoltaic system are enormous. Previous studies have shown that the payback period of the photovoltaic Trombe Wall project exceeds the equipment’s operating life [52].

Further, the Trombe Wall serves as a single building component, and it is imperative to examine the impact and modulation patterns resulting from the coupling effect between it and other building components from a building-integration perspective.

4. Conclusions

The network visual and quantitative analysis of research countries, research institutions, research authors, published journals, co-cited references, involved disciplines, and keywords were made based on 537 papers related to the Trombe Wall in the WOS (1991–2023) to analyze its research status as well as popular research topics and to further explore its research hotspots and potential development trends.

The analysis shows that the research in the relevant fields related to the Trombe Wall has progressed significantly over the past 32 years, with a consistent increase in the number of publications, indicating a phase of comprehensive development. Through the analysis of research enthusiasm trends, this paper identifies three distinct development stages related to the Trombe Wall: (1) the “preliminary development period”, spanning from 1991 to 2006; (2) the “rapid development period”, spanning from 2007 to 2018; and (3) the current “comprehensive development period” that began in 2019 and continues to the present.

According to statistics, there are scholars from 64 countries who have published articles related to the Trombe Wall. China has published the largest number of papers, with a total of 206, accounting for 38.36% of the total. However, its research institutions are numerous and scattered, and the research by related companies in the industry requires further promotion compared with colleges and universities. France, China, Japan, and other countries exhibited strong centrality, indicating that their academic research is closely integrated with that of other countries. In recent years, the United States, France, and other Western developed countries have had relatively close cooperation.

By analyzing the current status of the countries researching the Trombe Wall, it can be concluded that (1) there is a notable geographical alignment between the geographical distribution of regions conducting research about the Trombe Wall and solar radiation resources; (2) developing countries demonstrate the highest proportion of research papers in areas with extremely rich or relatively rich solar resources, indicating that the Trombe Wall exhibits immense potential for development in economically underdeveloped regions; and (3) while developed countries such as the United States and Canada have started employing advanced computer simulation programs and computer optimization algorithms for their research, there is still a technological research gap between developing and developed countries.

Among the published journals on the Trombe Wall, ENERGY AND BUILDINGS has had the most significant impact, followed by ENERGY, SOLAR ENERGY, RENEWABLE ENERGY. The reference published in 2007, “PCM thermal storage in buildings: A state of art” [35], which is authored by Tyagi, Vineet V from Europe, has the highest citation frequency, with 726 citations. According to statistics, from 1991 to 2023, there were 200 researchers in the fields of related research on the Trombe Wall, and Ji, Jie from China is the core research author with the largest number of publications.

In recent years, the research on the Trombe Wall has developed rapidly, primarily in the fields of energy and fuel science, building technology, civil engineering, and thermodynamics. Recently, with the emergence of life cycle theory, the proportion of research papers in building construction technology, green sustainability science, and environmental science h increased significantly. This development shows that the Trombe Wall plays an important role in achieving sustainable green buildings with solar energy as the primary clean energy.

From 2019 to 2023, the hot research topics in relevant fields related to the Trombe Wall have included performance, energy optimization, LCA, thermal behavior, thermal comfort, PCM, photocatalytic oxidation, water heat storage, etc. Based on the analysis of research trends, the performance, optimization, thermal comfort, BLC, PCM-Trombe Wall, PV-Trombe Wall, BIPV system, and other directions will be the key research topics in the fields of Trombe-Wall-related research in the future.

At present, there are still shortcomings in the research on and development of the Trombe Wall. The heat collection efficiency and thermal comfort of the Trombe Wall under different climates, application technologies, efficiencies, and durability of the Trombe Wall, as well as the economic cost of the Trombe Wall, should be studied, and solutions should be found. In addition, it is imperative to examine the impact and modulation patterns resulting from the coupling effect between it and other building components from a building-integration perspective.