Concentrating Solar Power Technologies: A Bibliometric Study of Past, Present and Future Trends in Concentrating Solar Power Research

Concentrating Solar Power Technologies: A Bibliometric Study of Past, Present and Future Trends in Concentrating Solar Power Research Alejandro Calderón 1 , Camila Barreneche 1 , Cristina Prieto 2 , Mercè Segarra 1 and A. Inés Fernández 1 * 1 DIOPMA, Department of Materials Science and Physical Chemistry, Universitat de Barcelona, Barcelona, Spain, 2 Department of Energy Engineering, Universidad de Sevilla, Sevilla, Spain Bibliometric analysis is a key study in order to elucidate the relevance of a research field. This study will point out the main evolution of the research performed in a topic considering the significance of each sub-topic. This study aims to perform a bibliometric analysis in concentrating solar power (CSP) technology. CSP had a first commercial wave between 1984 and 1995. Between 1995 and 2005 there was a period of commercial inactivity. Nevertheless, since 2005 CSP commercial implementation has been accelerating. This topic accounts for almost 6,300 publications on relevant journals and conference proceedings. Regional analysis showed that China is becoming the most relevant country for CSP research. The European Union still leads the development and will continue that way at least in the short term. Authors’keyword evolution analysis has been useful as an indirect measure of technological maturity. It was found that conference proceedings have an important effect in the overall CSP publications and need to be consider when working within this field. However, the journal publications have more impact since they are more frequently cited. Funding analysis had shown a direct relationship between funding and research output. Finally, it was concluded that inside the CSP field, parabolic trough has shown to have great relevance in the research output. Keywords: bibliometric, concentrating solar power, solar tower, parabolic trough, stirling dish, renewable energies index INTRODUCTION Meeting the world energy demand constitutes one of the greatest challenges of our generation, it has grown constantly until 2019 with a 0.9% annual growth. Even with the 2020 pandemic, renewable energy growth achieved 1% during 2020. For developed countries as well as for developing ones, energy has a major role for achieving the desired wellness; 67% energy demand growth since 2000 has been driven by China, Russia, India, Brazil, and South Africa (Enerdata, 2020;International Energy Agency, 2020). During the last few decades, use of fossil fuels as main source of energy to reach energy demand has resulted in a continuous CO 2 emissions growth related to it. (Dincer, 2000; International Energy Agency, 2018a). However, the emissions concern has resulted in the implementation of renewable energy and low emissions policies for several decades and, as a consequence, has contributed toward the reduction of emissions per the amount of energy produced (Dincer, 2000;Ürge-Vorsatz et al., 2015). Edited by: Valerio D’Alessandro, Marche Polytechnic University, Italy Reviewed by: A K M Monjur Morshed, Bangladesh University of Engineering and Technology, Bangladesh Amine Moussaoui, Faculty of Sciences, Mohamed Premier University, Morocco *Correspondence: A. Inés Fernández [email protected] Specialty section: This article was submitted to Thermal and Mass Transport, a section of the journal Frontiers in Mechanical Engineering Received: 18 March 2021 Accepted: 30 April 2021 Published: 18 June 2021 Citation: Calderón A, Barreneche C, Prieto C, Segarra M and Fernández AI (2021) Concentrating Solar Power Technologies: A Bibliometric Study of Past, Present and Future Trends in Concentrating Solar Power Research. Front. Mech. Eng 7:682592. doi: 10.3389/fmech.2021.682592 Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825921 REVIEW published: 18 June 2021 doi: 10.3389/fmech.2021.682592 Renewable energies have a main role in today’s global energy strategy. Nowadays photovoltaic and wind power have become the main drivers on renewable energy implementation (International Energy Agency, 2018b). Nevertheless, their development is limited by the mismatch between resource availability and energy demand. This mismatch cannot be fully countervailed, for now, with economically and technical viable solutions (like energy storage or interconnection) (International Energy Agency, 2014a). Solar thermal energy offers the advantage of storing heat and/ or cold, which is much simpler than storing electricity, even if the thermal energy is converted later to electricity. Concentrating Solar Power (CSP) offers a solution for harvesting solar energy and integrating it with thermal storage for high enthalpy applications (such as industrial heat or electricity generation) (International Energy Agency, 2014b). CSP technologies have been under constant development in recent years; it is expected that, with the adequate support, thermal CSP can constitute up to 11% of electricity generation by 2050 (International Energy Agency, 2010;International Energy Agency, 2014b). Notice that 11 TWh were produced by CSP during 2017 compared with 1 TWh in 2000, and these numbers are expected to grow to 180 TWh in 2030 (International Renewable Energy Agency, 2016;International Energy Agency, 2018b;International Energy Agency, 2020). Different strategies are in the research stage in order to make more technologically and economically viable thermal solar power, such as increasing the low and high temperatures (Ho and Iverson, 2014;Laughlin, 2017;Calderón et al., 2018), using latent heat on heat transfer fluid (HTF) and thermal energy storage (TES) media (Liu et al., 2012), studying thermochemical reactions for long term energy storage capabilities (Felderhoff et al., 2013), studying ways to improve current HTF-TES and power plant operation conditions (Casati et al., 2015;Liu et al., 2017), new supercritical power conversion cycles with improved efficiency, solar field components optimization for cost reductions (Cha et al., 2015;Guo et al., 2018), etc. Over the years, CSP progress has been closely related to scientific research, to finally impact CSP commercial applications, until each technology research has enough technological maturity (Liu et al., 2016;Hussain et al., 2017;Ahmadi et al., 2018). By studying research efforts and trends in order to monitor topic relevance and management of knowledge, Bibliometrics had become an important part of information sciences. Several studies have been made during years for several fields, such as health, marketing, entrepreneurship, production and operations management, innovation, etc. (Merigó et al., 2015). In the same way, bibliometric studies have been recently published for wind power price (Gao et al., 2016), for alternative energy research during 1994 and 2013 (Mao et al., 2015), and for TES technologies (Calderón et al., 2020). For CSP field, there are no deep bibliometric analysis that studied CSP technologies. Bibliometrics use mathematical and statistical methods for measuring science output and relate it to quality. These methods can be applied to individuals, research groups, institutions, countries, and regions (Góngora Orjuela, 2010;Ellegaard and Wallin, 2015). These analyses have proven useful for governmental and funding decision-makers, for building strategic plans on the long term, measuring research productivity, project potential evaluation, and for determining which research activities match the economic and political objectives of a region (Matcharashvili et al., 2014). The aim of this bibliometric study is to provide an overview of CSP research and development evolution by using bibliometric methods. Finding research and development trends can be of great interest for CSP developers and researchers, for energy policy stakeholders, and for everyone involved in or starting in this field. Several regional efforts, policies, and financing options were identified over the years from the knowledge creation perspective. Finally, advancements and technological maturity were studied for the main CSP harvesting configurations: parabolic trough, solar tower, Stirling dish and Fresnel systems. SCOPE Bibliometric methods are widely used in the social sciences for the analysis of the different outstanding research fields of which specific and objective information is required for their study. These methods are also used to determine potential strengths and weaknesses that a field of study has from an objective point of view. Bibliometrics had not been used extensively in technological or formal science fields, although in recent years they have been adapted to apply these methods to these fields. Currently, there are various scientific journals in the field of renewable energy and energy storage, among many others that have begun to publish bibliometric studies in high-impact scientific journals. The bibliometric methodology is used to discern from a specific technical field and find objective information to advance in the research associated with that field. The current paper presents a bibliometric study of the technologies related to the power generation concentrating solar plants, trying to shed light on the present, past, and future trends in research related to CSP plants. Therefore, authors have developed the analysis of different key points that have been identified, once the bibliometric search that is explained in the methodology section has been done. The results have been analyzed from different points of view. The first is a global analysis of the publications produced per year that represents the importance that the field holds. The keywords used in the field are analyzed in order to discern the maturity of the technology. The location of the publications is relevant in order to understand where is more relevant to the field under study. In addition, the top 10 authors are of the CSP field listed in this paper as well as the communities and interaction between them to discern who is leading the research in this field. Finally, the configuration of the technologies are identified separately in order to understand which is the most important for CSP at the present time. METHODOLOGY The procedures and tools used for performing the analysis of CSP research evolution are described in this section. Formally, Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825922 Calderón et al. Trends in CSP Research Bibliometrics is a science dedicated to study the scientific publications of a specific research field by using statistical tools; these studies can be extended to a wide number of scientificfields (Pritchard, 1969). Bibliometrics had been considered to bring a general picture of the progress of a particular research field, detect the top researchers as individuals or as communities, find the most outstanding journals on that specificfield, and the interactions between authors and countries, among other specific analyses (Bjork and Hedlund, 2015). In order to use the most significant information sources and indicators, Web of Science (WoS) Core Collection was used as the source of data. This database includes more than 14,000 highquality journals and provides enough information about the publications, such as authors name, affiliations, abstract, keywords, references, funding information, etc. All this information allowed us to extract high value analyses that could not be performed using other databases. For creating the database, authors searched in the topic, abstract or keywords sections the research query: (“solar tower”)OR[“solar field”AND (“concentrated”)] NOT (“pv” OR “photovoltaic”)OR(“parabolic trough”)OR(“concentrated solar”AND [“power”OR “thermal”)] OR (“fresnel reflect OR *”) OR (“dish stirling”)OR[“molten salt*”AND (“solar”OR “CSP”)] NOT (“pv”OR “photovoltaic”OR “polymer”OR “barrier”OR “lithium”OR “solar-cells”)OR[“heat transfer fluid”AND (“concentrated”)] OR [“heliostat*’” AND (“tower”OR “CSP” OR “solar”)]. A common problem for building up a database for bibliometric analysis is that terminology selected by authors is different according to the specific approach they are using. This occurs also in the CSP field. Therefore, a keyword map was used in order to build a more complex and inclusive search, allowing as well as omitting several keywords from the result to avoid all subjects not related to CSP. Establishing boundaries and selecting the best keyword combination was challenging, since there is no convention on CSP classification by stages or by components or even by the terminology. Table 1 shows these keywords that were used to collect the CSP database. In this keyword map a main phrase search is only included if at least one of the complement phrases in the same row are present in the different searched sections (title, keywords, abstract, etc.), with the exception of those publications that have at least one of the exclusion phrases in that same row. As a result, the keyword map allowed the procurement of, in an improved way, a reliable database of CSP publications. The total scientific articles selected by each row search are also shown in Table 1.Itis remarkable that several publications were selected by more than one row search and the sum of all the articles selected per row is higher than the total publications. Eight two thousand fiftysix publications were found between 1969 and 2020. For yearly analyses only periods until 2019 were considered, since the database was created at mid-2020. For deeper analysis, the database extracted from WoS was analyzed for studying specific CSP technologies which included parabolic trough, solar tower, Stirling dish, and Fresnel collectors. Most of the relevant authors, journals, regional cooperation and other relevant information was assessed and is presented in the following sections. Bibliometric analyses were carried out by python coding and graphic tools. Additional reports were done using VOS Viewer ® scientific visualization tools (Center for Science and Technology Studies, 2018) used for bibliometric network analysis and Complexity Lab Barcelona (CLabB) (Department de Fisica Fonamental, 2019) software. RESULTS Publication Bibliometric Analyses Based on the publications included in this new bibliometric database search, the total publications per year regarding the whole concentrating solar power field are presented as bars in Figure 1 for the last 20 years. Also, the number of citations can be appreciated by the green line in Figure 1. Therefore, the number of publications has grown considerably during the last decade, even though not every year increases the number of publications. Since proceeding papers have been included, they influenced the change of tendency from one year to another but not to the tendency on a greater time span. Citations of the publications are low for the few last years; nevertheless, this was expected since the most recent papers have not been cited yet. Indeed, publications will not reach their full citable potential until some years after their publication date. General tendency remarks that the CSP field is growing up along with its market development in the near future (International Energy Agency, 2018b). TABLE 1 | CSP keyword map search. Exclusion phrases Main phrases Complement phrases Total Solar tower 797 pv photovoltaic Solar field Concentrated 187 Parabolic trough 2,845 Concentrated solar Power Thermal 3,364 Fresnel reflector* 311 Dish stirling 244 pv Photovoltaic Polymer Barrier Lithium Molten salt* Solar csp 1,466 Heat transfer fluid Concentrated 430 Heliostat* Tower csp Solar 1,130 Note that proceedings were included in the extraction because authors noticed the relevance that this kind of publication has in the field. Note that phrases ending in * use this symbol as a wildcard, it represents any character, group of characters, or no character. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825923 Calderón et al. Trends in CSP Research Figure 2 shows the top trend keywords for CSP field. It can be seen that the most used keywords are “performance”,“design”, “solar energy”,“energy”,“optimization”,“systems”,“parabolic trough”, and “concentrated solar power”. Moreover, “optimization”and “concentrated solar power”have been used more recently in 2016 compared with “performance”,“design”,or “system”in 2014–2015; this reflects the maturity of the technology, since these keywords are most commonly used for optimization of commercial solutions. This is a visible example that optimization prevails over the design learning process, since FIGURE 1 | CSP publications and citations during the last 2 decades. FIGURE 2 | Evolution of trend keywords on CSP publications. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825924 Calderón et al. Trends in CSP Research CSP is a mature technology, even though it is under constant improvement. The oldest top keywords are “solar energy”, “energy”, and “parabolic trough”which were more frequently used in 2013. In Table 2 the 20 most cited articles can be appreciated. It is understandable that 12 of the top 20 articles are reviews. Also, several molten salts publications are in the top 20 since this topic is very attractive as HTF and TES media for its high service temperature limit. Kalogirou also denotes as one of the most cited authors as well as Steinfeld. This stays in line with the current CSP tendency to incorporate TES for optimizing CSP plants, which allows the consideration of the high manageability of CSP as a main attribute that adds flexibility to electric networks and to the energy mix. Country Bibliometric Evolution Regional evolution is of great interest for design and evaluation of research policies focused on particular technologies. In Figure 3, TABLE 2 | CSP Most cited publications. Authors Title Journal Type Cites Year Country Kalogirou, SA Solar thermal collectors and applications Progress in energy and combustion science Review 1405 2004 Cyprus Gil, Antoni; Medrano, Marc; Martorell, Ingrid; Lazaro, Ana; Dolado, Pablo; Zalba, Belen; Cabeza, Luisa F State of the art on high temperature thermal energy storage for power generation. Part 1- Concepts, materials and modellization Renewable and sustainable energy reviews Review 824 2010 Spain Steinfeld, Aldo Solar thermochemical production of hydrogen-a review Solar energy Review 817 2005 Switzerland Jacobson, mark Z Review of solutions to global warming, air pollution, and energy security Energy and environmental science Review 712 2009 United States Tian, Y.; Zhao, C. Y A review of solar collectors and thermal energy storage in solar thermal applications Applied energy Review 694 2013 United Kingdom, China Jacobson, mark Z.; Delucchi, mark A Providing all global energy with wind, water, and solar power, Part I: Technologies, energy resources, quantities and areas of infrastructure, and materials Energy policy Article 690 2011 United States Papageorgiou, N; Athanassov, Y; Armand, M; Bonhote, P; Pettersson, H; Azam, A; Gratzel, M The performance and stability of ambient temperature molten salts for solar cell applications Journal of the electrochemical society Article 669 1996 France, Switzerland Fernandez-Garcia, A.; Zarza, E.; Valenzuela, L.; Perez, M Parabolic-trough solar collectors and their applications Renewable and sustainable energy reviews Review 568 2010 Spain Kalogirou, SA Artificial neural networks in renewable energy systems applications: a Review Renewable and sustainable energy reviews Review 551 2001 Cyprus Kalogirou, SA Applications of artificial neural-networks for energy systems Applied energy Proceeding 527 2000 Cyprus Price, H; Lupfert, E; Kearney, D; Zarza, E; Cohen, G; Gee, R; Mahoney, R Advances in parabolic trough solar power technology Journal of solar energy engineering-transactions of the Asme Article 519 2002 Spain, United States Steinfeld, Aldo Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions International journal of hydrogen energy Article 480 2002 Switzerland Mills, D Advances in solar thermal electricity technology Solar energy Proceeding 470 2004 Australia Zhang, H. L.; Baeyens, J.; Degreve, J.; Caceres, G Concentrated solar power plants: Review and design methodology Renewable and sustainable energy reviews Review 432 2013 Belgium, United Kingdom, Chile Kenisarin, Murat M.; Van Valkenburg, ME; Vaughn, RL; Williams, M; Wilkes, JS Thermochemistry of ionic liquid heattransfer fluids Thermochimica acta Proceeding 419 2005 United States Liu, Ming; Saman, Wasim; Bruno, Frank Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems Renewable and sustainable energy reviews Review 349 2012 Australia Herrmann, U; Kelly, B; price, H Two-tank molten salt storage for parabolic trough solar power plants Energy Proceeding 349 2004 Germany, United States Selvakumar, N.; Barshilia, Harish C Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications Solar energy materials and solar cells Review 339 2012 India [’Behar, omar’,’khellaf, abdallah’, ’mohammedi, kamal’] A review of studies on central receiver solar thermal power plants Renewable and sustainable energy reviews Review 331 2013 Algeria Romero, Manuel; Steinfeld, Aldo Concentrating solar thermal power and thermochemical fuels Energy and environmental science Review 317 2012 Spain, Switzerland Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825925 Calderón et al. Trends in CSP Research total publications over time for the top 20 countries are presented. United States stands at the top as the country with the most publications, followed by China, Spain, and Germany. When considering regional efforts Spain, Germany, Italy, France, United Kingdom, Greece, and Portugal suggest that the biggest effort has been made by European Union. Also, emerging economies such as India and China outstand in this list. Figure 4A) shows the publication evolution per country over the last 2 decades. Notice that a high number of publications per country does not imply that countries produce a high relevance output, as can be observed in the number of citations in recent years. United States and almost every European country’s research literature was constant over the last five years, while growth during these years was mainly driven by China, India, United Kingdom, and Australia. This is related to different strong programs that have been implemented by the European Union, the Department of Energy (DoE) of United States, and the Chinese government (Visiongain Energy, 2018;National Renewable Energy Laboratory, n.d.). Furthermore, Figure 4B) presents the total publications for the top five countries divided by CSP technologies. The main ways that are considered to harvest concentrated sunlight are parabolic trough, solar tower, Stirling dish, and Fresnel lens. Total publications for each of these technologies are presented in Figure 4B) and compared to the total publications of CSP field. It can be observed that China leads the total publications for parabolic trough, solar tower, and Stirling dish, while Spain remains as the first one for Fresnel and second one in parabolic trough and solar tower. Figure 4B) also reflects the higher interest on parabolic trough and solar tower compared to both remaining technologies. This is understandable since parabolic trough and solar tower are the first and second most implemented CSP technologies, respectively. Based on current trends, it is expected that China will lead CSP research over the next years, followed by a significant increase of India participation in CSP deployment. China technological development policy is being implemented in the CSP field as it has been implemented in other technological areas, which includes use of foreign experts in order to achieve expertize combined with strong and well-funded homeland research and innovation programs (European Commission, 2017;Kennedy, 2017). This, in the long-term, will potentially allow China to be the most developed country in CSP. This tendency is replicated in Table 3 where publication rate is compared to their total papers accumulated up to the previous year. This information clearly elucidates that China, India, United Kingdom, and Australia were growing during recent years. Also, Spain, Italy, and France maintained their publications trend at a constant rate. Citations are very interesting to analyze, since it can reflect the relevance of the published work. In Table 4 the performance ratio, defined as the total number of citations divided by the number of publications, is presented for the top 20 countries. Again, the performance ratio is low for recent years’papers. Most productive period was registered for Spain between 2009–2011; nevertheless, it was reduced during recent year publications compared to other countries like Switzerland, Iran, or Greece. The case of Spain could be explained due to changes in the public regulations regarding solar energy in that country during 2013–2018 (Boletin Oficial del Estado, 2013;Boletin Oficial del Estado, 2015;Gabaldon-Estevan et al., 2018). In addition, the case of Switzerland’s performance ratio over the last ten years is remarkable, since it was the most prolific one, achieving about 35 citations per paper for the whole decade. Together with Spain, FIGURE 3 | Top 20 publishing countries in the CSP field by number of publications and cites. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825926 Calderón et al. Trends in CSP Research The United States decreased citations per paper over the last decade. Finally, India’s performance ratio was the lowest during the last decade of all the top ten countries, caused mainly by the low citations during the 2014–2016 period. Co-authorship between countries has also been studied in the CSP field (see Figure 5). Each country is represented by a circle in this figure; the size of the circle represents the number of publications, while thickness of the curved lines represents the FIGURE 4 | (A) Publication evolution regarding the CSP field in the last 2 decades by countries; (B) Total publications in CSP field and its sub research fields of the top five countries. TABLE 3 | Country increment of CSP publications compared to the accumulated from previous year. Country 2010 (%) 2011 (%) 2012 (%) 2013 (%) 2014 (%) 2015 (%) 2016 (%) 2017 (%) 2018 (%) 2019 (%) China 44.0 38.9 38.0 34.3 32.4 29.1 23.4 34.8 21.3 22.4 United States 23.7 20.0 41.8 18.7 33.3 13.5 17.8 14.2 12.6 10.1 Spain 13.8 18.5 25.2 26.1 48.3 29.1 28.6 24.0 23.2 13.5 Germany 14.7 14.2 18.3 14.0 30.7 22.8 20.5 17.2 14.3 8.0 India 16.7 23.8 53.8 77.5 47.9 36.2 46.2 29.2 40.4 16.9 Italy 38.2 46.8 31.9 29.7 38.1 35.0 26.8 22.9 19.2 9.3 France 20.0 31.7 17.7 23.7 36.5 24.8 25.5 27.6 17.2 10.6 Australia 7.1 16.7 37.1 45.8 40.0 42.9 42.1 32.2 21.3 16.0 United Kingdom 50.0 55.6 57.1 36.4 53.3 43.5 27.3 46.4 34.1 30.9 Switzerland 9.4 11.8 16.3 7.4 24.6 9.9 9.6 9.2 6.1 6.6 Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825927 Calderón et al. Trends in CSP Research interaction strength between countries. Colors represent the most relevant year of interaction between countries. Thereby, the oldest research cooperation exists between United States and Germany, followed by a newer relation between Spain and Italy. Also, Spain and Germany have the biggest relationship of all, leading the European cluster; followed by the United States-China relationship. This is consistent with known research cooperation between CIEMAT (Spain) and DLR (Germany) with the PSA (Plataforma Solar de Almeria), and ENEA Group (Spain) and DLR molten slat trough projects. In TABLE 4 | Country performance ratio of CSP publications per year for the last 10 years. Country 2011 2011 2012 2013 2014 2015 2016 2017 2018 2019 Publications Cites China 34.8 39.3 21.1 28.7 17.8 16.1 17.1 17.4 10.3 4.1 1,311 18,881 United States 23.5 63.1 17.0 30.9 15.5 20.7 8.9 12.8 6.4 3.1 1,296 23,702 Spain 113.8 67.0 33.3 33.6 19.9 16.2 13.2 9.7 5.4 3.4 1,048 19,223 Germany 21.3 42.8 25.5 26.3 21.1 13.2 8.8 12.8 4.5 3.9 665 12,609 India 64.0 19.8 50.0 32.5 11.8 15.2 12.0 10.9 5.0 2.6 490 5,934 Italy 39.8 53.6 28.6 25.4 17.1 15.6 15.9 8.8 6.4 4.2 474 7,432 France 20.8 34.6 24.9 41.8 21.3 12.1 14.5 12.3 6.4 5.5 430 7,350 Australia 36.0 14.8 74.1 24.9 18.4 21.8 17.3 12.2 11.9 3.3 399 7,754 United Kingdom 25.7 58.6 26.3 157.5 15.5 19.2 22.2 13.9 10.5 3.9 253 4,232 Switzerland 46.1 46.8 62.4 25.0 18.4 25.1 13.4 7.8 9.5 3.9 245 8,803 Iran 57.7 0.0 36.0 46.8 34.9 47.3 28.5 15.8 14.8 5.3 245 4,104 Japan 10.0 41.7 6.2 9.2 7.8 12.1 10.3 5.9 5.2 2.4 168 3,053 Mexico 25.0 28.9 6.0 53.0 5.6 14.9 9.1 12.5 3.4 0.6 152 1,941 Saudi Arabia 0.0 50.3 34.0 33.0 15.9 22.0 14.6 13.0 6.4 4.1 141 1,894 South Africa 2.0 1.2 18.0 41.6 16.5 22.2 10.9 7.3 4.8 1.6 140 1,936 Greece 31.7 52.0 29.5 26.7 37.0 23.0 42.9 21.0 15.8 6.6 140 2,993 Turkey 0.0 20.7 18.0 12.8 17.8 8.7 18.5 13.5 9.8 2.7 140 1,378 Portugal 25.7 17.0 13.5 13.9 10.6 8.0 13.9 6.1 3.9 2.4 135 1,107 Canada 20.3 53.5 34.6 26.6 18.1 12.3 28.9 12.4 14.9 3.1 133 2,021 United Arab Emirates 23.5 11.0 4.0 12.0 7.2 17.7 6.6 5.0 13.8 4.9 130 1,162 FIGURE 5 | Country co-authorship interaction collaboration of the Top20 countries. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825928 Calderón et al. Trends in CSP Research addition, China-United States co-authorship is in line with the NREL (United States) and Sandia National Laboratories (United States) cooperation with China. Other known cooperative projects that are coherent with the bibliometric data are the MIT (United States)–Masdar (UAE) projects as well as solid particle technologies for solar tower between DLR (Germany), Sandia National Laboratories (United States) and CNRS (France) (Calderón et al., 2018;National Renewable Energy Laboratory, n.d.;Calderón et al., 2019). One outstanding cooperation opportunity is the cooperation between China and the Germany-Spain cluster. Besides, Middle Eastern countries were the newest relevance and cooperation of the top countries, showing a growing relevance in the world’s stage. Finally, the lack of African and South American countries shows an area of opportunity for deployment, since a lot of those countries can find CSP implementation very favourable (International Energy Agency, 2010). A special highlight is the case of Chile. This country did a huge step forward in renewables implementation around the country and built the biggest CSP plant in South America in 2018 (110 MW and 17.5 h of storage) being one of the most ambitious projects looking within the future regarding the CSP field (Cerro Dominador, n.d.). Authorship Bibliometric Evolution The accumulated author publications profile is shown in Figure 6 by taking into account the grouped publication of each author profile available in the WoS-core collection database, and authors were checked for repeated WoS profiles although it is difficult to do this revision in every single profile. This analysis does not remark the authorship order, only if they are in the author list. As Figure 6 shows, Dr Aldo Steinfeld is the top-one researcher publishing in CSP field followed by Dr. Zhifeng, Dr. Pitz-Paal, and Dr. Flamant, who are the most representative prestigious researchers in the CSP field. The CSP h index can also be appreciated, this h index was calculated by taking into account only the CSP database. The H index was defined by Prof. J.E. Hirsch from University of California as “The number of papers with citation number higher or equal to h, as a useful index to characterize the scientific output of a researcher”(Hirsch, 2010). In the CSP field,Dr. Aldo Steinfeld obtained an h index of 42, while Dr. Zhifeng Wang and Dr. Giles Flamant had 22 each. These results must be interpreted with caution, since some research centers include the scientific directors of the centers into the publication’s author list. While in other centers only the persons involved directly in the projects are in the publication author list. Some known experts in the CSP field that are present in the current top 20-author list include Dr. Clifford Ho, Dr. Eduardo Zarza, among others. The H-index has been used by academic societies for measuring and quantifying the scientific output. It has become a fundamental standard method to determine the efficiency of the researchers, scientists, institutions, departments, and universities based on their publications and citations (Pavlova, 2018;Youssef et al., 2021). There have been some efforts to develop other indicators for overcoming the flaws of the h-index, however, it is still the most used parameter for measuring author scientific output (Grech and Rizk, 2018). For this analysis, the h-index has been calculated, taking into the account only the research area or sub-area under study. Therefore, the h-index reported in this paper only considers the CSP field. In Table 5, the top 20 authors by number of CSP publications are listed with the number of citations and the CSP h-index. The most cited author and the one with the highest CSP h-index is Dr. Aldo Steinfeld. Other relevant authors that can be denoted are Dr. Zhifeng Wang and Dr. Giles Flamant with a CSP h-index of 22. When considering countries of FIGURE 6 | Cumulative author evolution by publications for the last 2 decades. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 6825929 Calderón et al. Trends in CSP Research The increment of the number of publications of solar tower compared to the total from previous years is presented inn Table 10. The most relevant authors in the solar tower CSP configuration field and their affiliations are listed in Table 11 where the total publications, the total cites, and the h-index of these top 10 authors are also included. Finally, Table 12 shows the most relevant journals in the solar tower topic along with the total publications per journal, the total citations, the performance ration, the impact factor of each journal, and the quartile scores. Stirling Dish In the case of the Stirling dish, the evolution per year over the last 20 years is presented in Figure 14A. The trend is exponential until 2018 where a decrement is accounted. The trend needs to be studied in the next years in order to understand if it is an exception or if it is stagnation in the field. Figure 14B shows the total Stirling dish papers by countryover the last 20 years.China, India, andIran are standing out since 2014–2015. This is remarkable since before this period United States or Spain occupied these top positions. Table 13 summarizes the country increment in the Stirling dish field over the last decade in percentages. Figure 14C shows publication evolution over the last 20 years by authors in the Stirling dish CSP field. The publication evolution was linear since Charles E. Andraka started detaching in 2005. However, the most significant standing up author is Mohammad Hossein Ahmadi that started in 2013 the scaling up until today. Figure 14D shows the most prolific publishing resources regarding the Stirling TABLE 10 | Country/region increment of solar tower publications compared to those accumulated from the previous year. Country 2010 (%) 2011 (%) 2012 (%) 2013 (%) 2014 (%) 2015 (%) 2016 (%) 2017 (%) 2018 (%) 2019 (%) United States 19.7 13.2 33.0 8.8 33.6 11.6 18.9 9.1 9.7 7.9 China 28.9 24.5 32.8 23.5 23.0 15.4 20.4 20.5 23.3 18.5 Spain 18.8 13.2 11.6 14.6 56.4 32.6 25.4 31.5 26.1 13.1 Germany 18.6 21.6 14.5 5.6 32.0 19.2 26.3 19.5 12.9 6.0 Australia 0.0 7.7 14.3 31.3 47.6 48.4 32.6 24.6 22.4 16.1 France 26.7 10.5 19.0 24.0 38.7 25.6 27.8 30.4 8.9 6.1 Italy 0.0 66.7 40.0 38.1 27.6 13.5 28.6 27.8 30.4 10.0 India 14.3 12.5 11.1 50.0 40.0 52.4 31.3 11.9 21.3 10.5 Canada 5.3 10.0 9.1 8.3 26.9 12.1 8.1 12.5 17.8 13.2 United Kingdom 150.0 20.0 33.3 37.5 18.2 46.2 15.8 18.2 23.1 28.1 TABLE 11 | Top 10 authors in Solar Tower field. Author Affiliation Total publications Total cites h-index Wang, Zhifeng Hebei University of technology. China 37 649 15 Buck, Reiner German Aerospace Center (DLR). Germany 25 582 10 Ho, Clifford K Sandia National Laboratory, Department of energy. United States 16 29 3 Kribus, Abraham Tel Aviv University. Israel 14 192 6 Pitz-Paal, Robert German Aerospace Center (DLR). Germany 12 238 6 Santana, Domingo Universidad Carlos III de Madrid. Spain 11 87 5 Romero, Manuel IMDEA energy Institute. Spain 11 329 6 Roeb, Martin German Aerospace Center (DLR). Germany 10 149 6 Yang, Yongping North China electric power University. China 10 46 4 Arancibia-Bulnes, Camilo A Universidad Nacional Autonoma de Mexico. Mexico 10 40 4 TABLE 12 | Top 10 journals in solar tower field. Journal Total publications Total cites Perf. Ratio Impact factor Quartile scores Solarpaces conference (Proceedings) 137 333 2 N/A N/A Solar energy 118 2382 20 4.374 Q1 Journal of solar energy engineering 45 809 18 1.367 Q2 Renewable energy 37 831 22 4.9 Q1 Energy 30 414 14 4.968 Q1 Energy sustainability conference (Proceedings) 28 19 1 N/A N/A Applied energy 28 304 11 7.9 Q1 Renewable and sustainable energy reviews 23 1,215 53 9.184 Q1 Applied thermal engineering 20 379 19 3.771 Q1 Solar world congress (Proceedings) 18 26 1 N/A N/A Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 68259216 Calderón et al. Trends in CSP Research FIGURE 14 | (A) Total publication evolution in the Stirling dish field per year; (B) Total publications in Stirling dish per country; (C) Cumulative publication evolution per year of the top ten authors in the Stirling dish field; (D) Publication evolution per journal/source in the Stirling dish field by number of publications. TABLE 13 | Country/region increment of Stirling dish publications compared to those accumulated from the previous year. Country 2010 2011 2012 2013 2014 2015 (%) 2016 (%) 2017 (%) 2018 (%) 2019 (%) China 80.0% 33.3% 41.7% 29.4% 27.3% 17.9 27.3 28.6 18.5 12.5 United States 5.9% 38.9% 52.0% 10.5% 19.0% 10.0 12.7 11.3 5.8 5.5 India −−0.0% −50.0% 22.2 72.7 57.9 43.3 30.2 Iran − −−−40.0% 71.4 66.7 35.0 37.0 10.8 Italy 0.0% 0.0% −50.0% −57.1 9.1 75.0 9.5 30.4 Spain 0.0% 14.3% 50.0% 8.3% 30.8% 11.8 10.5 19.0 8.0 3.7 United Kingdom 100.0% 0.0% 0.0% 0.0% 25.0% 60.0 62.5 15.4 26.7 15.8 Australia 0.0% 66.7% 40.0% 42.9% 0.0% 10.0 9.1 16.7 7.1 26.7 Turkey 0.0% 0.0% 0.0% 66.7% 0.0% 0.0 60.0 62.5 23.1 18.8 France 0.0% 0.0% 0.0% 75.0% 14.3% 12.5 11.1 30.0 15.4 20.0 TABLE 14 | Top 10 authors in Stirling Dish field. Author Affiliation Dish publications Total dish cites Dish h-index Ahmadi, Mohammad Hossein University of Derby. United Kingdom 25 655 13 Pourfayaz, Fathollah University of Tehran. Iran 10 185 7 Laumert, Bjorn Royal Institute of technology. Sweden 8 31 3 Andraka, Charles E Sandia National Laboratory, Department of energy. United states 7 28 2 Bellos, Evangelos National technical University of Athens. Greece 7 27 3 Sayyaadi, Hoseyn K. N. Toosi University of technology. Iran 6 284 6 Ahmadi, Mohammad Ali Amirkabir University of technology. Iran 6 164 5 Tzivanidis, Christos National technical University of Athens. Greece 5 17 2 Bidi, Mokhtar Shahid Beheshti University. Iran 5 47 3 Wang, Fuqiang Harbin Institute of technology, China 4 136 4 Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 68259217 Calderón et al. Trends in CSP Research dish CSP configuration. Thesource publishing the most is the Energy Conversion and Management journal, since 2015. The second one again is Solar PACES proceedings followed by the Journal of Solar Energy Engineering. The top 10 authors in the Stirling dish field are listed in Table 14. Moreover, the total publication of each author as well as the citations and calculated h-index for the Stirling dish field are also included in this table. Table 15 lists the most relevant journals for the Stirling dish field. Moreover, this table includes the total publications per journal, the total citations of each journal, the performance ration, the impact factor, and the quartile of each journal. Fresnel Reflector InthecaseofFresnelreflector, the evolution per year since 1999 is presented in Figure 15A. The trend was clearly exponential until 2014 and a stagnation trend followed until 2017. This was mainly caused because the development and advantages of parabolic trough collectors. Nevertheless, in 2018, linear Fresnel gained interest for the process of steam generation. Figure 15B shows the total number of publications per country over the last 20 years. It has shown a growth by China since 2011 in concordance with China’s gross domestic product (GDP) growth. TABLE 15 | Top 10 journals in the Stirling Dish field. Journal Total publications Total cites Perf. Ratio Impact factor Quartile scores Energy conversion and management 25 648 26 6.377 Q1 Journal of solar energy engineering 16 336 21 1.367 Q2 Renewable energy 14 230 16 4.9 Q1 Solarpaces conference (Proceedings) 13 56 4 N/A N/A Solar energy 12 168 14 4.374 Q1 Energy 11 149 14 4.968 Q1 Renewable and sustainable energy reviews 11 438 40 9.184 Q1 Applied thermal engineering 10 160 16 3.771 Q1 Applied energy 7 193 28 7.9 Q1 Energy sustainability conference (Proceedings) 6 2 0 N/A N/A FIGURE 15 | (A) Total publication evolution in the Fresnel reflector field per year; (B) Total publications in the Fresnel reflector field per country; (C) Cumulative publication evolution per authors of the top ten authors in the Fresnel reflector field; (D) Publication evolution per year of journals publishing in the Fresnel reflector field. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 68259218 Calderón et al. Trends in CSP Research Figure 15C shows the publication evolution over the last 20 years regarding the top 10 authors in the Fresnel reflector field. The most published author is Ruben Abbas who has an outstanding publication trend, followed by Jose M. Martínez- Val. The h-index of each author is also listed in this figure. The journal publishing the most in the Fresnel reflector field is Solar Energy followed by Solar PACES proceding. Solar Energy journal has an outstanding trend for the last 10 years. Figure 15D shows the journal publication evolution over the last 2 decades considering the h-index of each journal. Increment of the publications in the Fresnel reflector field compared to the previous 10 years is presented in Table 16. The amount of publications are presented in percentages and the country publishing the most papers in 2018 was India, followed by Spain and the United Kingdom. Leaders like the EU, China, or the United States are growing at important rates. Other minor leaders such as Canada, India, Australia, and Saudi Arabia are growing at rates that can make them top leaders in the following years. The top ten authors in the Fresnel reflector field are listed in Table 17 as well as their affiliations. The most published authors are from the Polytechnic University of Madrid, Spain. In addition, the total publication, the total citations, and the h-index of each author calculated in the Fresnel reflector field is also listed in Table 17. Table 18 shows the most relevant journals for the Fresnel reflector field along with their Fresnel reflector performance index, showing that the Solar Energy journal is the leader with a good performance. Nevertheless, Renewable and Sustainable Energy Reviews has the highest performance for non-review journals showing higher quality on their publications in Fresnel reflector. In addition, the total publication, the total citation, the performance ration, the impact factor of the journal as well as the quartile score are listed in Table 18. CONCLUSION This CSP scientific research field bibliometric revision has studied almost 6,300 publications on relevant journals and conference proceedings. This is the first time a bibliometric study of CSP has been performed with current scope and detail. This study presents the publication evolution in the CSP field over the last 2 decades. Several metrics were used to analyze regional efforts, relevant authors, author communities, relevant journals, keywords evolution, and funding effort effects on the research output. Different parameters were used to measure the grade of cooperation between countries/authors and the relevance of authors/journals (h-index and impact factor). Some of these metrics were applied also within the main four CSP technologies: parabolic trough, solar tower, Stirling dish, and linear Fresnel. Regional analysis showed that China is becoming the most relevant country for CSP research. Nevertheless, the European Union still leads the development and will continue that way at least in the short term. Also, the United States have an important role in CSP development, as it has occurred in the past, in the TABLE 16 | Country/region increment of Fresnel publications compared to those accumulated from previous year. Country 2010 2011 2012 2013 2014 2015 (%) 2016 (%) 2017 (%) 2018 (%) 2019 (%) Spain 28.6% 33.3% 16.7% 57.1% 54.5% 26.5 25.6 29.6 20.0 9.5 India −−−83.3% 72.7% 47.4 32.1 13.5 28.6 16.7 China 50.0% −50.0% 66.7% 26.7% 31.6 16.0 41.4 19.5 14.3 Italy −−−33.3% −72.7 57.9 23.3 13.5 16.7 Germany −−−37.5% 90.9% 42.9 16.7 5.7 8.1 5.0 United States 0.0% 75.0% −13.3% 29.4% 13.6 20.0 10.0 18.2 5.1 France 100.0% 16.7% 0.0% 14.3% 75.0% 35.7 15.8 9.1 12.5 3.7 Australia 0.0% 0.0% 37.5% 27.3% 21.4% 11.8 15.8 0.0 13.6 4.0 Portugal −−0.0% −71.4% 16.7 42.9 15.0 8.7 0.0 United Kingdom −−−28.6% 22.2% 0.0 27.3 7.1 20.0 16.7 Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 68259219 Calderón et al. Trends in CSP Research foreseen future. Due to their energy needs it is expected that China and India will become the main boosters for CSP development for the years to come. The main interest in CSP research development is expected to continue more vigorously in regions with good solar resources. A research communities study allowed the identification of cooperation opportunities. Also, cooperation between regional and research communities is in line with known cooperation agreements between countries and institutions. Author analysis helped to identify individual CSP research stakeholders. However, each author should be considered individually, since several institutions add center managers into the author lists. This is actually not a negative issue, since usually these managers are important technology stakeholders, but it gets mixed with the technological and technical experts. Authors keyword evolution analysis has been useful as an indirect measure of technological maturity. China and India driven research communities can be expected to gain relevance during the following years due to the increase in scientific output in recent years. Main journals and conferences wereidentified and contrasted. It was found that conference proceedings have an important effect in the overall CSP publications. However, the journal publications used to have more impact since they were more frequently cited. High quality journals (mainlyQ1 and Q2) specializing in renewable energy, solar energy, and energy engineering and applications are expected to continue leading CSP research publications. Funding analysis had shown a direct relationship between funding and research output. This first approach could help to make deeper analysis in the future to measure the effectiveness by country or by funding agency. Inside the CSP field, parabolic trough has shown to have great relevance in the research output. This also has been reflected in the cost reductions achieved by this technology. Solar tower technology is under constant research evolution. Nevertheless, it needs to be optimized to compete with parabolic trough, as well to solve current technical difficulties.Newapproachesonthistechnology like the solid particle solar tower are currently under very active research activity. Finally, Stirling dish and linear Fresnel are still emerging within the CSP research field with an accelerated research output. These results are coherent with CSP scalability, since utility size power generation from CSP are the ones expected to grow. Parabolic trough and solar tower have shown viability in utility-scale power plants and its research is related to increasing its performance and TES and HTF materials for increasing service temperature and energy density. It can be expected that in the following decades CSP research development will boost green energy transition by making utilityscale power more competitive and viable, especially for the countries with more solar resources. AUTHOR CONTRIBUTIONS AC created the data base where all the information required from the papers in the literature. In addition, he developed the software and produced the first files. He led the writing of the original draft of this TABLE 17 | Top 10 authors in the Fresnel field. Author Affiliation Total publications Total cites h-index Abbas, Ruben Polytechnic University of Madrid. Spain 18 293 8 Martinez-Val, Jose M Polytechnic University of Madrid. Spain 13 257 8 Munoz-Anton, Javier Polytechnic University of Madrid. Spain 8 56 3 Desai, Nishith B Pandit Deendayal Petroleum University. India 7 104 5 Bandyopadhyay, Santanu Indian Institute of technology (IIT)–Bombay. India 7 107 5 Bellos, Evangelos National technical University of Athens. Greece 7 19 2 Kedare, Shireesh B Indian Institute of technology (IIT)-Bombay. India 6 82 5 Singh, Suneet Indian Institute of technology (IIT)-Bombay. India 6 76 5 Dai, Yanjun Shanghai Jiao Tong University. China 6 92 3 Reddy, K. S Indian Institute of technology (IIT)-Madras. India 6 50 3 TABLE 18 | Top 10 journals in the Fresnel field. Journal Total publications Total cites Perf. Ratio Impact factor Quartile scores Solar energy 42 929 22 4.374 Q1 Solarpaces conference (Proceedings) 18 96 5 N/A N/A Applied energy 16 351 22 7.9 Q1 Energy conversion and management 16 319 20 6.377 Q1 Renewable energy 14 117 8 4.9 Q1 Journal of solar energy engineering 11 103 9 1.367 Q2 Applied thermal engineering 10 245 25 3.771 Q1 Energy 10 187 19 4.968 Q1 Renewable and sustainable energy reviews 9 310 34 9.184 Q1 Solar world congress (Proceedings) 7 24 3 N/A N/A Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 68259220 Calderón et al. Trends in CSP Research paper. CB improved the information of the files and graphs this paper has and produce the information collectedinthispaperaswellashelp to write specificpartsofthepaperandtheconclusionssection.She revised the draft to improve it, and writing review and editing of the final manuscript. IF organizes the data to be included, supervised the paper, and did the project administration and funding acquisition. CP did the validation of the data collected in this paper from the industrial point of view and conceptualization of the information stated in this paper. MS organizes all the information required to produce the files. In addition, she supervised the work produced in this paper and did a data curation in order to improve the quality of this paper. FUNDING The research leading to these results is partially funded by the Spanish government RTI 2018–093849-B-C32, MCIU/AEI/FEDER, UE, and by the Agencia Estatal de Investigación (AEI) of the Ministerio de Ciencia, Innovación y Universidades (RED 2018–102431-T). ACKNOWLEDGMENTS The authors would like to thank the Catalan Government for the quality accreditation given to their research groups DIOPMA (2017 SGR118), and Consejo Nacional de Ciencia y Tecnología CONACYT from México for the doctoral grant 546692. SUPPLEMENTARY MATERIAL The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmech.2021.682592/ full#supplementary-material REFERENCES Ahmadi, M. H., Ghazvini, M., Sadeghzadeh, M., Alhuyi Nazari, M., Kumar, R., Naeimi, A., et al. (2018). Solar Power Technology for Electricity Generation: A Critical Review. ENERGY Sci. Eng. 6, 340–361. doi:10.1002/ese3.239 Ahn, Y., Bae, S. J., Kim, M., Cho, S. K., Baik, S., Lee, J. 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This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Frontiers in Mechanical Engineering | www.frontiersin.org June 2021 | Volume 7 | Article 68259222 Calderón et al. Trends in CSP Research