Recent Trends on Liquid Air Energy Storage: A Bibliometric Analysis

: The increasing penetration of renewable energy has led electrical energy storage systems to have a key role in balancing and increasing the e ﬃ ciency of the grid. Liquid air energy storage (LAES) is a promising technology, mainly proposed for large scale applications, which uses cryogen (liquid air) as energy vector. Compared to other similar large-scale technologies such as compressed air energy storage or pumped hydroelectric energy storage, the use of liquid air as a storage medium allows a high energy density to be reached and overcomes the problem related to geological constraints. Furthermore, when integrated with high-grade waste cold / waste heat resources such as the liqueﬁed natural gas regasiﬁcation process and hot combustion gases discharged to the atmosphere, LAES has the capacity to signiﬁcantly increase the round-trip e ﬃ ciency. Although the ﬁrst document in the literature on the topic of LAES appeared in 1974, this technology has gained the attention of many researchers around the world only in recent years, leading to a rapid increase in a scientiﬁc production and the realization of two system prototype located in the United Kingdom (UK). This study aims to report the current status of the scientiﬁc progress through a bibliometric analysis, deﬁning the hotspots and research trends of LAES technology. The results can be used by researchers and manufacturers involved in this entering technology to understand the state of art, the trend of scientiﬁc production, the current networks of worldwide institutions, and the authors connected through the LAES. Our conclusions report useful advice for the future research, highlighting the research trend and the current gaps.


Introduction
Energy in whatever form is an essential source that guarantees a high-quality standard of life in modern society. Due to the concomitant effect of population growth and the rapid development of emerging countries, energy demand is dramatically increasing year by year. 2018 witnessed a remarkable trend for energy demand, increasing at almost twice the average rate for the decade since 2010, with a remarkable fossil fuel share of 81% [1]. Unfortunately, fossil fuel combustion has long been recognized as the main cause for some serious environmental issues including the greenhouse effect, ozone layer depletion and acid rain [2] as well as social costs linked with combustion emissions [3]. assist the liquefaction process during the charge phase. The high pressure air gas will then be further heated up at the superheaters (SHs) by means of the heat coming from a so called high grade warm storage (HGWS), which stores the heat of the compression released during the charge phase. The high pressure and high temperature gaseous air is then re-heated in a four stage expansion process to achieve a quasi-isothermal expansion. The characteristics and advantages related to the LAES has attracted the attention of many researchers, which has led to a rapid increase in scientific production focused on this technology. However, the potential of LAES has not fully demonstrated yet. This shows the possibilities of new challenges that can bring this technology to the next level. However, advancing in the current technological level is not straightforward, requiring significant investments in this technology and a strong cooperation between both academia and industry. To evaluate the research trend and the future perspectives on LAES, bibliometric analysis techniques represent a statistical method that allows the growth and the scientific progress of a certain topic to be evaluated, giving an overall view of today's research. Furthermore, bibliometric analysis enhances science as a knowledge-generating system [13]. The simplicity and the effectiveness allow for bibliometric analysis to be applied to different scientific areas. Mao et al. [14] analyzed the trend on biomass energy from 1998 to 2013 through a bibliometric technique. The analysis was based on the Science Citation Index (SCI) and the Web of Science database. Results showed how the research in biomass has increased in the past 16 years with an actual strong focus on bioenergy applications. Calderon et al. [15] used bibliometric analysis to study the evolution of the research in the field of thermal energy storage (TES). The study based on the Web of Science core collection database included a survey of different TES system categories. Bartolini et al. [16] integrated a bibliometric analysis into a systematic review of green warehousing to evaluate the current state of research. The study analyzed the most common themes investigated, providing in the conclusions some research recommendations that considered the lack in the topic and a possible interesting study that could be conducted in the field of green warehousing. Du et al. [17] analyzed the literature outputs related to solar energy from 1992 to 2011 through a bibliometric analysis based on social sciences and the citation index. The study identified the main trend in the solar energy research field such as hydrogen production, desalination, drying, heat pumps, biomass, and air conditioning, and provided recommendations for future research that included nanoscience and molecular biology. Therefore, to define the prospects and the new challenges in storing the energy through the use of liquid air (with the LAES technology), this paper analyzed by means of bibliometric analysis techniques, the trend of the scientific research involved in the study of LAES. The results of the bibliometric analysis proposed in this study can be useful to inspire researchers and manufacturers interested in LAES technology in helping to understand the state-of-the-art in the literature, highlighting the hotspots linked to LAES of today and future research.

Methodology
In the bibliometric analysis studied in this paper, the database Scopus was used as a reference, since it includes a large number of works referring to technological topics compared to other databases such as Web of Science [18]. Furthermore, Google Scholar was not taken into account in the literature search due to the poor accuracy of results, as reported by Falagas et al. [19]. Figure 2 shows a schematic of the methodology followed for the bibliometric analysis. The formulation of the query was based on key terms frequently used in the keywords and abstract contained in the paper related to LAES. It is worth mentioning that although some authors in the literature have often referred to LAES technology using the term cryogenic energy storage (CES) [20], terms such as "cryogenic" were not included in the query since it led to papers related to cryogeny (or low temperature applications), but not related to LAES. The literature search presented in this work was based on the following query: ("liquid air energy storage" OR ("energy storage" AND ("liquid air")) AND NOT ("graphene hydrogel*" OR "supercapacitor*" OR "polymer*" OR "biosensor*" OR "hot working" OR "laser power system*" OR "LNGES")) with the last access on November 2019. The results were elaborated to obtain the trend and number of works distributed by type of journal, subject, countries, institutions, and authors. An analysis of the keywords was carried out to identify the hot topics and the main thematic related to LAES to help to define the research trend and future prospective. To identify the link between authors, countries, institutions, and keywords, results were graphically elaborated by means of the software VOSviewer [21].

Results and Discussion
This section reports the results of the bibliometric analysis conducted with the methodology described above. The database Scopus, with the query selected, gave a total of 387 documents. An analysis of this first output resulted in the inclusion of papers that were not related to the topic of LAES. Restricting the query research only to title, abstract, and keywords, (TITLE-ABS-KEY ("query")), the database Scopus gave a total number of 132 documents. As mentioned in the methodology, the possibility that some documents were not caught with the query selected was due to the absence of key terms in the keywords and the abstract that mentioned the use of liquid air as energy storage. This highlights the importance of the use of proper terminology in the abstract and a proper selection of keywords. Analyzing the output, the documents given were strictly related to the LAES topic, confirming a correct formulation of the query. The results of the bibliometric analysis shown in this section were obtained elaborating and analyzing the data of all 132 papers collected from the Scopus database. Furthermore, some of the works reported in the reference section were selected and cited to support the discussion of results and exemplify the analysis of the keywords used to describe the state-of-art and the hotspots in the topic of LAES. Figure 3 shows the trend of the publications in the Scopus database year by year. From the figure, it is possible to notice that the first publication that included the first concept of LAES appeared in 1974, authored by Sheperd [22], with one more publication in 1977 by Smith [23], and one in 1978 by de Marchi Desenzani [24]. After one additional paper was published in 1982 by Dooley et al. [25], no documents regarding liquid air as energy storage were published until the year 1998 in Japan by Mitsubishi-Hitachi [26][27][28][29][30][31], with no further developments after 2002.  [32]. The first experimental results of the pilot plant, located now at the University of Birmingham, were reported in a work published by Morgan et al. [33] in 2015 that, according to the Scopus database, has the highest number of citations amongst the publications related to LAES.  Figure 4 shows the number of publications contained in the Scopus database, divided by type of document. The figure clearly shows that the highest number of publications were categorized as articles and conference papers (75 and 48 documents, respectively). Reviews, book chapters, conference reviews, and notes covered only a small portion of the total documents published.

Document Type of Publications
The results of the bibliometric analysis with the query selected gave a total number of 38 different journals that contained publications related to LAES. Figure 5 shows the number of publications of journals and proceedings that contained at least two documents. Applied Energy was the journal that contained more publications related to LAES (21 publications), followed by Energy Procedia (18 publications). Other journals with a relevant number of publications were Energy (10 publications) and Applied Thermal Engineering (six publications). The four studies published in the Transaction of the Japan Society of Mechanical Engineering included the one carried out between the years 1998 and 2002 by Mitsubishi-Hitachi.   Table 1 shows the main characteristics of the journals and proceedings that contained at least two articles. Most articles were published in high-quality journals (Q1) without open access. Transactions of the Japan Society of Mechanical Engineers had the lowest category (Q4). Applied Energy, which contained the most publications, had the highest impact factor amongst the other journals shown in Table 1. It is worth noticing that a high number of documents on LAES have been published in conference proceedings, highlighting the fact that a portion of the material available can be checked out in open access form. However, open access journals excluded from the proceedings (Energies and Entropy) contained only a few publications. Figure 6 shows the distribution of the publications by subject area based on the output given by Scopus. The subject was mainly affected by the type of journal in which each document was published. Engineering and energy represented the main subjects covering 33% and 29%, respectively, of the total output. Another large portion relied on environmental science, which covered 15%, followed by physics and astronomy (8%). Only a smaller portion (3%) included computer science, material science, and chemical engineering.

Distribution of Publications by Institution
As above-mentioned, the first studies regarding LAES appeared between 1974 and 2002. Until 2010, when the trend of publication started to grow, only a few institutions worked on the topic of LAES. It is worth commenting that it was not possible to find the institution of some of the authors involved in the earliest publications (from the year 1974 to 1982). Table 2 shows the top 10 institutions with more documents published on LAES. From the table, it is possible to note that most of the institutions included in the top 10 were universities, but the presence of Hitachi Ltd. must be pointed out as they authored all publications from 1998 to 2002. It is worth mentioning that Yonsei University and Cornell University are not reported in Table 2. Indeed, these institutions, active in the topic of LAES [34,35], often use the terminology "cryogenic energy storage" to name the same technology, which, as mentioned in the methodology, was not included in the query selected.    Figure 8 shows a world map highlighting the countries that published more studies on LAES in the period related to 1974-2020 (updated to November 2019) using the Scopus database. The results showed that only a few countries are involved in the topic selected. The United Kingdom had the strongest output of publications, highlighted by the darker color in the map. Indeed, in recent years, the United Kingdom has been identified as the core of the LAES technology, and hosts the first 350 kW/2.5 MWh pilot plant in the world, developed first by Highview Power Storage at Slough and now relocated to the University of Birmingham [32,33]. Furthermore, another grid-scale 5 MW/15 MWh plant was launched in 2018 in Bury (Manchester) by Highview Power and Viridor [36]. The second country with the highest number of publications highlighted by the map in Figure 8 was China, followed by Italy and Germany.    Figure 10 shows the results obtained by elaborating on the data from Scopus with the software VOSviewer. In particular, the figure shows the links between the different countries and their relation in terms of co-authorship on the work published. This figure shows that there were a lot of countries that were not linked to each other. In the center of the figure, two main clusters can be noted. The first cluster (in red) includes the United Kingdom and China, who were connected with a strong relationship marked with a thick line. The second cluster (in green) included the main countries of Italy and Singapore that, also in this case, they were strongly linked together. It is interesting to note that these two clusters were linked together, connecting the United Kingdom with both Italy and Singapore. A third smaller cluster can be detected at the bottom of the figure in blue, linking Germany, Greece, and Poland.

Authorship Analysis
The top 10 authors who have published on the topic of LAES, according to the Scopus database, are reported in Table 3. In this case, some of the authors were present in the database with two different names, therefore in order to have a fair comparison, they were identified and, if they corresponded to the same author, grouped together. In this case, the real h-index reported in Scopus and the number of documents cannot correspond to the real one due to the different names. Table 3 shows that authors belonging to institutions from the UK published the majority of the documents present in the database. Yulong Ding leads the ranking with 16 documents, followed by Yongliang Li with 13 documents. These two authors belong to the same institution, the University of Birmingham, and have a total number of 351 and 76 documents and an h-index of 54 and 18, respectively. The third ranking author was Alessandro Romagnoli from Nanyang Technological University of Singapore (and Energy Research institute @NTU) with 12 publications. It is also possible to note that for some of the authors (Xiaodong Peng, Alessio Tafone, Emiliano Borri, and K. Chino), the total number of publications almost corresponded to the number of publications on the topic of LAES, indicating that this is their main research field. Figure 11 shows the links between the different authors with a minimum of two publications, obtained by elaborating the data with the help of the software VOSviewer. In this case, it was possible to identify two main clusters. In the first cluster (in red), there were authors linked to Yulong Ding, where the strongest links were with Yongliang Li, Xiaohui She, and Xiaodong Peng from the University of Birmingham [37][38][39]. In the second cluster (in purple), Alessandro Romagnoli was strongly related to Gabriele Comodi and Emiliano Borri from Università Politecnica delle Marche, and Alessio Tafone [40,41] from Nanyang Technological University, which highlights the strong link between Italy and Singapore. The other cluster included Chino K. and Araki H. from to Hitachi Ltd. [29] (in pink); Giglioli R., Desideri U., Paganucci F., Barsali S., Pasini G., and Antonelli, M. [42][43][44] from the University of Pisa (in yellow); and Evans J., Damas A., Brown T., Negro D., Varga L. and Hu Y. Fikiin K. from different institutions that belong to the CryoHub project [45].  Figure 11. Network of the different authors elaborated with VOSviewer.

Analysis of Keywords
The analysis of keywords was an important aspect of this research to identify the state-of-the-art, scope, and the objectives of the research that had been done and future prospective; moreover, research gaps could be identified. Table 4 shows the keywords found in the Scopus database from all of the documents considered for the present bibliometric analysis. In this case, the keywords reported in the table were divided into four sections. From the keywords, it was possible to observe that between the keywords related to energy storage, it appeared as compressed air energy storage (CAES). Indeed, this technology is often used to compare the LAES [46,47]. From the keywords related to the type of study, it was possible to note that most of the approaches were mainly theoretical such as "thermodynamics", "exergy analysis", and "economic analysis". With regard to energy systems and components, it was possible to find some keywords related to thermal energy storage such as "packed bed" and "storage (materials)".  Figure 12 shows the co-occurrence map of the keywords obtained by elaborating the output from the database Scopus with VOSviewer. The different density of colors and size of the character in the map highlights the frequency that the keyword appears in the different publications. The figure shows that the keyword "liquid air energy storage" had less relevance than the word "energy storage" and "liquefied gases". This can probably be attributed to the presence of the keyword "cryogenic energy storage", which is sometimes used to represent the same technology. Other frequent keywords that can be identified from the map shown in Figure 12, are "renewable energy", "liquid air", "compressed air", "energy efficiency", "exergy", "compressed air energy storage", "thermodynamics", and "liquefaction". The link between the different keywords is shown in Figure 13. In particular, Figure 13 shows that the keywords are divided into different clusters, highlighted by the different colors. In particular, the keywords "liquefied gases", "liquid air energy storage", and "cryogenic energy storage" were all included in the same cluster with a strong relation. Highlighting the keyword "liquid air energy storage" (Figure 14), it is possible to note that this keyword was strongly related to the keywords "energy storage", "cryogenic energy storage", "liquefied gases", "compressed air", "compressed air energy storage", and "energy efficiency". It is also interesting to note that "liquid air energy storage" had a strong relation with "exergy" and "thermodynamics". Indeed, thermodynamics analysis and exergy analysis represent the most common approaches in the study of LAES systems, and terms such as thermodynamic efficiency and exergetic efficiency are often used as the main index of comparison with other energy storage technologies. Furthermore, increasing the thermodynamic efficiency of the LAES is an important target of the technology feasibility, since it represents the main disadvantage when compared to other technologies. Indeed, roundtrip efficiency for large scale systems can be estimated to be around 50% [36], which is lower than other large scale technologies such as compressed air energy storage and pumped electric energy storage, whose roundtrip efficiency can be estimated around 70%.
Analyzing the keyword "energy storage", as shown in Figure 15, it is interesting to see the keywords "alternative energy", "liquefaction", "natural gas", "liquefied natural gas", and "electric power generation" in the same cluster. The link between these keywords can be explained since the LAES is often classified as an alternative storage solution to the actual large scale energy storage technologies. Furthermore, since cryogenics is involved as a storage energy vector, LAES is characterized by the advantage that it can be integrated with waste cold energy resources such as the regasification of liquefied natural gas [8,37,48]. Other connections, external to the energy storage cluster, were with "electric energy storage", "liquefaction", "renewable energy", and "thermodynamics".   Figure 16 shows the overlay visualization of the different keywords contained in the documents. This is an interesting approach to describe the frequency of the keywords in the different years (from 2015 to 2018) for the query used. From the figure, it is possible to see that during 2015/2016, the most popular keywords were "energy storage", "liquefaction", "electric energy storage", "electric power generation", and "gas turbine". The keywords "thermodynamics", "compressed air energy storage", "waste heat", and "economic analysis" were popular in documents published around 2017.
These keywords highlight that most of the work published in that period investigated the LAES mainly through thermodynamic analysis [49,50], with some studies that considered the economic aspect [51]. Recent publications on LAES contained keywords such as "exergy analysis", "packed bed", "cold storage", "exergy efficiency", and "organic Rankine cycle". The first keyword points out that recent studies have focused on a different thermodynamic approach based on second law analysis with the aim to evaluate the exergy efficiency of the system [52,53]. It is worth noting that the words "organic Rankine cycle", "packed bed", and "cold storage" are all concepts related to the use of thermal energy. Indeed, all the studies affirm that the efficiency of the LAES can be increased by recovering the thermal energy wasted by the system [39,54]. Furthermore, the thermal energy released is both at high temperature (energy wasted during the compression when LAES is charging) and cryogenic temperature (energy wasted the liquid air regasification process when LAES is discharging). In some LAES systems proposed in the literature, the waste heat at high temperature is recovered and used in the same system [49]. The keyword "organic Rankine cycle", linked to works published after 2018, highlights an interesting trend in the research of LAES with studies that aim for the integration of the two systems as a waste heat recovery solution [41,55]. On the other hand, the recovery of the cold energy by LAES is crucial to achieve a reasonable value of efficiency [12]. Indeed, this is mainly reused to help the liquefaction of the liquid air when the system is charged, thus increasing the efficiency. The presence of the keywords "packed bed", and "cold storage", linked to studies after 2018, highlights an interesting trend to investigate efficient cold storage solutions (including packed bed configuration) integrated with the LAES technology [56,57].

Conclusions
In recent years, LAES has gained momentum as an alternative energy storage technology characterized by high energy density and no geological constraints with the ability to be integrated with high-grade waste cold or waste heat resources. This has led to a rapid increase of scientific production on the topic of LAES, with a growing number of worldwide institutions investigating the potential of this technology. Although the first publication on LAES appeared on 1974, the trend on scientific literature started to rapidly increase after a project started in 2008 with the realization of the first LAES pilot plant located at the University of Birmingham in the United Kingdom. Nevertheless, this was the institution with the highest number of works published on LAES. The highest number of publications on this topic was contained in Q1 scientific journals and conference proceedings in the category of "engineering" and "energy". Applying bibliometric techniques in the analysis of the keywords, the most relevant aspects of the current research were analyzed to identify research gaps and future trends that could be used to inspire future research.
The main conclusions that can be drawn regarding the research on LAES, supported by the bibliometric analysis carried out in this review, can be summarized as follows: • Most of the approaches in the study of LAES have been based on thermodynamic and exergy analysis of the system with the aim to estimate the efficiency of the LAES and propose solutions to increase the system performance, which is currently the major drawback of this technology.

•
The keyword analysis also showed that part of the research on the topic of LAES was interested in investigating the economic feasibility of the system. However, the relevance of keywords related to the economic aspect was lower compared to terms related to the thermodynamic analysis, identifying the first research gap.

•
The last trend in the research of LAES aims for the integration of this technology with other external systems that allow for the exploitation of waste thermal energy. The most common system integrated to the LAES as a waste heat recovery solution is the organic Rankine cycle (ORC), while at low temperatures, different studies in the literature have used the LAES technology to exploit the waste cold thermal energy from cryogenic processes such as in liquefied natural gas regasification plants. Studies on how to efficiently store the cold and the hot energy wasted from the LAES represent another trend in the last research. Indeed, recovering cold energy is demonstrated to be crucial to increase the efficiency of the liquefaction process. On the other hand, the recovery of the waste heat can improve the discharge performance. Therefore, the research of new storage materials and thermal energy storage designed to efficiently store the cold and hot energy, represents a hotspot that future research should take into account to improve the LAES technology.

•
The analysis of the keywords lacked terms related to experimental studies that demonstrate the real performance of the system. This shows that this aspect is weak and has not been deeply investigated. This represents the main gap that future scientific research should fill in to reach a significant milestone in the LAES technology.