# Recent Trends on Liquid Air Energy Storage: A Bibliometric Analysis
## 3 Results and Discussion
### 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. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Related to Energy Storage**} & \multirow{2}{*}{**Related to the Type of Study**} & **Related to the Energy Systems and Components** & \multirow{2}{*}{**Others**} \\ \cline{3-3}  & & & **Components** & \\ \hline Cryogenic energy storage & Thermodynamics & Rankine cycle & Liquefied gases \\ \hline Energy storage & Exergy analysis & Storage (materials) & Waste heat \\ \hline Electric energy storage & Economic analysis & Pressure vessels & Combustion \\ \hline Liquid air energy storage & Optimization & Packed bed & Liquefied natural gas \\ \hline Compressed air energy storage & Cost benefit analysis & Gas turbine & — \\ \hline — & Environmental impact & Heat exchangers & — \\ \hline — & Sustainable development & Cooling systems & — \\ \hline — & Sensitivity analysis & Pilot plants & — \\ \hline — & — & Organic Rankine cycle & — \\ \hline \hline \end{tabular}
\end{table} 
Table 4: List of the main keywords divided in different categories from the Scopus database. Figure 12: Co-occurrence of the keywords elaborated with VOSviewer. 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.