# Recent Trends on Liquid Air Energy Storage: A Bibliometric Analysis
## 3 Results and Discussion
### Analysis of Keywords
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 13: Co-occurrence and link of the keywords elaborated with VOSviewer. 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. Figure 14: Co-occurrence and link of the keyword “liquid air energy storage” elaborated with VOSviewer. Figure 15: Co-occurrence and link of the keyword “energy storage” elaborated with VOSviewer. 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.