# Recent Trends on Liquid Air Energy Storage: A Bibliometric Analysis
## 1 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]. Moreover, despite the commitment of many countries to reach an early peak in emissions related to fossil fuel consumption, in 2018, the energy-related carbon dioxide (CO\({}_{2}\)) emissions reached the highest annual increase since 2013 (+1.9%) [1]. Nowadays, a key role in solving the environmental challenges posed by traditional fossil fuel depletion is being played by renewable energy sources (RESs), namely energy from sources that are naturally replenishing, but limited in the amount of energy that is available per unit of time. As a consequence, the use of renewable energy systems, namely technologies harnessing the energy from RESs, increased significantly during the early 2000s due to the mix of supporting policies and rapidly falling costs (\(-\)70% for solar PV and \(-\)25% for wind), and according to the Stated Policies Scenario [1], in 2018, the share of renewables in global electricity generation achieved nearly 26%, with a 2040 projection up to 44%. The rapid development and penetration of renewable energy sources in electricity grids influence the whole system's reliability and stability. Unlike most conventional power plants, renewable power ones are generally smaller in size and are not capable of supplying demand at any time due to renewable energy source intermittency. In fact, highly dependent on weather conditions, most of the renewable energy sources (wind and solar primarily) cannot be dispatched: if not stored, they must be utilized as soon as they are generated. As a consequence, their integration into the existing grid and in stand-alone mode represents a serious challenge for grid balance to meet the energy supply and demand through the chain of generation, transmission, distribution, and end-use. Amongst all the viable solutions to deal with this issue, electrical energy storage (EES) is recognized as one of the most promising technologies. EES technology refers to the process of converting energy from one form (mainly electrical energy) to a storable form and reserving it in various mediums; then the stored energy can be converted back into electrical energy when needed. Indeed, future electricity grids are planned to use large proportions of intermittent energy sources. Therefore, an increasing scale-up of energy storage is necessary to match the supply with the electricity demand profiles. Reflecting this, the International Energy Agency [4] has projected that 310 GW of additional grid-connected electricity storage capacity will be necessary in the United States, Europe, China, and India. EES technologies are available for different applications at different scales [5] with the predominance of batteries, pumped hydroelectricity storage (PHS), and compressed air energy storage (CAES). Among the large scale EES technologies, liquid air energy storage (LAES) has attracted significant attention in recent years due to several advantages. Indeed, LAES is a promising and novel long term cryogenic energy storage technology, suitable from mid to large scale applications. Compared to other energy storage systems, LAES guarantees higher volumetric energy density (214 Wh/kg) and no geographical constrains [6]. The system relies on well-established technologies that limit possible development risks and ensure long life to the system (30-40 years) [7].