# Increasing Compressed Gas Energy Storage Density Using CO\({}_{2}\)-N\({}_{2}\) Gas Mixture
## 1 Introduction  
Energy storage is a central piece in the efforts to modernize the electric grid, reduce cost of electricity to both utilities and end users, and to enable higher penetration of renewable energy technologies. Various technologies have been invented for energy storage with different degrees of maturity. An extensive review of the available energy storage technologies, their cost, performance and maturity is presented in [1]. Large-scale electric energy storage is dominated by Pumped Storage Hydropower (PSH) with a share of approximately 99.5% of the total installed capacity worldwide on a GW basis [2]. Large-scale non-PSH electric energy storage is dominated by Compressed Air (CAES) followed closely by Lithium-ion batteries as shown in Figure 1.  
PSH is a mature technology with a relatively high round-trip efficiency [3]. However, it is only economical at a scale of 100 MWe and higher [4] and it has poor expansion potential in the US since most suitable sites have already been exploited. The geographical and scale limitations of the PSH makes it unsuitable for behind the meter storage in buildings. Compressed air energy storage (CAES) is another large-scale storage that uses turbomachinery to store and retrieve energy from high-pressure reservoirs. Conventionally, CAES uses natural caerns as storage vessels. Conventional CAES is, similar to PSH, geographically limited by the availability of naturally occurring caerns [5]. Above-ground CAES, that uses pressure vessels rather than caerns, have been proposed and smaller scale systems built [4, 6]. The round trip efficiency of CAES systems is in the 40-50% range [4, 7]. More advanced configurations of CAES have been investigated and were claimed to have significantly higher round trip efficiencies [8]. Above-ground CAES is amenable to installations in buildings behind the meter. However, their relatively low round trip efficiency hurts their economic viability. Liquefied Air Energy Storage (LAES), similar to CAES, stores electric energy in the form of high pressure, low-temperatureliquefied air. Different possible configurations of LAES systems were analyzed in [9]. The study proposed hybridizing the LAES with another energy storage technology of faster response time, such as a Lithium-ion battery, to mitigate the slow intrinsic response of LAES. Round trip efficiency in the range of 50% to 62% has been reported in the literature for LAES [10; 11]. Batteries are the most ubiquitous electricity storage device due to their excellent modularity and scalability from fractional- to Mega- watt scale. Batteries have high energy density (ED) owing to the high energy levels that can be stored in chemical bonds. They have been typically manufactured with capacity in the range of few kilowatt-hours. However, recently, Megawatt-hour scale battery systems have started to emerge as an option for grid-scale energy storage. Despite their ubiquity and high energy density, battery systems still face serious challenges to their adoption for large-scale electricity storage. Batteries have limited life and deep cycling them accelerates their deterioration [4]. They require replacement every few years and they are made of hazardous chemicals. The process to dispose of them safely is environmentally and financially costly. A great deal of research and development effort has been invested in reducing the cost, increasing the lifetime, and improving management systems of grid-scale battery systems [4; 12; 13]. Lithium-ion batteries are so far the most mature and readily available for large-scale storage. Current research is focused on better chemistries [14; 15; 16] for these batteries. The Ground-Level Integrated Diverse Energy Storage (GLIDES) is a recently invented modular technology that utilizes gas compression for electric energy storage [17]. Its scalability makes it suitable for behind the meter storage in buildings as well as for grid-scale storage. GLIDES relies on hydraulic compression of gas inside a vessel to store energy.