# A Solar-Thermal-Assisted Adiabatic Compressed Air Energy Storage System and Its Efficiency Analysis
## 1 Introduction  
Modern cities are facing environmental and energy shortage issues, and the large-scale development and utilization of renewable energy sources (RES) in smart cities is a primary strategy for addressing those issues [1]. However, much wind and solar energy is curtailed due to a lack of reasonable planning and compatibility with conventional energy [2; 3; 4]. Notably, in 2017, the average curtailment ratios of wind and solar power in Gansu Province, China reached 25% and 20%, respectively [5]. Moreover, RES will enlarge the peak to valley difference of power demands. These facts have posed great challenges to the smart grid and caused crises in secure operation [6]. Therefore, strategies to ensure the large-scale utilization of RES in a safe, efficient, and economical way has become a crucial issue in the construction of today's smart grid and city. Energy storage has been recognized as an effective means of RES integration and peak-shaving [7]. Currently, compressed air energy storage (CAES) and pumped storage systems (PHS) are the major large-scale energy storage technologies [8; 9]. Compared with PHS, CAES has broad application prospects because the geographic conditions or water resources hardly constrain it. Nevertheless,commercial CAES plants, including Huntorf (E.N Kraftwerke, Huntorf, Germany, 1978) and McIntosh (Alabama, America, 1991), strongly depend on natural gas to enhance electricity storage efficiency [10; 11], which is a restriction in areas without abundant natural gas resources. Integrated with thermal energy storage (TES), adiabatic CAES (A-CAES) is the main trend of CAES that is independent of fossil fuels. Several A-CAES projects have been announced, are under construction, or are in operation [12]. Germany has planned to build a large-scale A-CAES power plant with a target cycle power of 70% [13]. China's first 500 kW A-CAES demonstration system successfully realized energy storage and power generation in 2014 [14; 15]. In addition, a national A-CAES demonstration power station led by Tsinghua University, with rated power of 60 MW and rated capacity of 300 MWh, is under construction in China [16]. However, constrained by the efficiency and structure of compressors and the effectiveness of multi-stage heat exchangers, the heat medium of TES that depend on compression heat cannot reach a high temperature [17]. Consequently, it is difficult to enlarge electricity production in the discharge process, which may not balance the load demand and confine system efficiency. It is an effective measure to replace the TES of A-CAES with solar thermal collection and storage (STC) of concentrating solar power to overcome deficient heat capacity. Hybrid A-CAES (HA-CAES) notably enhances the storage heat temperature and capacity and can also improve the comprehensive utilization level of clean energy [18; 19]. Therefore, researchers have recently begun to focus on essential technologies of HA-CAES such as process design, efficiency analysis, and key parameter optimization to improve the system efficiency. A gas turbine combined solar energy and CAES focusing on process design and efficiency analysis has been proposed, and characteristics of its off-design were studied in [20]. To cope with the issue of wind and solar curtailment, a novel scheme of wind-energy complementary A-CAES was presented and its thermodynamic characteristics were analyzed in [21]. A HA-CAES design scheme to optimize cycle efficiency and efficiency for microturbines, solar dish collectors, and A-CAES was described in [22; 23]. Modeling and analysis of combined cooling heating and power (CCHP) integrated solar thermal energy for its cogeneration characteristics were established in [24]. Because of the multi-carrier poly-generation capability, an HA-CAES-based multi-carrier clean energy hub is proposed and modeled in [25] to realize the comprehensive consumption of the renewable energy. A HA-CAES is utilized as an energy hub in a solar-based smart micro-energy grid in [26].