## 1 Introduction  
The transformation of the electrical energy system from conventional to renewable energy sources enforces significant extensions of grid and storage capacities due to the fluctuating nature of wind and solar power. Relevant contributions for reaching this goal and for securing of energy supply are offered by large-scale storages that allow grid-compatible extraction and supply of electrical energy [1]. Today, the electrical energy storage systems (EES) used, with high powers of up to 1.5 GW, several hours of storage capacity, and high round trip efficiencies, are pumped storage power plants (PSPP). However, further extensions of this technology are limited because of geological restrictions and high capital costs of up 600-2000 $/kW [2; 3]. As an alternative commercial technology, compressed air energy storage systems (CAES) offer the potential for high storage capacities and powers with lower overground geological impacts [4; 5; 6; 7]. The first power plant was built at Huntorf in Germany in 1978 [8], followed, in 1991, by a power plant in McIntosh, USA [9]. Here, in times of low grid load, electrical energy is used to compress and store air in underground cavers. During times of required peak loads, the cold pressurized air is expanded in a modified gas turbine and heated via natural gas combustion. Due to the gas firing, lower round trip efficiencies are achieved with CAES systems compared with PSPP. Aside from these concepts, alternative large-scale technologies with high storage capacities have been investigated in several research and development activities [10; 11]. Promising configurations are based on flow batteries [12; 13], hydrogen [14], pumped heat energy storage (PHES) [15], and modifications of the CAES system towards an adiabatic operation (A-CAES) offering increased round trip efficiencies [16; 17; 18]. Considering the technology-dependent scientific and economic challenges of these mentioned concepts, relating to lifetime, efficiency, electrical power, and investment costs, only two EES technologies show marketability and potential in short-term realization: the A-CAES and PHES systems. Both require thermal energy storage systems (TES) as central elements to increase total round trip efficiency and to allow a local zero-emission operation. Heat - while compression - is transferred to the storage medium and reintegrated into the gaseous working fluid during the discharging period to supply the gas turbine with the required thermal energy and to avoid natural gas combustion. With the integration of TES systems into the cyclic operation procedure, additional parasitics, in terms of pressure losses, are involved. In particular, PHES systems are highly affected by pressure losses leading to significant decreasing total round trip efficiencies [13; 15; 19; 20; 21; 22; 23]. Therefore, extensive design calculations embedded in overall thermodynamic system simulations are needed in order to identify efficient configurations with low parasitics. In contrast, A-CAES systems allow robust configurations due to their lower sensitivity to pressure losses, allowing increased design freedoms in TES arrangements. In principle, all heat storage technologies offering minimal exergy losses during heat transfer have been considered [24]. However, because of the gaseous heat transfer medium and the elevated temperatures inside the cyclic operation, storage media based on solid and liquid inventory materials meet the requirements of thermal efficiency and costs particularly well. Alternative TES concepts based on phase change materials (PCM) were investigated in a study [25] to reduce the amount of entropy generated from the heat exchange process, leading to an improved overall efficiency of the system. In order to increase the performance of CAES configurations, different system extensions have been investigated to utilize the waste heat carried in turbine exhaust during discharge [26] or to allow cooling, heating, and electrical generation [27].