# A Solar-Thermal-Assisted Adiabatic Compressed Air Energy Storage System and Its Efficiency Analysis
## 4 Results and Discussion  
### Typical Operational Conditions  
In this section, the performance of ST-CAES under typical operational conditions is analyzed. Table 3 lists the design parameters of the system. In this case, the durations of charging and discharging were the same (4 h), and the mass flow rate of the air was fixed at 0.56 kg/s. The adiabatic efficiency of the air compressors was 86%. The pressure of the outlet air of the throating valve (state A10), which is defined as the minimum SPT pressure, is approximately equal to the pressure of the inlet air of the first-stage air turbine (state A11). From the calculation presented in this section, the compression and expansion pressures were selected as 10 and 7 MPa, respectively. The effects of the compression and expansion pressures on system performance are discussed in Section 4.2. In the TUR subsystem, the adiabatic efficiency of the turbines was selected at 85%. The other parameters of the STC were selected by previous studies [31; 32]. \begin{table}
\begin{tabular}{c c c} \hline
**Subsystem** & \(\mathbf{Ex_{in}}\) & \(\mathbf{Ex_{out}}\) \\ \hline AC & \(W_{COM}+Ex_{AR3}+Ex_{AR5}+Ex_{AR7}\) & \(Ex_{AR2}+Ex_{AR4}+Ex_{AR6}+Ex_{AR8}\) \\ \hline \multirow{2}{*}{HEX of COM} & \(Ex_{AR2}+Ex_{AR4}+Ex_{AR6}+Ex_{AR8}+Ex_{WA1}\) & \(Ex_{AR3}+Ex_{AR5}+Ex_{AR7}+Ex_{AR9}  
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Table 4 lists the main simulation results of the ST-CAES system under typical operational conditions. Tables 4 and 5 list the stream parameters of air and other fluids, respectively. As shown in Table 3, the total compression power consumption was 329.9 kW, and the heat energy collected from solar was 213.5 kW. The output power of the turbine was 231.5 kW. The mass flow rate associated with the consumption of cooling water was 1.36 kg/s, and the consumed solar thermal power to heat the inlet air of the turbine was 189.67 kW. The ESE, RTE, and exergy efficiency of the proposed system were 70.2%, 61%, and 50%, respectively. As listed in Table 4, the temperatures of supply and return water for the heating load were 80 and 30 \({}^{\circ}\)C. Under average solar irradiation operational conditions, the system needed about 6 h to raise 11.7 tons of VP-1 from 115 to 305 \({}^{\circ}\)C. Output power of the turbine is strongly influenced by the VP-1 temperature provided by collected and stored solar energy, which is discussed in Section 4.2. A-CAES performance depends on the key technology of A-CAES, which is to recover and store compression heat in thermal energy storage (TES) [33]. This is greatly constrained by structure of compressor and multi-stage heat exchanger effectiveness. The use of solar thermal energy can eliminate the high-temperature limit of the compressor and complex heat regeneration subsystem, which can greatly simplify the structure of A-CAES [14]. For a 4-h discharge duration with total power generation capacity of 955.4 kWh, Tables 5 and 6 list the stream thermodynamic parameters of air, water, and Therminol VP-1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Unit** & **Values** \\ \hline Compressor power consumption & kW & 329.9 \\ Air turbine electricity generation & kW & 231.5 \\ Collection power of PTC & kW & 213.3 \\ Mass flow rate of VP-1 to TUR & kg/s & 2.75 \\ Mass flow rate of cooling water & kg/s & 1.36 \\ Production the mass of hot water & ton & 20 \\ Electricity storage efficiency & \% & 70.2 \\ Round-trip efficiency & \% & 61 \\ Exergy efficiency & \% & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Simulation results of the ST-CAES system under typical operational conditions.