## 3 Results and Discussion  
### Analytical Model Results  
The analytical model has been simulated for 30 cycles. The air is compressed from atmospheric conditions to 8 MPa in the first cycle. In the following cycles, the air pressure varies from 5 to 8 MPa at a stable mass flow rate considering an injected air temperature of 310 K and daily compression and decompression cycles. During an operation cycle, the air is compressed for 8 h at a mass flow rate of 0.22 kg s\({}^{-1}\) and stored for 6 h. Then, the air is released for 4 h at a mass flow rate of \(-\)0.45 kg s\({}^{-1}\) and stored for 6 h.  
The variations in temperature for the air, sealing layer and concrete lining are shown in Figure 4 using FRP as sealing layer for 30 continuous cycles, which is equivalent to one month of operation. The results are also presented more in detail for the first cycle (Figure 4a) and the 30 th cycle (Figure 4c). The air temperature increases up to 322 K during the first compression cycle. However, due to mixing with the injected air and heat exchange between the compressed air and the lining, the air temperature within the \begin{table}
\begin{tabular}{c c c c} \hline
**Mesh Cells** & **Thermal Energy** & **Error** & **Computational Time** \\ \(\times\)**10\({}^{6}\)** & **(kWh)** & **(\%)** & **(h)** \\ \hline
1.44 & 53.96 & 7.14 & 56 \\
2.01 & 56.76 & 2.33 & 78 \\
2.69 & 57.72 & 0.68 & 116 \\
3.47 & 58.11 & - & 147 \\ \hline \end{tabular}
\end{table} 
Table 2: Mesh sensitivity analysis. reservoir decreases to 307 K from the fifth cycle (five days). During the discharge period the air temperature is also stable, reaching minimum values of 294 K. The temperature of the FRP and concrete during the charge period reach 305 and 303 K, respectively, decreasing to 296 and 300 K during the discharge phase. As shown in Figure 5, the air pressure varies from 5 to 8 MPa over the entire 30 days. Because of air temperature, the pressure decreases to 7.8 MPa during the storage period in the first cycle (Figure 5a). This reduction decreases to 7.9 MPa from the fifth cycle. The air pressure increases slightly in the storage period after the decompression stage. Figure 4: Temperature of air, fiber-reinforced plastic (FRP) and concrete lining for 30 charging and discharging cycles considering FRP as sealing layer. (**a**) First cycle; (**b**) 30 days of operation; (**c**) 30 th cycle. Figure 5: Air pressure for 30 charging and discharging cycles considering FRP as sealing layer. (**a**) First cycle; (**b**) 30 days of operation; (**c**) 30 th cycle. The surface heat flux by convection between the compressed air and the FRP and between the FRP and concrete is shown in Figure 6. Note the thickness of the sealing layer (20 mm). Due to the temperature effect, the surface heat flux increases in the first cycle to 150 and 140 W m\({}^{-2}\) for FRP and concrete lining, respectively. The heat flux decreases to \(-90\) W m\({}^{-2}\) in the decompression stage. As observe in Figure 6c in more detail, from second cycle the heat flux through the FRP and concrete is stable, reaching maximum values of 50 and \(-100\) W m\({}^{-2}\) in the charge and discharge periods, respectively. The surface heat flux is decreasing rapidly during the storage periods. Due to the thickness of the sealing layer and the long air charging and discharging time, the surface heat flux in the sealing layer and the concrete lining is very similar. A comparative analysis for the air temperature, pressure and surface heat flux is shown in Figure 7 for the first cycle considering both FRP and steel as sealing layers. The results are also presented for cycle 30 th in Figure 8. The air temperature fluctuation between the compression and decompression periods decreases when steel is employed as sealing layer. Because of the thermal conductivity, the temperature in the contact surface between the compressed air and sealing layer is lower when steel is employed as sealing layer (Figure 8a).