## 3 Results and Discussion
### Numerical Model Results  
To validate the results of the analytical model, a 3D CFD numerical model has been performed and the results are shown in Figure 9. The numerical model has been simulated for 5 cycles, considering an injected air temperature of 310 K and stable air mass flow rates of 50 kg s\({}^{-1}\) and \(-\)75 kg s\({}^{-1}\) in the charge and discharge periods, respectively. The model geometry and thermal properties of air and solids are the same as those used in the analytic model. As presented in Figure 9a, the maximum air temperature decreases from 410 to 340 K between the first and fifth cycle. The air temperature fluctuation between air charging and discharging reaches 45 K in the fifth cycle. Likewise, the FRP temperature decreases from 385 to 322 K. However, the concrete lining temperature is more constant throughout the process. The surface heat flux for FRP and concrete is shown in Figure 9c. Due to convective effects, the heat flux increases in FRP during the first cycle. Then, from third cycle is stabilized in a maximum value of 1000 W m\({}^{-2}\) in the compression period and  
Figure 8: Comparative analysis between FRP and steel in the 30 th cycle. (**a**) Temperature of air, FRP and steel; (**b**) Air pressure; (**c**) Surface heat flux of FRP and steel. Figure 7: Comparative analysis between FRP and steel in the first cycle. (**a**) Temperature of air, FRP and steel; (**b**) Air pressure; (**c**) Surface heat flux of FRP and steel. \(-1000\) W m\({}^{-2}\) in the decompression period. The numerical model results using steel as sealing layer are shown for five cycles in Figure 10.  
As in the analytical model, the air temperature fluctuation between air compression and decompression periods is reduced down to 41 K when steel is employed as sealing layer (Figure 10a). In general, the air temperature is much lower than the previous scenario. The surface heat flux is presented in Figure 10c, varying in the steel surface between 2000 W m\({}^{-2}\) and \(-2000\) W m\({}^{-2}\) for the charge and discharge periods, respectively. Due to the high thermal conductivity (45 W m\({}^{-1}\) K\({}^{-1}\)), the heat flux through the sealing layer is greater, and therefore the temperature in the steel surface is lower. The temperature in the steel coincides with the temperature on the concrete surface, reaching a maximum value of 320 K in the first cycle. Figure 10: Numerical model results for 5 charging and discharging cycles considering steel as sealing layer. (**a**) Temperature of air, steel and concrete lining; (**b**) Air pressure; (**c**) Surface heat flux of steel and concrete lining. Figure 9: Numerical model results for 5 charging and discharging cycles considering FRP as sealing layer. (**a**) Temperature of air, FRP and concrete lining; (**b**) Air pressure; (**c**) Surface heat flux of FRP and concrete lining. The distribution of the air temperature within de reservoir and the detail of the temperature in the sealing layer are shown in Figure 11 after the decompression period at 5 MPa in the first cycle, using FRP as sealing layer. Due to the low thermal conductivity of FRP, the temperature reaches 390 K at the top of the reservoir. However, the temperature in the FRP decreases to 330 K at the bottom. A minimal increase in temperature is observed in the concrete lining. The distribution of the air temperature within de reservoir and the detail of the temperature in the sealing layer are shown in Figure 12 after the decompression period at 5 MPa in the first cycle, using steel as sealing layer. The temperature in the steel reaches 330 K at the top of reservoir. In this scenario, a temperature increase up to 320 K has been obtained for the concrete lining. The distribution of the surface heat flux around the compressed air is shown in Figure 13 after the first compression cycle at 8 MPa for both FRP and steel sealing layers. The surface heat flux reaches a maximum value of 7500 W m\({}^{-2}\) in the steel surface, while in the FRP surface the maximum value is 3000 W m\({}^{-2}\).