In this expression, pa is the atmosphere pressure and is the stored pressure. The above results show that energy density increases with storage pressure. However, increasing pressure also requires increased tower thickness since the inner volume now serves as a pressurized vessel. To determine the most efficient usage of tower thickness for energy storage requires consideration of safety limits for both fatigue and bursting. The primary concern, when the tower is pressurized, is the fatigue strength of the horizontal welds. Herein, the fatigue and bursting pressures [19] can be computed as