Data centres are facilities used to house computer systems and associated components which aim for performance. The increment of these facilities is translated into a huge increment on energy consumption representing a total energy consumption of 2 % worldwide, where up to a half is attributed to cooling energy. In the present work, a real case will be analysed in order to reduce the electricity bill, the carbon footprint and also, the energy consumption. The system is modelled using TRNSYS and optimized using GenOpt. In order to optimize the operational cost, the implementation of thermal energy storage is studied. The decision variables of the study are the chiller temperatures performance and the storage tank volume. Two main scenarios are analysed according to different investment functions and it can be concluded that the optimal desired temperature, in both cases, is the least possible. Besides, we realized that if the investment cost is lower, the optimum tank volume is bigger allowing a higher difference of the chiller temperature performance.