A long term mission in a lunar base presents technological challenges that have not been solved yet. The electricity production and energy management are some of the most crucial elements of the life support system. The technical solutions used today in space exploration are not suitable for a long-term mission on a manned outpost. The Thermal Energy Storage (TES) system is a technical solution proposed for the above mentioned problem. In this project the TES design is evaluated, studied and modified to satisfy the main goal of the thesis, the inclusion of the phase-change in the system. The multiphase approach is fundamental to improve the heat transfer and the energy storage. Several CFD simulations are performed using the software ANSYS Fluent with the goal to validate the multiphase approach, as well as to obtain data to validate the system design over the mission requirements. The project development starts with an analytical study to find the most suitable model for the multiphase problem. This is followed by a series of simulations to model and to obtain information on the individual elements of the system. With the collected information, a full system design is presented and tested by means of numerical simulations, trying to satisfy the mission requirements. The results are compared with those obtained in previous projects for validation. These demonstrate that the increase in technical complexity is compensated by the huge potential advantage derived from the use of multiphase flows