With a view towards better efficiency in renewable energy utilisation, particularly solar energy, the authors study a long-term solar thermal energy storage based on water absorption by a lithium bromide aqueous solution. After a description of the process, the system dynamic simulation model is detailed and used to investigate the influence of certain parameters (heat exchanger size, solution flow rate, absorption percentage) and operating conditions (heat supply temperature to the building, crystallisation ratio, heat need) on the system performance (storage density, thermal efficiency, etc.). The analysis of simulations made for a low-consumption building in Chambéry shows that the solution flow rate is a critical parameter in the process performance. It also appears that crystallisation in the solution storage tank increases the storage density more than three times. The simulation results are used in the design of a prototype that is under experimentation for validation of the model.