This paper describes a thermochemical seasonal storage with emphasis on the development of a reaction zone for an absorption/desorption unit. The heat and mass exchange is modelled and the design of a suitable reaction zone is explained. A tube bundle concept is presented for the heat and mass exchangers and the most demanding working conditions they should fulfil are modelled and discussed. To estimate the performance of such a reaction zone and to design it, numerical models were developed and are described in this paper. Several parameters influencing these models were tested such as the sensitivity of the models to the correlation used to calculate the heat and mass exchanges, the tube diameter and the tube pitch influence. The final contribution of the tube bundle modelling is to size and design the heat and mass exchanger constituting the reaction zone. This work will be used as a basis for the reaction zone construction of an aqueous sodium hydroxide seasonal thermal energy storage prototype.