Seasonal storage using sensible thermal energy in materials (usually water) has two main disadvantages: comparably high thermal losses and a low volumetric energy density. A process involving absorption and desorption could be a solution to improve both aspects. In this paper, the theoretical and experimental investigation of the reaction zone construction of an absorption/desorption concept with sodium hydroxide (NaOH) and water is described. Both heat and mass exchangers constituting this reaction zone are falling film tube bundles working under reduced pressure (evacuated container). Preliminary experimental assessments showing the proper working of the reaction zone are presented.