Thermodynamic studies of water absorption of ternary systems containing zinc bromide applied in solar heat storage process

E. Lefebvre (1) , S. Bennici (1) , E. Gagniere , D. Mangin , A. Auroux (1)

1 ENERGIE - ENERGIE

Thermal solar energy storage is an interesting way of reducing gas emissions. This storage can be achieved using water vapour absorption-desorption in binary or ternary systems with a desiccant salt like lithium bromide, lithium chloride, zinc bromide, zinc chloride, etc. As illustrated in the figure below, in summer, solar energy dehydrates a concentrated solution of salt in the generator. The water vapour is condensed and stored in the water tank. The resulting more concentrated solution is stored in a semi crystallized form in a second tank. In winter, the previous condensed water is evaporated at low temperature, using geothermal source, and absorbed by the highly concentrated salt solution in the absorber, thus generating thermal energy. A previous successful work [1] was realised using lithium bromide as absorbent. In spite of its good thermal properties, its high price prevented it from use in the industrial process. Therefore, i n order to design the most powerful system, it is important to determine its thermal behaviour compared to LiBr. In this work, dissolution enthalpies of various anhydrous salts (LiBr, LiCl, ZnCl2, ZnBr2, KOH, NaOH, KCHO2 and KC2H3O2) were determined. The experiments were performed by titration calorimetry at 25°C. The plot of cumulated enthalpies versus coefficient a= n_salt/n_solute allowed determining the dissolution and dilution enthalpies [2]. According to this study, zinc bromide presents the highest dissolution enthalpy. Nevertheless, literature data indicates a low activity coefficient of ZnBr2 in water which implies a weak ability of water absorption. Moreover, in the binary system ZnBr2/H2O, zinc hydroxide can be created and could gravitate in pipes. In order to improve the efficiency of a system comprising ZnBr2, this salt was associated to the previously cited anhydrous salt, in water or mixed solvents such as alcohol-water. Dissolution and dilution enthalpies at various ratios of salts of this new system were determined like previously. Furthermore, measurement of the vapour pressure above concentrated solution indicated the rate of water absorption and allowed determine the most suitable system for the process. [1] K.E. N’Tsoukpoe, N. Le Pierrčs, L. Luo, Energy 37 (2012) 346-358 [2] M.H. Hamedi, B. Laurent, J-P.E. Grolier, Thermochim. Acta 445 (2006) 70-74

Domaines: Catalyse; Environnement et Société

E. Lefebvre, S. Bennici, E. Gagniere, D. Mangin, A. Auroux. Thermodynamic studies of water absorption of ternary systems containing zinc bromide applied in solar heat storage process. International Workshop on New Working Fluids for Absorption Heat Pumps and Refrigeration Systems, Jul 2013, Tarragone, Spain. ⟨hal-00846480⟩