Principle And Experimental Verification Of A Three-phase Sorption Cycle For Thermal Energy Storage

As an extended application of absorption/adsorption refrigeration technology, recently sorption thermal storage has attracted extensive attentions. Compared with conventional storage methods like sensible and latent storage, sorption thermal storage offers advantages of high energy storage density, long-term heat preservation ability and combing heat and cold storage in one system.For absorption thermal storage systems with water as sorbate, to further increase the storage density, crystallization problem will be confronted; for adsorption thermal storage systems using inorganic salt and water as working pair, deliquescence problem will be encountered. The two phenomenons are related to the mechanism for interactions between water vapor and inorganic salts. To further enhance the sorption thermal storage density, research attentions should be focused on crystallization and deliquescence phenomenons. Researches on water sorbents for sorption thermal storage are mainly focused on searching for new inorganic salts with great water uptake and high storage density. However, most studies have failed to carry out a deep analysis on the water sorption mechanism of inorganic salts- giving a clear clarification between absorption, adsorption, crystallization and deliquescence. Effects of porous matrixes on water sorption properties of inorganic salts and solutions for preventing and solving the deliquescence problem are rarely involved. In allusion to these issues, this study focuses on the sorption working pairs with combinations of highly hygroscopic salts and water and starts with the water sorption mechanism of highly hygroscopic salts. LiCl/H2 O was then perceived as a promising choice. Around this pair, thermal storage cycle analysis, choice of matrix, consolidation of sorbent and building of a lab-scale sorption thermal storage prototype were carried out in sequence. The main research can be described as follows:（1） First, the features of sorption storage were compared with conventional sensible and latent methods. Important progress in the last several decades in sorption thermal storage was summarized. The future challenge was pointed out.（2） Through the analysis of the mechanism for interactions between water vapor and inorganic salts, the relations between different water sorption phenomenons were described clearly. Thee interactions including liquid/gas absorption, solid/liquid/gas deliquescence/crystallization and solid/gas hydration/dehydration reaction are all possible in the water sorption/desorption process, depending on working conditions. The cycle covering all the above processes was named three-phase sorption cycle. Cycle paths of cold storage in summer and heat storage in winter for LiCl, CaCl₂ and Li Br were depicted in their P-T-x phase diagrams. In heat storage mode, when condensation pressure is 1.71 k Pa, the calculated crystallization temperatures are 56.1 oC for LiCl, 47.9 oC for CaCl₂ and 70.4 oC for Li Br and the predicted heat storage densities are 4500 k J/kg for LiCl, 1559 k J/kg for CaCl₂ and 1757 k J/kg for Li Br. Thus, LiCl is perceived as the most promising salt. Two possible implementation solutions for the cycle were proposed and discussed based on the conventional absorption and adsorption refrigeration systems. Using porous matrix to carry LiCl was determined to be the final solution to solve crystallization problem.（3） To find suitable porous matrix for LiCl, preliminary screening of several common porous materials were carried out, showing that zeolite and expanded natural graphite（ENG） were not suitable. Intensive study was then focused on silica gel. Factors which may infect the sorbent performance during preparation was investigated and it was shown that impregnation under ambient or under vacuum caused no significant difference on the silica gel-LiCl composites. A test rig for sorption measurement was set up based on a Rubotherm suspension TGA balance. Sorption equilibrium isobars for silica gel-LiCl composites and pure silica gel were measured with this instrument. It was observed that shapes of the isobars could be divided into three sections and no obvious sorption platform was observed. The experimental data were fitted with the sorption potential theory and it was found that instead of using a single DA equation, three equations should be developed for different sections in the sorption characteristic curve to describe the equilibrium data. Possible solutions for deliquescence problem were proposed and compared, i.e., forced deliquescence, adjusting the salt content in composite sorbent and increasing pore volume.（4） Activated carbon-LiCl composite sorbent was prepared using activated carbon as matrix. Related parameters such as pore structure, equilibrium and kinetic water sorption characteristics, TGA-DSC simultaneous thermal analysis were investigated. It was found that impregnation process for activated carbon-LiCl composite should be proceeded in vacuum condition. Results showed that water uptake of activated carbon-LiCl was apparently greater than silica gel-LiCl composite, illustrating that activated carbon is conductive to water sorption performance of LiCl thus it was a better choice of matrix for LiCl. Thus, activated carbon is determined to be the final choice of matrix.（5） Consolidated composite sorbent was developed by adding expanded natural graphite treated with sulfuric acid（ENG-TSA） to increase heat transfer and adding silica solution（SS） as a binder to enhance mechanical strength. Important parameters including equilibrium and kinetic water sorption properties, thermal conductivity, permeability were experimentally investigated. In addition to enhancing heat transfer, another advantage brought by consolidated sorbent is avoiding the risk caused by salt deliquescence: the capillary force generated within the macropore structure between activated carbon particles could hold more extra LiCl solution, thus the total LiCl solution amount which the matrix could accommodate is increased greatly.（6） A 1 k Wh（3600 k J） lab-scale prototype for sorption thermal storage was designed and established. A two-dimensional kinetic simulation model considering the combined heat and mass transfer was developed for the sorption reactor with COMSOL software and temperature and water uptake fields were obtained. Experiments were carried out for heat storage mode. When the desorption, sorption, evaporation/condensation temperatures are 85 oC, 40 oC, 15 oC respectively, the total released heat is 2517 k J, in which the contribution of sorption heat is 57 %, i.e., 1438 k J. The heat storage efficiency is 93%. The heat storage density is 874 k J/kg for the consolidated sorbent and 2622 k J/kg for LiCl. The established two-dimensional model proves to be effective since the general evolution trends of experimental and simulated outlet fluid temperatures are similar. After peak values, an average gap of about 0.4 oC between the experimental and simulated outlet temperatures were observed.Theoretical analysis in this study shows that LiCl offers a heat storage density over 1 k Wh/kg, indicating that the three-phase sorption cycle, which includes three possible interactions, namely liquid/gas absorption, solid/liquid/gas deliquescence/crystallization and solid/gas hydration/dehydration reaction, shows a promising potential in energy storage application. To carry out the cycle, some attempts have been made on options of porous matrix and design of sorption reactor. It was found that activated carbon was a better choice of matrix for LiCl compared with silica gel. Preparing consolidated sorbent by adding ENGTSA to the AC-LiCl composite could be considered an effective way to enhance thermal conductivity and avoid the hazard caused by salt deliquescence. The experimental sorption heat storage result of a lab-scale prototype has achieved 80% of its simulated value. To bright the three-phase sorption thermal energy storage cycle to practical use, more intensive studies in reactor design, combination solution of consolidated sorbent with heat exchanger surface, material stability are still required.