Theoretical And Experimental Study Of Inorganic Salt/water Sorption Thermal Energy Storage

Thermal energy storage is a critical technology in effective thermal management to achieve the goal of energy conservation and emission reduction.It is capable to alleviate the time-difference and space-difference between energy supply and the demand for solar heat and industrial waste heat utilization.Compared with conventional sensible and latent heat storage,sorption thermal storage has the advantages of high energy storage density,small heat loss and combing heat and cold storage in one system.In recent years,sorption thermal storage has attracted extensive attentions.Inorganic salt/water sorption thermal storage technology is suitable for spacing heating and cooling driven by low-temperature solar heat or industrial waste heat,for its high energy density,environmental friendly and simple structure.However,there exist several problems in its application,including material loss and corrosion of containers caused by deliquesce of the hygroscopic salts,low thermal conductivities,poor mass transfer performance and stability of hygroscopic salts,and the lack of experience in the experimental validation.It is difficult to design an effective,efficient and compact salt/water sorption thermal storage system.To solve the above problems,water sorption mechanism of inorganic salt,thermal storage cycles of inorganic salts/water,development of composite consolidated materials,investigation of thermal storage systems and evaluation of thermal storage systems were carried out in this paper.First,the mechanism for interactions between water vapor and inorganic salts in closed system were illustrated and the hygroscopic salts were classified into moderate hygroscopic salts and strong hygroscopic salts by the appearance of deliquesce or not.The sorption process of moderate hygroscopic salts in closed system is based on hydration reaction.While the sorption process of strong hygroscopic salts in closed system is based on hydration reaction,deliquesce and absorption and it is called three-phase sorption process.SrBr₂ and LiCl are selected as the representatives of two types of hygroscopic salts to store low-grade heat?<100^oC?.The cycle paths of heat storage in winter and cold storage in summer were depicted in the P-T or P-T-x phase diagrams.The estimated energy density of SrBr₂can achieve 618 kWh/m³ and the estimated energy density of LiCl can achieve 668kWh/m³.The possible solutions for the design of systems were proposed.Fin-tube heat exchanger was suitable for SrBr₂ system and tray type plate heat exchanger was suitable for LiCl system.After preliminary screening of several common porous materials,a new type of expanded graphite-expanded graphite treated with sulfuric acid was selected as the host matrix for its good heat transfer performance and stability.SrBr₂ and LiCl consolidated composite sorbents were prepared by using cold pressing method.The composite LiCl sorbent was added 5 wt.%silica solution to increase the mechanical strength.The sorption characteristic,heat and mass transfer properties of consolidated sorbent samples were investigated by testing the TG-DSC curves,equilibrium sorption performance,kinetic sorption performance,thermal conductivities,permeability and porosity.The optimal SrBr₂ sorbent and LiCl sorbent were selected for practical applications.A 1 kWh lab-scale prototype for sorption thermal storage was designed and established by employing SrBr₂ consolidated composite sorbent.A two-dimension kinetic simulation model considering heat and mass transfer was developed for the sorption reactor with COMSOl software in the design phase.The heat storage and cold storage performances were simulated.Experiments were conducted in lab and the results showed that the heat storage and cold storage performances were acceptable.The heat storage capacity reached 63%of the simulation value and the cold storage capacity reached 58%of the simulation value.A 13.8 kWh lab-scale prototype for sorption thermal storage was designed and established by employing LiCl consolidated composite sorbent.Similarly,a two-dimension simulation model was developed for the sorption reactor with COMSOl software to investigate the heat storage performances.By comparing the performances of LiCl sorption heat storage prototype and 300 L hot water tank,it was clear that the energy storage density of LiCl prototype was about two times of that of hot water tank.By comparing the experimental and simulation results,the heat storage in practical was not very satisfying.The heat storage capacity reached40%of the simulation value.The main reasons were the incomplete reaction in real system and the poor mass transfer performance of LiCl consolidate composite sorbent during sorption process.The evaluation criteria of thermal energy storage system was proposed.LiCl and SrBr₂ sorption heat storage prototypes were compared according the evaluation criteria.LiCl prototype has the benefits of high energy density and good storage performance,also some drawbacks of poor durability,severe performance degradation,complex design and high requirement of vacuum degree.SrBr₂prototype has the benefits of simple design,stable performance and good cold storage performance.Currently,when choosing a hygroscopic salt/water sorption heat storage system,SrBr₂ prototype as the representative of moderate hygroscopic salt systems is recommended for its stability and simple structure.While if the stability and durability could be intensified,LiCl prototype as the representative of strong hygroscopic salt systems is more competitive.