Theoretical And Experimental Research Of Thermochemical Sorption Heat Storage At Middle–low Temperature

As one of the thermochemical heat storage ways,thermochemical sorption heat storage not only has the high thermal energy storage density and tend to realize the long–term storage of thermal energy,but also can implement the coinstantaneous storage of cold and heat.Hence,thermochemical sorption heat storage has attracted considerable interest in recent years.In this paper,a laboratory test system of thermochemical sorption heat storage cycle with the working pair chlorides and ammonia was constructed.This novel thermochemical sorption heat storage system combines the thermochemical adsorption with resorption,in which two chemical reactive salts with different working temperature regions were employed as the chemical sorbents.The consolidated composite sorbents were fabricated by impregnating the expanded graphite matrix with the chlorides.The expanded graphite was used to improve the heat and mass transfer performance of chemical sorbents.The experimental system mainly includes three subsystems: two separate adsorption units and a resorption unit.The adsorption units employ the SrCl2/NH3 and MnCl2/NH3 as the working pairs,respectively.These units can work at the different operation modes,such as the direct thermal energy storage and the coinstantaneous storage of cold and heat.The resorption unit utilizes the MnCl2 as the high–temperature reactive salt and SrCl2 as the low–temperature reactive salt.The resorption unit connects the two adsorption units.Meanwhile,two chemical reactive salts with different working temperature regions are linked together.For the middle–low temperature heat storage of thermochemical sorption,the theoretical analysis and experimental investigation on three thermochemical sorption heat storage cycles were carried out.The main research work of this paper are as follows:Firstly,this paper theoretically analyses the various thermochemical sorption heat storage cycles and contrasts their advantages and disadvantages,from which it can be pointed out that the thermochemical sorption heat storage cycle,which coupled with thermochemical adsorption and resorption,has great flexibility.It can be not only converted into thermochemical adsorption and resorption heat storage cycle according to the specific requirements of heat users but also more effectively elevate the thermal energy quality.In addition,it can achieve the temperature–change regulation according to the external heat source temperature and heat users' needs.This thermochemical sorption heat storage cycle can greatly reduce the driving temperature of external heat source for desorption of high–temperature reactive salts during the thermal energy storage stage through the resorption technology.This measure is not only beneficial to broaden the available working temperature range of low–grade thermal energy,but also can enhance significantly energy upgrade capacity.Furthermore,it overcomes the drawback of the heat output temperature fluctuation arisen from the oscillation of working pressure caused by the mismatch of the decomposition reaction rate of low–temperature reactive salt and the synthesis reaction rate of high–temperature reactive salt during the thermal energy release stage via the adsorption technology.Therefore,it can ensure the stability of system pressure and then further provide a strong guarantee for the eligible heat supply quality.Secondly,this paper summarized the screening principle of thermochemical sorption heat storage materials.It has been emphasized that the dehydration process of chloride hydrate must be taken into account because the preparation of the consolidated composite sorbents widely employs the impregnation method.Moreover,it is indispensable that the detrimental effects generated by the possible side reaction should be evaluated and eliminated to ensure the cycle stability of thermochemical sorption heat storage materials.The experimental research on the thermal energy storage performance of the thermochemical adsorption and resorption heat storage systems has been conducted.The results show that the maximum value of total thermal energy storage efficiency is 92.74% under the test conditions for the thermochemical adsorption heat storage system using the SrCl2/NH3 as the working pair.Meanwhile,the sorption heat storage efficiency reaches a maximum of 68.71%.The optimum sorption heat storage density obtained is 1386.16 kJ/kg consolidated sorbent or 1630.78 kJ/kg SrCl2.The maximum of total heat storage density is 1871.03 kJ/kg consolidated sorbent or 2201.21 kJ/kg SrCl2.For the thermochemical adsorption heat storage system using the MnCl2/NH3 as the working pair,the maximum value of total thermal energy storage efficiency and sorption heat storage efficiency is 93.86% and 47.84% under the test conditions,respectively.The optimum sorption heat storage density obtained is 1391.34 kJ/kg consolidated sorbent or 1636.87 kJ/kg MnCl2.The maximum of total heat storage density is 2729.82 kJ/kg consolidated sorbent or 3211.56 kJ/kg MnCl2.As for the thermochemical resorption heat storage system using the MnCl2–SrCl2/NH3 as the working pair,the maximum value of total thermal energy storage efficiency and sorption heat storage efficiency is 94.67% and 48.34% under the test conditions,respectively.At the same time,the optimum sorption heat storage density is 2027.74 kJ/kg consolidated sorbent or 2385.58 kJ/kg MnCl2.The maximum of total heat storage density is 3971.47 kJ/kg consolidated sorbent or 4672.32 kJ/kg MnCl2.Then,the experimental research on the thermochemical sorption heat storage system coupling the thermochemical adsorption with resorption using MnCl2/SrCl2/NH3 as the working pair has been carried out.Under the operation condition of the charging temperature of 135 oC for the desorption of MnCl2 and the temperature of 12 oC for the adsorption of SrCl2;the desorption temperature of 90 oC for the regeneration of SrCl2,ammonia condensation/evaporation temperature of 12 oC,the discharging temperature of 40 oC for MnCl2 and SrCl2,the obtained total thermal energy storage efficiency is 93.31% and the sorption heat storage efficiency is 53.36%.The total heat storage density of MnCl2–based composite sorbents is 3734.36 kJ/kg consolidated sorbents or 4393.36 kJ/kg MnCl2.Meanwhile,the total heat storage density of SrCl2–based composite sorbents is 1655.19 kJ/kg consolidated sorbents or 1947.28 kJ/kg SrCl2.Finally,this paper puts forward that adsorption–resorption coupling and dual–salt crosslinking between high–temperature and low–temperature reactive salt are the good way to realize thermochemical resorption cycle based on experimental research.Meanwhile,it points out that the resorption is not only a single effect but also embodies essentially the comprehensive effect through coupling with the adsorption.Furthermore,it also proposes that desorption/adsorption reactions between chemical reactive salts and ammonia should be divided into three regions: active zone,transition zone and inert zone.Besides,the obtained sorption heat storage density of thermal energy storage materials and sorption heat storage efficiency of thermal energy storage system can offer helpful references and experiences for the trans–seasonal heat storage of thermal energy using the thermochemical sorption method.The thermal energy storage density of thermochemical sorption heat storage materials is as 10 to 20 times as that of the sensible heat storage and phase change heat storage materials.Therefore,thermochemical sorption heat storage is feasible and a very potential way in storing heat.It can be used for recovering the low–grade thermal energy to promote the highly efficient utilization of thermal energy and then provide the forceful technical support for the comprehensive utilization of thermal energy.