Experimental study of ammonia sorption of halide salts and thermodynamic analysis of ammoniated salts based gas-solid sorption systems

Abstract

The gas-solid sorption system offers a probable choice to harness the thermal energy available in the form of solar energy or waste heat. One of the fascinating features of gas-solid sorption system is the utilization of pollution-free natural refrigerants like methanol, ethanol, water, and ammonia. The operation of gas-solid sorption system does not cause depletion of the ozone layer or contribute to the global warming. The easily available gas-solid pair of ammonia and halide salt, which also provides a wide range of working temperatures, strengthens the applicability of ammoniated salt based gas-solid sorption devices. Various halide salts have distinct ammonia sorption properties according to which ammoniated salt based sorption systems can be used to produce cooling and heating effects, store thermal energy or generate power. Thus, it is important to understand the ammonia adsorption/desorption behavior of halide salt and evaluation of thermodynamic properties during ammonia sorption, i.e. adsorption capacity, equilibrium pressure and temperature, and reaction enthalpy. Simultaneously, it is also essential to thermodynamically analyze the different sorption systems for the selection of best halide salt and optimization of operating conditions and performance.The ammonia adsorption/desorption Pressure-Concentration Isotherms (PCIs) of NaBr, NH4Cl, KI, CaCl2, SrCl2, MnCl2, and FeCl2 are measured at different temperatures. Significant hysteresis is noticed between ammonia adsorption and desorption of halide salt which is mainly because of dimensional modification of lattice atoms during adsorption and desorption. Among chosen halide salts, NaBr, NH4Cl and KI are categorized as low-temperature, CaCl2 and SrCl2 as medium-temperature, and MnCl2 and FeCl2 as high-temperature saltdepending upon their ammonia desorption temperatures. The Pressure-Temperature equilibrium lines (P-T line) of ammonia adsorption/desorption of halide salt are constructed on the van’t Hoff plot to estimate the reaction enthalpy during adsorption and desorption.The thermodynamic analyses of various sorption systems, namely; adsorption cooling system, resorption cooling system, advanced sorption system, and thermal energy storage system, are performed. The halide salt for different applications is selected based on its thermodynamic properties. It is not possible to use low-temperature salt as adsorber material in adsorption cooling system due to its high adsorption pressures at ambient temperature. The performance of the adsorption cooling system is greatly affected by the hysteresis which increases the desorption enthalpy of halide salt and decreases the desorption pressure of halide salt at a specific temperature. The coefficient of performance of adsorption cooling system based on medium-temperature salt is better than that of high-temperature salt because of high adsorption capacity, low desorption enthalpy and low desorption temperature of medium-temperature salts. The maximum theoretical value of the coefficient of performance (COP) of adsorption cooling system is obtained as 0.3 while using CaCl2 as adsorber material. As the low-temperature salt can desorb ammonia at low temperatures, the ammonia desorption enthalpy of low-temperature salt is used to produce the cooling effect in resorption cooling system. A favorable regenerating material in the resorption cooling system is chosen from the medium-temperature or high-temperature saltThe effect of sensible mass is more in resorption cooling system than in adsorption because of the use of two salt adsorbers in the former. The highest value of the theoretical coefficient of performance (COP) of resorption cooling system is 0.43 for the pair of NH4Cl–CaCl2. The improvement in the coefficient of performance of adsorption and resorption cooling system is achieved by allowing heat-recovery process between high-temperature and medium-temperature salt in a single system. It is feasible to use the adsorption heat released by the high-temperature salt to desorb the ammonia from medium-temperature salt to develop the heat-recovery sorption system. The theoreticalcoefficient of performance (COP) of Heat-Recovery Adsorption Cooling System (HRACS) is noticed as 0.51 for the combinations of NH3–CaCl2–MnCl2 or NH3–SrCl2–MnCl2 and of Heat-Recovery Resorption Cooling System (HRRCS) is 0.64 for the combinations NH4Cl–CaCl2/NaBr–MnCl2 and NH4Cl–SrCl2/NaBr–MnCl2.Thermal energy can be stored as the desorption enthalpy of ammonia from the halide salt. As the stored thermal energy is recovered during the adsorption of ammonia by the halide salt, the energy can be stored for an indefinite period, unlike sensible heat and latent heat storage system. The storage of fluctuating thermal energy seems to be possible due to the wide range of working temperatures of different ammoniated salts. Also, the stored thermal energy can be recovered with heat up-gradation by forming the combination of suitable halide salts. The maximum values of theoretical energy storage density are observed as 1806.7 kJ kg-1 for MnCl2–NH3 Adsorption Thermal Energy Storage (ATES) system and 1165.6 kJ kg-1 for FeCl2–SrCl2 Resorption Thermal Energy Storage (RTES) system. The maximum heat up-gradation of stored thermal energy is observed as 40 ºC in case of MnCl2–CaCl2 RTES system. The thermal energy available in the broad temperature range of 90–200 ºC can be stored by pairing different halide salts in RTES system, while in ATES system, the thermal energy available in the very narrow ranges, 100-120 and 170-200 ºC, can be stored.Hence in this thesis, the ammonia sorption characteristics of halide salt have been studied, and the thermodynamic sorption properties have been evaluated. Also, various sorption devices based on ammoniated salt have been thermodynamically analyzed using the measured thermodynamic properties. The performance of different sorption systems using various combinations of halide salts has been estimated and compared. Thus, this thesis provides knowledge about the ammonia sorption properties of halide salt and selection of halide salt for a sorption device at different operating conditions.