Thermalchemical Resorption Refrigeration Driven By Low Grade Thermal Energy

Resorption is an optim um cycle of therm o-chemical adsorption. It adopts two adsorbent beds filled with two dif ferent reactive salts. Since under the sam e working pres sure the equilibrium temperatures of the t wo employed reactive salts reactin g with the sam e gas (ammonia) are different, these two salts are named as high tem perature salt (HTS) and lo w temperature salt (LTS). The decomposition of the LTS complex yields the cooling effect. Due to the decomposition heat lar ger than vaporiz ation enthalpy of refrigerant, the cooling capacity of the resorption system is theoretically lar ger than that of the conversional adsorption system. Besides, the fact that th e working pressure in resorption system is relatively lower comparing to the traditiona l adsorption system, mitigates the dangerous introduced by the high pressure; the application of resorption system can be extended to the vehicle, fish boat and even the spacecraft because there is no liquid phase refrigerant inside, the system is not sensitive to the vibration as well as the gravity.Rational and wise selection of the working pair s is one of the key strategies for enhancing the utilization rate of the low grad e thermal energy source and the co oling efficiency of the resorption systems. This work firstly carried out the inve stigated on the characteristics , thermodynamic equilibrium and reaction k inetic properties of ammoni ate complexes, thus some guides for seeking proper reactive salts acco rding to a certain practical requirem ent of resorption refrigeration were proposed.Afterwards, with the combination of the selection criterions of the working pairs and the aim for cooling at 0℃or lower than 0℃, three reactive salts BaCl₂, NaBr, and NH₄Cl were picked up as LTS to coupling-work with MnCl₂ acting as HTS. They comprised three kinds of resorption prototypes, the performance of wh ich were investigated and com pared under different conditions, leading to the conclusi on that, when the coo ling temperature was required at 0℃or lower than 0℃, NH₄Cl was the best choice of LTS when MnCl₂ was HTS. The prototype with NH 4Cl achieved the highest COP and SCP among all the prototypes studied.Since NH₄Cl was a novel salt for sorption cycle and has been rarely reported so far, in order to built the global modelling of the resorption system employing NH₄Cl/ MnCl₂, the basic physical characteristics of NH 4Cl compound sorbent with expanded graphite as well as the kinetic model were experim ental studied in this work. Due to take the influence of a pseudo-equilibrium zone on the reaction process into consideration, a revised kinetic model based on the classic ones for chemisorptions and with both equilibrium temperature drop and equilibrium pressure drop being involved into was proposed expres sly for NH₄Cl complex. The reasonable results obtained by the global model using such a kinetic equation had good agreement with the experimental results.The resorption system using NH₄Cl as LTS and MnCl₂ as HTS was applied to a refrigerator with a 33 L cold storage box, with the objective of validating the advantage of NH₄Cl as LTS. The refrigerator provided two levels of cool ing effect: the bottom zone insid e of the refrigerator box the minimal temperature reached -16 -14℃and the air was kept under -10 oC for about 3 h; the upper zone inside could be cooled down to -1 6℃. However, the drawbacks exposed by this refrigerator em phasized the n ecessity of system optimization, including reduction of m etallic heat load p roportion and enhancement of heat transfer efficiency.Based on the proposed optim ization strategies, a bench-scale therm ochemical resorption system with the workin g pair MnC l2/NH3 and NH 4Cl/NH3 was built up and inve stigated under different conditions to demonstrate the feasibility of sub-zero refrigeration. When the flow resistance of the reactiv e gas could be neglected, th e resorption system achieved COP and SCP, respectively, 0.24 0.31 and 260 350 W per kg of NH₄Cl when refrigeration temperature was at -15 5℃. Such cooling level achieved by resorption system s has been never reported before, moreover, the COP was increased by 1.55 tim es. If the transm ission resistance of the reactive gas was high e nough to produce pronounced pressure drop, the system could output cooling ef fect at a certain point m ore steady and longer though, at the same time it sacrificed some cooling capacity. Additionally, a match performance between HTS and L TS complexes when refrigerating at 0℃was discussed based on the adsorption/desorption capacity and thermodynamic equilibrium property. A mathematical expression of the match behavior was proposed and validated by the simulative results of the global system modeling.