Theoretical And Experimental Investigation Of NH₃ -H₂O Falling Film Absorption And Its Application In Two-Stage Air-Cooled Absorption Refrigeration

Heat driven solar cooling systems have big advantages in load shifting, energy saving and environmental protection. Solar collector and heat driven absorption chiller are the main components of these kinds of systems, which are expected to have good performance when driven by low-grade heat. The traditional solar absorption/adsorption cooling applications usually need relatively higher driven temperature. The systems are also difficult to realize air-cooled. Because of these drawbacks, solar cooling system of small capacity is usually with high initial cost and low electrical COP, which the maintenance of the vacuum system (water as refrigerant) is also a tough problem. In order to achieve solar cooling systems for residential uses, two-stage air-cooled ammonia-water absorption cycle is proposed and studied. Low-grade solar energy driven and air-cooled could be combined together in the system, which could also satisfy extreme summer working conditions. The intial cost and maintenance fee can also be much lower. In the other hand, absorber is usually the main difficulty when designing absorption refrigeration system. The researches of absorption behaviors of ammonia-water as working pair are less, even the evaluation methods are not unified. So, it is important to investigate the main resistance of transfer process based on the absorption mechanism. Try to construct a new evaluation method which has solid theoretical foundation. The new method should be used in the thermal design of air-cooled absorber. Two-stage air-cooled ammonia-water absorption chiller could then be constructed and tested. The following aspects are studied in this thesis:The simulation of first law of thermodynamics shows that driven hot water at 85oC, cooling air at 35oC, evaporation temperature at 10oC, thermal COP of the system is above 0.3 and electrical COP could be as high as 10. The system performances are stable under variable working conditions. For the air-cooled equipment, the condenser should be arranged in front of absorber to get an optimum system performance. Rectifier is not required because the purity of ammonia vapor is already above 99%. If the working condition permits, two-stage system could be switched to single-stage to obtain a higher COP. The simulation of second law of thermodynamics shows that low pressure and middle pressure absorbers have the biggest exergy destructions. The theory of exergy splitting makes the exergy destruction into four parts: endogenous, exogenous, avoidable and unavoidable. The air-cooled absorbers have the biggest avoidable exergy destructions. The theory also shows the importance of solution heat exchanger. In the contrary, the refrigeration sub-cooler could be omitted due to little contribution.An experimental set-up has been established to investigate falling film absorption performance on inner tube surface with low solution flow rates. The traditional evaluation method based on log-mean-temperature and concentration difference is criticized for its lack of theoretical basis when simultaneous heat and mass transfer process occurs. So it is important to derive a uniform evaluation method for ammonia-water falling film absorption. Reasonable assumptions are made for the absorption process. Coupled heat and mass transfer differential equations are established from the conservation and transfer equations. Eigenvalues, eigenvectors and simplified method are used to solve the equations. Both traditional and new methods have been used to evaluate the experimental data. The new method has a relatively smaller error with better consistency, especially in heat transfer aspect. The effect of solution inlet sub-cooling could be evaluated correctly. All the experimental data could be fitted as Nusselt and Sherwood correlations respectively. The transfer rates, Nusselt and Sherwood numbers all increase with the developing of solution Reynolds numbers. The absorption performances could be enhanced by decreasing the cooling temperature and increasing the absorption pressure. The experimental results of coulpled heat and mass transfer process could be used in the thermal design of two-stage air-cooled absorber. The experimental set-up can also be used to testify the adiabatic falling film absorption. The potential of absorption is only driven by solution inlet sub-cooling which is adjusted by the heat exchanger. The calculation method of Sherwood number is derived from the mass transfer differential equation. Mass transfer rates and Sherwood numbers increase with the developing of solution Reynolds numbers. They could be enhanced by increasing absorption pressure and inlet sub-cooling. The mass transfer performance of coupled heat and mass transfer combined with inlet sub-cooling is higher than inlet sub-cooling or coupled heat and mass transfer only. Sherwood correlation has been fitted to evaluate the adiabatic falling film absorption.Based on the Nusselt and Sherwood correlations obtained by the experiments of falling film absorption, the mathematical model is formed with the conservation equations of mass, component and energy while simultaneously heat and mass transfer processes are considered to both vapor and liquid side. The simulation results are used for the thermal design of the air-cooled absorber. Temperature, concentration profiles and transfer resistances are also investigated. The results show that low pressure absorber should be placed in front of middle pressure absorber to get an optimum absorption length. Configuration of circular finned tube bundles of air-cooled equipment is also simulated and suggested. The heat and mass transfer process is controlled by heat transfer between falling film and inner tube wall, mass transfer between interface and falling film and heat transfer process of cooling air side. The assumptions used in the derivation of coupled heat and mass transfer method are checked and verified. In the beginning section of the absorption process, water desorption phenomenon is discovered. In most other section, both ammonia and water vapor are absorbed from vapor side to liquid side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also studied.Experimental prototype has been built to investigate the two-stage air-cooled ammonia-water absorption refrigeration. The heat exchangers and solution pumps are carefully designed and chosen to achieve miniaturization of the system. The air-cooled equipment is of falling film type which is designed by absorption experimental correlations. Threaded shaped solution distributer is also desiged for the absorber. The specified concentration of solution is filled into each component. The step of starting process is also discussed. The system design is successful. It can work as both single-stage and two-stage cycle according to different ambient temperatures. The system is without cooling water tower and rectifier. 70-85oC solar hot water is enough to drive the system. Thermal COP ranges from 0.2 (two-stage) to 0.45 (single-stage). Electrical COP ranges from 5 to 12 respectively. Variable working conditions are performed, which are also compared with the simulation results. The novel designed threaded shape solution distributer is also effective in the middle pressure absorber. The main drawback is that the efficiencies of rotating machines are still too low which further limits the electrical COP. Both simulation and experimental studies show that two-stage air-cooled ammonia-water absorption refrigeration system is technically feasible in practical solar cooling applications.