Research On Internal Heat Recovery Of Ammonia Water Absorption Refrigeration Systems

Refrigeration and air-conditioning technologies are applied more and more widely in industrial and residential fields for the improvement of people’s living. However, they also bring considerable energy consumption. Absorption refrigeration technology which can be driven by low grade thermal energy such as solar energy, geothermal energy and industrial waste heat, etc. decreases power consumption and increases the overall energy efficiency. Compared to LiBr-H2 O absorption refrigeration, ammonia-water absorption refrigeration technology has many advantages such as applications below 0 oC, easier maintenance, no crystallization problem and easy to be air-cooled, etc. Also, there are some drawbacks such as low system performance, a necessary rectification process, big bulk and high initial investment, etc. In order to extend the applications, the drawbacks must be improved. The internal heat recovery has significant positive influence on the system performance, thus this thesis conducted a comprehensive theory, numerical and experimental research on the internal heat recovery of ammonia water absorption refrigeration systems.A graphical method using temperature- heat load(T-Q) diagram has been been applied to deduce the optimal internal heat recovery cycle of an ammonia water absorption refrigeration system from pinch technology with a low evaporation temperature working condition as an example. Saturated feed condition, rectification heat recovery by a branch of the strong solution and a generation process heated by the weak solution are the features of the optimal cycle. The optimal cycle is not unique and it is determined according to the actual operating conditions. When there is a temperature overlap between the absorption and generation processes, the optimal cycle deduced from pinch technology is exactly the GAX cycle. Reducing the heat flow rate in the pocket of the background process and saturated feed condition are the two key points on the internal heat recovery. The analysis on two-stage ammonia water absorption refrigeration cycle with pinch technology has proved the method valid on the internal heat recovery issue.First law analysis is applied to explain the performance improvement of the optimal cycle. The performance improvement is significant and the performance improvement ratio(ε) increases with high temperature increasing. The parameter has a maximum value with mid temperature increasing and it decreases with evaporation temperature increasing first and then increases rapidly after GAX cycle is applied. The performance of the optimal cycle is at least 20% higher than the conventional cycle. The parameter named heat recovery potential ratio(π) can be effectively used to explain the heat recovery potential compared to the conventional cycle. First law analys is shows the performance improvement of the optimal cycle based on the quantity of energy.The exergy loss of each process of the ammonia water absorption cycle can be shown clearly in a Carnot factor- heat load(η-Q) diagram. According to the graphical comparison, the average temperature differences of the heat transfer processes are reduced due to the cycle construction thus the overall exergy loss of the system is reduced. The drawback of traditional exergy loss analysis is pointed out and the evitable exergy loss should contain the exergy loss due to the cycle construction. The results of quantitative exergy analysis show that the exergy loss of the internal heat transfer process of the optimal cycle is larger than that of the conventional cycle. Nevertheless, the exergy loss of the external heat transfer process of the optimal cycle is smaller than that of the conventional cycle. Since the average temperatures of the heat source and the environment are constant, thus the reduced exergy loss is exactly from the cycle construction. Second law analys is explains the essence of the improvement of the optimal cycle.A modified heat integrated structure referred to the optimal cycle is proposed to be applied in an actual ammonia water absorption refrigeration system according to the operating conditions. In order to investigate the heat and mass transfer mechanism and get an effective design of the structure, a numerical study is conducted. Based on double film theory, the physical and mathematical models are established with considering both heat and mass transfer resistances in both vapor and liquid phases. The nonlinear equations are solved by finite difference method with the help of a computer program. The length,the conditions at the inlet and outlet of the structure and the temperature and concentration distribution profiles along the structure can be obtained from the results. The analysis shows the heat transfer res istance is mainly in the vapor phase while the mass transfer res istance is dominant in the liquid phase. Therefore, heat transfer enhancement should be concerned in vapor phase and mass transfer enhancement in liquid phase. The temperature of the bulk vapor is higher than the saturated temperature at the vapor outlet which means the length satisfies the mass transfer requirement if it satisfies the heat transfer requirement.According to the modified heat recovery cycle, an actual heat integrated experimental set-up is built with spiral tube exchangers to verify the feasibility and investigate the features. A single pressure ammonia water absorption has been designed which can simulate the high pressure processes of an actual system. The experimental results show the structure can be operated stably and effectively. And the job of each component goes well. The The split ratio of the strong solution and the heat loss to the envioment have significant influences on the heat transfer processes and the system performance. The results verified the feas ibility and availability of the multi-flow heat integrated structure.An actual application of an ammonia water absorption chiller is conducted to valid the optimized theory of internal heat recovery and its feasibility and reliability. The ammonia water absorption chiller is designed according to the modified cycle consider ing the operating conditions. Moreover, the conditions of swaying, vibration, corrosion and big bulk are fully considered. The test system is designed based on heat balance. The experimental results show the chiller can be operated steady. The generation process is stable even the temperature of the waste gas varies sharply. No terrible conditions such as flood occur and the requirements of design have been achieved. The results show the chiller outputs the cooling capacity between 25 kW and 35 kW and the COP between 0.4 and 0.6 when the evaporation temperature is between-22 and-14 oC, the condensation temperature is between 25 and 34 oC. Under a typical operating condition of the evaporation temperature of-19 oC and the condensation temperature of 29 oC, the average cooling capacity is 27.4kW and the average COP is 0.5. The experimental results proves the feasibility and effectiveness of the heat recovery cycle optimization.