Development of a falling-film desorber combining vapor generation and purification for ammonia-water absorption chiller (Modeling and Experiments)

Summary :

The need to meet the growing demand for cold production, as well as to favor the use of fatal heat as a source of energy are among the main challenges of the energy transition. Absorption machines can be used to address these two problems, by producing cold for the industry or air-conditioning of buildings, by recovering heat, coming from abundant and low costs sources such as the sun and the heat networks. Among the existing working-pair, ammonia water has interesting physical properties, in addition to having a very low environmental impact. One of the constraints for better performance of a NH3-H2O absorption machine is the purity of the ammonia vapor generated, which must be high. However, the small volatility gap between ammonia and water induces the presence of traces of water in ammonia vapor.The work carried out in this thesis presents the development of a new combined generator, combining the generation and the rectification of ammonia vapor in a falling-film and plate exchanger, and the impact of its implementation on the performances of a small capacity NH3-H2O absorption chiller designed for refreshment.First of all, a numerical model is developed, allowing simulating the desorption and absorption processes, as well as the coupled heat and mass transfer operating within the falling-film, in co-current or counter-current configuration. Adiabatic plates are placed above the heated plates, allowing increasing the fraction of ammonia in the vapor generated. The impact of the input variables and the geometry of the plates is studied, in order to determine the ideal input conditions, and the most compact geometry, allowing generating the purest ammonia vapor possible at lower energy costs. Then, the combined generator, characterized by new correlations establishing its mass, thermal and species effectiveness, is implemented in a NH3-H2O absorption chiller model to study its impact on the machine's performance. The influence of the input parameters of the combined generator is analyzed. The new combined generator is then experimentally designed and implemented in a single-effect NH3-H2O absorption chiller of 5 kW cold capacity. Parametric studies are carried out on the operating conditions of the machine, (in particular through a design of experiments), which allows characterizing its operation including the new combined generator, and defining the operating limits and optimums in terms of coefficient of performance and cold production. Finally, according to the operating conditions of the absorption chiller, a comparative analysis of the combined generator performance is carried out between modeling and experiments. Two modes of operation are studied: with falling-film only, or with the third bottom flooded and seat of boiling phenomena. A boiler (formerly implemented in the machine) is also modelled and its numerical performance is compared to the measurements. The comparison of the performance and effectiveness of the three generator models showed that the combined falling-film combined generator produces purer ammonia vapor for a better cycle performance.