The development of a model for the absorber component of an aqua-ammonia absorption heat pump
Van Niekerk, Eleana
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In recent years, South Africa as a country has increasingly faced challenges in regards to electricity provision. Modern society is highly dependent on refrigeration technology in order to refrigerate food, not only industrially, but also domestically. Humans are also dependent on air-conditioning, since they now inhabit parts of the world, which were previously less habitable. The electricity woes of South Africa are obviously a threat to refrigeration capability, as most refrigeration systems use the vapour-compression cycle, which in turn uses electricity from the national grid. An alternative method of refrigeration is thus needed in order to make provision for future problems as well as becoming less dependent on the national electricity provider. One alternative system, which can potentially satisfy this need, is the aqua-ammonia heat pump, which incorporates different principles than the conventional vapour-compression heat pump cycle. This desorption/absorption system utilizes heat input rather than electricity input, and is thus an ideal candidate. In recent years, work has indeed been done on this system, as it consists of several working components. The absorber component is a critical component of this system, and needs to be investigated and designed carefully. All the absorber designs considered in this study, have some component present or some undesirable aspect which make these designs unsuitable for a system which needs to work with thermal syphoning only. Thus, it was necessary to investigate an absorber component variation which avoid these pitfalls. Several theoretical heat transfer models were developed for an absorber and compared with each other. Surrounding issues such as heat generated in situ were addressed and incorporated into the standard ε-NTU method for solving heat exchangers. The developed EES-programs can now solve with only minimal (but realistic) input parameters given. Theoretical issues with ammonia‘s solubility in water were also addressed, in order to enhance insight into the system. A few physical experiments were done in order to obtain values for parameters, which come into play in the practical design. The absorber component was characterised using the different models developed. The different cooling tube configurations were compared and the best one chosen. The influence on the absorber performance of input parameters such as cooling water inlet temperature and system pressure were investigated. Absorber performance for winter and summer conditions and inputs was also investigated. This study resulted in a much better overall understanding of the absorber component in the aqua-ammonia heat pump, which is planned to be built. Problem areas and physical sensitivities were identified and solubility misunderstandings cleared up. Working computer models were developed which will enable future designers of the overall cycle, as well as the absorber component to make predictions. The study also enabled a first iteration for a design.
- Engineering