Design and simulation of solar absorption cooling systems
Creators: name: NURZIA, Giovanni
Publisher: name: Universit� degli studi di Bergamo
Year: 2008

description: The aim of this work is to improve the modeling of single stage absorption chillers and to asses a design procedure for solar cooling systems based on computer simulations. The absorption technology is reviewed in its fundamentals, with an effort on the phenomena that might be considered or not in a physical model. A basic version of the absorption chiller model, which assumes that the heat transfer characteristics of the standard components do not depend on working conditions, is applied to simulate generic commercial chillers. The procedure outlined requires the knowledge of the nominal working condition of the chiller, a data which is usually available from manufacturers. The model applied to the simulation of a chiller produced by the Thermax company shows good agreement with the off design curves declared by the manufacturer. The hypotheses of this basic model are then reviewed, in order to catch some important aspects in the simulation of the Yazaki WFC-10 chiller. It is the first attempt, and therefore susceptible of improvement, to model the entire Yazaki chiller taking into account the behaviour of its heat exchangers. A compromise between theoretical modeling and experimental correlations has been adopted, due to the complexity of the chiller and its barely accessible patented design. Some encouraging results have been obtained, but they point out that further research on the heat and mass transfer phenomena in the Yazaki absorption chiller is needed. The absorption chiller modeling is then exploited in a computer tool that gives valuable information on the planning of solar cooling systems. The design of these systems involves the choice of the main components, their sizing and an adequate control strategy of the whole. Several installations are present worldwide, but there is a lack of standard planning methodology. In the present thesis the focus is on the choice of the plant elements and their sizing. Energy savings and economic competitiveness are regarded as essential in guiding the design process. To find out an ideal sizing of the system the optimization-simulation approach is introduced. Once a typical system layout is chosen the TRNSYS environment is employed to see how different sizes of the system components effect the overall energy performance. The main design variables considered are the orientation of the solar collector array, the collectors area, the nominal capacity of the absorption chiller, the nominal capacity of the back up compression chiller, the volume of the thermal storages. Several simulations are launched by an external optimization programme, that retrieves the value of an objective function and changes the variables consequently. The procedure has been applied to a real planning case, carried out in two different installation sites. The results show that the economic convenience of solar cooling systems is really not easy going in the present scenario. Primary energy savings and reduction of electrical peak loads during hot season. Finally a section of the present work describes how the absorption cycle model can be exploited to design a novel chiller. The allocation of the heat transfer area among the standard components of a chiller is analysed to get the best compromise between cost and thermodynamic performance. The geometry of each exchanger is optimized looking for a trade off between primary cost of the tubes and running cost of the pump employed in the external circuits.