A design tool for sizing thermosyphon solar water heaters

Today, thermosyphon solar water heaters are the most popular type of solar water heaters for providing households with the required hot water for domestic purposes. However, sizing and designing these systems is still based on experience or on trial and errors methods, which are based more on intuition rather than scientific methods. The present study is aimed at addressing this problem (sizing thermosyphon systems) through the development of a design tool that can be used by engineers and manufacturers to arrive at optimised systems designed according to the weather and operating conditions of particular geographical locations. The design tool developed during the course of this study is based on the TRNSYS simulation programme for evaluating the thermal performance of thermosyphon systems, and on the genetic algorithm approach for the purpose of optimising selected design parameters of thermosyphon systems. A thorough literature review of the available models and software packages that are capable of evaluating the performance of thermosyphon systems has shown that the best available programme is TRNSYS, namely the component Type 45 (thermosyphon collector-storage component). However, the component Type 45 in its current form cannot be used directly for the purpose of optimisation, because this component relies on information that must be determined experimentally. This means that the component is mainly used for evaluating the thermal performance of already-made and tested systems under varying operating and climatic conditions. For this reason, two components developed in this research have been added to the TRNSYS suite to account for information that would otherwise have to be determined experimentally. The new components are: solar collector characteristics component Type 210; and pipes - tank heat loss coefficients Type 211. Furthermore, the component Type 45 is also modified to accept as, inputs, the outputs from the previous two new components. The modified component is named as modified thermosyphon component Type 245. The new components are validated experimentally and by using reports of tests conducted according to the appropriate European standard. The modified component Type 245 does not require any validation, as no changes were made in the main programme, except that of altering the experimentally-determined information from being parameters to instead being inputs in the TRNSYS terminology. The newly validated components were added to the original TRNSYS model so as to constitute a modified TRNSYS model which is used throughout this work. The modified TRNSYS model was· then used to perform a parametric study of the design parameters of thermosyphon systems. A genetic algorithm routine for constrained single objective optimisation problem was used, and the constraints are handled by using the stochastic ranking procedure. The genetic algorithm programme is combined with the modified TRNSYS model to constitute the final design tool. The design tool is used in this study to find the optimum thermosyphon system design that best suits Libyan families (as a case study in this research) according to the weather conditions of Tripoli and a simple, but representative, hot water load pattern. The design tool is shown to have significant potential, and with further development and validation would be capable of commercial application.