Design, development and optimisation of a novel thermo-syphon system for domestic applications

In order to decrease reliance on fossil fuels, renewable energy has become an important topic of research in recent years. The development in the renewable energy source will help in meeting the requirements of limiting greenhouse-gas effects, and conserve the environment from pollution, global warming, ozone layer depletion, etc. There are various naturally available renewable energy sources. One of these sources is solar energy. Solar energy is available in abundance throughout the world and is the cleanest of all known energy sources. There are various devices that can be used to harness solar energy. One of such devices is a thermo-syphon. Thermo-syphon converts the solar energy obtained from the Sun into thermal energy of a working fluid. This thermal energy in the working fluid can be used for various industrial and household activities. In a closed loop thermo-syphon system, the working fluid circulates within the thermo-syphon loop via natural convection phenomenon and does not need any external devices, such as a pump. Therefore, it is considered to be one of the most efficient devices for the heat transfer. Moreover, the absence of a pumping device reduces the manufacturing and maintenance costs of a thermo-syphon system.

The heat exchange process in the thermo-syphon is a complicated process, which considers the heat convection phenomenon. Therefore, to understand the natural convection process in the thermo-syphon and their effect on the thermal performance of the system a Computational Fluid Dynamics (CFD) based techniques have been used. Numerical results obtained have been verified against the experimental results, and they match closely with each other. The comparison between the CFD and experimental result, suggest that CFD can be used as an effective tool to analyse the performance of a thermo-syphon with reasonable accuracy. In order to investigate the flow structure within the thermo-syphon system, detailed qualitative and quantitative analyses have been carried out in the present study. The qualitative analysis of the flow field includes descriptions of the velocity magnitude and the static temperature distributions contours within the closed loop thermo-syphon system. Furthermore, the variation in the temperature of water within the storage tank, temperature of the working fluid, heat transfer coefficient, wall shear stress, and local velocity and temperature distribution of the working fluid within thermo-syphon loop have been quantified as a function of time. In addition, numerical studies have been conducted to identify the effects of various geometrical parameters, which include the number of the riser pipes, length-to-diameter ratio of the riser pipe on the thermal performance of a closed loop thermo-syphon system. Moreover, a further investigation has been carried out to analyse the effect of various heat flux conditions and different transient thermal loadings on the thermal performance of a closed loop thermo-syphon system. Based on these analyses some novel semi-empirical relations have been developed to predict the thermal performance of the thermo-syphon, which is one of the focal points of this research.

Another goal of the current study is to improve the thermal performance characteristic of thermo-syphon solar water heating system using an enhancement device to improve the heat transfer. This aspect of the work focuses on the increasing energy conversion from the riser pipes to the working fluid within the thermo-syphon loop. This is accomplished by increasing the surface area of riser pipes by employing several design modifications, such as straight, wavy and helical pipes, within the riser pipes, while maintaining the amount of the working fluid constant within the closed loop thermo-syphon system. In this study, a comparative analysis has been carried out for these new design modifications to identify the best in terms of heat transfer coefficient, heat gain in collector etc., as an indication of thermal performance. According to the findings of this analysis, the model comprising of pipe inside the riser pipe depict better thermal performance as compared to other models. After defining the best design modification, a further detailed investigation has been carried out between the traditional and modified design (straight pipe inside the riser pipe) using experimental and numerical method.

Established methods regarding the design process of thermo-syphons are very limited, and they are severely limited in estimating important design parameters, such as useful heat gain and heat transfer coefficient, which have a significant impact on the thermal performance of thermo-syphon system/loop. A design methodology has been developed to enrich the design process of a closed loop thermo-syphon solar water heating system. The developed methodology is more efficient and reliable since it is capable of estimating various geometrical and thermal parameters, such as collector area, diameter and length of the riser pipes, distance between the centers of the riser pipes, heat transfer coefficient, temperature of the working fluid and the mass flow rate. This design methodology is user friendly and robust.