Heat transfer and air-flow circulation processes in a solar water-heating polygeneration system

Abstract

Relevance: In the context of the rapid development of solar water-heating systems as a stable and environmentally safe source of energy supply, increasing their thermal efficiency and ensuring uninterrupted heat delivery to end-users are becoming particularly important. Heat transfer processes—especially the dependence of convective heat exchange on the Nusselt number-represent key factors that determine the operational performance of such systems. Although numerous studies on solar water heaters exist, the fundamental physical mechanisms of heat transfer, the influence of flow regimes on temperature distribution, and the relationship between flow behavior and the thermal output at the collector outlet have not been sufficiently explored. In particular, more accurate and reliable correlations are needed for real operating conditions such as changes in mass flow rate, absorber geometry, thermo-physical properties of the working fluid, and fluctuations in solar irradiance. From this perspective, the present study is highly relevant as it proposes new methodological approaches for characterizing heat-exchange processes, improves the accuracy of analytical and empirical models, and enables optimization of structural solutions in solar water-heating systems through advanced numerical simulations. The obtained results are of significant practical value for ensuring thermal stability, reducing energy losses, and enhancing the overall efficiency of solar-thermal systems.

Aim: The primary aim of this study is to conduct an in-depth analysis of the heat-transfer processes and the natural and forced circulation phenomena occurring in solar water-heating polygeneration systems, to determine their thermo-hydraulic characteristics, and to develop new scientific approaches aimed at improving system performance. Special emphasis is placed on examining the formation of natural thermosiphon flow from the solar collector to the storage tank, temperature-driven water stratification, the development of pressure differences, and the dependence of circulation intensity on the Nusselt number, Reynolds number, and density variations of the working fluid.

Methods: The heat-transfer mechanisms and the natural and forced circulation of liquid and air flows in the solar water-heating polygeneration system were investigated through theoretical analysis, computational modelling, and numerical simulation. Initially, natural thermosiphon circulation was analyzed based on water heating in the collector, density reduction, downward movement of cooler water, and vertical stratification within the storage tank. The pressure difference driving circulation was determined as a function of the height difference between the collector and tank and the density variation of the fluid. Convective heat transfer was evaluated using correlations based on the Nusselt, Reynolds, and Prandtl numbers. Under turbulent flow conditions, heat-transfer enhancement in the absorber channels was achieved through the application of surface roughness elements and turbulence-promoting devices. Thermo-hydraulic performance was assessed through the ratio of pressure losses to the heat-transfer coefficient. Experimental measurements were compared with CFD simulations performed in ANSYS Fluent to validate the theoretical models.

Results: This study investigated heat-transfer and circulation processes in a solar water-heating polygeneration system using theoretical analysis, numerical modelling, and simulation tools. Natural circulation was shown to arise from the reduction of water density during heating, downward movement of colder water, and stratification within the storage tank. The pressure difference driving the circulation was evaluated from the height difference and density change of the working fluid. Heat-transfer processes were quantified using Nusselt-, Reynolds-, and Prandtl-number-based correlations. Under turbulent conditions, heat-transfer enhancement was achieved using surface roughness and turbulators inside the absorber channels. Thermo-hydraulic performance was evaluated through the balance between pressure drop and heat-transfer coefficient. Experimental observations showed good agreement with CFD (ANSYS Fluent) simulation results, confirming the accuracy of the proposed models.