Estudo de um coletor solar de tubo a vácuo com baixas inclinações através de CFD e proposta de correlações de transferência de calor

Abstract

Global warming is driven by the consumption of fossil fuels for energy generation, where only 7% of energy comes from renewable sources (IEA, 2020a). In Brazil part of the energy generated is used for heating water and of the 43.1% of households that heat water only 0.96% use solar energy for this purpose (ELETROBRAS, 2020). An evacuated tube consists of two concentric tubes closed together, where a vacuum is applied. The outer wall of the inner tube receives a layer of a material that absorbs solar radiation. The commercial software ANSYS Fluent was used to perform this numerical study, adopting a three-dimensional model governed by the equations of conservation of mass, momentum, and energy. A hexahedral mesh was used with refinement in regions of the largest fluid dynamic and thermal gradients. The generated model was validated by the Grid Convergence Index method and with literature results. A case study was performed considering the variation of the gradient angle, fluid inlet temperature and heat flux. The mass flow rate was evaluated, where in the tube inclination of θ = 5° the maximum value is at a distance L = 0.17 m from the open end, the temperature variation along the tube, being the maximum value found of 3.172 K, the profiles of heat transfer coefficient by natural convection, h, in relation to the inlet temperature and the temperature variation, where it was observed that the inclination angle can reduce on average 13.3% the h, and the Nusselt number. A correlation was proposed for the Nusselt number, with a coefficient of determination of R² = 0.996, and for the temperature variation at the open end of the evacuated tube manifold, with a coefficient of determination of R² = 0.946.