Modelagem numérica e simulação computacional do processo de fusão de PCM (Phase-Change Material) em um tanque de armazenamento térmico acoplado a um coletor solar

Abstract

A cylindrical thermal storage tank, present in a heating system, is studied in this work. The use of a material that undergoes a phase change in an energy storage tank makes it possible to store energy using latent heat. To predict the diurnal behavior of the tank, during which energy is continuously stored, a model from the literature based on the enthalpy method is used. With changes in the numerical resolution strategy found in the study taken as a starting point, it was proposed a correction in the equation used for the enthalpy to make it continuous and also allow the direct integration of the system of algebraic-differential equations, resulting from the model discretization in two spatial directions in finite volumes. The integration routine implemented in MATLAB and used in the present work allows the control of errors inherent to numerical integration, which is more difficult to be performed using the Implicit Euler method previously used by other authors. Furthermore, in this new methodology, it is no longer necessary to use constant properties (specific heats and thermal conductivities). The simulated system employs CaCl2.6H2O (Calcium Chloride Hexahydrate) in the storage tank and water as the heat transfer fluid. The simulated time of complete melting was used for the mesh convergence analysis and the solution with 30 divisions in the radial direction and 15 in the axial direction was considered converged to a uniform mesh. For the non-uniform mesh, the solution with 18 divisions in both directions was considered converged. In the simulation, for the converged uniform mesh, a computational time was observed approximately 12 times greater in relation to the simulation with non-uniform mesh. The difference of about 15% in the time to complete fusion in relation to that calculated in a previous study may be associated with the use of a 1st order method for temporal integration by the authors. Temporal, radial and axial profiles of temperature in the tank are presented and it was verified that all are physically coherent.