Modelling of liquid flat-plate solar collector operation in transient states

The article presents a one-dimensional mathematical model for simulating the transient processes which occur in liquid flat-plate solar collectors. The proposed method considers a collector model with distributed parameters. In the model, the boundary conditions can be time dependent. The proposed model is based on solving equations which describe the energy conservation for the glass cover, air gap between cover and absorber, absorber, working fluid, and insulation. The differential equations derived were solved using the implicit finite-difference method in an iterative scheme. All thermo-physical properties of the fluid, absorber and air gap are computed in real time. The time-spatial distributions of heat transfer coefficients are also computed in the on-line mode. In order to experimentally verify the proposed method, a test bench was built and measurements were carried out. Comparing the measurement results of the transient fluid temperature at the collector outlet with computational results, satisfactory convergence is found. The proposed method is appropriate for the verification of the effectiveness of various absorbers and their surface coatings, without the need to carry out research on existing collectors. It allows the influence of fluid mass flowrate on the collector performance to be analysed and collector time constant to be determined. The presented model is suitable for collectors working in a parallel or in a serpentine tube arrangement with single or double covers.