Flat-Plate Solar Collector in Transient Operation: Modeling and Measurements

Abstract

Solar energy is a viable alternative to limited fossil fuel resources. One of the simplest and most direct applications of this energy is the conversion of solar radiation into thermal energy with a flat-plate solar collector which can be used in water-heating systems. This paper presents a mathematical model for simulating the transient processes which occur in liquid flat-plate solar collectors. A discrete nodal model that represents the flat-plate solar collector’s layers and the storage tank is employed. The model is based on solving a system of coupled differential equations which describe the energy conservation for the glass cover, air gap, absorber, fluid, insulation, and the storage tank. Inputs to the model include the time-varying liquid flow rate, incident solar radiation, and ambient air temperature, as well as the volume of liquid in the storage tank and initial temperature of the solar collector and tank. The system of differential equations is solved iteratively using an implicit, finite-difference formulation executed with MATLAB software. In order to verify the proposed method, an experiment was designed and conducted on different days with variable ambient conditions and flow rates. The comparison between the time-varying computed and measured fluid temperature at the collector outlet shows good agreement. The proposed method is extremely general and flexible accounting for variable ambient conditions and flow rates, as well as allowing for a geometrical and thermophysical description of all essential components of the solar collector system, including the storage tank. The validated and verified, general model is suitable to investigate the effectiveness of various components without the necessity of carrying out experimental work, and the flexible computational scheme is useful for transient simulations of energy systems.