  This work establishes a closed-form analysis for two-dimensional transient
heat transfer in an absorber plate based on the thermal wave model when the
collector starts to work. The method of separation of variables is employed to
determine the exact thermal response during the starting of a solar collector.
The thermal field is represented by 2-D thermal contours and 3-D surface plots
to provide an accurate prediction compared to the 1-D analysis using the
time-lag theory. An influence of Fourier number on the heat flow in the
absorber plate is studied for different thermal relaxation times. The present
analysis highlights that the magnitude of plate temperature depends
significantly on the physical geometry. Differences in two models of heat
propagation, viz. classical and thermal wave, in the absorber plate have an
alteration in predicted collector performances. The present study also develops
another design situation for heat transfer in the absorber plate when the
collector shutdowns to the ambient state from its steady condition. Laplace
Transform Method (LTM) in combination with the product solution method uses to
determine the temperature with the initial condition represented as a steady
case. Due to the lagging nature of thermal behavior, a significant difference
between the classical and thermal wave models for the propagation of heat was
observed, and also this study determines the exact time to be taken to reach
the dead state for the complete shutdown of the collector.
