The flat-plate solar collector is an important component in solar-thermal systems, and its heat transfer optimization is of great significance in terms of the efficiency of energy utilization. However, most existing flat-plate collectors adopt metallic absorber plates with uniform thickness, which often works against energy conservation. In this paper, to achieve the optimal heat transfer performance, we optimized the thickness distribution of the absorber with the constraint of fixed total material volume employing entransy theory. We first established the correspondence between the collector efficiency and the loss of entransy, and then proposed the constrained extreme-value problem and deduced the optimization criterion, namely a uniform temperature gradient, employing a variational method. Finally, on the basis of the optimization criterion, we carried out numerical simulations, with the results showing remarkable optimization effects. When irradiation, the ambient temperature and the wind speed are 800 W/m 2, 300 K and 3 m/s, respectively, the collector efficiency is enhanced by 8.8% through optimization, which is equivalent to a copper saving of 30%. We also applied the thickness distribution optimized for wind speed of 3 m/s in heat transfer analysis with different wind speed conditions, and the collector efficiency was remarkably better than that for an absorber with uniform thickness.
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