An analytical solution to the radiation-convective heat transfer in a parallel-plates direct absorption solar heat collection duct

Parallel-plates direct absorption solar heat collection (DASC) duct, as a basic and elementary structure for photo-thermal conversion, has great potentials in the applications of building solar heat collection. Traditionally, performance analysis of such a duct needs to solve the complete radiation-convection heat transfer equations in the duct, which requires considerable computational costs. Besides, it is difficult to be used in system optimization because of the unclear roles played by the numerous influencing factors. To solve this problem, in this study, a dimensionless parameter, conductive to convective heat transport ratio (the ratio of ability for thermal conductions to that for thermal convections in the duct, θ), is proposed. In analogy to common heat exchanger ducts, an analytical model based on heat collecting effectiveness-conductive to convective heat transport ratio, is solved for the radiation-convective heat transfer in a DASC duct. Different forms of effectiveness-conductive to convective heat transport ratios are depended on different duct penetration ratios (α) and substrate absorption coefficients (χ_plate). Then, the effectiveness-conductive to convective heat transport ratio correlations are validated by numerical simulations and experimental results. The largest relative deviation between the new model and the traditional method is smaller than 5%, which shows the excellent accuracy of the new model. Lastly, parametric analysis is conducted to evaluate the influences of operating parameters, nanofluids and substrate properties on system performance.