New biaxial approach to evaluate the optical performance of evacuated tube solar thermal collector

Solar thermal collectors are devices that convert solar energy into useful heat gain. It is of great importance to understand the optical performance variation in response to solar movement. The biaxial incidence angle modifier is a widely adopted method for evaluating the optical performance of 2D collectors. However, the current method is time-consuming and labor-intensive because the biaxial modifier is determined using two sets of optical efficiency data in the transverse and longitudinal directions. This study proposes a new biaxial model composed of two analytical correlations to characterize the angular dependence of the optical performance of evacuated tube collector. This model can account for shading and material optical attenuation as a function of collector geometry without the effort of determining the optical efficiencies angle by angle. The proposed model's accuracy and feasibility were verified through rational geometrical derivation and experimental validation. It was found that the relative deviations for daily collector performance between simulated and experimental results were kept within 10%. Moreover, the relative deviations in the optical performance evaluation between the new biaxial model and the traditional incidence angle modifier remained within 4.5%. Compared with the traditional method, the new biaxial model provides a simpler and more intuitive way to describe the optical performance of evacuated tube collector with comparable accuracy, which would be helpful, particularly in the design stage.