Recently, the direct-absorption parabolic-trough solar collector (DAPTSC) using nanofluid has been proposed, and its thermal efficiency has been reported to be 5-10% point higher than the conventional surface-based parabolic-trough solar collector (SBPTSC). However, the inner tubes of the receivers of the existing DAPTSCs are all transparent such that sun rays entering the inner tube can only travel once through the nanofluid. Due to the approximately linear relationship between the absorption coefficient and the particle concentration, a higher absorption coefficient is likely to cause particle agglomeration, having detrimental effects on efforts to maintain stable collector performance. In the current study, the transparent DAPTSC is improved by applying a reflective coating on the upper half of the inner glass tube outer surface such that the optical path length is doubled compared to that of the transparent DAPTSC. This allows a reduction in the absorption coefficient of the nanofluid. The coated DAPTSC is found to have an obvious advantage compared to the transparent DAPTSC at absorption coefficients below 0.5 cm(-1) for a receiver with an inner glass tube diameter of 7 cm. (C) 2019 Elsevier Ltd. All rights reserved.