  A parabolic trough solar collector is a dominant technology for
high-temperature industrial applications, but efficient use of a conventional
surface-based parabolic trough solar collector (SBPTSC) is limited by its high
radiation loss due to the high surface temperature. Recently, direct-absorption
parabolic trough solar collector (DAPTSC) using nanofluids has been proposed,
and its thermal efficiency has been reported to be 5-10$\%$ higher than the
conventional SBPTSC for inlet temperature up to 250$^\circ$C. However, the
inner tubes of the receivers of the existing DAPTSCs are all transparent, so
the sun rays entering the inner tube can only travel once through the
nanofluids. As a result, the optical path length for the sun rays is limited by
the inner tube size, which in turn requires high value of the absorption
coefficient of nanofluids. Due to the approximately linear relation between the
absorption coefficient and the particle concentration, higher absorption
coefficient is likely to cause particle agglomeration, leading to detrimental
effects on maintaining stable collector performance. In the current study, the
transparent DAPTSC is improved by applying a reflective coating on the upper
half of the inner tube outer surface, such that the optical path length is
doubled compared to the transparent DAPTSC; thus, the absorption coefficient of
the nanofluids can be reduced accordingly. The coated DAPTSC is found to have
obvious advantage compared to the transparent DAPTSC at absorption coefficient
below 0.5 cm$^{-1}$ for a receiver with inner tube diameter of 7 cm. In
addition, performance of the transparent DAPTSC, the coated DAPTSC and the
SBPTSC with black chrome coating have been compared to explore their
advantageous operation conditions, such as inner tube diameter, flow rate, and
inlet temperature, with or without a glass envelope for vacuum evacuation.
