Large temperature gradients and serious nanoparticles aggregation are the two vital issues, which lead to a low photothermal conversion efficiency of Nanofluids-Based Direct Absorption Solar Collectors (NB-DASCs) in the solar thermal utilization. It is reported that reverse illumination is a good way to address the above issues of the NB-DASCs, which can change the way of energy transport by Rayleigh-Bénard convection. However, the previous studies are based on simulation and few experiments are found. In this paper, a novel test rig is designed

for the first time to experimentally study the reverse illumination NB-DASCs. Uniform temperature distribution inside nanofluids is obtained. It is found that the photothermal conversion efficiency of the reverse illumination NB-DASCs is up to 51.9% in a continuous test of 10,000 s, which is 99.6% higher than that of the conventional NB-DASCs. Furthermore, the spontaneous convection, resulted from reverse illumination, can effectively alleviate the sedimentation and agglomeration of nanoparticles, which enhances the stability of nanofluids. In addition, the mechanism is also studied in detail. This paper provides a practical avenue for improving the solar thermal conversion efficiency of the direct absorption solar collectors.