A nanoparticle-based direct absorption system provides a promising alternative to conventional solar collectors. This work investigates experimentally the photothermal conversion characteristics of one of the plasmonic nanoparticles, i.e., silver, under realistic conditions. Stable silver nanofiuids are formulated through a high-pressure homogenizer and the experiments are conducted under sunlight on a rooftop with tests running continuously for similar to 10 h. The results show that silver particles have excellent photothermal conversion capability even under very low concentrations. Up to 144% enhancement in the stored thermal energy can be obtained at the peak temperature for a particle concentration of 6.5 ppm. The photothermal conversion performance shows a transient behavior and is best achieved at the initial radiation period due to the low heat loss and strong surface plasmon resonance effect of silver nanofluids. Nearly constant initial specific absorption rate (SAR), similar to 0.6 kW/g, is obtained for nanoparticle concentrations up to 6.5 ppm, but it decreases significantly at higher concentrations, which is associated with increased particle-particle interactions. (C) 2014 Elsevier Ltd. All rights reserved.