The largest source of renewable energy in the world comes in the form of solar energy with an average amount of power incident on the earth's surface of 178,000 terra Watts. One method for harvesting the power of the sun is solar thermal energy conversion. In order to potentially improve the efficiency of solar thermal energy generation the use of nanofluids, liquid-nanoparticle suspensions, has been proposed for directly absorbing the solar energy within the fluid volume. Using a micro-solar thermal collector developed for the production of hydrogen, the use of nanofluids as the working fluid, as well as the absorber, is investigated. Experiments were conducted to investigate the impact of particle size, particle shape, and volume fraction on the efficiency of the solar collector as well as the stagnation temperature. In addition the experimental data were compared with a numerical model of a solar collector with direct absorption nanofluids. The experimental and numerical results demonstrate an initial rapid increase in efficiency with volume fraction, followed by a leveling off in efficiency as volume fraction continues to increase. Results also indicate the importance of size and shape on collector efficiency.