In this study, low-flux direct absorption solar collectors (DASCs) with nanofluid volume absorbers were modeled, analyzed, and optimized. The Rayleigh scattering approximation with size-dependent effects was used in order to determine nanofluid optical properties. Upon validating the mathematical model resulting from numerically solving and coupling the energy conservation equation with the radiative transfer equation, effects of internal bottom-surface optical boundary condition and base-fluid type on nanofluid temperature homogeneity and collector first- and second-law efficiencies were studied for different particle loadings, film thicknesses, and nanoparticle materials. Non-linear multi-variable constrained single- and multi-objective global optimization studies were conducted to find the optimal design vectors with respect to first- and/or second-law objective functions. The type of bottom surface in a DASC was shown to significantly affect its performance, particularly for relatively low particle loadings. Beyond a critical nanoparticle volume fraction value, collector performance was independent of bottom surface type and a DASC operates similar to a surface absorber. It has been also found that regardless of particle loading and bottom surface type, collectors with thinner nanofluid films always had a lower efficiency compared to collectors with thicker films. Water-based nanofluids were shown to offer stronger radiation absorption than therminol (R)-based ones up to a nanoparticle volume fraction of about 0.005%, at which level, therminol (R) becomes the stronger solar absorber. However, it was established that a nanofluid exhibiting stronger photo-thermal conversion does not necessarily lead to a higher collector efficiency. Finally, it was shown that optimizing with respect to a normalized combination of energy and exergy efficiencies (as opposed to only energy or exergy efficiencies) results in more reasonable design vectors with a balance between collector power and temperature gains.