In the present work, entropy generation analysis on double-diffusive natural convection of nanofluid in a rectangular enclosure is conducted to deepen our insights into the performance of solar thermal systems. The effects of thermal Rayleigh number, ratio of buoyancy forces, nanoparticle volume fraction and aspect ratio of enclosure are discussed in details. It is found that entropy generation will be enhanced more intensively in turbulent regimes. The total entropy generation will reach its minimum when the ratio of buoyancy forces is unity. The total entropy generation is a monotonic decreasing function of nanoparticle volume fraction. It is interesting that the irreversibility due to thermal diffusion, viscous dissipation and concentration diffusion will change about the same rate with nanoparticle volume fraction. The differences and similarities between nanofluid and pure base fluid are also analyzed. In some scenarios, the data based on the second law of thermodynamics are better than that based on the first law of thermodynamics to describe the difference between heat and mass transfer intensity. Moreover, we find a kind of self-organized phenomenon, which we firstly observed in double-diffusive natural convection of air, also exists in its nanofluid counterpart. Especially, even in fully turbulent regimes, there appear order structures in the picture of relative entropy generation. Such phenomenon can be used as an indicator to find order in complicated flow. (C) 2015 Elsevier Ltd. All rights reserved.