Dear Reza Afsharian Zadeh. When using nanofluids in solar collectors, the addition of nanoparticles affects two properties of the liquid. The first is an increase in the intensity of heat transfer due to some improvement in the thermal conductivity of the liquid. In my opinion, this factor can be taken into account through the Nusselt criterion, which includes thermal conductivity. The heat transfer coefficient can be calculated through the corresponding criterion relation, which will also depend on the geometry of the solar collector channels, etc. I believe that an increase in thermal conductivity will not make a significant contribution, since the heat transfer coefficient linearly depends on the thermal conductivity of the liquid. In my opinion, nanoparticles have the greatest effect on the optical properties of a liquid. The presence of nanoparticles creates large local gradients of refractive index in the liquid and allows better absorption of solar energy. Local heating of a liquid near nanoparticles (clusters of nanoparticles) will intensify convection in their local vicinity. It is important to note that nanoparticles will not float in nanofluids separately from each other; they always tend to combine into clusters. This is the nature of nanoparticles. I believe that in this problem, key attention should be paid not to a change in the thermal conductivity of a liquid, but to an increase in the absorption of solar energy by clusters of nanoparticles.

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Hi, This paper may help you:

An approach to the simulation of PCMs in building applications using TRNSYS