Optical Absorption and Material-Cost Trade-Offs in Nanofluids for Solar Steam Generation

Selecting photothermal nanofluids requires balancing solar absorption against nanoparticle demand and material cost. We compare Ag, Au, Cu, MoS2, WS2, TaS2, TiN, and Ti3C2 using a common Mie-theory framework for homogeneous spherical particles in water. Calculations over 300–2500 nm and particle radii of 2–200 nm relate spectral absorption and scattering to nanoparticle loading. Solar-weighted absorptance is combined with a minimum-loading criterion for prescribed absorption targets and a cost-normalized absorbed power metric. Under the adopted optical and price assumptions, MoS2, WS2, TiN, and Ti3C2 show favorable cost-normalized responses, whereas the assigned price penalizes TaS2, illustrating why high optical absorption alone does not determine material selection. The absorption metric represents the combined water–particle system and should not be interpreted as evaporation or device efficiency. Its cost normalization includes only screening-level nanoparticle material cost and does not account for synthesis, purification, size selection, surface modification, colloidal stabilization, equipment, lifetime replacement, or water production costs. It also does not isolate nanoparticle-induced heating from absorption by water. The framework therefore provides a material-level screening tool for direct solar absorbers relevant to steam generation, rather than a prediction of device performance or industrial desalination cost.