Nanofluid cooling of a concentrated photovoltaic thermal (CPVT) receiver was simulated by a sub-continuous lattice Boltzmann model with the effective thermal conductivity (ETC) and the effective viscosity (EV) nonlinearly related to both nanoparticle concentration and size. AlO-water nanofluid cooling efficiencies for various solar irradiance are compared with those of pure water cooling. Flow and temperature fields are simulated for nanofluids with the nanoparticle concentration from 1% to 10%, particle size less than 120 nm, and flow rate over a range of 0.17–3.34 L/min (i.e., the inlet velocity from 1/10 to 2 times of the natural convection velocity scale). In dimensionless form, the parameters are described by concentration, Knudsen number and Richardson number. The enhancement ratios of Nusselt numbers, drag coefficients, and power coefficients due to the application of nanofluids compared to water are presented. An objective enhancement function is defined as the ratio of the Nusselt number to the power coefficient. The maximum enhancement ratio is 1.14 for nanoparticle concentration at 8%, Knudsen number at 0.1 (AlO nanoparticle size 6 nm), and Richardson number 10 (the inlet velocity about 1/3 of the natural convection velocity scale), respectively. This study provides a practical tool for optimal nanofluid cooling enhancement of CPVT solar receivers.