In the current study, thermal efficiency of a direct absorption solar collector, using single-walled carbon nanohorn (SWCNH)-water nanofluid as the working fluid, is numerically investigated. An in-house, parallel, multirelaxation time, lattice Boltzmann method (MRT-LBM) code is developed, and the effects of nanofluid concentration, presence of aluminum absorber sheet and its position on design parameters of the collector are discussed. Results are discussed for two cases: with and without the absorber plate, and the effects of nanoparticles concentration (phi), nanofluid's mass flow rate (m), collector height (H), absorber plat position (Y) and radiation heat flux () on efficiency and maximum temperature of the collector are studied. The importance of these parameters have been proven and it was shown that, in some cases, optimal working conditions exist. It has been demonstrated that low concentrations of nanoparticles can improve the collector efficiency, while its larger amounts can lead to negative performance. The absorber plate highly improved the collector performance especially when the working fluid is the pure water. Simultaneous use of nanofluid with the absorber plate is not recommended except for low mass flow rates. Also, the bottom of the collector is the best position for the absorber plate in order to gain the maximum efficiency.