Nanofluids have been used to improve the efficiency of direct absorption solar thermal collectors (DASC) as working fluids because of their excellent heat transfer and optical absorption properties. However, aggregates could be generated in nanofluids especially in the two-step preparation. A theoretical approach for calculating the extinction coefficients of nanofluids with particle agglomeration is proposed based on diffusion limited cluster aggregation (DLCA) simulation and generalized multi-particle Mie solution (GMM) method, in which the dependent scattering effect between particles is considered. The influences of particle agglomeration, particle diameter and particle volume fraction on the extinction coefficient of nanofluids with particle agglomeration are investigated. Moreover, the harvested solar energy under the real solar irradiation at sea level for the nanofluids with particle agglomeration is also studied. The results show that the particle agglomeration reduces the absorption peak in the short-wavelength as well as enhances the long-wavelength scattering. The nanofluids with particle agglomeration have a higher performance in solar energy harvest. (C) 2015 Elsevier Ltd. All rights reserved.