Solar thermal collectors are crucial for decarbonizing thermal energy needs in both residential and industrial sectors. However, improving thermal efficiency remains a key challenge for this technology, as it is significantly affected by the conductive and convective thermal resistances between the working fluid and the absorber. Additionally, reducing investment costs is necessary for widespread adoption. To overcome these issues, Direct Absorption Solar Collectors (DASCs) using nanofluids with tailored optical properties have been proposed. In DASCs, the working fluid directly absorbs the solar radiation and converts it into heat, which simplifies the system's design and improves the temperature distribution within the fluid, therefore enhancing the overall thermal efficiency. The present study involves numerical simulations in ANSYS Fluent to evaluate the thermal performance of two DASCs: a flat rectangular and an evacuated tube configuration. Both systems operate with carbon nanofluids, specifically Single-Wall-Carbon-NanoHorns (SWCNHs) suspended in deionized water. The impact of nanofluid temperature and mass flow rate, nanoparticles’ concentration, glass properties and geometrical features on thermal efficiency is thoroughly analyzed. The optimization of DASC geometry, proper material selection and tuning of nanoparticles’ concentration are found to be crucial for the future deployment of DASCs in the building sector, ensuring higher performance and cost-effectiveness.