Achieving affordable and clean energy is one of the SDGs proposed by the United Nations, and that is linked to promoting the implementation of renewable energies and improving energy efficiency. Solar thermal energy combines a high number of applications (domestic heating, industrial thermal processes, electricity generation, etc.) with a high availability of solar energy. Conventional thermal solar collectors, with a great implantation in the market, convert the incoming solar energy into useful thermal energy, however, they present numerous thermal losses that reduce their efficiency. The development in recent years of nanotechnology has allowed a new type of collector, direct absorption solar collectors (DASC), where a small amount of nanoparticles dispersed in their working fluids (nanofluids), directly absorb solar radiation. This paradigm shift in solar thermal collectors, which goes from conventional surface absorption to volumetric absorption, increases the efficiency of these devices. However, to boost the economic and commercial viability of DASCs, certain practical problems that concern the industry remain: nanofluid stability, useful life and cost of installation, efficiency optimization, etc.). The blackNF2DASC project proposes to address these problems through a systematic study that, by combining processing, experimentation, modeling and optimization, obtains the most suitable carbon-based nanofluids (for price, stability, absorption, photothermal efficiency, etc.) for low and medium temperature DASC (i.e. with base fluids such as water, ethylene glycol thermal oils).