Direct absorption nanofluid-based solar collectors for low and medium temperatures. A review

Solar energy is expected to play an important role in the decarbonization of the energy and industrial sectors. Low and medium temperature (<400 °C) solar thermal collectors have proved to be a reliable solution to supply heat and decarbonize the industrial sector, with over 800 Solar Heat for Industrial Processes (SHIP) plants put in operation in the last decade. Governmental support policy is a key factor for solar thermal energy to play a major role in CO2 emission reduction, which require improving the efficiency of solar collectors and reducing costs. Recent studies have demonstrated the potential of nanoparticles to enhance the optical properties of heat transfer fluids for direct absorption solar collectors (DASC). In a DASC the transfer fluid absorbs volumetrically the incident radiation, resulting in a more homogeneous temperature distribution and less heat losses than in conventional surface collectors. In this paper, the current state-of-the-art of SHIP installations and conventional surface collectors is presented, and a critical literature review dedicated to nanofluid-based DASC for both concentrating and non-concentrating collectors is provided. The key findings and the challenges to be overcome toward promoting the development of nanofluid-based DASC for SHIP applications are discussed.