Ray-tracing-based numerical study of direct absorption solar collectors with carbon nanofluids

Solar energy is one of the most abundant renewable resources for earth and a promising means to achieve the decarbonization goals: solar thermal collectors are used to harness it. In conventional flat-plate collectors, incident solar radiation is collected by an absorber plate and used to heat up a working fluid: however, that system is characterized by major thermal losses, low outlet temperatures and thermal efficiency. A valid alternative is the employment of direct absorption solar collectors (DASCs) where a nanofluid, a base fluid with a suspension of carbon nanometric particles, is used to directly absorb incident solar radiation within the volume of the carrier fluid. In this work, evacuated tube collectors working with nanofluids are considered and coupled with concentrating mirrors: different geometrical configurations of vacuum tubes and designs of reflectors are analysed. The dissertation includes a study of the irradiance distribution, made by these concentrators, on the tube surfaces, obtained by using Optisworks software. Additionally, a three-dimensional numerical simulation of the nanofluid-based solar receiver is performed by using the commercial CFD software FLUENT, which allows to evaluate the temperature distribution and the thermal efficiency of the solar thermal collector.