Numerical study for the design of direct absorption solar collectors operating with nanofluids

With the rapid increase in energy consumption, the need for renewable and clean energy has become urgent. Solar energy can be considered the best target. Furthermore, equipment that can efficiently capture and utilize solar energy is also in substantial demand. Nowadays, solar energy utilization focuses mainly on converting solar energy into other energy types that can be used directly, such as electricity and heat. Solar collectors are a type of equipment that can absorb incident solar radiation and convert solar energy into thermal energy. In most of the applications, the thermal energy is produced with solar collectors in which the solar radiation is first absorbed by a metallic material and then transfer to the working fluid. The process is characterized by convective, conductive and radiative thermal losses and therefore this type of collectors presents low thermal efficiencies. Direct absorption solar collectors (DASC) represent a possible alternative. In this type of solar collectors, colloidal suspensions of nanometric sized carbon particles can be employed to directly absorb the solar radiation within the volume of fluid. One of the most promising nanofluid, tested in the past by the University of Padua and whose characteristics are considered for the present numerical model, is a suspension of functionalized SWCNHs (Single-Wall-Carbon-NanoHorns) nanoparticles in deionized water. This type of nanofluid has enhanced optical properties relative to the base fluid. The main issue with carbon nanofluids is related to the long-term stability of the fluid, it is a fundamental aspect and one of the main challenges to overcome. The nanofluids are subjected to aggregation and deposition because the particles are subjects to forces such as the attractive force of Van der Wall, gravitational force, floating thrusts, electrostatic repulsion forces. The goal of the utilisation of DASCs with nanofluids is the improvement of the collector’s efficiency. In previous test campaigns a DASC was designed and experimentally studied in the Solar Energy Conversion Laboratory of the Department of Industrial Engineering of the University of Padova, mainly to study the optical properties of carbon nanofluids. The results obtained, however, show low performances of the DASC tested with the increasing mean reduced temperature and therefore must be re-designed to improve the performances. In this thesis, a numerical study on two different DASC geometries is performed. In Chapter 1, a brief introduction on the subject of the thesis is presented. In Chapter 2 the two geometries designed and studied in this thesis will be described In Chapter 3, the materials implemented in the numerical model will be. In Chapter 4, a theory part regarding numerical modelling will be presented In Chapter 5 will be described the numerical model, the equations implemented, the implemented boundary conditions and the analysis of the numerical results. In Chapter 6 the procedure done to validate the numerical model will be reported and described. In Chapter 7 a comparison between the two geometries regarding fluid distribution and outlet temperature homogeneity will be presented and discussed, then the results obtained studying the incidence of mass flow rate, nanofluid concentration, beam path length and glass emissivity on the efficiency of both the geometries will be reported. In Chapter 8 there will be a discussion on the numerical results. Finally, in Chapter 9, will be presented the conclusions regarding the study done in the thesis.