| dc.description.abstract | Solar energy is the most promising source of renewable energy. However, the solar energy harvesting process has relatively low efficiency, while the use of solar energy is challenging. Direct Absorption Solar Collectors (DASC) have been proved to be effective for a variety of applications, such as water heating. At the same time, a challenge with this technology is the collector efficiency limitation due to the absorption properties of typical working fluids. Nevertheless, mixing nanoparticles with a base fluid has shown dramatic effect on the fluids thermophysical properties. Moreover, nanoparticles also has the potential to improve radiative properties, thus increasing the efficiency of a direct absorption solar collector. In this thesis, a numerical study of the inter-phase fluid-particle interactions and efficiency optimisation of a nanofluid direct absorption solar collector was performed using Computational Fluid Dynamics (CFD). A flat-plate DASC with incident light on the top surface was simulated using an Eulerian-Eulerian two-phase model. Theoretical calculations predicted the particle behaviour and magnitude of the applied forces. Validating the model against experimental results showed low discrepancies. The first simulations were done with no momentum except for gravity working on the nanoparticles, and various volume fractions of nanoparticles were tested . Next, Brownian force and thermophoretic force were added to the model. After evaluating how these forces affected the flow, the drag force was updated to include the retardation factor, for both the thermophoretic and Brownian model. Later, the models were tested for an updated heat transfer coefficient. Investigation of the particle concentration showed that the optimum value for enhancing efficiency was obtained at volume fractions of 0.3%. The highest efficiency (65%) was obtained for the model including Brownian motion and a corrected heat transfer coefficient. However, thermophoretic model with corrected heat transfer coefficient was in best correlation with the experimental results, so it was chosen as a base case for further study. The base case simulation was developed, and has a qualitatively similar evolution of thermal efficiency, an optimal absorption of radiant heat and low discrepancy from experiments. This base case was used in a parametric analysis to optimise the performance of the collector. Collector height, nanofluid velocity and black surface absorbers were investigated. As the collector height was reduced, the outlet temperature increased. A maximum temperature of 49˚C was observed for a 50 µm nanofluid film. The maximum efficiency (67%) was observed for collector height equal to 300 µm. Next, a high flow velocity of 3 cm/s gave a maximum efficiency of 88%. Nevertheless, this high velocity results in a high pressure loss through the collector. Lastly, properties of the top and bottom surface were investigated. An efficiency of 67% was obtained for a water-filled collector with a black absorbing bottom. This efficiency is surprisingly high, and lead to a further investigation of these black absorbing bottom collectors. Using nanofluids, and adjusting the collector height resulted in better collector performance for lower collector heights. Finally, design recommendations based on the performed theoretical and numerical analysis were presented. | en_US |
