Enhanced Harvest of Solar Energy with the Utilization of Nanofluids and Biodegradable Fluids

Abstract

Efficiency improvement opportunities for decarbonization of fossil fuel-dominated sectors are crucial when the energy end-use sector generating significant annual greenhouse gas emissions are attributed to heating. Given the forecast increases in global population and energy demand, effectively utilizing the available renewable energy resources is a pressing issue for minimizing the greenhouse gas emissions from energy production. Solar energy, the most prevalent renewable energy source, has not yet reached its full potential. The primary explanation is the inefficiencies of the solar technology available. Due to significant thermal losses, conventional solar thermal collectors have been suboptimal systems for harvesting the immense energy provided by the Sun. Proposed mechanisms limiting the thermal resistance and using engineered fluids have been contemplated. Despite the last decades' fascination with nanoparticle-based fluids demonstrating superior heat transfer characteristics, its drawbacks, such as toxicity and clogging, have stimulated the development of more bio-sustainable fluids. In this thesis, an experimental and numerical analysis considered the thermal performance of both carbon-based nanofluids and biodegradable fluid composed of coffee colloids in a direct absorption solar collector (DASC). The constructed experimental set-up investigated the thermal enhancement within the working fluids of study under the same system configuration. The working fluid composition consisted of carbon-black nanoparticle concentrations ranging from 0.005 to 0.020 wt.% and biodegradable fluid with a coffee colloid concentration of 0.05 g/ml. As for biodegradable fluid, a supplementary stability investigation was carried out. Additional experimental investigation of the extinction coefficient for corresponding working fluids dictates the expected heat absorption by the fluid in simulations. Further, a numerical analysis using computational fluid dynamics (CFD) software was implemented to study a recreation of the experimental DASC. Two simplified approaches were applied to examine the working fluids' thermal performance. The experimental results were validated against the simulated models. The overall findings indicated a concurrence: nanofluids exhibited the best thermal performance. Moreover, it was experimentally observed a decreasing thermal performance for increasing nanoparticle concentration. According to experimental results, the optimum nanoparticle concentration of 0.010 wt.% carbon black and SDS yielded a 102 % thermal enhancement from the base fluid, whereas the biodegradable fluids experienced a 12 % diminution. As for the simulations, the experimentally estimated extinction coefficient approximated the thermal enhancement for both approaches. While the biodegradable fluids experienced no distinct performance enhancement, the optimal nanofluid concentration disclosed a 76.6 - 90.9 % increase compared to the base fluid. However, the heat generation from simulation underpredicts the experimental results, indicating a deficiency in heat transfer in the model considerations. The approaches for examining the volumetric heat absorption and findings suggest that model adjustment is necessary for grasping the fundamental heat transfer principles of the system.