Modélisation de systèmes photovoltaïques-thermiques (PVT) basés sur des nanofluides et des matériaux à changement de phase

Abstract

The escalation in global growth has resulted in a heightened reliance on fossil fuels for energy generation. This significantly affects the amount of greenhouse gases released into the atmosphere. Researchers worldwide are exploring novel, cost-effective, and sustainable energy alternatives. An alternative strategy is to enhance the current energy storage systems, which are equally crucial to the development of new energy sources. Thermal energy is abundantly present in nature as a secondary result of many energy conversion facilities. Thermal energy may be stored utilising latent, sensible, and thermo-chemical storage techniques. The usage of phase Transition materials in the latent heat thermal energy storage method is considered the most capable strategy for energy storage. The approach has a high energy storage density and is distinguished by minimal temperature fluctuations throughout the energy storage process. The limited thermal conductivity of phase change materials (PCMs) is a significant disadvantage that restricts their use as materials for storing thermal energy via latent heat. The poor thermal conductivity of Phase Change Materials (PCMs) prolongs their thermal energy storage and release durations. The current research examines several methods based on phase change materials (PCMs) to improve the overall thermal efficiency of storage systems. The solutions presented in this thesis include a novel design of metallic fins and the incorporation of nanoparticles and varying HTF temperature into the heat exchanger. Computing simulations are performed to assess the suggested solutions by monitoring the melting process. This publication is organised into two primary parts that provide the findings of the research aimed at enhancing the efficiency of latent thermal energy storage systems using phase change materials (PCMs): In the first part of this study paper offers the findings of a study focusing on four various types of stepped fins, including two-downward step wings, two-upward step wings, an ascendent and descending stepped fin, and two-stepped wings opposite one another. The calculation of the force of buoyancy in the vertical thermal energy storage system, which is filled with rubitherm (RT27) phase change material and Cu nano-additives, was performed using the Boussinesq approach. The numerical simulation of the governing equation was performed using the enthalpy porosity approach, namely the Generalized Finite Element Method (GFEM). The distinctive nature of the thermal and functional entropies for two-ascendent stepped fins and 5 two-descending stepped fins may be attributed to the effect of acceleration caused by gravity, the mass of phase transition material, and the orientation of the intake fins. The container with two downward-sloped fins facilitated a melting time of no less than 71 minutes. There is a 6.58% variance in the charging time among instances 1 and 2. Nonetheless, a notable disparity of 26.76% is seen in the period required for charging. The second phase of this work included the implementation of Y-shaped fins integrated with nano-improved phase transition material. The study included examining and contrasting three distinct setups of the TESS system: case 1, which served as the reference without any fins, case 2, which had two Y- formed fins linked to the tubes, and case 3, which featured four Y-formed fins attached to the tubes. The system's governing equations are discretized using the finite element approach. In addition to examining the impact of the TESS arrangement, we also investigated the effects of the HTF temperature (338 and 348 K) and the volume percentage of the nanoparticles (ranging from 0 to 0.08). The analysis focuses on the changes in temperature contours and liquid fractions of three distinct designs when subjected to two different HTF temperatures. The results indicated that the use of nanoparticles with a volume fraction of 8% increased the thermal conductivity by 19% during the process of melting. Raising the HTF temperature to 348 K resulted in an 87% acceleration of the melting process. Ultimately, it was determined that TESS, equipped with four Y-shaped fins, proved to be the most efficient. It managed to decrease the changing time by 48% when related to the base case, also known as case 1.