• Energy Research

Abstract This paper concerns the optimum design of double elliptical latent heat storage units during the charging process using three-dimensional numerical simulation. Water and RT35 are employed as the heat transfer fluid and phase change material, respectively. The orientations of both inner and outer elliptical tubes, the number of inner tubes, and the comparison between the straight and wavy configurations for the inner tubes are examined to find the maximum melting rate. Moreover, the sensitivity analysis on the operating conditions, including the Reynolds number and inlet water temperature, is performed. The results show that the best performance is found when the inner and outer tubes are oriented vertically and horizontally, respectively. The performance of the unit enhances as the distance between the inner tube and the bottom wall of the outer tube reduces. Besides, the optimum place for positioning the inner tube is where the minimum distance of the ellipses is 2 mm. It is found that the implementation of double wavy inner pipes increases the heat transfer surface area, which accelerates the melting time by 2.17. The delivered energy rate to the PCM using double wavy inner tubes is 218.75 W/kg, while it is 180.4 W/kg using double straight inner tubes. Eventually, the sensitivity analysis confirms the system is more sensitive to the variations of inlet temperature compared to the Reynolds number regarding the tested operating conditions.

Abstract In this study, an experimental assessment is presented on the effects of employing serpentine tubes with three different cross-sections of circular, triangular, and rectangular, on the characteristics of a photovoltaic/thermal (PV/T) unit in terms of energy and exergy efficiencies compared with a conventional PV system. The influences of adding magnetite nanoparticle to the base-fluid and employing a higher mass flow rate of the cooling fluid are examined. The results demonstrated that by comparing PV and PV/T units with a circular serpentine tube, the electrical efficiency improves by almost 12% due to adding coolant tube in addition of getting 22.6W extra thermal energy power. Furthermore, by changing the cooling tube configuration from conventional circular form to rectangular, electrical efficiency eases by 2% to reducing PV module temperature. Moreover, it was found that by adding nanoparticle to pure water to employ nanofluid as coolant fluid, overall energy and exergy efficiencies enhance by 6.6% and 0.7%, respectively, using nanoparticles with a volume concentration of 2% for the case of the rectangular serpentine tube for the flow rate of 20 kg/h. Furthermore, enhancing the mass flow rate has a positive trend on the PV/T performance in terms of both energy and exergy efficiencies.

Abstract High-temperature metal hydride (MH), such as magnesium hydride, is considered as one of the most promising technology to store hydrogen. However, there are two main bottlenecks, including the low rate of hydrogen absorption and low capacity of the MH reactor. In this regard, heat removal from the MH tank plays a crucial role in the hydrogen storage process. In the present study, to increase the hydrogen absorption performance, a novel configuration of the MH reactor is proposed and simulated using computational fluid dynamics (CFD). The study aims to assess the geometrical parameters of the proposed heat exchanger. Besides, a sensitivity analysis of the operating parameters of the reactor, including the Reynolds number and temperature of the air as well as hydrogen supply pressure is performed. The results indicate that the charging time drops significantly by increasing the number of air passages since the heat transfer rate improves dramatically. By raising the heat transfer fluid initial temperature, the charging time increases; however, as the heat transfer fluid Reynolds number and the inlet pressure of hydrogen increase, the absorption process accelerates. The recommended configuration is introduced by considering both charging time and manufacturing limitations. It is shown that the loading is approximately 30 minutes for the new multi-zone hydrogen energy storage using four passages for the air which provides a more applicable hydrogen fuel system.

Abstract To obtain maximum exergy and energy efficiencies of photovoltaic-thermal (PV-T) systems, innovative configurations of coolant tubes are proposed and simulated numerically. The tubes’ configuration is modified using non-uniform wavy tube concept, which forms different styles, including ascending and descending amplitude of coolant tubes. Besides, the influences of geometrical parameters, extending PV panel length, sensitivity analysis on the operating conditions and a comprehensive investigation of different types of heat transfer fluids are analysed for the innovative systems. The results demonstrate that the PV-T performance develops in terms of electrical, thermal, and exergy efficiencies using ascending wavy tubes compared with straight, uniform wavy and descending wavy tubes. The electrical and thermal efficiencies are promoted from 10.94% to 61.04% for the straight tubes to 11.32% and 65.21%, respectively, for the system in which ascending wavy tubes are utilised. A comprehensive study of different coolant fluids proves that when SiC and MPCM-28 are simultaneously employed, the best cooling fluid is achieved, leading to a 0.4% higher electrical efficiency than the case in which water is used.

Abstract Hydrogen energy storage through Metal Hydride (MH) reactors has various applications in concentrated solar powers and fuel cells for stationary applications in renewable energy systems. Hydrogen storage performance and consumption of these systems are strongly dependent on heat and mass transfer characteristics. Incorporating Phase Change Materials (PCMs) and spiral tube heat exchangers into metal hydride reactors improves storage performance significantly. The present paper includes a numerical investigation on the storage performance of a novel Porous Metal Hydride Tank (PMHT) integrated with PCM as a passive heat transfer system. Moreover, a heat exchanger with newly-offered Flat Spiral Tube Planes (FSTPs) is mounted into a reactor for heat transfer enhancement. The air as Heat Transfer Fluid (HTF) flows into the metal reactor through a spiral tube to enhance the heat and mass transfer. The effects of various spiral configurations, different shapes, and thicknesses of PCM jackets on the absorption/desorption process of hydrogen are investigated in detail. Also, various inlet temperatures are applied to the system to examine its impact on the absorption/desorption process. According to the results, increasing the number of FSTPs along the PMHT reduces the absorption duration by 8%. Moreover, compared with PMHT without PCM, the absorption and desorption performance of PCM-based PMHT is improved by 44% and 20%, respectively. Furthermore, increasing and decreasing the air inlet temperature leads to a 43% and 47% reduction in total desorption and absorption duration, respectively. Finally, conical-shaped PCM-jackets are found to be capable of demonstrating better absorption and desorption performances. They offer the advantage of less PCM usage to provide even a higher performance than annulus PCM-jackets. In this regard, the conical-shaped PCM provides the means for the PMHT to reach its maximum hydrogen absorption by only 85% usage of its PCM volume. In comparison, annulus-shaped PCM uses 98% of its PCM volume to help the PMHT reach its maximum hydrogen absorption capability.

Abstract To develop a more efficient water-cooled photovoltaic-thermal system, energy and exergy analysis of a photovoltaic-thermal system with wavy tubes are investigated numerically using different coolant fluids. A comparison between the straight tube and wavy tubes is conducted for various wavelengths and wave amplitudes. The geometrical parameters of the wavy tubes as well as the velocity of the coolant fluid are examined. Besides, the consequences of coolant fluid including water, Ag/water nanofluid, microencapsulated phase change material slurry, and also a new type of cooling fluid called microencapsulated phase change material nano-slurry are studied. The results show that the electrical, thermal, and exergy efficiencies of the photovoltaic-thermal module enhance by using the wavy tubes compared with the corresponding straight tubes. By declining wavelength in a constant wavelength/amplitude, the heat absorbed by the heat transfer fluid raises. For the best configuration, the primary and exergy efficiencies of the module increase by 6.06% and 4.25%, respectively, for the wavy tubes system compared with those for the straight unit. Furthermore, in both configurations, by increasing the inlet velocity, the overall performance of the photovoltaic-thermal module increases due to a higher heat transfer rate. The results also reveal that among different types of cooling fluids, the microencapsulated phase change material nano-slurry has higher performance in terms of both energy and exergy efficiencies due to having higher thermal conductivity and heat capacity. By employing the wavy tube and the novel proposed coolant fluid, the primary and exergy efficiencies increase in comparison with a typical photovoltaic-thermal module.