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The evacuated tube solar collector is considered an efficient, convenient, and economical option used to convert solar energy into heat. In this work, enhancement of evacuated tubes solar collector performance and the potential for energy storage by using Al2O3 water-based nanofluid embedded in Graphite as a saturated porous media was presented and studied theoretically. The Governing equations derived from the principles of conservation of mass, momentum, and energy were written in a dimensionless form, and solved these equations analytically for the fully developed region, and numerically throughout the entrance region, and the heat flux data for Amman, Jordan was used in this study. The analysis of the effect of different parameters as porosity, pore diameter, nanoparticles solid volume fraction, pressure, and radius of conduit on temperature variation through the pipe and Nusselt number was done. The results show that the flow of nanofluid in porous media enhances the evacuated tube's performance compared to just using water in the same medium under the same conditions. In August, the temperature variation in the evacuated tube at the same condition for Al2O3 water-based nanofluid for 1.5% volume fraction reach 42° and 73° for 6% volume fraction. In contrast, In January for the 1.5% volume fraction the temperature variation reached 25° and 43° for 6% volume fraction. On the other hand, the stored energy approximately reached 30 kJ/kg and 45 kJ/kg for solid volume fraction 1.5% and 6% respectively at the same conditions. [ABSTRACT FROM AUTHOR]

Thermal energy storage systems are used mainly in buildings and industrial processes. In this study, solar energy storage by using a circular conduit filled with porous media that is saturated by a non-Newtonian fluid at constant heat flux was represented. The fully developed region was studied by solving the equations analytically, the non- Newtonian fluid parameters used in this model are carboxymethyl cellulose (CMC) properties. In addition, graphite was used as porous media. The heat flux data for Amman city was used in the equations in this study. The effect of Porosity and particle diameter and pressure on the performance of the model were discussed and sketched. As a result, the temperature of storage filled with CMC fluid is better than water in porous media. It is found that the power index of the fluid, porosity, particle diameter, pressure drop, and conduit radius effect inversely the temperature in energy storage. In January, the temperature variation in conduit at the same conditions reach for CMC-1 to 35°C for n = 0.724 and to 85°C for n = 0.7182 and to 190°C for n = 0.7122. CMC-2 has a higher consistency index at the same concentration which means higher viscosity and less power index than CMC-1 so the temperature variation in conduit reaches 50°C for n = 0.599 and 200°C for n = 0.57. The stored energy of CMC-1 for n = 0.724, n = 0.7182, and n = 0.7122 is approximately 120 kJ, 300 kJ, and more than 600 kJ respectively, and the stored energy of CMC-2 n = 0.599 and n = 0.57 is 200 kJ and approach to 800 kJ at the same conditions and conduit. [ABSTRACT FROM AUTHOR]