Abstract
The aim of this paper is to study the influence of enclosure size in latent heat thermal energy storage systems embedded in a porous medium for domestic usage of latent heat thermal energy storage heat exchangers. A 2-D rectangular enclosure is considered as the computational domain to study the heat transfer improvement for a phase change material embedded in a copper foam considering a constant heat flux from the bottom surface. Different dimensions of the composite system are examined compared with a system without a porous medium. The thermal non-equilibrium model with enthalpy-porosity method is employed for the effects of porous medium and phase change in the governing equations, respectively. The phase change material liquid fraction, temperature, velocity, stream lines and the rate of heat transfer are studied. The presence of a porous medium increases the heat transfer significantly, but the improvement in melting performance is strongly related to the system's dimensions. For the dimensions of 200 × 100 mm (W × H), the melting time of porous-phase change material with the porosity of 95% is reduced by 17% compared with phase change material-only system. For the same storage volume and total amount of thermal energy added, the melting time is lower for the system with a lower height, especially for the phase change material-only system due to a higher area of the input heat. The non-dimensional analysis results in curve-fitting correlations between the liquid fraction and Fo.Ste.Ra −0.02 for rectangular latent heat thermal energy storage systems for both phase change material-only and composite-phase change material systems within the parameter range of 1.16 < Ste < 37.13, 0 < Fo < 1.5, 2.9 × 104 < Ra < 9.5 × 108, 0 < L f < 1 and 0 < Fo.Ste.Ra −0.02 < 0.57. Over a range of system's volume, heat flux and surface area of the input heat flux, the benefit of composite phase change material is variable and, in some cases, is negligible compared with the phase change material-only system.
Highlights
latent heat thermal energy storage (LHTES) systems are used due to having a high capacity of heat storage, typically 5 to 14 times higher than sensible heat storage systems, with the added advantage of almost constant temperature during the solidification/melting process [1, 2]
Zhang et al 32 studied numerically different metal foams of copper and nickel in a latent heat solar energy storage system using molten salt phase change material (PCM). They showed the reduced effect of natural convection in the presence of composite metal foam due to the flow resistance. They showed that due to the large difference between the thermal conductivity of metal foam and PCM, a considerable temperature gradient exists between the porous medium and PCM and the thermal non-equilibrium model should be correctly considered in the simulation
5.1.Effect of porous-PCM compared with PCM only The presence of the porous medium inside the LHTES system results in the enhancement of heat transfer rate in the domain
Summary
LHTES systems are used due to having a high capacity of heat storage, typically 5 to 14 times higher than sensible heat storage systems, with the added advantage of almost constant temperature during the solidification/melting process [1, 2]. They showed the reduced effect of natural convection in the presence of composite metal foam due to the flow resistance. They showed that due to the large difference between the thermal conductivity of metal foam and PCM, a considerable temperature gradient exists between the porous medium and PCM and the thermal non-equilibrium model should be correctly considered in the simulation. Mahdi et al studied a triplex-tube LHTES system using both Al2O3 nanoparticles and copper foam to increase the melting time of the heat exchanger They showed that simultaneous use of porous medium and nanoparticles can improve the melting of PCM significantly. By employing 95% porosity copper metal foam, the melting time reduces from 162 minutes for the non-porous case min to 18 minutes; employing 5% nanoparticle just reduced the melting time to 130 min
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