Experimental demonstration, modeling and analysis of a novel latent-heat thermal energy storage unit with a helical fin

This paper presents a novel configuration suitable for latent-heat thermal energy storage systems based on the use of phase-change materials (PCMs). It is a double-pipe unit with a helical fin attached to the inner tube in which a heat-transfer fluid (HTF) flows. Experiments in the unit are performed both for regular conditions, when its shell is exposed to ambient air, and for a slightly heated shell which allows to achieve close-contact melting (CCM). It is clearly shown that CCM in the suggested unit is possible and shortens the melting time by a factor of three. In addition to the experimental demonstration, the paper includes comprehensive modeling of the unit. Both analytical and numerical models are included, describing the phenomenon of close-contact melting on a helical surface. The analytical model, made possible under the assumption that the fin is isothermal, accounts for other main features of the process and also reveals its governing dimensionless parameters, including the Fourier, Stefan, Archimedes and Prandtl numbers, along with the group representing the unit geometry. The numerical model includes a complete solution for the time-dependent fin temperature distribution, and can account also for the sensible heat of the PCM. A very good agreement of the numerical predictions and experimental findings is achieved. Then, a dimensional analysis, based on the analytical model, is applied to the numerical results. Generalized behavior is obtained for such parameters as the temperature difference between the heat-transfer fluid and the PCM, and the helical fin pitch. The analysis is then extended to a non-isothermal fin, suggesting an additional physically-meaningful dimensionless group which completely generalizes the results for different fin radii, fin thicknesses and fin-to-PCM volume ratios. It is argued that the suggested configuration may have a number of advantages in comparison with the existing fin-array systems. These advantages include enhanced melting, prevention of increased pressures in melting (or solidification if the material is anomalous like water) and voids in solidification, and highly-convenient handling and maintenance of the system. Also, the helical fin might be attached to the outer tube (shell), and a configuration in which the HTF flows across the shell is possible while preserving all important features of the unit explored.