This paper is aimed to propose an analytical and numerical procedure for design thermal storage systems employing Phase Change Materials PCM as medium. The use of PCM for thermal energy storage is a really challenging opportunity because of their high heat capacity. It is assessed that heat capacity per unit volume of a PCM medium can be 5-14 times more than traditional sensible heat storage materials such as water, masonry or rock. In addition, PCM absorbs and release heat at a nearly constant temperature. Therefore, PCM represent a valuable choice for those energy systems needing a thermal storage section, such as solar thermo-dynamic plants. Latent heat storage is based on the heat absorption or release when the storage material undergoes a phase change from solid to liquid or vice versa. A complete understanding of the phase change phenomenon involves an analysis of the various processes that accompany it. The most important of these processes, from a macroscopic point of view, is the heat transfer process. This is complicated by the release, or absorption, of the latent heat of fusion at the "moving" solid-liquid interface. A numerical procedure to define design criteria for engineered thermal storage units is proposed in this study. It is mainly based on the following steps: i) assessment of the total mass/volume of a chosen PCM by the energy balance solution; ii) analytical computation of the transient position of the phase front by a transcendental equation solution applied to outward-directed melting/freezing of a hollow cylinder due to a temperature/thermal flux imposed at the internal radius; iii) determination of the geometrical properties of the storage unit (length, number and relative arrangement of tubes in the array); iv) FE-based simulation of a portion of the storage unit for estimating the total transient time needed by the melting/freezing process and the energy effectively stored by the designed system. Numerical simulations were carried-out adopting the enthalpy formulation for the energy equation, that allows to solve one single equation for heat transfer both in solid and liquid phase. The applied methodology was firstly validated by comparison with literature results. As test-case for the numerical tool built-up, we analysed the melting process of a Lithium Nitrate storage unit serving a 1 MW Direct Steam Generation solar plant. The present work has been developed in the framework of the FREeSun, supported by a grant from the Italian Ministry of the Economic Development.
|Titolo:||A numerical procedure for pcm thermal storage design in solar plants|
|Data di pubblicazione:||2013|
|Appare nelle tipologie:||1.1 Articolo in rivista|