A NUMERICAL PROCEDURE FOR PCM THERMAL STORAGE DESIGN IN SOLAR PLANTS

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