I. Rodriguez, R. Consul, A. Oliva, C. Orozco

Centre TecnOlogic de Transferbncia de Calor (CTTC)
Lab. de Termotecnia i Energetica
Universitat Politecnica de Catalunya (UPC)
labtie@labtie. mmt. upc. es, www. cttc. upc. edu

The aim of this work is to present the numerical tools developed by our Center, to be employed in the design and optimization of hybrid latent/sensible storage devices. Two main kind of codes are distinguished: i) a general purpose Computational Fluid Dynamic (CFD) code used in the detailed multidimensional simulation of these equip­ment; ii) and a group of subroutines, based on global or one-dimensional mass and energy analysis, used to determine the store thermal performance, and to be em­ployed in long term thermal solar systems simulation codes. Illustrative results of the application of these numerical tools are presented.

Introduction

In thermal solar energy systems, heat storage devices help to match the delay between energy production and consumption. Among the different alternatives of thermal energy storage, sensible heat stores of liquid water are the most extended ones.

A promising alternative is the use of phase change materials (PCMs), with melting tem­peratures within the range of working temperatures, as a medium to store thermal energy by latent heat. The advantage of latent heat over sensible heat storage is its high thermal storage capacity at relative low temperatures. This property allows to store energy in a com­pact manner if appropriated heat transfer mechanisms between PCM and the fluid flow in the system loops are designed.

There are several proposals in the literature to take benefit of the attractive thermal prop­erties of the PCMs, among them, can be found what are defined as hybrid latent/sensible storage devices [9]. These stores consist in the classical storage tank that incorporate some PCM modules.

On the design and optimisation of these hybrid equipment, detailed numerical simula­tions of heat transfer and fluid flow using Computational Fluid Dynamics (CFD) codes, can be used as a powerful tool. CFD simulations can provide the designer with a way to test im­proved designs virtually. Mathematical models employed may be appropriate to reproduce heat transfer, fluid flow and solid/liquid phase change. Experimental validation plays here an important role.

Complementing detailed numerical simulations, simplified numerical models, based on global or one-dimensional mass and energy analysis, can help to predict the global thermal and fluid-dynamic behaviour of the hybrid devices. This kind of models can be used both to determine the store thermal performance (i. e. store parameters identification, ENV 12977­3), and to be included in long term thermal solar systems simulation codes.

The aim of this work is to present illustrative results of the employment of this two main kind of numerical tools in the design and optimisation of hybrid latent/sensible storage tanks. Main features of the numerical tools employed in these studies are described.