J. Cadafalch, R. Consul, X. Trias, C. D. Pbrez-Segarra

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

This paper presents a CFD model for the study of a rectangular water store forming part of a integrated collector storage system. The govening equations (mass, momen­tum and energy conservation) are solved assuming bidimensional and laminar flow. Details of the mathematical model (governing equations and boundary conditions) and of the numerical model (discretization of the equations, solver, numerical param­eters…) are described. A set of numerical solutions of a reference case representing 24 hours of exposure to outdoor weather conditions and without draw-off, have been obtained in order to investigate the sensitivity of the numerical solutions on the tem­poral and spatial discretization adopted. A final validation process by comparison of the numerical solutions with experimental data from the litterature and with direct numerical simulation, DNS, (high level simulation that considers turbulence effects), obtained by the authors is also presented.

Introduction

The integrated collector storage ICS is one of the simplest and cheapest solar thermal systems. The first thermal solar systems constructed were ICSs and during all the XXth century these kind of systems were commercial all over the world.

In the ICS the store is directly exposed to the ambient. Therefore, the system heat losses are high, especially during night periods or during periods with low ambient temperature. This technical problem has limited the applicability of the ICSs up to now to low temperature applications in hot climates. Future improvements of the design of ICSs should be focus on solving this problem, for example by means of transparently insulated covers, or using new configurations of the store.

As in all other thermal engineering areas, the use of Computational Fluid Dynamics tech­niques will play a key roll in the development of new components for solar thermal applica­tions. In this work, a two-dimensional transient numerical model for the evaluation of heat transfer and fluid flow in a rectangular storage element of a ICS system is presented. The model solves the governing equations (mass, momentum and energy conservation) assum­ing two dimensional laminar flow. With this model is possible to evaluate a complete cycle of 24 hours of charging (heating up during sunny hours) and discharging process (cooling down) of the ICSs on a standard Personal Computer in a few hours. The mathematical formulation is discussed including both the governing equations and the boundary condi­tions. Details on the numerical model are given, and the required numerical parameters (discretization, numerical schemes, convergence criteria..) are evaluated. In order to vali­date the results of the numerical model, the store elements has been analysed under two different working conditions. In one of them, results are compared to experimental values from other authors. In the other, solutions are compared to numerical solutions obtained by a more detailed direct numerical simulation (DNS) model in which the hypothesis of laminar flow is not assumed.