R. Consul, I. Rodriguez, K. Claramunt and A. Oliva

Centre TecnOlogic 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 work presents the numerical studies performed by means of multidimensional Complutational Fluid Dynamics (CFD) simulations to analyse the thermal behaviour of solar water storage tanks operated by rigid distribution manifolds. Advantatges and limitations of the use of these kinds of diffusers to improve thermal stratification in storage tanks are discussed. A test procedure to characterise the level of mix­ing produced by natural convection when fluid entering the tank is colder than the surrounding fluid is proposed.

Introduction

The importance of developing improved storage devices is a key aspect of the thermal optimisation of many energy systems that are characterised by the delay between energy production and consumption. One of the most illustrative cases are thermal solar energy systems, where a good performance of the storage devices means a considerable increase in the overall efficiency of the installation.

Within the wide range of storage equipment, sensible heat storage tanks of liquid water are routinely used in standard thermal solar systems (i. e. solar domestic hot water and heating). The good thermal properties of water and its relatively low cost make the water storage tank the most traditional and extended storing equipment.

Even though the configuration of storage tanks is apparently simple, the extreme weak­ness of the level of temperature stratification (one of the most desirable properties in these equipments) make optimised designs very difficult. The store performance is dominated by mixing and diffusion processes over the interface between hot and cold fluid layers, but one of the major causes of loss of stratification in storage tanks is produced by the inlet mass flow rates (from collector and load loops) due to the enhancement of mixing and diffusion phenomena.

In order to improve the level of temperature stratification inside the storage devices, dif­ferent constructive possibilities have been explored in the literature such as the study of the position of the inlet/outlet ports, different kinds of materials, diffusers [1,2, 3, 4, 5], etc. Some of these improvements are being implemented in commercial products. Special interest has motivated the design and optimization of distribution manifolds [5]. These kinds of diffusers are used to reduce the mixing due to natural convection that occurs when the fluid entering the storage tank from the collector loop is colder than the surrounding fluid. Their principle of operation is based on the reduction of the momentum of water entering the tank, allowing buoyancy forces to direct the fluid to the location at which the resident store fluid temperature most closely matches the inlet fluid temperature.

In this work, and by means of multi-dimensional transient numerical simulations of heat transfer and fluid flow, the thermal performance of such internal elements will be virtually analysed. Special attention will be given to the advantages and limitations of available com­mercial designs, discussing possible improvements and/or alternatives to be considered.