Simon Furbo and Soren Knudsen
Department of Civil Engineering
Technical University of Denmark
Building 118, DK-2800 Kgs. Lyngby
Denmark

Fax: +45 45 93 17 55

1. INTRODUCTION

Investigations have shown that small SDHW systems are best designed as low flow systems with a vertical mantle tank [1], [2], [3], [4], [5], see Fig. 1.

A simulation model, MANTLSIM, for small low flow SDHW systems with a vertical mantle tank was originally developed and later modified at the Technical University of Denmark [6], [7], [8], [9], [10]. MANTLSIM can be used to calculate the yearly thermal performance of a solar heating system based on weather data from the Danish Test Reference Year TRY [11]. Recently the model was further improved and validated [12], [13]. The improvements were based on detailed studies of the fluid patterns and the heat transfer, both in the vertical mantle and in the inner domestic hot water tank. The studies were carried out by means of CFD (Computational Fluid Dynamics) models. These models were validated by means of experiments, both with a mantle tank in a heat storage test facility and by means of PIV (Particle Image Velocimetry) measurements with a transparent glass mantle tank.

With the CFD models parameter analyses were carried out for differently designed mantle tanks under typical operation conditions. Based on the analyses a number of Nusselt-Reynolds-Rayleigh heat transfer correlations were developed for the heat transfer between the solar collector fluid in the mantle and the inner and outer
mantle walls and between the tank wall and the domestic water in the hot water tank.

Thermal stratification is built up in the hot water tank due to natural convection in the tank. By means of CFD calculations for typical operation conditions, a method was developed to determine the heat transfer in the hot water tank caused by the natural convection.

Based on CFD calculations for typical operation conditions, a method to determine the mixing inside the mantle caused by the incoming solar collector fluid was developed.

The developed heat transfer correlations and methods to determine the heat transfer in the hot water tank caused by natural convection and to determine the mixing in the mantle were utilized in MANTLSIM. MANTLSIM was validated by means of measurements in a test facility for solar heating systems for two low flow solar heating systems with mantle tanks — one with the mantle inlet at the top of the mantle and one with the mantle inlet placed with a distance from the top of the mantle of about one fourth of the mantle height.

The measurements of the thermal performance of the two low flow systems as well as results of calculations of the yearly net utilized solar energy of low flow SDHW systems with differently designed mantle tanks will be presented in this paper.