Simon Furbo, Elsa Andersen, Alexander Thur, Louise Jivan Shah, Karin Dyhr Andersen

Department of Civil Engineering
Technical University of Denmark
Building 118, DK-2800 Kgs. Lyngby
Denmark

Email: sf@byg. dtu. dk,

Fax: +45 45 93 17 55

1 INTRODUCTION

The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat storage tank. The thermal performance is increasing for increasing thermal stratification in the heat storage, [1].

Thermal stratification in solar storage tanks is normally established in two ways:

— During charge periods, where heat from the auxiliary energy supply system or from the solar collectors is transferred to the “right” level of the tank. That is, the heat from the auxiliary energy supply system is normally transferred to the top of the tank and the solar heat is transferred to the level in the storage tank, where the tank temperature is close to the temperature of the incoming fluid transferring the solar heat to the tank. For small SDHW systems this is with advantage done by means of a vertical mantle heat exchanger, [2], [3], [4]. For large SDHW systems and for solar combi systems this is with advantage done by means of inlet stratifiers [5],

[6] , [7].

— During discharge periods where heat is discharged from a fixed level of the tank, for instance from the top of the tank for SDHW systems or from a level just above the lower level of the auxiliary volume in a storage tank for solar combi systems. Thermal stratification is best established during discharge if cold water enters the bottom of the tank in SDHW systems during draw-offs, and if the returning water from the heating system enters the tank through inlet stratifiers in solar combi systems [5].

Thermal stratification can be built up to a greater extent than normally if the solar tank is discharged from more than one level, [8], [9]. For instance, a hot water tank for SDHW systems can be equipped with two draw-off pipes, one at the very top of the tank and one at the middle of the tank. In periods where the temperature at the top of the tank is higher than the required hot water temperature, a part of the hot water can with advantage be tapped at a lower temperature through the lower draw­off pipe. In this way the volume of the cold water entering the tank during draw-off is increased, resulting in increased solar collector efficiency, decreased tank heat loss, decreased auxiliary energy supply and consequently increased thermal performance of the SDHW system.

This paper presents the results of theoretical as well as experimental investigations of the thermal advantage of discharge from different levels in solar storage tanks, both for SDHW systems and for solar combi systems.