C. A. Cruickshank1* and S. J. Harrison1

1 Solar Calorimetry Laboratory, Department of Mechanical and Materials Engineering, Queen’s University,

Kingston, ON, K7L 3N6, CANADA

Corresponding Author, cruickshank@mc. quccnsu. ca

Abstract

The thermal response of a series-connected multi-tank thermal storage was studied under variable charge conditions. Tests were conducted on an experimental apparatus consisting of three 270 L storage tanks, connected in series and each charged through a coupled, side-arm, natural convection heat exchanger. Both energy storage rates and temperature profiles were experimentally measured during charge periods consisting of two consecutive clear days or combinations of a clear and overcast day. Of particular interest was the effect of rising and falling charge loop temperatures and collector-loop flow rate on storage tank stratification levels. Results of this study show that the series connected thermal storage reaches high levels of temperature stratification in the storage tanks during periods of rising charge temperatures and limits destratification during periods of falling charge temperature. This feature is a consequence of the series connected configuration that allows sequential stratification to occur in the component tanks and energy distribution according to temperature level. A further aspect of the study investigated the effect of increasing charge — loop flow rate on the temperature distribution within the storage system during a simulated charge sequence. Temperature distributions measured during high flowrate tests were found to be similar to those obtained during parallel charging of the storage system. This final result is consistent with expected theory. A disadvantage for both the high-flow series — connected and parallel connected multi-tank storage is that falling charge-loop temperatures that normally occur in the afternoon tend to mix and destratify the storage tanks.

Keywords: Solar energy, thermal storage, stratification, multi-tank

1. Introduction

This paper builds on previous studies undertaken to investigate the operation of medium capacity, multi-tank thermal storages connected in both series and parallel configurations. Multi-tank storage schemes have been previously investigated [1-5]. A multi-tank thermal storage with immersed-coil heat exchangers was originally proposed by Mather et al., [1] and showed to sequentially thermally stratify during charging. More recently, a series-connected thermal storage, Fig. 1, in which the individual storage tanks are charged through natural convection heat exchangers (NCHE’s), has been studied [2-5]. In this arrangement, each NCHE can charge its associated storage tank in a fashion similar to a traditional thermosyphon [2]. As well, the charging of the individual storage tanks is self-regulating (i. e., each acts as a thermal diode) and self­distributing (i. e., the energy from lower temperature charge periods is directed to lower temperature downstream tanks). This sequential stratification maintains higher temperatures in the thermal storage by avoiding destratification during cooler charge periods or the removal of energy from the upstream tanks to the downstream storage tanks (i. e., “thermal carry-over”). Under ideal

operation, maximum thermal stratification is obtained in the storage system. In real systems, however, non-ideal conditions exist including: a finite number of storage tanks; limited heat transfer rates in the heat exchangers; and non-adiabatic storage tank walls and piping, etc. In addition, reverse thermosyphoning due to adverse temperature gradients in the heat exchanger or thermosyphon loop may result in destratification. Thermal carry-over may occur during periods of falling charge temperatures, i. e., in the afternoon or during periods of low solar input.

To investigate these effects, a multi-tank thermal storage, Fig.2, was assembled in a laboratory environment to allow tests to be conducted under controlled charge conditions representative of combinations of clear and cloudy days. Data obtained included temperature profiles within the storage tanks (recorded as a function of charge duration) and energy transfer rates in the individual heat exchangers. The description of the system, the test apparatus and results are presented below.