As described previously, the storage tanks were seen to stratify in a sequential manner as well as individually during the charge sequences. During periods of declining charge temperatures, there was some evidence of mixing, resulting in a slight temperature drop in the upper sections of the storage tanks. As well, it was noted that, during the intervals corresponding to low charge temperatures, the storage tanks appear to be slowly dropping in temperature. This effect is a result of a number of factors including: standby heat losses from the tank walls to the surrounding environment; heat losses from the natural convection loop; and reverse thermosyphoning caused by discharge or carry-over of heat from a high temperature storage to a lower temperature storage. A careful inspection of Figs. 8, 11 and 14 during these intervals show that the bottoms of the tanks are actually increasing in temperature as the upper sections are dropping.

Time (hour) Fig. 16. Case B, Series: Temperature profile of storage tanks during charging for 2-day high/low input power test.	Time (hour) Fig. 18. Case B, Parallel: Temperature profile of storage tanks during charging for 2-day high/low input power test.
Fig. 17. Case B, Series: Individual charge rates across each heat exchanger for 2-day high/low input power test.	Fig. 19. Case B, Parallel: Individual charge rates across each heat exchanger for 2-day high/low input power test.

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Figure 20 shows the effect of the collector loop flowrate on the magnitude of the heat transfer rate for Day 1. This figure indicates that at lower charge loop flowrates, higher degrees of stratification are obtained in the storage tanks. That is, at lower flowrates, more energy is transferred in Tank 1, driving it to a higher temperature. At higher charge loop flowrates, less energy is transferred to Tank 1 and more is carried over to Tanks 2 and 3. This result is expected as the temperatures in the storages will tend towards a uniform temperature at higher flowrates. In the extreme case, the storages will behave as though the tanks were connected in parallel. This conclusion is supported by results shown in Figs. 16 to 19

1st International Congress on Heating, Cooling, and Buildings, 7th to 10th October, Lisbon — Portugal /

0 2 4 6 8 10 Time (hour)

Fig. 20. Individual charge rates across each heat exchanger for variable input power tests at various charge flowrates.

3. Conclusion

Laboratory tests were conducted on a series-connected modular thermal storage to measure the unit’s thermal performance and temperature profiles under specified charge conditions. In particular, tests were performed to study the interaction of the individual sequentially-connected tanks and to investigate the effects of rising and falling charge loop temperatures on temperature profiles in the storage tanks. Preliminary results indicate that sequential stratification was observed for the charge profiles studied however a small amount of mixing was observed in the upper section of the storage tanks due to falling charge temperatures in the afternoon period. In addition, results indicate that a small amount of heat was also carried over from the high temperature storage to the lower temperature, downstream storages during charging. Furthermore, the dependence of the heat transfer rate on the magnitude of the charge loop flowrate was also shown. Finally as predicted by theory, as collector loop flow rate is increased, the distinction between the series and parallel connected storages disappears.

Acknowledgements

This study was supported by the Canadian Solar Buildings Research Network, the Natural Science and Engineering Research Council of Canada and EnerWorks Inc.

References

[1] Mather, D. W., Hollands, K. G. T. and Wright, J. L. 2002. Single — and Multi-tank Energy Storage for Solar Heating Systems: Fundamentals, Solar Energy 73, pp. 3-13.

[2] Cruickshank, C. A. and Harrison, S. J., 2006. Experimental Characterization of a Natural Convection Heat Exchanger for Solar Domestic Hot Water Systems, Proceedings of ISEC 2006, International Solar Energy Conference, Denver, Colorado, USA.

[3] Cruickshank, C. A. and Harrison, S. J. 2006. Simulation and Testing of Stratified Multi-tank, Thermal Storages for Solar Heating Systems, Proceedings of the Eurosun 2006 Conference. Glasgow, Scotland.

[4] Cruickshank, C. A. and Harrison, S. J. 2006. An Experimental Test Apparatus for the Evaluation of Multi­Tank Thermal Storage Systems, Proceedings of the Joint Conference of the Canadian Solar Buildings Research Network and Solar Energy Society of Canada Inc. (SESCI), Montreal, Quebec.

[5] Cruickshank, C. A. and Harrison, S. J. 2008. Experimental Evaluation of a Multi-tank Thermal Storage Under Variable Charge Conditions, Accepted for Publication in the Proceedings of the Joint Conference of the Canadian Solar Buildings Research Network and Solar Energy Society of Canada Inc. (SESCI). Fredericton, New Brunswick.