Figures 15 to 20 show results from the simulations of the CTCFR, CTVFR, and VTVFR cases. The duration of each simulation was in the order of 4.5 to 5.5 hours. Only the “hot” tank has been simulated in these tests.

The combination of the Energy Response and Exergy Charge Response curves indicate that the original STTS design, Tank 1, produces a high rate of mixing in all cases studied. This is evidenced by a slow climb of the Exergy Charge Response along with an early decline of the Enerhy Reponse. This observation was also evident in the initial study by the authors [6]. The high values of the цш curves during the VTVFR simulations, however, illustrate the difficulties in assessing closed-loop systems using only First Law characterization methods.

From the Exergy Charge Response curves, we see that Tanks 6 and 7 have initially very high values of П(ск. This must be attributed to the tanks’ separation of the internal volume into smaller nodes, as the incoming fluid has less of a distance to travel downstream before a thermocline can develop. Tank 7, which incorporates the slotted plenum at the inlet region, exhibits only a slight performance advantage over Tank 6. We also see, however, temporal dips in the curves for both Tanks 6 and 7, most evident in Figure 16. These dips occur at each instance that hot fluid reaches a new node, initially mixing until a new thermocline develops. Tank 4, with a less immediate increase to its n^CR curves eventually outperforms Tanks 6 and 7 once its thermocline develops. It should also be noted that the eventual fall of the Energy Response also occurs at a lower rate for Tank 4, indicating improved energy storage characteristics. Tanks 2 and 3 illustrate the storage effectiveness gained solely from repositioning the STTS inlets, irrespective of a center baffle. For the Operational Exergy Charge Response in Figure 20, we can see the impact of the system’s flow rate limitations. This characteristic will be discussed further in work currently in development by the authors. The assessment of Tank 5 will also be done within a future study.

2. Conclusion

This study has presented an early investigation into a new method for characterizing thermal energy stores using the 2nd Law of Thermodynamics. Although initial findings have shown that the new index, the Exergy Charge Response, may be useful in the assessment of thermal mixing at the initial stages of charging, a future study will continue to examine its applicability for various charging/discharging applications, as well as account for existing 2nd-Law characterization indices.

References

[1] Panthalookaran, V., (2007). CFD-Assisted Characterizationand Design of Hot Water Seasonal Heat Stores.

Ph. D. Thesis. University of Stuttgart, Germany

[2] Shah, L. J., and Furbo, S., (2003). Entrance Effects in Solar Storage Tanks. Solar Energy, 75(4) pp. 337­48.

[3] Sibbitt, B., Onno, T., McClenehan, D., Thornton, J., Brunger, A., Kokko, J., Wong, B. (2007). The Drake Landing Solar Community Project — Early Results. Proceedings of the 2nd Annual Canadian Solar Buildings Research Network. Calgary, AB, Canada.

[4] Rysanek, A., Harrison, S. J., (2008). On the Characterization of Thermal Energy Stores using CFD: A Case Study of the Drake Landing Solar Community. Proceedings of the 3rd Annual Canadian Solar Buildings Conference. Fredericton, NB, Canada.

[5] Moran M. J., Shapiro H. N. (2004). Fundamentals of Engineering Thermodynamics. 5th Edition.

John Wiley and Sons, Inc., USA

[6] Krane, R. J., (1987). A Second Law Analysis of the Optimum Design and Operation of Thermal Energy Storage Systems. International Journal of Heat and Mass Transfer. 30(1) pp.43-57.

[7] Rosen, M. A., and Dincer, I., (2003). Exergy methods for assessing and comparing thermal storage systems,

Transport Phenomena in Energy systems and Processes. Wiley. UK. 27, pp. 415-30

[8] Haller, M. Y., Cruickshank, C. Streicher, W., Harrison, S., Andersen, E., Furbo, S. (2008). Methods to determine stratification efficiency of thermal energy storage processes — review and theoretical comparison.

Solar Energy. (in publication)

[9] Han, Y. M., Wang, R. Z., Dai Y. J., (2008). Thermal stratification within the water tank. Renewable and Sustainable Energy Reviews (in publication)