15.07.2015   EuroSun2008-10

Comparison between tank-in-tank and bikini solar combisystems

In order to compare bikini and tank-in-tank solar combisystems, the best tank-in-tank model system from Fig. 5 is compared to a bikini tank system. Thus, Model 7 is the tank which performs best among the tank-in-tank models with different space heating loads and the Task 26 DHW profile. Both bikini and tank-in-tank system have the same H/D ratio of 4. Fig. 8 shows the net utilized solar energy of the solar combisystem installed in the old house, the house with medium space heating load and the low energy house for collector areas of 3, 5 and 8 m2. The smaller the collector area, the better the bikini solar combisystems performs compared to tank-in-tank solar combisystems. For a collector area of 3 m2, the net utilized solar energy increases for the tank-in-tank solar combisystem and decreases for the bikini solar combisystem as the space heating load increases. For bikini solar combisystem, the set point temperature for the auxiliary heater is increased from 55°C for the low energy house to 60°C and 75°C for the houses with medium space heating demand and high space heating demand. For the tank — in-tank system, the auxiliary set point temperature remains constant at 59°C. It can be seen that bikini solar combisystems have higher thermal performance than tank-in-tank solar combisystem as long as the collector area is small. When the collector area increases, the thermal advantage of bikini tank is decreased especially for houses with high space heating demand. The results are explained by the different required set point temperature for the auxiliary energy supply system and the better thermal stratification in the bikini tank compared to the thermal stratification in the tank-in-tank system.

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Fig. 8. Net utilized solar energy versus space heating demand for Task 32 DHW profile 4. Conclusion

Solar combisystems based on bikini tanks and tank-in-tank solar combisystems have been studied theoretically. Bikini tank systems require, if installed in low energy buildings, low auxiliary volume set point temperatures resulting in high thermal performances. High auxiliary volume set point temperatures are required for bikini tank systems installed in houses with a high space heating demand. This results in a relatively low thermal performance. Bikini tank systems are therefore suitable for low energy buildings, while tank-in-tank combisystems are more suitable for normal and old houses. Tank- in-tank stores with domestic hot water tanks with large auxiliary volumes are recommended for normal variable domestic hot water consumption patterns in order to achieve a high thermal performance.

References

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[4] E. Yazdanshenas, S. Furbo. Solar combi system based on a mantle tank. Proceedings of ISES Solar World Congress 2007, Beijing, China, ( 2007).

[5] TRNSYS 16, User Manual, University of Wisconsin, Solar Energy Laboratory, 2005.

[6] S. Knudsen, (2002). Heat Transfer in a ‘Tank-in-Tank’ Combi Store, Department of Civil Engineering, Technical University of Denmark, Report No. R-025.

[7] H. Druck, (2006). MULTIPORT Store — Model, Type 340 for TRNSYS. Institut fur Thermodynamik und Warmetechnik, Universitat Stuttgart.

[8] U. Jordan, K. Vajen, (2001). Realistic Domestic Hot-Water Profiles in Different Time Scales, FB. Physik,

FG. Solar, Universitat Marburg, D-35032 Marburg.