The measurements showed that the used lifeline has poor thermal characteristics. The heat loss of the hot pipe is not much lower than the heat loss of the hot 10/8 mm pipe, while the cold pipe in the lifeline has a lower heat loss than the cold 10/8 mm pipe. The decreased pipe heat loss will however only slightly increase the thermal performance of the solar heating system.

Therefore, in July 2008 the 16 m lifeline was replaced by 33 m 6.35/4.85 mm copper pipes with 9 mm INSUL-TUBE insulation. At the same time the positions of the pumps were changed from the cold side of the solar collector loop to the hot side of the loop. That is: The solar collector fluid is now passing the pumps just before the fluid enters the mantles. In this way the pump power which heats up the fluid is utilized in a better way than if the fluid is heated before it enters the solar collectors. Further, the SOLAR 15-40 pump was replaced by a normal circulation pump, type UPS 15-40, since this is the circulation pump used in most Danish marketed solar heating systems today.

The results of the measurements for the period July 30 — August 12, 2008 are shown in table 3. The test period was a rainy period resulting in relative low solar fractions.

Table 3. Measured energy quantities in the test period July 30 — August 12, 2008.

The thermal performance of the system with the 6.35/4.85 mm copper pipes and the SOLAR 15-65 pump is 6% higher than the thermal performance of the solar heating system with the normal solar collector loop and the UPS 15-40 pump for the test period of 2 weeks. The extra pump energy for the SOLAR 15-65 pump is now lower than the extra net utilized solar energy for the system with the SOLAR 15-65 pump. Consequently, the use of the SOLAR 15-65 pump together with the small separate pipes in the solar collector loop is now justified from a thermal performance point of view. However, the results might be different after a long test period. The measurements will therefore be continued for a long period.

4. Conclusion

Side-by-side laboratory tests for two small low flow SDHW systems have shown that a solar collector loop based on a marketed lifeline with a SOLAR 15-65 pump has no thermal advantage compared to a solar collector loop based on normal pipes and a normal solar circulation pump. The reason is the poor thermal characteristics of the lifeline.

The laboratory measurements will be continued with an improved solar collector loop for the SDHW system with the SOLAR 15-65 pump. The lifeline was in July 2008 replaced with small well insulated separate copper pipes. Further measurements will elucidate if the thermal advantage by using such small pipes is large enough to compensate for the extra pump energy required by the SOLAR 15-65 pump.

References

[1] S. Furbo (1990). Small Low Flow DHW Solar Heating Systems — Status. Thermal Insulation Laboratory, Technical University of Denmark, report no. 90-13.

[2] S. Furbo (1993). Tests of the components of the Solar Boiler system from Thermo Dynamics Ltd.

Thermal Insulation Laboratory, Technical University of Denmark, report no. 93-20.