An insert-type two-phase closed loop thermosyphon for split-type solar water heaters

• An insert-type loop thermosyphon is proposed for split-type solar water heaters.
• The mechanism behind this loop thermosyphon is analyzed experimentally.
• The proposed loop thermosyphon has a 100% water heating speed improvement.

The requirement for improving the capacity, efficiency and reliability of a solar water heater typically increases its cost and installation difficulty in most situations. In this work, a novel insert-type two-phase closed loop thermosyphon (or called gravity return loop heat pipe) is introduced for reducing the cost and the installation/maintenance difficulty, especially for a split-type solar water heater. A series of systematic visualization experiments were conducted to explore the operation characteristics of the proposed loop thermosyphon at different filling ratios, different heat loads and different cooling conditions. The results show the filling ratio has a critical impact on the thermal resistance, flow patterns and start-up behavior of the loop thermosyphon. The practical comparative study indicates that the proposed insert-type loop thermosyphon is superior to a conventional thermosyphon in terms of thermal transportation limit and start-up speed through a prototype demonstration. Meanwhile, the transition conditions including stable operation, interval operation and dry-out operation were also observed and discussed accordingly.