Thermosiphon heat pump for pipe freeze preventing of domestic passive solar water heater

Potable water supply/return pipe freeze problem of domestic passive solar water heaters in chilly spells is a major concern which limits the reliability of this renewable energy in building applications. Thus, a thermosiphon heat pump-piping system was introduced that pumps indoor room air heat to the outdoor section for pipe freeze protection. In the outdoor section, the hot pipe (return) has been embedded in supply (cold) tube, while the indoor section mainly consists of a metal heat absorber pipe as supplier. Room air heat is transferred from indoor to outdoor section via a naturally driven water loop due to buoyant pressure. A prototype has been fabricated and tested in a freezer with the base temperature of −22 ± 2 °C (as the outdoors ambient), while the indoor section had been placed in a separate chamber with adjustable ambient temperature in the range of 15-40 °C (simulating the indoors air). Performance tests have been conducted in draw-off, for different chamber temperatures, heat absorber tube lengths, and filled/empty water storage tank conditions. In the filled tank experiment, threshold pipe freezing was observed where chamber temperature was as low as 15.5 °C compatible with residential condition. Moreover, the heat pumping rate decreases linearly with room (chamber) temperature and approach to 10 W at pipe freeze threshold. Thermosiphon, low room air temperature performance, slight parasitic energy loss (0.25–0.45 kWh/day), low price, and its simplicity are the promising aspects of this heat pump for prevention of pipe freeze for domestic passive solar water heaters and sustainable development of solar energy in residential buildings.