Thermosiphon solar water heating systems

Thermosiphon systems are a kind of passive solar water heating that use some of the passive components of active systems, such as flat plate solar collectors. The solar panels usually are mounted at a lower elevation than the storage water to be heated. Thermosiphon systems can circulate potable water or utilize a heat exchanger and heat exchange fluid.

For potable water systems, the cooler water at the bottom of the storage tank is thermally siphoned to the hotter water near the solar collector by the raising temperature and volume of the warmer water, initiating a circulation of the storage water through the collectors fluid passage ways back into the top of the storage tank. The circulation continues until the temperature at the bottom of the storage tank is about the same as the temperature of the outlet pipe at the top the solar collector.

Thermosiphon systems using a heat exchanger work on the same principle by circulating a nonfreezing solar exchange fluid through an internal heat exchanger installed inside the solar storage tank, instead of circulating potable water. This type of system is usually installed in climates that experience freezing temperatures to prevent freeze damage of the solar collectors and siphon pipes.

strengths

Thermosiphon systems, like other passive solar systems, usually cost less than active solar systems to purchase and maintain.

weaknesses

Regardless of the thermosiphon system used, care needs to be taken that the water feed pipes connecting the solar collectors to the water heater inside the house or building do not freeze. Insulation of the feed pipes does not guarantee against freezing. We recommend a manual drain down of the pipes and tank preceding freezing conditions. Also, thermosiphon systems need special installation conditions not associated with the active solar systems. Open-loop thermosiphon systems do not work well with hard water.