Thermosiphon Solar Water Heater
From Swikipedia
Introduction
A thermosiphon solar water heater is the one that operates passively (through natural convection), circulating water through a solar collector and into an insulated storage tank situated above the collector. In this water heater, pumps and controls are not required.
Brief Description
It basically takes advantage of the fact that hot water rises and cool water sinks. As water is heated it expands and so, as gravity pulls down the relatively heavier cool water molecules, the warmer molecules rise up. This phenomenon is known as natural convection. In this type of installation, the tank must be above the collector.
As water in the collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, cooler water in the tank flows down pipes to the bottom of the collector, causing circulation throughout the system. The storage tank is attached to the top of the collector so that thermosiphoning can occur. Thus, the intended purpose of a thermosiphon is to simplify the pumping of water and/or heat transfer, by avoiding the cost and complexity of a conventional water pump.
In a thermosiphon solar water heater, the water is heated passively by solar energy and relies on heat energy being transferred from the sun to a solar collector. The heat from the collector can be transferred to water in two ways: directly where water circulates through the collector, or indirectly where an anti-freeze solution carries the heat from the collector and transfers it to water in the tank via a heat exchanger. Convection allows for the movement of the heated liquid out of the solar collector to be replaced by colder liquid which is in turn heated. Due to this principle, it is necessary for the water to be stored in a tank above the collector.
References:
http://vonlowendesigns.com/blog/green-glossary/
http://en.wikipedia.org/wiki/Thermosiphon
