Less popular are the forced circulation systems, in which a mains pressure tank is located at ground level with the collector on the roof. In these systems, a pump is activated when the sun shines and cold water is pushed through the collector. Force Circulation systems are more expensive to purchase than either the close coupled or gravity feed systems, and electricity is required to provide power for the circulating pump. Batch Water Heaters Batch water heaters, also known as 'breadboxes' are very simple passive systems for heating water using solar energy and have been used since the early 1900's. Batch systems consist of black storage tanks contained within an insulated box that has a transparent cover.
Cold water is added to the hot water stored in the tanks whenever hot water is removed.
Modern batch systems are used as preheating systems, where the water is then heated further by conventional gas, electric or wood systems. To retain the heat within the water, the system requires insulated covering to be placed over the glazing at night to prevent the heat being lost to the environment. Figure 2 shows a typical Breadbox water heater. Thermosyphoning Thermosyphoning utilises flat plate collector to heat the water which is then returned to a storage tank, located above the collector. Cold, denser water flows through the collector heating up and is then returned to the tank.