Passive solar water heaters do not use additional energy from pumps, but rather rely on gravity, to circulate the water. While often more reliable, long-lasting and cheaper, a passive solar water heating system is less efficient than an active system. There are two types of passive solar water heating systems:
Photo Credit: U.S. Department of Energy
Integral Collector-Storage Systems
Also called batch systems, ICS systems have storage tanks or tubes inside an insulated box, with the glazed side facing the sun. Cold water passes through the solar collector, which warms the water. The water then continues to the conventional backup water heater. This type is the least expensive and least efficient solar water heating system.
Because the outdoor pipes could freeze in very cold weather, solar batch heaters work best in milder climates. Since most heat loss occurs at night, batch systems work better in households that use most water during the day and evening, rather than the morning.
A thermosyphon system takes advantage of the fact that warm water rises. The tank is placed above the collector, and as water in the collector heats up, it naturally rises to the tank. The colder water flows downward into the pipes at the bottom of the collector. Learn more about thermosyphon systems.