Believe it or not, you can be aggressive about solar water heating while remaining completely passive. The trick is to scrap all those pumps, fans and blowers. Just let gravity, convection, and the unending energy of the sun do the work.
As you've likely deduced by now, passive solar water heaters (as opposed to active systems) are "passive" in that they need no external power to operate - no electric pumps or controls. Variations on this concept have been around for centuries upon centuries. In today's solar market, there are two types of passive solar water heating systems used the vast majority of the time.
Also called 'batch-collector' systems, Integral Collector Storage water heaters are unique. The water itself is the collector of solar heat and it collects this heat within its storage tank. The water does not pass through a separate collector to be heated and then passed to a storage tank to await use - a timely process.
In an ICS system, one or more black tanks or tubes rest in a glazed enclosure. The black tank with water absorbs solar radiation that is allowed entrance, and then traps it with the exterior glazing. The water within the tank is heated. Due to convection, the warmer water will rise to the top of the tank. So hot water for use is drawn from the top of the tank, while cool water pouring in to replenish the system arrives at the base of the storage tank.
ICS systems are relatively simple solar hot water systems and are often built by the homeowners themselves. Of course, considerations must be made before building your own system. Just one consideration lies in the weight of the storage tank. The tanks, or tubes, usually hold about 30 to 50 gallons of water, which gets very heavy. This demands some serious structural considerations, whether the system is to be mounted on the home or on the ground.
Integral collector storage systems are best suited for warmer climates. Because the water itself is the collector, freezing would be a major problem for an ICS system. Sunlight should be readily available (near year-round if possible), to make such a system truly worthwhile. Although, in many cases, the ICS is used to preheat water for a conventional backup system in cooler months. This ensures that the home will have hot water all year and eases the heating load on the grid- or gas-powered system.
Thermosyphons are similar to ICS systems in that the water to be heated flows directly through the system and eventually into the home for use in a shower, sink or tub. Thermosyphons have a unique design. They do utilize separate collectors, but contrary to other systems, the collectors are located below the storage tank.
This is because thermosyphon systems rely heavily on convection, or a warmer liquid's tendency to rise. As water flows through the system of collectors, the cool water sinks while the warm water rises into the storage tank, where it is taken to the house. A thermosyphon system's rather backward design requires some structural considerations similar to those inherent to an ICS system. Because the tank must be situated above the collectors, it is almost always mounted on the roof, which requires added work for the contractor to secure the roof, and thus, adds cost to the system.
Thermosyphon systems, like their ICS counterpart, are best used in warm, non-freezing climates and typically require a backup system to handle the cooler times. They can be very effective, however, especially for off-grid homes with relatively small demands for hot water.
Passive Solar Water Heating / Active Solar Water Heating