Passive systems
“Passive” systems solar hot water systems do not have a pump or other moving parts. These heating systems rely on temperature changes in the water located in the solar collectors on the roof to move the water through the system. They are typically less expensive than systems having a pump (active systems) because they have no mechanical parts, but they are usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems: batch and thermosiphon.
Batch or Integrated collector-storage (ICS) systems. These systems work best in areas where temperatures rarely fall below freezing. They also work well in buildings with significant daytime and evening hot-water needs. Batch collectors (Figure 3.3) or ICS, use one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector and is preheated. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. This type of collector should be installed only in mild-freeze climates because the outdoor pipes can freeze in severely cold weather.
Thermosiphon systems. Thermosiphon systems (Figure 3.4) move water through the system due to density differences (warm water rises as cooler water sinks). Neither pumps or electricity are used. However, the collector must be installed below the storage tank so that warm water can rise into the tank. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. Although they are usually more expensive than ICS systems, they can be used in areas with less sunshine.
Passive solar water heating systems are used on individual buildings or for a single heating demand. They are not for central heating systems that service several buildings. They are also inefficient in cooler climates. Since the purpose of this design guide is to focus on systems that can serve multiple buildings, these systems will not be further discussed.
