GUIDE to SOLAR WATER HEATING SYSTEMS
Solar water heating systems can be either ACTIVE or or PASSIVE.
Most common are active systems, which rely on pumps to move the liquid between the solar panel (collector) and the water storage tank. Passive systems, on the other hand, rely on gravity and the tendency for water to naturally circulate as it is heated.
ACTIVE SOLAR WATER HEATING SYSTEMS
Active solar water heaters rely on circulating pumps, valves and controllers to circulate water, or other heat-transfer fluids (usually a propylene-glycol mixture) through the collectors.
There are three types of active solar water-heating systems:
1. Direct circulation systems (or open systems)use pumps to circulate water through the solar panels. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water.
2. Indirect circulation systems (or closed systems)pump heat-transfer fluids, such as a mixture of glycol and water antifreeze, through solar panels. Heat exchangers transfer the heat from the fluid to the potable water stored in the tanks. Some indirect systems have overheat protection, which protects the solar panels and the glycol fluid from becoming super-heated when the load is low and the intensity of incoming solar radiation is high.
3. Drainback systems, a type of indirect system,use pumps to circulate water through the solar panels. The water in the collector loop drains into a reservoir tank when the pumps stop. This makes drainback systems a good choice in colder climates. Drainback systems must be carefully installed to assure that the piping always slopes downward, so that the water will completely drain from the piping. This can be difficult to achieve in some circumstances.
PASSIVE SOLAR WATER HEATING SYSTEMSare typically less expensive than active systems, but they are usually not as efficient. Passive solar water heaters rely on gravity and the tendency for water to naturally circulate as it is heated. Because they contain no electrical components, passive systems are generally more reliable, easier to maintain and possibly have a longer work life than active systems.
1. Integral collector storage systemsconsist of one or more storage tanks placed in an insulated box with a glazed side facing the sun. During the winter, they must be drained or protected from freezing. These solar collectors may be best suited for areas where temperatures rarely go below freezing. They are also good in households with significant daytime and evening hot-water needs; but they do not work well in households with predominantly morning draws because they lose most of the collected energy overnight.
2. Thermosyphon systemsare an economical and reliable choice, especially in new homes. These systems rely on the natural convection of warm water rising to circulate water through the collectors and to the tank (located above the collector). As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the cooler water flows down the pipes to the bottom of the collector, enhancing the circulation. Some manufacturers place the storage tank in the house's attic, concealing it from view. Indirect thermosyphons (that use a glycol fluid in the collector loop) can be installed in freeze-prone climates if the piping in the unconditioned space is adequately protected.
Solar water heating systems almost always require a backup system for cloudy days and times of increased demand.Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a demand (tankless or instantaneous) water heater for backup.
