Types of Solar Water Heaters
Solar water heaters can be either active or passive. An active system uses an electric pump to circulate the heat-transfer fluid; a passive system has no pump. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors.
Solar water heaters are also characterized as open loop (also called “direct”) or closed loop (also called “indirect”). An open-loop system circulates household (potable) water through the collector. A closed-loop system uses a heat-transfer fluid (water or diluted antifreeze, for example) to collect heat and a heat exchanger to transfer the heat to household water.
Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but are also more efficient. Active systems are usually easier to retrofit than passive systems because their storage
tanks do not need to be installed above or close to the collectors. But because they use electricity, they will not function in a power outage. Active systems range in price from about ,000 to ,000 installed.
Open-Loop Active Systems
Open-loop active systems use pumps to circulate household water through the collectors. This design is efficient and lowers operating costs but is not appropriate if your water is hard or acidic because scale and corrosion quickly disable the system.
These open-loop systems are popular in nonfreezing climates such as Hawaii. They should never be installed in climates that experience freezing temperatures for sustained periods. You can install them in mild but occasionally freezing climates, but you must consider freeze protection.
Recirculation systems are a specific type of open-loop system that provide freeze protection. They use the system pump to circulate warm water from storage tanks through collectors and exposed piping when temperatures approach freezing. Consider recirculation systems only where mild freezes occur once or twice a year at most. Activating the freeze protection more frequently wastes electricity and stored heat.
Of course, when the power is out, the pump will not work and the system will freeze. To guard against this, a freeze valve can be installed to provide additional protection in the event the pump doesn’t operate. In freezing weather, the valve dribbles warmer water through the collector to prevent freezing.
Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Double-walled heat exchangers prevent contamination of household water. Some codes require double walls when the heattransfer fluid is anything other than household water. Closed-loop glycol systems are popular in areas subject to extended freezing temperatures because they offer good freeze
However, glycol antifreeze systems are a bit more expensive to buy and install, and the glycol must be checked each year and changed every 3 to 10 years, depending on glycol quality and system temperatures. Drainback systems use water as the heattransfer fluid in the collector loop. A pump circulates the water through the collectors.
The water drains by gravity to the storage tank and heat exchanger; there are no valves to fail. When the pumps are off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.
Pumps in Active Systems
The pumps in solar water heaters have low power requirements, and some companies now include direct current (DC) pumps powered by small solar-electric (photovoltaic, or PV) panels. PV panels convert sunlight into DC electricity. Such systems cost nothing to operate and continue to function during power outages.
Passive systems move household water or a heat-transfer fluid through the system without pumps. Passive systems have no electric components to break. This makes them generally more reliable, easier to maintain, and possibly longer lasting than active systems.
Passive systems can be less expensive than active systems, but they can also be less efficient. Installed costs for passive systems range from about ,000 to ,000, depending on whether it is a simple batch heater or a sophisticated thermosiphon system.
Batch heaters (also known as “bread box” or integral collector storage systems) are simple passive systems consisting of one or more storage tanks placed in an insulated box that has a glazed side facing the sun. Batch heaters are inexpensive and have few components—in other words, less maintenance and fewer failures. A batch heater is mounted on the ground or on the roof (make sure your roof structure is strong enough to support it). Some batch heaters use “selective” surfaces on the tank(s).
These surfaces absorb sun well but inhibit radiative loss. In climates where freezing occurs, batch heaters must either be protected from freezing or drained for the winter. In welldesigned systems, the most vulnerable components for freezing are the pipes, if located in uninsulated areas, that lead to the solar water heater.
If these pipes are well insulated, the warmth from the tank will prevent freezing. Certified systems clearly state the temperature level that can cause damage. In addition, you can install heat tape (electrical plug-in tape to wrap around the pipes to keep them from freezing), insulate exposed pipes, or both.
Remember, heat tape requires electricity, so the combination of freezing weather and a power outage can lead to burst pipes. If you live in an area where freezing is infrequent, you can use plastic pipe that does not crack or burst when it freezes. Keep in mind, though, that some of these pipes can’t withstand unlimited freeze/thaw cycles before they crack.
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