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Types of Solar Water Heating Systems
Active Systems and Passive Systems
- Passive Systems.
Passive systems move household water or a heat-transfer fluid through the system without pumps. Passive systems have the advantage that electricity outage and electric pump breakdown are not issues. This makes passive systems more dependable, easier to maintain, and perhaps longer lasting than active systems. Passive systems are cheaper than active systems, but are less efficient due to the slower water flow rates through the system.
- Thermosiphon Systems.
Thermosiphon Systems rely on warm water rising through a property known as natural convection. The natural convection circulates water through the solar absorber and to the tank.
- Batch Heaters.
Batch heaters are simple passive systems consisting of one or more storage tanks places in an insulated box that has a glazed side facing the sun. Batch heaters are cheaper and have fewer parts, but are only heat efficient when the water is warm.
- Active Systems.
Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They cost more than passive systems but are more efficient.
- Open Loop Active Systems.
Open loop active systems use pumps to circulate household potable water through the collectors. This design is efficient and lowers operating costs unless the water is hard or acidic because corrosion and scale will damage the components. Flat plate open-loop systems should not experience freezing temperatures for prolonged periods of time.
- Closed Loop Active Systems.
Closed loop active systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through the solar water heater. Heat exchangers transmit the heat from the fluid to the water that is stored in the tanks. Double-lined heat exchangers or twin coil solar tanks prevent contamination of household water. Closed-loop glycol systems are popular in areas subject to prolonged freezing periods because they will not break from the cold. These systems are more expensive then open loop systems and their fluids must be changed every few years.
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