How Do Solar Collectors Work?
Solar thermal collectors are designed to harness and transfer heat from the sun's rays to other areas of a building. Unlike photovoltaic systems that produce electricity, the main purpose of solar collectors is to heat water and air. Collectors are generally installed on rooftops or locations where they can be positioned to gather maximum sunlight. When installed on homes, collectors typically heat water for household use.
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Enclosure Design
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The solar heating process begins with the collector, which is normally a shallow rectangular metal-framed enclosure. The top cover, or glazing, is transparent to allow sunlight to pass through in order to heat metal tubing laid down inside in a serpentine pattern. The backing and tubing are often painted black or a dark color to absorb as much heat as possible. The tubing contains a liquid, heated by the sun, and pumped through copper tubing to the home's hot water tank. The enclosure is sealed, while the sides and back are fitted with insulation to minimize heat loss.
Glazing Material
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The solar collector enclosure glazing is the window, which allows sunlight through the top. Glazing's are often made of low iron-tempered glass or polycarbonate plastic. Low-iron tempered glass allows a high percentage of the sun's energy to enter the enclosure, therefore increasing heat transfer. Polycarbonate material allows for heat transfer and is impact and scratch resistant to prevent damage from hailstorms and severe weather.
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Circulation System
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In an active solar thermal collector system, heated fluid circulates from the solar collector on the roof through copper tubing to the hot water tank and then back to the enclosure. An electronic controller uses sensors to monitor fluid temperatures near the collector and hot water tank. The controller activates a pump to allow fluid to flow when the temperature differential is high enough. Temperature and pressure regulators and an expansion tank prevent heated fluid pressure from reaching unsafe levels. A tempering valve mixes hot and cool water before it reaches the water taps to prevent scalding.
Tubing and Fluids
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Solar collector systems use soft copper tubing because it is a good heat conductor and flexible enough to be run easily throughout the home. Liquid pumped through the copper tubes is normally a water-glycol mix, which prevents freezing in colder climates. If properly maintained, a glycol solution will last a few years, but requires regular checks for degradation. In warmer climates, potable water can run directly through an open loop system.
Water Storage
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Water heating takes place as hot water or glycol solution from the solar collector passes through a closed loop heat exchanger inside the water storage tank. The heat exchanger's spiral shaped copper tubing design transfers heat to the cooler water in the bottom of the tank. Solar collector systems normally use steel storage tanks, with a thin metal cover holding on foam insulation, and an enamel inner lining. The lining protects the tank from water corrosion while the insulation reduces heat loss. Insulation is very important in maintaining hot water temperature to reduce or eliminate the use of backup gas or electric water heating.
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References
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