Solar Thermal Collection and Delivery

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Unlike photovoltaic systems, which convert the sun’s energy into electricity, active solar thermal systems transform sun-heated fluid into space heat and hot water. Systems consist of electric pumps, fans, complex controls, storage tanks and collectors. Collecting Solar Radiation Flat-plate collectors Two types of collectors gather solar rays: flat-plate and evacuated-tube. Flat-plate is the more common type and has been around longer. Evacuated-tube is a more recent design. A flat-plate collector consists of an insulated, waterproof metal box. The top side faces the sun and has a glass or plastic cover that acts like a greenhouse, letting in the rays and holding in the heat. Inside the box is a specially coated dark-colored absorber plate and pipes containing circulating fluid. Evacuated-tube collectors include a dark-colored absorber plate as well as parallel rows of vacuum tubes, all connected to one header pipe. Tube designs include glass, glass-metal and glass with fluid flow paths. Absorber plates typically include long strips of metal covered by a special coating. New technologies now produce coatings that are highly efficient. The absorbed radiation heats the circulating fluid. Solar System Concerns Flat-plate collectors have a simple, sturdy design. However, that design does allow them to lose heat through convection and radiation. Wind loads, which can carry away the heat, must be calculated. Evacuated tubes can operate even on cloudier days. They use somewhat fragile glass—think of an insulated vacuum-sealed bottle. However, they may have some problems where snow or ice may accumulate and need to be removed. There can also be problems if breakage or damage destroys the all-important vacuum or from heat buildup if there is not an adequate draw on the system.

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