Types of Solar Collectors
A variety of solar collectors are currently available. With the solar field continually evolving, technology will continue to improve. Generally, three types of solar collectors for solar thermal energy are available: Glazed flat plate, unglazed flat and evacuated tube. Collectors for pool heating typically do not need glazing or insulation since they are utilized during times of the year when solar radiation and ambient temperatures are relatively high. This allows for a simpler design that is far less expensive than domestic hot water collectors. In fact, many collectors are made of heavy duty plastic which is treated with an ultraviolet light inhibitor to extend the life of the panels. The advantages of plastic collectors are that they weigh less than metal collectors and they are significantly less expensive.
Glazed Flat Plate Collector
The glazed flat plate collector appears like typical solar panel consisting of a durable aluminum framed box with a tempered glass aperture area. Harp or serpentine style copper tubing welded to a copper and chrome absorber plate are inside the box. The dimensions can range from 3″-6″ deep, 3′-4′ wide and 6′-10′ high. These types of collectors are durable, have high pressure ratings and low probability that components that can fail.
Unglazed collectors are unglazed because the absorber surface is not enclosed by glass. Constructed from black polyethylene plastic or EPDM rubber, they heat water very efficiently to high temperatures when there is enough insolation (sunlight). Large volumes of water can be heated but they cannot be pressurized. They are very light weight and come in a wide variety of dimensions. They are ideal for heating an outdoor swimming pool. A significant amount of collector area is needed though the panels are very affordable.
Black flat-plate, flexible mats and black pipes are considered low temperature collectors because the water is heated to a temperature lower than the temperature required for domestic hot water.
Flat-plate collectors intended for pools include a large diameter header at each end with numerous small fluid passageways running through the plate portion.
Evacuated tube collectors differ from flat collectors in a number of ways however the overall amount of heat produced per sq ft of collector area is nearly identical to flat plates. The tube is essentially two glass tubes that are fused at the top and bottom. The inner tube has a solar absorbing coating, and the space between the two tubes is evacuated to form a vacuum. Each performs better under certain conditions. Two types of evacuated tubes are the water tube and heat pipe. In a water tube collector the heat transfer fluid, such as propylene glycol, circulates through the evacuated tube. Water tubes cannot be used in a drain back systems and contain very small passages for the fluid resulting in losses and possible points of failure. Heat pipes carry heat out of the vacuum using a medium such as a refrigerant to a point outside of the tube which is then immersed in the heat transfer fluid. The top of the heat pipe can exceed 400 degrees quickly when left dry in the sun.