a strong frame. The collector mounting hardware is fastened to the frame, and strength is very important because the collectors must be able to withstand high wind conditions without breaking. These frames are almost exclusively made of extruded aluminum, although some are made of rolled aluminum or extruded fiberglass. The heaviest-duty collectors use heavy, thick, extruded aluminum frames. The extruded aluminum frames have channels, or flanges, built into them that the mounting hardware fastens to. Because these flanges go completely around the collector, great flexibility in mounting options is available.
Another important component to look at when considering a collector is the kind of fasteners used to assemble the collector. All fasteners should be made of stainless steel. It is critical to use compatible metals where they are attached to each other. Aluminum and stainless steel are compatible; aluminum and plain or galvanized steel are not. This must be applied not only to the construction of the collector but also to the mounting hardware. Because each manufacturer makes its own mounting hardware, and because each collector is tested with its specific hardware, you should always purchase your mounting hardware to match your collectors.
Glazing
All kinds of plastics have been used as glazing material for collectors, but they have all failed under direct, constant exposure to the sun. Only low iron tempered glass has stood the test of time. Iron in glass causes some of the solar radiation to be absorbed by the glass, diminishing the solar radiation hitting the absorber plate. The glass is usually patterned on one side to reduce glare and reflection. A rubber gasket is fitted to the edges of the glass plate both to protect the edge and to create a good seal where it sits against the collector frame. Some collectors use a silicon caulk to seat the glass against the frame. Though this method does last, it makes it almost impossible to remove the glass when making repairs.
Note that if you ever have to take the glazing off a collector, the edge of the tempered glass is very fragile. If you even tap the edge or side of a tempered-glass pane, it can literally explode, so be very careful and always wear safety glasses and gloves when handling glass.
Some early collector models used either a double pane of glass or a thermopane in an attempt to minimize heat losses from the front of the collector. However, over time we have learned that the second sheet of glazing actually lowered collector efficiency because it reduced the amount of solar radiation that could reach the absorber plate. To our knowledge, no flat plate collectors are made this way any more, but you may still encounter them in service calls.
Insulation
To minimize heat losses, flat plate collectors have insulation on the back and sides of the collector. Common insulation types are polyisocyanurate, rigid expanded polyurethane (PUR) and mineral wool (fiberglass, rock wool). All are commonly used and are capable of withstanding the prolonged high temperatures experienced inside the collector.
Absorber Coating
Absorber plates are necessary to conduct the solar radiation to solar fluid. How the absorber has been coated will directly affect the efficiency of this process. Absorber coatings are rated along two parameters: absorptance and emittance. The former refers to the percentage of solar radiation that can be absorbed, and the latter is the percentage of heat that is emitted back from the absorber plate. To get the net heat gain, you need to subtract what is emitted from what is absorbed. Most coatings will have similar absorptance ratings in the range of 90 to 98 percent, but will vary in their levels of emittance. Traditional flat or selective black paints will emit anywhere from 15 to 30 percent of their heat, and the modern high-tech methods, such as sputtering, physical vapor deposition (PVD), black chrome or black crystal, can reduce the emittance levels to 5 to 10 percent.
Evacuated Tube Collectors
When you look at a flat plate collector you will see that the sides and back are well insulated. However, there is no way to insulate that large pane of glass because it would block all the solar radiation. This obvious limitation has led to the development of evacuated tube collectors. Inventors sought a way to permit the transmittance of solar radiation while still insulating.
Evacuated tube collectors are constructed of a series of glass tubes. Each tube is made of annealed borosilicate (Pyrex) glass and has an absorber plate within the tube. During the manufacturing process, a vacuum is created inside the glass tube. The absence of air in the tube creates excellent insulation, allowing higher temperatures to be achieved at the absorber plate by minimizing heat losses. Air is the medium in which convective heat is transferred. If all the air is removed from the tube, this method of heat movement is interrupted.
Because evacuated tube collectors are able to minimize heat losses they are able to achieve higher temperatures than other collector types. This can be an advantage or a disadvantage depending on how the system has been designed and/or what climate the collector is installed in. For applications that require high temperatures, such as solar cooling, evacuated tubes are the most common choice. However, care must be taken when planning and installing these collectors to ensure that the fluid does not overheat and boil during periods of stagnation or when the load on the system is low. Whenever the sun is out, you need to ensure that a fluid is flowing through the system to prevent overheating.
Evacuated tube collectors vary widely in how they are constructed and how they heat a fluid. The principal distinctions are how many layers of glass they have and whether they heat the solar fluid directly or use a heat pipe.
Figure 3.2: Evacuated tube
Glazing
The collectors can be constructed of either a single or double tube of glass. In single-tube versions, the absorber plate and riser tube rest directly inside the vacuum. The configuration means that the solar radiation has to pass through only one layer of glass, but it requires a seal where the riser tube leaves the vacuum. This seal needs to be able to withstand the high temperatures experienced within the vacuum tube, but it must also remain pliable enough to withstand the expansion and contraction of the copper pipe extending out the end as it heats and cools.
Double-tube versions go by many names, Sydney tubes, Dewar tubes or Twin tubes, but all versions are principally the same. A tube is placed inside a larger tube, and the two are sealed together at the ends. The vacuum is then drawn into the space between the layers of glass. The absorber plate is either coated on the underside of the inside layer glass or placed within the vacuumed space. In the latter case, the heat will need to transfer through the inner layer of glass. Having the double tube eliminates the metal/glass connection, which may extend the longevity of the vacuum but requires energy transfer through two layers of glass.
Heat Transfer
Most evacuated tubes on the market today use a heat pipe to absorb and transfer the solar heat. A heat pipe is a sealed hollow tube that is filled with a small amount of fluid. This fluid can be water, an alcohol/water mix, ammonia or some sort of proprietary blend. A vacuum is then created inside the pipe and it is sealed. When the atmospheric pressure is reduced inside the pipe, the vaporization point of the liquid is lowered so that when the pipe gets hot the liquid vaporizes at a lower-than-normal temperature. When a liquid changes state to a gas, a lot of energy is absorbed. When a gas condenses back to a liquid, a lot of energy is released. This energy transfer when a material changes state is called
