Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
All you have to do is install the system and you can start saving today.
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TYPES OF SOLAR COLLECTORS
FROM THIS POINT ON, this book will deal with the nuts and bolts of solar water heating and space heating systems. We think it is appropriate to first let you know our perspective. We both live in Wisconsin, where the weather can be severe, particularly in regard to cold temperatures and snow that stays for months. Our experiences with solar thermal systems have been heavily influenced by the weather here. It is interesting to note that approximately 90 percent of the population of the world lives in a warmer climate than we do. Green Bay, Wisconsin, just 50 miles away from our homes and at the same latitude, is the coldest city in the continental US with a population over 100,000. The average coldest temperature in Wisconsin is–44°F. (The record is–56°F.) In a cold climate like this we have seen everything go wrong with a solar thermal system than can possibly happen because of cold weather. We also get a lot of snow each winter, and this snow typically stays on the ground and on roofs for at least four months without melting. Also know that it gets plenty hot here during the summer, so we have seen that side of things as well. The bottom line is that there is no room for error around here in either system design or installation, and this has influenced our perspective significantly.
So we are picky when it comes to system designs. If you follow our suggestions that are based on “worst-case scenarios” you will have success, even if you are in a warmer or less severe climate than ours. The next question is, are we suggesting designs that are overkill? The answer is no, because it does not actually cost more to design and install a system properly whether you are in a hot, cold or in-between type of climate. Having lived through the oil embargo years and the ensuing solar boom, we have seen many systems installed that were not appropriate for the local climate or were sub-standard in other ways. These systems required significant repair costs that, when added to the original cost of the system, resulted in a more costly system than systems that were properly designed and installed in the first place.
Flat Plate Collectors
Flat plate collectors are the most widely used kind of collector in the world for domestic solar water heating and solar space heating applications. These collectors have an operating range from well below 0°F to about 180°F, which is precisely the operating range required for these applications. They are durable and effective. They are the standard to which all other kinds of collectors are compared.
Figure 3.1: Flat plate collector
Flat plate collectors are rectangular shallow boxes that typically are 4 feet wide, 8 or 10 feet long and 4 to 6 inches deep, but they also come in other sizes. A common metric size is 1 meter by 2 meters. These collectors are made with a strong frame, a glazed (glass) front, a solid back and insulation on the sides and back. An absorber plate lies just beneath the glazing. In most cases this absorber plate has manifolds that run across the top and bottom of the collector, just inside the frame. These manifolds are usually ¾-inch or 1-inch-diameter copper pipe and extend out both sides of the collector through large rubber grommets.
These collectors, called internally manifolded collectors, can be easily ganged together to make large arrays. Smaller riser tubes, typically ½-inch copper pipes, run vertically, usually brazed to the manifolds above and below, and are spaced 3 inches to 6 inches apart (the closer the better). Another type of absorber plate has a serpentine tube that meanders back and forth continuously from the bottom of the collector to the top of the collector. A flat copper or aluminum fin is then attached to each riser to complete the absorber plate. The fin must make intimate contact with the riser tube to facilitate effective heat transfer from the fin to the tube. Soldering, welding or roll bonding the fins to the tubes makes the best connection. The plates are also usually dimpled or corrugated to increase absorptivity. The absorber plates are not attached to the frame; they just sit inside it and can expand or contract as they are heated or cooled without being restricted by the frame.
Collector Frame
It is very important that a flat plate collector have 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