Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
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coatings will influence the performance of a solar thermal collector, and the amount of this effect may be more critical depending on the climate the collector will be placed in. Some types of absorber coatings have a higher performance rating than others. In climates that experience a high number of sunny days per year and also experience relatively consistent warm temperatures, collectors can perform well with less efficient absorber coatings, whereas in climates with fewer sunny days per year and colder temperatures, collector coatings with higher efficiencies work best.
You may also need to consider the density of the absorber area compared to the overall footprint of the collector. Manufacturers and certification agencies use the terms gross collector area, net aperture area and absorber area. Gross collector is the entire area including the frame; aperture area is the size of the glass; and absorber area is the amount of surface that will actually absorb solar radiation. In flat plate collectors virtually the whole collector area equals the absorber area, with only about an inch around the edge being frame and not absorber. In an evacuated tube collector, there is space between each tube that is not absorber area. In most instances a flat plate collector will take more than 25 percent less space than an evacuated tube collector that has an equivalent absorber area. This may be an important factor because a common limiting factor in siting collectors is the amount of available mounting space. In other words, you want to have enough space for the amount of collector area you want, so to get as much heat as you can, you want to have a type of collector that has the best gross-to-net absorber ratio.
Note that the amount of solar energy that falls on a square foot of the earth is a constant and cannot be changed by the type of collector used. The primary way to increase the amount of energy collected is to increase the absorber area.
By now it has probably become clear that we are not completely impartial when it comes to collector selection. It is difficult to remain unbiased while still trying to provide the knowledge we have gathered through years of experience installing, maintaining and designing these systems. We have seen the best performance from flat plate collectors, and we want you to have the same success. We have no doubt that evacuated tube collectors have a secure place in the solar thermal industry, especially in high-temperature applications. For instance, the emerging solar thermal-powered air conditioning systems that use evacuated tube collectors to drive single-or double-effect chillers hold great promise. However, for the majority of domestic water and space heating applications, flat plate collectors have a proven track record.
Air Collectors
Up to this point, all the kinds of collectors we have talked about have used a liquid as the heat transfer medium. Air can also be used as the heat transfer mechanism in a solar collector. Air collectors are flat plate collectors and share all the same characteristics of liquid-type flat plate collectors in size and construction. Instead of an absorber plate made of copper piping and copper fins, the absorber plate in an air collector is typically made of a solid sheet of aluminum. The aluminum absorber plate is coated with a selective surface or black paint and is usually dimpled to increase efficiency. When the sun shines on the absorber plate, it gets hot. Air is drawn from the building and is blown across the back of the absorber plate and heated. The hot air is then delivered to the building through ductwork. A blower circulates the air through the system.
Figure 3.11: Air collector
Air collectors can come in standard sizes that are very similar to flat plate collectors and they can also be site built to fit a particular building. We personally manufactured site-built air collector solar space heating systems in the early and mid-1980s and had great success. These collectors should be less expensive than liquid-type solar thermal collectors because the absorber plates in them are much simpler and are typically made of aluminum, which is less expensive than copper. These collectors can produce the same Btus per square foot as any other collector, so they can have very attractive returns on investment.
Another kind of air collector is a transpired collector. This collector is an unglazed flat plate collector. It draws fresh air through tiny holes that are punched in the absorber plate. This fresh air is heated as it passes through the collector and is delivered into the building. This kind of collector is used for heating makeup air in buildings that require many air changes per day. As these collectors are even simpler, with no glazing and very simple frames, their cost is very attractive, and these systems have very good returns on investment.
4
OTHER SYSTEM COMPONENTS
THIS CHAPTER WILL DETAIL a number of the other components that are used in solar water heating systems. This is not an exhaustive list of components; it includes only those that are most commonly used. Components that are designed for solar systems are typically preferable.
Storage Tanks
Because of the path of the sun, there is a limited time in which we can harness solar energy. But we aren’t willing to use hot water only when it is sunny. For instance, we mostly shower in the morning or at night. Therefore, all solar water heating systems will require some form of storage tank, where the solar-heated water is stored until needed. These tanks typically range in capacity from 40 gallons to 120 gallons but can be much larger with space heating systems. Determining the right size of storage tank will be covered in Chapter 7. As with most things, there are a number of options to consider when choosing a storage tank. The type of system you are planning will determine some of the specifications of your storage tank.
But first, a quick discussion on tanks in general. The most common and traditional tank used today is a steel tank. The steel tank is encased in foam insulation to reduce heat loss and has a light-gauge steel jacket on the outside to protect the insulation. It looks just like a regular water heater. The inside of the tank should be coated with an enamel layer, often called glass lining, which is typically baked on. This lining helps reduce corrosion and significantly prolongs the life of the tank. High-quality steel tanks should also be fitted with an anode, or sacrificial, rod, which is screwed into a fitting on the top of the tank and extends down into the tank. An anode rod helps reduce tank corrosion by rusting before any of the system components do. Anode rods actually wear away, and their life expectancy varies depending on the conditions at your location. The anode rod should be checked every five to ten years. Steel tanks typically last 15 to 30 years, depending on the environment at your location and the quality of your water.
Fiberglass and plastic tanks are the new kids on the block. Modern developments in fiberglass and thermoplastic technology have enabled engineers to create a cost-competitive alternative to steel tanks. These tanks are constructed much like the steel tanks except that the tank itself is made of fiberglass or plastic and the jacket is plastic. They have a huge advantage over steel tanks because they will not deteriorate because of rust or corrosion. When using these types of tanks, a few special precautions should be taken. First, when screwing fittings into the tank, it is important not to over-tighten the fittings. Many installers tend to crank fittings very tight when working on steel tanks, and there is no problem with that, but on plastic or fiberglass tanks you can break the fitting by applying too much force. So be sure to use plenty of pipe dope, sealant or Teflon tape on the fitting, and do not over-tighten. Second, install a vacuum breaker on the top of the tank to facilitate safe drainage. Whenever a tank is being drained and the tank is sealed at the top (all faucets above it are closed), a vacuum is created at the top of the tank. This is no problem with a steel tank, as steel is strong and rigid. Although a fiberglass or plastic tank is very strong in relation to outward pressure, it is weak when it comes to the inward pressure that would be caused by having a vacuum inside the tank.
If you choose to install a plastic tank, be sure that the plastic is rated to handle prolonged exposure to high temperatures. Most types of plastics will deteriorate under the constant hot conditions.