Solar Water Heating--Revised & Expanded Edition. Bob Ramlow

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       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.

       Tanks with Built-In Heat Exchangers

      Some tanks are constructed with a heat exchanger as an integral part. Commonly referred to as indirect tanks, these have been popularized by the heating and plumbing industry as a means to heat domestic hot water with a separate hydronic heat source, typically a boiler. It used to be the case that tanks with internal heat exchangers were a specialized product to the solar thermal industry, making them very expensive. However, their application in the more traditional heating and plumbing field has expanded their usage and, consequently, driven the price down significantly.

      There are several indirect tank configurations for you to consider. The most common setup is to have a knurled or rifled copper or stainless steel coil of tubing placed inside a steel tank. This provides the greatest surface area in contact between the heat exchange coil and the domestic water, increasing efficiency. However, over time the textured surface of the coil may become coated with scale and mineral deposits, decreasing efficiency. This is especially true in hard-water situations. As long as the tank is flushed at regular intervals or installed in systems with soft or softened domestic water, it will be successful.

To avoid issues with scaling, some manufacturers of indirect tanks place the coiled heat exchanger on the outside of the tank. An illustration of the wraparound method can be seen in Figure 4.1. This technique reduces the amount of surface area that is in contact with the water you are heating but ensures consistent and prolonged efficiency of the tank. Instead of a coil, a few manufacturers use a fluid-filled jacket surrounding the tank. Essentially, the domestic water tank is submersed inside a slightly larger tank. In areas with hard water either one of these methods would be preferable.

      Some tanks have an internal heat exchanger that is removable and is accessible through a bulkhead fitting on the side of the tank. You can also buy a heat exchange “wand” that can be installed through the top port of standard electric or gas water heater. A removable heat exchanger allows for easy access and cleaning, but may limit the total amount of heat exchange surface area that can fit into a single tank.

      Indirect types of solar storage tanks have several advantages. First, installation is significantly simplified because you don’t have to do the extra work of mounting and plumbing an external heat exchanger. Though indirect tanks are more expensive, you must remember that you are eliminating the cost of an external heat exchanger and the cost to install it. Second, the system will require only a single main circulating pump. Most external heat exchangers require a pump on the solar side and on the domestic hot water side. The second pump results in not only an initial installation cost but also increased parasite loads and utility bills over time. Finally, the tanks can also save space where quarters are tight. After years of working in cramped utility rooms, and sometimes closets, we have developed an understanding СКАЧАТЬ