Читать книгу Solar Water Heating--Revised & Expanded Edition - Bob Ramlow - Страница 36

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 of the value of floor space. Sometimes, this type of tank will be the only option.

Figure 4.1: Solar storage tank with wraparound heat exchanger

Figure 4.2: Solar storage tank with internal heat exchanger

Figure 4.3: Drainback tank with internal heat exchanger