Solar Water Heating: Overview

Solar Collectors & Storage Tanks

Flat Plate Collectors are the most commonly used collectors in domestic hot water applications. The collectors should last well over 20 years and can handle an operating temperature up to 180 degrees F. Surprisingly, a typical flat plate collector holds less than 2 gallons of water! You might think the more fluid-filled tubing the better; but the trick is getting the heat into the water, so the ratio of surface area to water volume is limited by the collector technology. There are many brands and sizes of collectors; most range in size from about 3-4 feet wide to 6-10 feet long and weigh from 90 to 160 pounds dry. It does require some planning (and some helpers) to get these collectors safely to the roof. Evacuated Tube Collectors are often used in commercial applications or in applications where hotter water is needed, since they are capable of generating temperatures above the boiling point of water. While evacuated tubes have a long life similar to flat plate collectors, they are composed of fragile tubes and are subject to breaking. In snowy climates, the evacuated tubes may not shed snow as well as their flat plate counterparts. Regardless of your collector choice, siting them in the appropriate location is important. Ideally, solar collectors should be south facing, or at least within 30 degrees of south, and have unshaded exposure between the hours of 10 am and 3 pm. As with solar electric modules, the tilt angle is also important: the more perpendicular the collectors are to the sun’s rays, the better. Since system size is usually based on summer temperature maximums and there is far less available solar energy in winter, mounting the collectors at a tilt angle equivalent to your latitude plus 15 degrees will help to balance annual hot water production.

Solar collectors are evaluated and rated by an independent organization called the Solar Rating and Certification Corporation (SRCC). Many states require that you install an SRCC-rated collector in order to qualify for state rebates and tax credits.

[pictures: flat plate collector (preferably one with snow) & evacuated tube collector; tilt angle for winter]

Solar storage tanks are larger and much better-insulated than your average water tanks. The additional insulation allows the water to stay hotter longer. The storage tank in a hot water system is like the batteries in a solar electric system, storing energy (in this case, as hot water) for later use. These are specially designed tanks with extra ports to allow connections to the solar collector piping loop or to external heat exchangers. A household of 1-3 people will typically need an 80 gallon tank. Larger households of up to six people would use a 120 gallon tank. These are big tanks! Make sure to measure the area where you intend to locate the tank and the doorway through which it will have to pass. The tanks are about 5 feet high and 2 to 2 ½ feet in diameter. Some storage tanks have an internal heat exchanger; others must be connected to an external heat exchanger. Choosing a tank with an internal heat exchanger reduces the plumbing work compared to assembly of an external heat exchanger system; but external heat exchangers are more efficient than internal units.

You may be able to use your existing water tank as part of the system; however, you may need an additional tank to achieve the minimum storage capacity. All solar storage tanks have an electric heating element for backup heating when necessary—gas-fired heating of the solar storage tank is not possible. But it is possible to avoid using the electric element by plumbing the hot water output of the solar tank to the cold input of your existing gas-fired tank. Such a system would provide more efficient “finish” heating, as needed, during winter or on cloudy days.
   
[picture: person standing next to solar storage tank for relative size comparison]