Siting Your Solar Water Heating System

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Before you buy and install a solar water heating system, you need to first consider the characteristics of your site: available roof or land area, the solar resource, shading by trees or buildings, as well as the optimal orientation and tilt of your solar collector. The efficiency and design of a solar water heating system depends on how much of the sun's energy reaches your building site.

Solar water heating systems use both direct and diffuse solar radiation. Even if you don't live in a climate that's warm and sunny most of the time -- like the southwestern United States -- your site still might have an adequate solar resource. If your building site has unshaded areas that generally face toward the equator (to the south in the US) , it's a good candidate for a solar water heating system. Your local solar system supplier or installer can perform a solar site analysis.

Shading by surrounding trees or other buildings should be avoided.  It is impossible to avoid all shading, but every effort should be made to avoid shading between the hours of 10 am and 2 pm and in winter, when the sun is lowest in the southern sky (in summer the sun is almost directly overhead and there is less shading).

The type, age, and condition of the roof are all important. Roofs covered with composite shingles are easier and less expensive to install solar on than roofs like wood shingles or tile roofs.  It is possible to re-roof around solar water heating collectors, but new solar systems should be installed on new or sound roofs that will not need to be replaced in the 25- year life of the solar system.  It is often necessary to reinforce the roof structure with blocking between rafters.  Care should be taken when installing roof rack stanchions and waterproof flashing- and that job is often done by a professional roofer.  Although the roof is the natural place to locate solar collectors, some are installed on ground foundations (piles) to avoid roof issues.

Both the orientation and tilt of the collector will affect your solar water heating system's performance. Your contractor should consider both factors while evaluating your site's solar resource and sizing your system.

Collector Orientation 

Solar hot water collectors should be oriented geographically to maximize the amount of daily and seasonal solar energy that they receive. In general, the optimum orientation for a solar collector in the northern hemisphere is true south. However, studies have shown that, depending on your location and collector tilt, your collector can face up to 45º east or west of true south without significantly decreasing its performance.

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You'll also want to consider factors such as roof orientation (if you plan to mount the collector on your roof), local landscape features that shade the collector daily or seasonally, and local weather conditions (foggy mornings or cloudy afternoons), as these factors may affect your collector's optimal orientation.

Collector Tilt

Today, most solar water heating collectors are mounted flat on the roof. This is more aesthetically pleasing than rack-mounted collectors, which stick up from the roof at odd angles. Thus, most collectors have the same tilt as the roof.

The optimal tilt angle for your collector that maximizes annual energy delivery with maximum delivery in spring and fall is an angle equal to your latitude.  However, because we often need more heat in winter (colder water coming in), it is often advisable to tilt water heating solar collectors up to a steeper tilt angle. This is in contrast to photovoltaic-type solar systems which are often mounted on a flat roof or low tilt angle. You will, however, want to take roof angle into account when sizing your system.

Solar liquid collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector.

The liquid flows rapidly, so its temperature only increases 10° to 20°F (5.6° to 11°C ) as it moves through the collector. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows to either a storage tank or a heat exchanger for immediate use. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls.

The flow rate depends on the heat transfer fluid. To learn more about types of liquid solar collectors, their sizing, maintenance, and other issues, see solar water heating.

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