Solar water heaters
Know options and facts before you installBy James Dulley
For a typical family of four, water heating can account for about 20 percent of its annual utility bills. Using solar energy to heat your water can reduce your water heating bill, but it's good to know your options first to determine the best system for your home.
Don't expect a solar water heating system to cut water heating costs to zero. A target savings of 50 percent often provides a good economic payback.
The two basic types of solar water heating systems are "active" and "passive." Active systems require a storage tank, electric pumps and controls to function. Sometimes 12-volt pumps can be powered by a photovoltaic solar panel located near the solar water heating collectors on the roof.
In cold climates, the system has to include some type of antifreeze working fluid and heat exchanger so it does not freeze at night during winter. Other systems that circulate the actual potable water through the collector need a draining system to empty the collectors at night during winter.
Passive water heating systems rely on the natural upward flow of less-dense warm water to move the water through the solar collector. In these systems, the warm water storage tank is located above the solar collector — usually on the roof or in the attic, so these systems have structural considerations.
They are less expensive than more sophisticated active systems but they tend to be less efficient, especially during cold weather.
There are many types of solar collector designs. The best one for your house depends on your climate, your hot water requirements, and your budget. They can be as simple as black copper tubes in an insulated box with a glass top to ones with vacuum tubes, concentrating reflectors, and heat pipe technology.
If you are thinking of building your own system, I suggest a passive system unless you are an accomplished craftsman. Trying to build an active system — with collectors on the roof, plumbing and control systems, and storage tanks — is beyond the skill level of most do-it-yourselfers. (I am a design mechanical engineer, and I don't think I could build an active system myself from scratch.) Consider building a passive "batch" system. It's a preheater for your existing water heater, with the simplest design called a "breadbox." It uses a horizontal metal water tank inside a box with a clear top. The sun shines through to heat the water. Another slightly more efficient option uses a tall box tilted at an angle to face the sun. This allows the warmer water to be drawn first from the top of the tank.
If you are getting an active system, in general look for an OG-300 rating from the Solar Rating and Certification Corporation (www.solar-rating.org) and a knowledgeable, qualified contractor for installation. Look for contractors certified by the North American Board of Certified Energy Practitioners (www.nabcep.org). And check the Database of State Incentives for Renewables and Efficiency (www.dsireusa.org) for local incentives on installing a solar water heating system, in addition to the federal tax credit — be sure to review specific program requirements on system types, sizing, certifications, installers, and the like to make sure your new system qualifies.