Solar water heaters

Weighing pros and cons
By Brian Sloboda
Solar water heaters

A technician (Doug Powell) from the U.S. Department of Energy’s National Renewable Energy Laboratory checks the piping on an experiment involving natural convection loops for pipe freeze protection in a solar water heating system. This system is an integral collector storage system. These natural convection loops would work for any system having potable water exposed to ambient air. (Photo: Warren Gretz/NREL)

Solar hot water systems are a time-tested and efficient method of harnessing the sun's rays and saving on energy costs.

Solar hot water systems are relatively simple: The sun shines on a dark-colored tank or collector and heats the water inside. They must be matched to a heating load — the volume of hot water needed for the size of your home and its number of occupants.

The most common use is to preheat domestic water — what goes to sinks and showers. Larger, much more complex systems can also supply hot water for a whole house, including washing clothes and dishes. These can also be a good option if you need to heat water for a swimming pool.

Heating domestic water typically accounts for about 20 percent of a household's energy bill, or $100 to $300 per year. But a well-designed solar water heating system can nearly eliminate that energy use.

A basic model costs anywhere from $4,000 to $8,000, with a payback of 10 to 20 years for most consumers. However, federal tax incentives (30 percent) and state tax credits (35 percent) can shorten that period significantly. Check the Database of State Incentives for Renewables and Efficiency at dsireusa.org.

There are two main choices when shopping for a solar hot water system:

Active versus passive: Active systems feature circulating pumps and controls that move water. Passive systems simply use pressure of the water system within the house to move water.

Direct vs. indirect: Direct (or "open loop") systems circulate household water through solar collectors and into the home. Direct systems can be used only in mild climates that do not experience freezing conditions. Indirect ("closed loop") systems circulate an antifreeze solution through the collectors to a heat exchanger, where the potable water absorbs the heat. They can also be designed so the water retreats back into the house at night. Indirect systems can be used in any climate.

One challenge is what to do with excess heat at the solar panel when there is no hot water demand — for example, in the summer if you go on vacation for an extended time. Drainback systems, if properly designed, can solve this problem, but other options include a "dummy load" to dump excess heat, night discharge controls, high-pressure operation to prevent boiling in the panel, and oversized expansion tanks that can handle the volume change when the fluid in the panel actually does boil. Local contractors can assist in choosing a setup that best addresses these issues.

Economics of solar hot water depend on several things, such as the cost of the fuel that the system replaces, when and how much hot water is used, incoming water temperature, and how much sunlight your location receives. The U.S. Department of Energy provides an online tool to estimate the initial costs and payback period for a solar hot water system at energy.gov/energysaver/articles/solar-water-heaters.

Consideration of PV panels

Consumers interested in going solar may want to consider the pros and cons of solar hot water versus deploying solar electric photovoltaic (PV) panels. Until recently, solar hot water was the more cost-effective option, but decreasing PV prices and economic and tax incentives have re-energized the debate between the two technologies. The decision depends on local factors, including hot water demand and climate.

For a family of four, solar hot water typically is the logical choice. However, electricity generated from PV panels can power your whole house and pump power back into the grid when not needed on site. Before investing in a PV or any other type of "backyard" renewable energy system, contact your local electric cooperative to discuss interconnection costs and other issues associated with it.

About the Author

Brian Sloboda is a program manager specializing in energy efficiency for the Cooperative Research Network, a service of the National Rural Electric Cooperative Association.

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