solar hot water for the home

Solar Hot Water for the Home
Technological advances and improved industry standards have improved both the performance and reliability of residential solar water heaters, making them competitive in cost and environmentally safe.

By Jane Davidson
and Byard Wood

Over the last 15 years, solar water heaters have become a familiar sight in many parts of the country. According to the Department of Energy, nearly one million homes in the United States, about one percent of all residences, use the sun’s energy to heat water. This number should grow thanks to lower prices, new initiatives, technological advances, and improved industry standards.

All systems include one or more solar collectors that capture and convert the sun’s energy to usable heat, as well as a tank to store heated water for later use, a means of moving the collected heat to the required location, and a method of regulating the collection, distribution, and storage of heat. Nearly all solar water heaters use flat-plate collectors with tempered-glass covers. The absorber is usually copper with either flat black paint or a wavelength-selective coating. The absorber plate and tubes that contain the working liquid are mounted in a box (typically aluminum) with insulated bottom and sides.

Forced circulation systems use a pump to circulate the fluid heated in the solar collector either to a heat exchanger or directly to the storage tank. Another design, known as thermosiphon, eliminates the need for a pump and replaces the conventional water storage tank and external heat exchanger with a wrap-around heat exchanger/tank combination. Integral collector storage water heaters combine the collection and storage functions in the solar collector. Since potable water is used as the heat-collecting fluid, these systems require neither pumps nor a heat exchanger.

Experience and analysis show that the optimum system supplies 60 to 70 percent of the annual energy required to heat water. Solar water heaters are economically competitive with conventional electric water heaters and have environmental advantages over both gas and electric water heaters.

Jane Davidson is an associate professor in the Department of Mechanical Engineering at the University of Minnesota in Minneapolis. Byard Wood is a professor in the Department of Mechanical & Aerospace Engineering at Arizona State University in Tempe.

The above was adapted from an article in the August issue of Mechanical Engineering magazine. To obtain a copy of this issue, click here.