If you’ve ever felt warm water flow from a garden hose that’s been stretched across a sunny lawn, you know how solar water heating works. And if you’ve ever paid a fuel or electric bill, you can understand why using the sun’s rays to warm water is a good idea. Up to 25 percent of a household utility bill goes to heating water for washing clothes, dishes, and ourselves. A solar hot-water system can reduce those costs by two thirds—without using any fossil fuels or causing any pollution. You don’t even need to live in a sunny climate to take advantage of this infinitely renewable source of free power. Thanks to advances in solar-collector technology, these systems have become practical even in places where the sky is gray more often than blue.
The basic setup consists of a heat-trapping solar collector sitting outdoors in an open, south-facing location—usually up on the roof—and a water-storage tank inside the house. In cold climates, a pump circulates an antifreeze-laced liquid through a closed loop of pipe connecting the rooftop array and the tank. A submerged coil inside the tank transfers the heat from the sun-warmed liquid to the household water supply. (In frost-free zones, potable water can be heated directly by the collector.)
Collectors come in two main types. The most popular are the so-called flat-plate collectors: insulated glass boxes with copper pipes attached to heat-trapping “absorber sheets.” Under ideal conditions, they can produce 150-degree water, well above the 125-degree water in a typical water heater. More efficient tube-type collectors encapsulate the absorber sheets and pipes in glass vacuum tubes for maximum insulating effect ( see slide 2 at left: Collecting Heat in a Vacuum). They can heat water up to 200 degrees. And because tubes can capture heat when the sun is not directly overhead and even on cloudy days, you don’t need a big array to get a lot of hot water. The downside is that they cost twice as much as flat plates.
