A MAJOR solar energy boom occurred in the United States in the early 20th century, with more than 100,000 solar water heaters operating in California and Florida.

In the late 1800s, heating water was a daily challenge for people in the Sunbelt. There wasn't much wood around and coal was expensive. A simple alternative was to stick a storage tank outside and let the sun heat some water for evening use.

Early efforts to improve this system included the "greenhouse effect." The storage tank was placed in a glass-covered pine box which trapped the incoming sun rays and the water temperature rose quickly.

In 1909, according to California historians Ken Butti and John Perlin, a new technology was introduced. William Bailey realized that it was inefficient to heat a whole tank of water at one time; it was better to do it little-by-little. His new design provided the foundation for today's solar hot water systems.

Bailey separated the solar collector from the storage tank. He led cold water from the tank to a coiled copper pipe, which was placed in a shallow, insulated box under glass. The sun heated the small amount of water flowing in the pipe, and this water circulated to the tank's top. The solar collector was placed on the roof, with the tank in the attic. This system was called the Day and Night Solar Heater.

Bailey's unit sold briskly in the Los Angeles area. In 1920, more than 1,000 heaters were installed. In the late 1920s, sales began to decline: in 1926, only 350 units were mounted, and by 1930, the number fell to 40.

The reason was natural gas. During these years, the new fuel was sent by pipeline throughout southern California. It was cheap and easy to use; solar technology couldn't compete.

But just as the business crashed in California, it took off in Florida. Bailey had sold the rights on his system in 1923 to the Solar Water Heater Company of Miami.

The new industry grew quickly in the 1920s. Many businesses -- mostly roofing and general construction outfits -- entered the field. Soon prices on the water heaters fell and demand increased. Prior to World War II, 100,000 solar water heaters were installed in Southern Florida.

Residential home owners often financed their purchase with low-interest government loans. In some government-built housing, solar water heaters were required; they were cheaper than the alternatives.

Due to materials restrictions, the solar industry halted during the war and didn't revive afterward. Four factors converged on the industry during the 1950s to destroy its competitive edge.

Electricity rates dropped dramatically; at the same time, solar costs rose due to higher material prices.

Many of the solar units, in service for more than 20 years, were rusted or unreliable. People replaced them with new types of water heaters.

A new force entered the housing market -- the builder-developer. Houses come fully equipped and buyers were unable to choose their appliances.

A major publicity drive was launched for electrification.Americans increasingly identified modernism with electricity, and the solar age was forgotten.

Ironically, however, this ability of the technology to vanish without a trace indicates one of its principal strengths.

The sun provided a portion of America's energy needs for almost 50 years. No pollution, radioactivity or raped landscapes remain as a legacy. The water heaters functioned quietly, cleanly and efficiently when no alternatives uere available. This suggests that the sun can have a renaissance, using technologies that are little different from those developed early in the 20th century.