rature operation, was the focus of his experimentation.To store the sun's energy, Willsie built large flat-plate collectors that heated hundreds of gallons of water, which he kept warm all night in a huge insulated basin. He then submerged a series of tubes, or vaporizing pipes, inside the basin to serve as boilers. When the acting medium--Willsie preferred sulfur dioxide to Tellier's ammonia--passed through the pipes, it transformed into a high-pressure vapor, which passed to the engine, operated it, and exhausted into a condensing tube, where it cooled, returned to a liquid state, and was reused.In 1904, confident that his design would produce continuous power, he built two plants, a 6-horsepower facility in St. Louis, Mo., and a 15-horsepower operation in Needles, Calif. And after several power trials, Willsie decided to test the storage capacity of the larger system. After darkness had fallen, he opened a valve that "allowed the solar-heated water to flow over the exchanger pipes and thus start up the engine." Willsie had created the first solar device that could operate at night using the heat gathered during the day. He also announced that the 15-horsepower machine was the most powerful arrangement constructed up to that time. Beside offering a way to provide continuous solar power production, Willsie also furnished detailed cost comparisons to justify his efforts: the solar plant exacted a two-year payback period, he claimed, an exceptional value even when compared with today's standards for alternative energy technology.Originally, like Ericsson and Eneas before him, Willsie planned to market his device for desert irrigation. But in his later patents Willsie wrote that the invention was "designed for furnishing power for electric light and power, refrigerating and ice making, for milling and pumping at mines, and for other purposes where large amounts of power are required."Willsie determined all that was left to do was to offer...
