162 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1915. 



This Needles plant used sulphur dioxide, and its results decided 

 them to build a larger plant, which Willsie speaks of as their third 

 sun-power plant, and describes as follows: 



A 20-horsepower slide-valve engine was connected to an open-air water-di'ip 

 condenser and to a fire-tube boiler 22 inches by 19 feet having fifty-two 1-inch 

 tubes. The solar-heated liquid flowed through the tubes giving up its heat to 

 the sulphur dioxide within the boiler. Boiler pressures of over 200 pounds were 

 easily obtained. The engine operated a centrifugal pump, lifting water from a 

 well 43 feet deep (sic), and also a compressor, in addition to two circulating 

 pumps. 



Their fourth plant was a rebuilding of the third, and they tried 

 the expedient of covering the heat-absorbing water Avith a layer of 

 oil, but the results were not so good as wdien a heat-absorbing liquid 

 (water, or oil, or a solution of chloride of calcium) was rapidly 

 circulated in a thin layer. The sun-heat absorber for this plant was 

 in two sections, one covered with one layer of glass and one with 

 two layers, and both on a slope, the liquid running from the first to 

 the second, and its temperature in the two sections being 150° F. and 

 180° F., respectively. The liquid at 180° F. was distributed over a 

 " heat exchanger " consisting of horizontal pipes about 3 inches in 

 diameter, arranged in a vertical plane, something like an air con- 

 denser. The pipes contained sulphur dioxide, and the heat-absorbing 

 liquid lost about 100° F. in its descent. The cooled liquid was 

 returned to the two sections of the absorber to be reheated. The heat 

 exchanger was inclosed in a glass-covered shed. Willsie says : 



The engine used in this experiment was a vertical automatic cut-off, which 

 at times, with a boiler pressure of 215 pounds, probably developed 15 horse- 

 power. The two-heater sections exposed an area of about 1,000 square feet to 

 the sun, but as the heat was taken from storage and not directly from the 

 heater, it is not fair to assume the above proportion of heater surface to horse- 

 power developed. 



The condenser consisted of G stacks of horizontal pipes, 12 pipes to the 

 stack. The cooling water, pumped from a well 43 feet deep, had a temperature 

 of 75° F. Only enough water was allowed to drip over the pipes to keep them 

 wet, and so great was the evaporation in the dry desert breeze that the cooling 

 water left the lower pipes at 64°. By using the cooling water over and over, 

 the condenser gave very satisfactory results. A shade of arrow weed, a straight 

 willbwlike shrub abundant along the Colorado River, kept the sunshine from 

 the condenser pipes and permitted a good air circulation. 



Willsie estimated the cost of his sun-power plant, complete with 

 engine, at £33 12s. per horsepower. 



With regard to Willsie's results, it is to be noted that 377 B. t. u. per 



hour means an efficiency of ^^y J ^y " 19 ~-*'^^ P^^ ^^^^' ^°^" ^^^ ^°^ 

 know that a maximum of only about 299 B. t. u. per square foot per 

 hour penetrate the atmosphere. The author agrees with the 50 per 

 cent efficiency given a little earlier by Willsie. 



