CONVERTERS OF SOLAR ENERGY — HOTTEL 155 



which the collected heat is to be put. If the objective is the max- 

 imum collection during the entire year, tilting should favor the sum- 

 mer season. If, on the other hand, the objective is to supply heat for 

 a load which varies throughout the year, the tilt should be chosen to 

 favor that part of the year in which the load is highest. 



As to the use of such collectors, it has already been indicated that 

 one must find first just what they can do. But speculation is permissi- 

 ble. One might visualize a large artificial lake with sloped sides 

 formed by throwing up an earthen ring around a surface-scraped cen- 

 ter, the bottom and sloping sides being surfaced with asphalt. Float- 

 ing on this lake, which is, say, 20 to 40 feet deep, is an enormous raft 

 covering it completely. On the raft is a layer of insulation, then a sys- 

 tem of flat-plate collectors. Forced circulation of lake water through 

 the collectors whenever they attain a temperature above the reser- 

 voir will produce a large body of hot water available for continuous 

 operation of a power plant. The working fluid in the engine might 

 be low-pressure steam or, to cut down engine size, a fluid which 

 boils at lower temperatures. It is not possible to state at this time 

 whether such an idea has possibilities. 



Another less ambitious use of flat-plate collectors might be that of 

 house heating in relatively cold but sunny climates, or summer air 

 conditioning. Some preliminary figures may indicate the prospects 

 in this direction. Consider house heating in New England, and 

 take as a basis the furnishing of one therm of heat throughout the 

 heating season — 100,000 B. t. u. : the heat obtained by burning 1 

 gallon of fuel oil with normal efficiency of combustion. If 1 square 

 foot of flat-plate receiver covered with three plates of glass and 

 tilted 40° southward is operated in connection with 1^/^ cubic feet 

 of water in a well-insulated tank, and the water is pumped from 

 the tank to the receiver and back whenever the receiver is hotter 

 than the tank, the combination will supply all but 15 percent of the 

 100,000 B. t. u. required during the season; the 15 percent has to be 

 supplied as auxiliary heat in December, January, and February. 

 The value of the heat saved is the cost of 0.85 gallons of fuel oil, or 

 about 6 cents. Capitalizing this at 6 percent gives only $1 available 

 to be spent on the roof collector and tank. This is plainly not enough, 

 but the answer is interesting because we have not determined the 

 optimum number of glass plates, or tilt, or ratio of roof to tank 

 area, or considered the possibility of some day having treated glass of 

 lower reflectivity. More particularly, the idea looks interesting for 

 localities where the ratio of winter to summer sunshine is somewhat 

 more favorable than in Boston, and the winter heating requirements 

 somewhat lower. According to a recent publication of Dr. Abbot's, 

 Dr. F, G. Cottrell has proposed a somewhat similar storage system 

 in which sand is to be used instead of water. Whether the ad- 



