SOLAR ENERGY FOR WATER HEATING BROOKS 169 



Size of absorber in relation to quantity of water to be heated. — 

 The average daily heat absorption on clear days in September can be 

 judged from the curve of figure 2 to be approximately 1,400 B. t. u. 

 per square foot of glass for a flat-tanker absorber. Such an absorber 

 although most efficient, is not practical because of its tendency to de- 

 form when subjected to internal pressure. A larger-area pipe 

 absorber is more economical, and shows a simultaneous daily heating 

 of approximately 1,000 B. t. u. per square foot of glass. If a supply 

 water temperature of 65° F. is assumed, each gallon of hot water will 

 require about 700 B. t. u. for heating. The night losses are about 

 100 B. t. u. per gallon of high-temperature water, making a total 

 daily requirement of about 800 B. t. u. per gallon. As some allowance 

 must be made for dust on the glass and for desired operation when 

 the sky is not entirely clear, an arbitrary figure of 1 square foot of 

 pipe-absorber glass area per gallon of hot water can be assumed 

 adequate for satisfactory solar-heater operation for 7 or 8 months of 

 the year. 



LENGTH or PIPE RUN FOR SATISFACTOKT THERMOSIPHON PERFORMANCE 



There is considerable difference in hot-water temperature at the top 

 of the tank when using single zizag and when using parallel pipe ab- 

 sorbers, especially over a short heating period. With the single-pipe 

 zigzag small quantities of water hot enough for the small demands are 

 available long before the whole tank temperature builds up with the 

 rapid-flow type. 



For the rapid-flow type where it is desired to heat the whole tank- 

 ful as quickly as possible, probably a 30° F. minimum temperature 

 rise should be obtained by noon. The experiments indicate that this 

 can be accomplished on cloudless days with single %-inch pipes 

 about 70 to 100 feet long when the absorber discharges into the stor- 

 age tank 71/2 feet above the center of the pipe absorber coils, and the 

 pipes to and from the storage tank are I14 inches for three or four 

 parallel pipes and li/^ for five or six parallel pipes. Large variations 

 will occur, determined by the number of return bends, the height of 

 storage-tank connections, and the weather conditions. When this 

 arbitrary criterion is applied to pipe absorbers containing 4 or 5 

 lineal feet of 34-inch pipe per square foot of glass area (and per gal- 

 lon of storage-tank capacity), with the bottom of the tank 214 feet 

 above the center of the absorber, a 45-gallon system should have three 

 or four pipes in parallel; a 60-gallon system, five or six pipes. In 

 larger systems the storage tank may be raised to obtain greater ther- 

 mal head. Each pipe run should be about 10 or 15 feet per foot of 

 effective riser height from center of absorber to discharge into storage 

 tank. 



