SOLAK ENERGY FOR WATER HEATING BROOKS 163 



somewhat above average air temperature, as would be the case in a 

 solar water heater. The total useful heat obtained per day, as indi- 

 cated by the area under the curve of figure 2, was 1,360 B. t. u. per 

 square foot. There are large heat losses from the hot absorber box 

 and considerable heat absorption by the insulation. The latter, though 

 not a true loss, delays and lowers the peak of the curve in relation to 

 the theoretical input. 



Exposed-tank solar water heater. — Simple, bare water boilers 

 mounted outdoors where they will not be shaded have long been used 

 during the summer for furnishing late afternoon hot showers. The 

 tanks are usually horizontal or vertical, but experiments described 

 later indicate that sloped mounting (pi. 2, fig. 1) is more effective. 

 These exposed tanks cool nearly to air temperature at night and are 

 useless before noon. 



The temperature rise in round-tank absorbers differs from that in 

 thin flat-tank absorbers mainly because of the large water quantity 

 associated with a given absorber area. The common 30-gallon hot- 

 water boiler 1 foot in diameter by 5 feet long has such a poor ratio of 

 area to volume that the water does not warm rapidly. Since larger- 

 capacity tanks have even poorer ratios of area to volume, and smaller 

 diameter would be special, this study by H. D. Lewis was confined to 

 the regular 30-gallon tanks. 



Figure 3 shows the performance of exposed horizontal and sloped 

 tanks which on calm days furnish enough hot water for two or three 

 hot showers at 102° F. The horizontal tank on days with average 

 air-day temperatures of 100° F. furnished 20 percent less hot water 

 than the sloped tank even on days 10° colder. The horizontal tank is 

 less efficient because all the sunshine falls on the hottest part of the 

 tank. If the tank is sloped (pi. 2, fig. 1), the sun shines on the lower, 

 cold end as well as on the upper hot end. In this case much of the 

 colder water is heated directly at low temperature and with small 

 heat loss. In all cases the average water temperature reached its 

 maximum before 4 p. m. 



The lowest curve (fig. 3) indicating the heat output on a cold sum- 

 mer day, shows the need for protecting the tank against cold air and 

 wind ; but even so there is ample water for one hot shower. 



When the quantity of hot water available from a 30-gallon tank 

 heater is not sufficient but the characteristic temperature performance 

 is satisfactory, a larger quantity is obtained by using several tanks 

 in parallel, with all the cold-water inlets connected together in one 

 direction and all the hot outlets connected together in the opposite 

 direction. The tanks should be spaced well apart to avoid the shading 

 of one by another in the early morning and late afternoon. 



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