166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 9 



enclosed triple tanks and the ideal thin, flat-tank absorber indicate an 

 approximate relative eflBciency of YO to 75 percent for daytime heating. 



The tanks in figure 4 were on 24-inch centers, and the inside width of 

 the box was 8 feet. 



Single-pipe absorber with storage tanh. — The usual "solar heater" 

 consists of a flat, glass-covered, zigzag pipe-coil absorber connected 

 for thermosiphon circulation with an insulated storage tank (pi. 3, 

 fig. 1). In this system when the storage tank is above the absorber 

 there is no appreciable reverse circulation at night, and the high day- 

 time temperature is conserved by the tank insulation so that tempera- 

 tures over 140° F. are available at all times if the system is properly 

 designed. 



Multiple-pipe absorber with storage tank. — Wlien the required 

 absorber area is too large for a single zigzag pipe, the flow resistance 

 can be decreased by installing several pipes in parallel (pi. 3, fig. 2). 

 The heat transfer operation is more effective than for the single-pipe 

 absorber of the same area because with the faster flow the tempera- 

 ture rise will be less and the heat losses from the absorber lower. 



Thermosiphon circulation and temperatures in pipe absorber with 

 storage tanh. — The faults of the round-tank absorbers — namely, 

 small absorption area in proportion to the tank capacity, and large 

 nocturnal losses — are remedied by separating the storage tank from 

 the absorber area. The absorber area can then be designed to satisfy 

 the heat requirements independently of tank size; and the storage 

 tank, being separate, is easily insulated to minimize heat losses. 



The separation of absorber and storage tank requires some means 

 of heat transfer from absorber to tank during the day and the pre- 

 vention of heat loss from tank to absorber during the night. This 

 transfer can be accomplished positively by forced circulation, using 

 a positive pump that is operated only during the heating period. 

 For domestic installations this system is objectionable because of ex- 

 pense, leaky packing glands, and the need for mechanical or electrical 

 power not directly available from sunshine. 



Solar heat itself can, however, be used for producing thermosiphon 

 circulation if the piping system from tank bottom to absorber and 

 back to tank top is properly designed. Water warmed in the absorber 

 becomes of lower density than the colder water in the pipe from the 

 tank bottom, which will flow into the absorber and push the heated 

 water into the top of the tank. The force available for this circula- 

 tion is proportional to the difference in density of the hot and the 

 cold water. 



If the absorber is well below the tank, it becomes a low, cold pocket 

 at night and thermosiphon circulation ceases. The night losses are 

 thus confined to the escape of heat through the tank insulation and 



