CONVERTERS OF SOLAR ENERGY — ^HOTTEL 153 



ture level. If the water enters 50° F. above the surrounding air 

 temperature and flows through fast enough hardly to rise in tem- 

 perature, there is the same interception and absorption of solar 

 energy by the plate ; but now much of it is used in keeping the plate 

 up to temperature; it is lost to the surroundings by radiation and 

 convection, and very little of the absorbed energy appears in the 

 water stream. To improve the efficiency the losses must be cut down. 

 There are several ways. The back side of the plate may be insulated, 

 since it never sees the sun. Or a plate of glass may be placed over 

 the metal plate and parallel to it, with an inch or so of air space 

 between. Then the plate receives and absorbs almost as much sun- 

 light as before — the glass transmits about 90 percent — ^but the losses 

 from the metal to the outer atmosphere are reduced : the convection 

 loss because of the imposed stagnation of the air, and the radiation 

 loss because glass, though transparent to the sun's rays, is opaque 

 to the long- wave infrared radiation emitted by the hot metal plate. 

 Variations of this idea include the use of several glass plates and of 

 glass vacuum chambers. Another method of cutting down losses is 

 to reduce the area at which losses occur relative to the area of the 

 interceptor of the energy to be collected. This may be done by 

 choosing the most favorable orientation of the plate, that is, normal 

 to the sun's rays, or by use of a concentrating device, such as a mirror, 

 which intercepts rays covering a large area and brings them to a 

 focus on an object of much smaller area where the heat loss is 

 consequently correspondingly small despite the high temperature. 

 From this discussion there emerges a threefold basis of classifica- 

 tion of solar energy collectors: (1) By nature of orientation of the 

 collector (whether and how completely it follows the sun), (2) by 

 amount of concentration achieved by mirrors, (3) by amount and type 

 of insulation of the receiver surface. It is perfectly obvious that 

 many of the early inventors and engineers in this field were familiar 

 with these principles in a general way. 



One might now ask, "With all this work, have not the possibilities 

 of energy production by conversion to heat been so thoroughly studied 

 as to yield a definite answer?" Unfortunately, no. Qualitative 

 familiarity with the principles involved, these men had certainly ; but 

 with the exception of the work of Dr. Abbot, their experiments and 

 records indicate inadequate quantitative knowledge of the problem. 

 As an exemple, consider the simplest possible collector, the flat plate 

 insulated with several air-spaced glass layers. Willsie's work at 

 Needles in 1909 indicated the possibilities of this simplest of solar 

 plants, but it left unanswered the question of merit relative to the 

 much more efficient— and more expensive— plant of Abbot, and did 

 not yield data permitting the design of such a plant for any given 



