SOLAR ENERGY FOR WATER HEATING — BROOKS 161 



A wliite surface out-of-doors acts partly as a one-way heat valve 

 because it reflects most of the sunshine and yet at night readily emits 

 ordinary heat waves to the cold sky. However, new galvanized iron, 

 for example, has a solar absorptivity of 0.65 ^° and a long-wave emis- 

 sivity of 0.23,^^ indicating high daytime heating and relatively low 

 nocturnal cooling by radiation. Polished metals, particularly alumi- 

 num, act as radiation shields, being excellent reflectors and also poor 

 emitters. Such properties are valuable for minimizing diurnal 

 temperature fluctuations. 



Glass is opaque to long-wave radiation and therefore acts as a solid 

 surface in emitting ordinary heat waves. Sunshine or short-wave 

 radiation is readily transmitted, only a few percent being absorbed by 

 the glass itself and a small amount reflected. Hence in the sunshine, 

 glass acts partly as a one-way heat valve in the opposite sense to the 

 cooling effect of white surfaces, although for a different reason, namely 

 transmission and opaqueness instead of reflection and emission for 

 short- and long-wave radiation respectively. 



The effect of angle of incidence on the amoii/nt of light tramsmitted 

 hy glass and dbsorhed hy a 'blaxih surface. — The total solar energy re- 

 ceived by fixed glass-covered absorbers is considerably less than the 

 total radiation impinging on a surface kept perpendicular to the direct 

 rays of the sun throughout the day. The essential difference is that 

 the effective reception area of a fixed surface is less than that of a 

 normal surface in proportion to the cosine of the angle of incidence. 

 There is also greater absorption by the glass and increasing reflection 

 from the glass and absorber surfaces as the angle of incidence increases, 



EXPERIMENTAL INVESTIGATIONS 



The useful heat output of a fixed solar-energy absorber depends pri- 

 marily upon the short-wave energy received, which changes rapidly 

 during the day and differs from day to day because the intensity of the 

 sunshine varies with the season and with the character of the atmos- 

 phere. Second, the useful heat output is affected by the heat losses of 

 the absorber, which vary with temperature and wind. No single-day 

 figure or curve, therefore, can be a standard. In order to compare the 

 results of different experiments on different days, three different types 

 of heaters (see pi. 1) were built by Charles Barbee and H. D. Lewis.^^ 

 Simultaneous observations were made throughout the late summer and 

 fall and are discussed in the following sections. 



Ideal thin^ fiat-tank solar-energy absorber. — Although some of the 

 solar energy falling in the space between pipes can be utilized indi- 



wFowle, Frederick E., Smithsonian physical tables, 8th rev. ed. Smithsonian Misc. Coll., 

 vol. 88, 686 pp. (see specifically p. 383), 1933. 



"McAdams, Wm. H., Heat transmission. 383 pp. (see siJeciflcally pp. 45-49). McGraw- 

 Hill Book Co.. New York, 1933. 



12 Staff members of the Agricultural Engineering Division, California Agricultural 

 Experiment Station, Davis, Calif. 



