152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



horsepower output drops to 490. These calculations have so far all 

 been on the assumption of normal incidence of the sun on the collector 

 systems. To achieve this the collector must be mounted to turn with 

 the sun and must be far enough from its neighbor not to shade the 

 latter in morning or afternoon. Introducing a ground-coverage fac- 

 tor of one-third to allow for this, the output is cut to 163 horsepower. 

 But this figure applies only to the hours when the sun shines with 

 full intensity. Converting to a 24-hour basis of operation on clear 

 days in summer in Arizona, the output drops to 83 ; or in winter to 

 46 horsepower ; or for the year to 68 horsepower. Passing on to the 

 average year of New York weather, the output is down to 30 horse- 

 power. Even if one stops at a reasonably attainable value of 50 

 horsepower in Arizona, that figure is one one-hundred-and-fiftieth 

 of the original 7,400 horsepower. 



For rough orientation as to the meaning of these figures, suppose 

 the possibility of a 50-horsepower steady output from an acre in 

 Arizona be accepted. To evaluate this power, let it be assumed that 

 electric power can be produced in a large modern steam plant at a 

 cost of 0.6 cents per kw.-hr., or $53 a kilowatt year, making the out- 

 put of our 1 acre worth $1,900 per year. In the absence of knowledge 

 of labor costs, maintenance, etc., one can only guess the capital value 

 of such an output. Capitalization at 15 percent is almost certainly 

 overoptimistic, and even that yields but $13,000 to spend on the en- 

 tire plant, or about $2.60 per square yard. Since the ground coverage 

 is but one-third, $8 are available to build each square yard of re- 

 flectors, mounts, and accessories. The result is one so often encoun- 

 tered in engineering projects : indecisive. It may be possible to build 

 a plant for such an amount ; much more exact knowledge of perform- 

 ance and costs is necessary than was at hand in making the above 

 rough estimate. What I have particularly wanted to emphasize by 

 this preliminary consideration is perfectly obvious to the engineer, 

 namely, that solar power is not there just for the taking! 



However, this preview has at least indicated that solar power is 

 not completely outside the realm of economic feasibility. It is worth- 

 while, then, to examine in more detail the problem of use of solar 

 energy by conversion to heat, a problem which has commanded the 

 attention of engineers for three-quarters of a century. 



First, a moment on some elementary principles of heat transmis- 

 sion. If a black metal plate is exposed to the sun, and cooling water 

 is run under the plate fast enough to keep the plate from rising 

 appreciably above the surrounding air temperature, substantially all 

 the energy of the sun's rays intercepted by the plate shows up as 

 energy in the water; the efficiency of collection of heat is nearly 100 

 percent, but the value of the heat is low because of its low tempera- 



