ARTIFICIAL CONVERTERS OF SOLAR ENERGY ^ 



By H. C. Hotter 

 Massachusetts Institute of Technology 



A study of the literature on solar energy utilization has convinced 

 me of the existence of an unalterable tradition among speakers and 

 writers on the subject. One must always begin such a discussion by 

 expressing the earth's reception of solar energy in units no one has 

 thought before to use, the more startling the better. In keeping 

 with this tradition, I shall mention a few old figures and add my 

 own. The earth and its atmosphere intercept the equivalent in 

 energy of 21 billion tons of coal per hour ; 6 million tons per second ; 

 the equivalent in 3 minutes of the annual American energy consump- 

 tion of about 1 billion tons; energy at a rate sufficient each year to 

 melt a layer of ice 114 feet thick ; on an acre at noon the equivalent 

 of the discharge of a healthy stream from a garden hose spouting 

 fuel oil instead of water. 



Having made the conventional beginning, let me add what many 

 of you know : that figures such as these are almost irrelevant to the 

 problem of practical utilization of solar energy. They have attracted 

 uncounted crank inventors who have approached the problem with 

 little more mental equipment than a rosy optimism. Now, an in- 

 formed pessimism is sometimes the healthiest mood in which to 

 approach an engineering problem ; and I want to use a little space in 

 an endeavor to put you in that mood. Consider a solar power plant 

 utilizing 1 acre of land, and operating on the principle of conversion 

 of solar energy to heat in steam used to run an engine. There is 

 incident at noon, normal to the sun's rays and outside the earth's 

 atmosphere, 7,400 horsepower of solar energy. On a clear day, of 

 this quantity about 5,000 horsepower arrives at the earth. Allowing 

 for the efficiency of collection of the sunlight as heat in the working 

 fluid to be used in the engine, the quantity drops to about 3,300 horse- 

 power. Utilizing the highest achieved efficiency of conversion of 

 solar heat to useful power (results of Dr. Abbot's experiments), the 



1 Presented before the symposium on Solar Energy, Harvard Chapter, Spring, 1940. Re- 

 printed by permission from Sigma Xi Quarterly, vol. 29, No. 1, April 1941. 



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