CONVERTERS OF SOLAR ENERGY — ^HOTTEL 157 



Plainly, the material should have a low thermal conductivity to 

 minimize the loss of heat flowing from the hot to the cold junction. 

 Moreover, the electrical conductivity should be as high as possible 

 in order not to dissipate an excessive amount of electrical energy 

 as heat within the "engine." The ratio of the two quantities, thermal 

 conductivity to electrical conductivity, is known as the Wiedemann- 

 Franz ratio; and it has just been shown that this ratio should be 

 as low as possible. A correlation of data from the literature and a 

 consideration of theoretical limitations indicate a sort of conspiracy 

 on the part of Nature to prevent the finding of any material with 

 a Wiedemann-Franz ratio less than a certain minimum value. A 

 study of the properties of zinc-antimony alloys indicates that the 

 thermoelectric power is a maximum for an alloy containing 36 per- 

 cent zinc, but that, owing to the extremely abnormal value of the 

 Wiedemann-Franz ratio in this alloy, there is an advantage in use 

 of an alloy containing 43 percent zinc, since the thermoelectric power 

 of such an alloy is almost as good as the best, and the Wiedemann- 

 Franz ratio is very much more favorable. 



An "engine" consisting of an alloy of zinc and antimony contain- 

 ing 43 percent zinc against the alloy copel has been found to produce 

 a 5 percent useful conversion of heat to electrical power in the 

 external circuit, when the temperature difference of the hot and cold 

 junctions of the system is maintained at 400° C. To the layman 

 this may not sound very imposing, but it is to be remembered that 

 25-percent efficiency is attained only in the best of modem steam 

 power plants and that 5 percent would not be considered bad for a 

 small engine. Moreover, it is to be remembered that a great many 

 alloys and compounds exist, the thermoelectric properties of which 

 are unknown, that it is not inconceivable that further study of the 

 problem may produce a material increase in efficiency in this kind 

 of an engine. With such an idea in mind, there has been initiated 

 at M. I. T. a program of study of the thermoelectric properties of 

 various compounds and alloys. The work is in too early a stage to 

 justify consideration at the present time. 



So far in this discussion only the so-called heat engine has been 

 considered as a means of conversion of solar enregy to useful power. 

 The term, to an engineer, means a device which receives energy as 

 heat at a certain temperature, converts part of that energy to useful 

 power and throws away the rest to a so-called heat sink at a second 

 lower temperature. That this discussion was concerned in the first 

 instance with the use of steam in the engine and in the second in- 

 stance with the use of a thermocouple for conversion to power is 

 immaterial; in each case the first step has been the conversion of 

 solar energy to heat. Now, there is available to the scientist and 

 engineer a powerful tool, known as the second law of thermodynam- 



