The physical characteristics, thermoelectric power, resistivity, specific heat conductivity, and the temperature difference between hot and cold junctions determine the efficiency of thermocouple materials. These data have been determined for Chromel P-constantan, bismuth alloys, and the intermetallic compound ZnSb, containing small amounts of added metals. The optimum characteristics of flat-plate type solar energy collectors have been evaluated in combination with thermocouples, designed for maximum energy conversion. Using flat-plate collectors with 2 glass panes, an efficiency of 0.63 percent was obtained with the ZnSb-type alloys in combination with a negative Bi-alloy. The commercially available ductile alloys Chromel P-constantan, produced ten times lower efficiency. Calculations indicate that the efficiency may be increased to 1.05 percent by using 4 panes of low-reflection characteristics. With solar energy concentrating means (lenses), the efficiency was increased to 3.35 percent, using the above mentioned best alloy combination. The maximum allowable cost of thermoelectric generators has been evaluated, by comparison with present methods of electric power generation. Solar thermoelectric generators may find useful application in the tropics, as they do not contain moving parts and require very little attention. Further development is needed to determine their economical value.

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The generation of electrical energy from thermal energy by thermoelectric means cannot be accomplished with thermocouples made of the available alloys generally used for temperature measurements, their efficiency being less than one percent. A review of the theoretical efficiency calculations shows that higher efficiencies can be attained with thermocouple materials to which the Wiedemann-Franz-Lorenz relation is applicable, when their thermoelectric power is greater than 200 microvolts/°C. Some zinc-antimony alloys with added metals approach the above conditions and have produced an experimental efficiency in excess of five percent, in accordance with the theoretical calculations. The criteria of higher efficiency are applied to semi-conductors, deriving the optimum conditions. Lead sulfide with excess lead was found to be the only suitable material at present, which in combination with the zinc-antimony alloy produces an efficiency of seven percent. Higher thermoelectric efficiencies can be produced only by developing new materials which can attain the theoretically required high values of thermoelectric power, low heat conductivity, and low specific resistance.

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