Thermoelectric power conversion in space

Thermoelectric power conversion systems have a broad potential for applicability to a large number of different classes of space missions. As research continues on thermoelectric materials, the potential for significantly improved performance is good.

With research also occurring in the power conversion field to improve configurations and specific designs, thermoelectric power conversion continues to show great promise for near- and long-term space missions. Previous applications of thermoelectricity to power generation date back more than a century and a half to Seebeck, who showed in 1822 that a current is obtained when the junctions of dissimilar materials forming a thermocouple loop are maintained at different temperatures.

Such a circuit is shown schematically in Figure 1. For example, two dissimilar materials, an n- and a p-type semiconductor, are joined at their ends by a metallic conductor. Heat is supplied to the hot junction from an external source; the other junction is maintained at a fixed lower temperature.

As a result of the temperature difference, a current, 1 , flows through the branches of the thermocouple. By allowing the current to flow through an external load resistance, inserted into the circuit between the cold junctions, the arrangement represents a direct conversion of heat into electrical energy.

The figure of merit, Z, measures how good a material is for thermoelectric energy conversion and is given by: Z = S[.sup.]2 /p[Lambda] where S is the Seebeck coefficient, p is the electrical resistivity, and A is the total thermal conductivity. The best materials to use for thermoelectric applications are semiconductors. This can clearly be seen in Figure 2 , where the Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit (which measures how good a material is for thermoelectric energy conversion) are plotted versus carrier concentration.

Insulators have a high Seebeck coefficient but low electrical conductivity (high resistivity) while metals have a high conductivity (low resistivity) but low Seebeck coefficient. Semiconductors with a carrier concentration in the 10 [.sup.]19- to-10 sup.]21- percm3 range have a reasonably high Seebeck coefficient and conductivity.