ABSTRACT
In a companion chapter, parametric and optimization analyses of Cascaded Thermoelectric Modules,
Advanced Radioisotope Power Systems (CTM-ARPSs) with four Genral Purpose Heat Source (GPHS)
bricks are performed (El-Genk and Saber, 2005). A GPHS brick contains four
PuO
fuel pellets, each
clad in iridium and generates 62.5 W of thermal power when using fresh fuel. Therefore, a GPHS brick
loaded with fresh fuel generates 250 W of thermal power as an average surface temperature of the Fine-
Weave Pierced Fabric (FWPF) aeroshell, in which the fuel pellets are encapsulated, of ,1305 K
(Carpenter, 1970). The thermal power from the GPHS bricks is transferred by radiation to the hot-side of
the CTMs to maintain a hot junction temperature of the SiGe unicouples in the top array of CTMs of
1273 K. The results presented by El-Genk and Saber (2005) firmly established that CTM-ARPSs are not
only more efficient but also higher than SOA-RTG for the same or almost the same beginning-of-
mission (BOM) electric power. A SOA-RTG for generating BOM power of 105 W
would require seven
PHS bricks. The reported performance of the CTM-ARPSs is on line with current expectations of
NASA’s Project Prometheus for ARPSs that are lighter and more efficient than SOA-RTGs. This chapter
extends the analyses by focusing on the development of CTM-ARPSs for BOM power of 108 W
, and
comparing the performance parameters and mass estimates with those of SOA-RTG for BOL power
of 105 W
. A CTM is comprised of a top array of SiGe unicouples and a bottom array of unicouples of
different, high thermoelectric materials (El-Genk and Saber, 2005). The top and bottom arrays are
thermally, but not electrically, coupled and operate at the same nominal terminal voltage but different
load currents. Thus, the number of unicouples in the top and the bottom arrays is different. The SiGe
unicouples in the top array of CTMs (Figure 54.1 and Figure 54.2a) are optimized for maximum
efficiency operation at a nominal hot junction temperature, T
, of 1273 K, constant input thermal
power per array, Q
, of 29.7 W, and constant cold junction temperature, T
, of either 780 or 980 K,
depending on the thermoelectric materials of the unicouples in the bottom array (Figure 54.2b-d). The
heat rejection by the SiGe top array, Q
, equals the thermal power input to the bottom array, in which
the hot junction temperature, T
, is constant and 15 K lower than the cold junction of the SiGe
unicouples in the top array, T
. The cold junction temperature, T
, of the unicouples in the bottom
array and the radiator base temperature, T
, surface area, and fin dimensions in CTM-ARPSs
(Figure 54.1b and c) are determined from the optimization of the unicouples in the bottom array of the
CTMs (Figure 54.2 and Figure 54.3).
