©€ should be as large as possible and the 
temperature difference, hot to.cold, should 
be as large as possible. If the thermal con- 
ductivity, K, is small then a smaller amount 
of heat will be required to maintain a given 
temperature difference. To minimize the 
amount of power consumed internally by the 
generator, the electrical resistivity,e , 
should be small. A figure of merit Z is 
defined where 
oc ® 
CK 
is an index of material efficiency. The 
higher the value of Z the better the thermo- 
electric material. 
Z= 
Now, with metals it is possible to obtain 
reasonable values of e and@, but it is not 
possible to get low values of K. Thus, the 
Z of metals is too low for useful power 
generating or cooling devices. With semi- 
conductors, however, even higher values of 
e are obtainable and by the proper addition 
of impurities (doping) the relationship 
between @ and K can be adjusted to give use- 
ful values of Z. 
Some of the intermetallic semi-conductor 
materials currently used are lead telluride, 
bismuth telluride, zine antimonide, silver 
antimony telluride, all with various and com- 
plex dopings. Extensive work is currently 
being done to improve these materials and to 
find new ones with higher figures of merit 
(Z) over wider temperature ranges. 
In addition to the problems of semicon- 
ductor material one of the most serious pro- 
blems facing the designer of a thermoelectric 
device falls into the realm of heat transfer. 
The heat input must be sufficient to main- 
tain the hot junction at the desired temper- 
ature and the cold dump mechanism must be 
sized to maintain the proper cold junction 
temperature. The power level of some gen- 
erator design concepts is directly limited 
by-the cold side heat transfer. In the case 
of generators for oceanographic research, 
however, the sea can usually be used as a 
large and efficient heat sink. 
A thermoelectric device is used to con- 
vert heat into electricity, or vice versa. 
No discussion of, these devices is complete 
without examining the heat sources that can 
be used. All thermal energy sources can be 
classified as: fossil fuels, solar, chemi- 
cal, geophysical and nuclear. All these 
energy sources can, and most are, being 
applied to thermoelectric converters. 
Missions in space vehicles are primarily 
considering solar and nuclear and chemical 
heat sources whileterrestrial or air-breath- 
ing applications are relying heavily upon 
fossil fuels. It is in these latter cate- 
291 
gories that generators for oceanographic 
research can be found. 
Since both the initial and operating cost 
of a thermoelectric device will depend on the 
power and voltage level required, some atten- 
tion to this requirement is essential In 
some applications a continuous constant 
power level is required and the generator 
will be directly coupled, with suitable 
voltage transformation and regulation, to 
the utilization equipment. There will, how- 
ever, be many applications where the power 
requirement will be intermittent and an 
average power analysis is required. Such 
applications may be periodic data trans- 
mission systems, alarm devices, flashing 
lights, etc. In these cases a low average 
power output may be stored in chemical 
batteries or capacitor systems, for use as 
required. Average power can be defined as 
Power consumption X 
Avg. Pwr. = time_of consumption 
Time of Cycle 
for example a data transmission system 
required 300 watts for 20 minutes every four 
hours 
300 X 20 
ROSE 25 watts average 
avg. pwr. 
A twenty-five watt power supply has consid- 
erable operating and capital cost economy 
compared to a 300 watt system. 
A second important consideration is 
selection of a heat source for a maximum 
economy, reliability, and availability. 
While nuclear and chemical heat sources have 
a most important role to play in undersea 
applications, fossil fuels, such as propane, 
are preferred in air breathing applications 
for economy reasons. In cases where volume 
is important in long life missions, a 
nuclear heat source can be used, if cost and 
hazard considerations are also in consonance. 
It would be interesting to consider the 
overall efficiency values that might realis- 
tically be expected from presently available 
fossil fueled devices and those that might 
reasonably be expected in the future. 
Near 
PFFICIENCY COMPONENT Present Future 
Thermoelectric 
converter 5% 8% 
Combustion 70% 75% 
Voltage conversion 90% 90% 
Energy Storage 80% 80% 
Overall 2.5% 4. 3% 
Since cost will always be a significant 
parameter in power sources, selection of 
a cost analysis for a typical application 
