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1
A Research of The Difference between the Design of
MMRTG and common TEG
Jue Bo
Feixiang Sun
ME 6950
Professor: Dr. HoSung Lee
2
Abstract
A radioisotope thermoelectric generator is an electrical generator that makes use of
series of thermocouples to convert the heat released by the decay of some radioactive
material into electricity. RTGs have already been used in space probes and some remote
facilities. Since they have no moving parts that will lose effectiveness or wear out, RTGs
have historically been considered as a highly reliable power option especially for space
application. This work is based on summary current researches, analysis the impact factors
of power output efficiency. As a result, due to the condition of usage and environment, an
optimal design is secondary under the most circumstances. The design must submit to the
load of the RTG.
Keywords: thermoelectric, generator, radioisotope, RTG, Radioisotope thermoelectric
generator, impact factor, optimal design
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Content
Content ....................................................................................................................... 3
Introduction ................................................................................................................. 4
Basic Model ................................................................................................................ 6
Assumption ................................................................................................................. 8
Procedure .................................................................................................................... 9
Conclusion ................................................................................................................ 13
Referance .................................................................................................................. 15
4
Introduction
RTG is a thermoelectric generator with radioisotope as heat source. Plutonium-238 is
used most widely as power source in lots kinds of devices. Such as Pacemaker, Artificial
aircraft and Lighthouse Beacons. What’s more, Pu-238 has some big advantages like long
life for power output with low reduction, low requirement of protection of radio and it has
most decay energy per unit mass among all known isotope. Compared with the traditional
thermoelectric generator, RTG has always been considered as a higher reliable selection.
Because it has no moving parts that can be changed. RTGs have been used in many different
ways and they are always been used in variable situations for lots of different missions. So
what we usually see in series devices is MMRTG which is Multi-mission Radioisotope
Thermoelectric Generator. What’s more, in this paper we will also talk about some methods
to improve the MMRTG.
5
Fig.1. Power output of Pu-238
PowerPerKg 540W
kg
Power mass lifetime( ) PowerPerKg mass1
2
lifetime
halflife
6
Basic Model
Fig. 2. Cut-away view of the MMRTG
Fig. 3. Schematic Diagram of an RTG System
7
In this simple RTG model provided for the pacemaker (Fig.3), Plutonium-238 as the
isotope heat source provide heat of decay for the hot junction. And of course, there is some
heat lost due to insulation, cracks and support structure. Then from the thermoelectric
generator module, this device output the electrical power finally. Also as we can see the
Cut-away view of the MMRTG in Fig.2, there are several modules with the heat source
which can be considered as the core of the device with the aluminum cover outside.
Furthermore, some additional components help the whole MMRTG device more reliable
as well. Now, we are facing some difficult situations like the scarcity of the Pu-238 fuel
and high associated cost. So increasing the efficiency of RTGs became a very serious issue.
For the same power to be generated from generators, this could allow less usage of the Pu-
238 fuel and present a lower payload weight to the launch vehicle.
MMRTG with Pu-238 used in satellite
8
Assumption
The TE (Thermoelectric) couples of the MMRTGs usually have one PbTe n-leg and
one segmented PbSnTe/TAGS-85 p-leg (see fig.4). Then we can make an assumption as:
While with the TE efficiency being related to the figure of merit (ZT), selecting some new
kinds of materials with those having higher figure of merit will be a good choice. What’s
more, giving a better chemical stability at higher temperatures can increase the efficiency
as well. We can call the new design of MMRTG eMMRTG which is enhanced Multi-
mission Radioisotope Thermoelectric Generator.
Fig.4. MMRTG and eMMRTG couples
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Procedure
Assume both MMRTG and eMMRTG are designed with maximum power output,
therefore the figure of merit Z can be reverse calculate by the maximum power
efficiency ηmp equation (Eq.1). [1]
ZTc, Figure of Merit at cold junction temperature
Tc, The temperature of cold junction temperature.
Th, The temperature of hot junction temperature.
By the design parameters of the MMRTG and eMMRTG system(Table I), when the
at cold junction temperature is 150°C while hot junction temperature is 525°C, the
figure of merit of MMRTG ZTc is 0.35 and the figure of merit of eMMRTG ZTc is 0.385-
0.434. Therefore, at the hot junction the figure of merit of MMRTG and eMMRTG are
0.778@525°C and 0.794-0.895@600°C, respectively, which is a great agreement with
the data in Fig.5.
mp
1Tc
Th
21
21
Tc
Th
4
Tc
Th
ZTc
10
Table I. the design parameter of the MMRTG and eMMRTG systems [2]
Design parameters MMRTG Enhanced MMRTG
Design-point QHS 1984 WTH TBD
TE hot-side temp 525°C 600 °C
TE cold-side temp 100-200°C 100-200°C
Initial Power ~120W ~145-170W
Initial system efficiency 6.0% 7.6-8.3%
Calculation
by Eq1
ZTc 0.35@150°C 0.385-0.434@150°C
ZTh 0.778@525°C 0.794-0.895@600°C
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Obviously, a higher hot-junction-temperature will provide a higher efficiency, but
it need an emissive coating implemented on the heat source isolation liner when
above 550°C to protect overheat damage and retard the aging of TE module. [2]
However the figure of merit Z can only be treated as constant in a small
temperature range, it is various in a large range, which makes it not a straight line but
a curve. At the same time, the ZT of many kinds of TE martial does not increase or
even decrease above some certain temperature as Fig.5. [3]
Fig.5. Thermoelectric figure-of-merit, ZT, of the materials used in the MMRTG and
proposed for the eMMRTG systems.
12
Note that the several increase output power point of Curiosity’s MMRTG (Fig.7),
in these time point, the environmental conditions on the Mars raise the temperature
of the fin root of the generator, which in turn raised the hot-junction-temperature
back into the designed temperature range. This phenomenon mention us that the
environment condition of the destination is also an important factor that must be
consider during the design process. As well as a large difference of temperature does
not mean a very low cold-junction-temperature. [4]
Fig 6
560 580 600 620 640
80
100
120
140
eMMRTG_Power Thi
Thi /°C
500 510 520 530 540 55080
100
120
140
160
180
MMRTG_Power Thi
Thi /°C
13
Conclusion
From what have been shown above, the assumption can be proved. While select the
new materials with those having higher figure of merit, the TE efficiency increased. What’s
more, from fig.5 we can directly see that giving a higher temperatures can increase the
figure of merit which could increase the TE efficiency as well.
A design of MMRTG includes many impact factors. By considering maximum power
output efficiency equation alone, lower cold side temperature, higher hot side temperature
and better thermoelectric material are the only three factors that impact the efficiency
Fig.7. Output power of Curiosity’s MMRTG during flight and on the surface of Mars
during its first Martian year. [5]
14
directly. However, due to the limitation of material property, a too high temperature will
bring a negative drop for certain material as well as increase the risk of overheat damage
of the load. Considering the space application is the most widely usage of MMRTG where
the equipment on it always needs good protection and has huge difficulty or even hardly to
repair, a simple high hot-side temperature or low cold-side temperature is only a better
design for the generator but not the entire system. The better way to increase the efficiency
of RTG system is using better and more reliable thermoelectric material.
15
Referance
[1] H. Lee, "Thermoelectric Generator," in Thermal Design: Heat Sinks,
Thermoelectrics, Heat Pipes, Compact Heat Exchangers, and Solar Cells, New
Jersey, John Wiley & Sons, Inc. .
[2] R. B. T. H. TIM C. HOLGATE, "Increasing the Efficiency of the Multi-
mission Radioisotope," The Minerals, Metals & Materials Society, vol. 44, pp.
1814-1821, 2015.
[3] X. W. Wang, H. Lee, Y. C. Lan, G. H. Zhu, G. Joshi, D. Z. Wang, J. Yang,
A. J. Muto, M. Y. Tang and J. Klatsky, "Enhanced thermoelectric figure of merit
in nanostructured n-type silicon germanium," Applied Physics Letters, 2008.
[4] B. Poudel, H. Qing, M. Yi, L. Yucheng, M. Austin , Y. Bo, Y. Xiao, W. Dezhi,
M. Andrew, V. Daryoosh, C. Xiaoyuan, L. Junming, D. S. Mildred, C. Gang and
R. Zhifeng, "High-Thermoelectric Performance of Nanostructured Bismuth
Antimony Tellurid Bulk Alloys," Science, pp. 634-638, May 2008.
[5] NASA, Curiosity, 2011-2015.