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A Research of The Difference between the Design of

MMRTG and common TEG

Jue Bo

Feixiang Sun

ME 6950

Professor: Dr. HoSung Lee

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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

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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.

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Fig.1. Power output of Pu-238

PowerPerKg 540W

kg

Power mass lifetime( ) PowerPerKg mass1

2

lifetime

halflife

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Basic Model

Fig. 2. Cut-away view of the MMRTG

Fig. 3. Schematic Diagram of an RTG System

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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

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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

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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.

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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

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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]

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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.

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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.