Thermoelectric Generation
1.1 Thermoelectric
1.1.1 Nuclear Thermoelectric
Nuclear devices such as thermoelectric generators have been
employed by the Soviets for decades powering many of their satellite systems.
Russia launched thirty-one low-powered fission reactors between 1967 and 1988.
The first thermoelectric generator
(RTG) was used in a space mission that was launched from a U.S. Navy Transit
navigation satellite in 1961. The RTG generates it electric power from the heat
of radioactive decay of an isotope. The plutonium isotope has a calculated 7%
efficiency for thermoelectric applications, and 12% for thermionic processing.
This system used plutonium-238 fuel and weigh 56 kg. There is 10.6 kg of
plutonium dioxide fuel is stored in individual modular units to minimize the
risk of radioactive release. There is a layer of iridium metal which is encased
in a high-strength graphite blocks. The efficiency of such systems has improved
greatly over the decades with the development of more efficient materials to
lower the weight requirements of such as unit. The figure below illustrates the
earlier thermoelectric power generators employed.[1]
Modern thermoelectric generators employ a different configuration. All
thermoelectric generators convert heat to electricity using the phenomenon
called the “Seebeck effect.”
The MMRTG generator is about 64 cm
(25 in) in diameter (fin tip to fin tip) by 66 cm (26 in) long and weighs about
45 kg (94 lb.). This power system has been developed for the Mars Science
Laboratory and would be ideal for use in a spacecraft as the power output is
approximately 2,000 watts of thermal power and 120 watts of electrical power
generation. The evolution of thermo-electric generators (TEG) has allowed
commercial applications of the TEG in every sector. Hybrid cars, solar powered
fuel cells, computer CPU cooling, home heating, and other applications have
applied the principles of thermo-electric power generations.
The MMRTG has led to the development of high efficiency,
Compact Thermoelectric Generator (TEG) that is more efficient than current
thermal power-generation technologies due to the fact that they must first
convert thermal energy to mechanical work before producing electricity.
Solid-state energy conversion method directly transforms thermal
energy into electricity by the application of using thermoelectric
transformation materials.[2]
The incorporation of micro cooling
units will improve the efficiency of thermo electric generators, by the
incorporation of carbon nanotube to improve heat dissipation and therefore
increase its overall efficiency. [3] The use of NanoCaTE will increase the
temperature range of up to 100° C to produce more efficient TEG-modules. The
increased efficiency will allow the production of heat from the waste heat
generated by the electronic equipment in the spacecraft.[4]
The advent of new material
semiconductor telluride has environmentally stable properties and can convert
15 to 20 percent of waste heat to electricity. A research team from
Northwestern University and Michigan State University have found that the waste
heat temperature range from 400° to 600° C. The incorporation of this material
into space born TMG will greatly improve the performance in space where this
type of temperature extremes is common.[5]
1.1.2 Thermo-Photovoltaic
The production of thermo-photovoltaic
is a conversion due to heat differentials to electricity by the use of thermal
emitters and a photovoltaic diode cell. Typical photovoltaic solar cells have
limited efficiency to convert sunlight into energy, especially in a space
environment. This is based on bandgap of the material that is used, known as
the Shockley-Queisser limit of 33.7 percent for standard solar cells. When
sunlight is absorbed by the absorber/emitter device heat is generated and emits
light tuned directly to the bandgap of the PV cell underneath. The efficiency
of heat to energy translation is elevated to over eighty percent.[6]
The high efficiency of the absorber
and emitter is achieved by the utilization of the entire solar spectrum. This
application of nanophotonic properties of the absorber, and the emitter surface
incorporates nanotube absorber, and a one-dimensional Si/SiO2
photonic-crystal emitter on the same substrate.[7]
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