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]

 

Figure 21 General purpose heat source (GPHS) ^[61]

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.

Figure 22 Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) ^[61]

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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[1] (Picsacane, 2008)

[2] (Liu, Chen, & Li, 2014)

[3] (Using Micro Cooling Unit To Improve Efficiency In Thermo Electric Generator, 2014)

[4] (Skovse, 201)

[5] (Quick, 2012)

[6] (Levitan, 2014)

[7] (Lenert & Bierman, 2013)