Applications of nuclear-powered thermoelectric generators in space

Abstract

The NASA spacecrafts Voyager 1 and 2 became household names overnight following the showing on television of the remarkable photographs of the rings of Saturn and more recently of Neptune and its mysterious moon, Triton. None the less remarkable is the source of electrical power which enables information to be transmitted from the craft back to Earth after a time period of more than a decade and at a distance of more than a billion miles. 1991 marks the thirty-second anniversary of the first public demonstration of this source of electrical power, in front of President Eisenhower, at the oval office of the White House. The power source known as an RTG (radioisotope thermoelectric generator) utilises the Seebeck effect in producing electricity from heat. In essence it consists of a large number of semiconductor thermocouples connected electrically in series and thermally in parallel. A temperature difference is maintained across the thermocouples by providing a heat source, which in the case of an RTG is a radioactive isotope, and the heat sink is space. The combination of an energy-conversion system, free of moving parts and a long-life, high energy-density heat source, provides a supply of electrical power typically in the range of tens to hundreds of watts and which operates reliably over extended periods of time. An electric power source, based upon thermoelectric conversion but which utilises a nuclear reactor as a heat source, has also been deployed in space and a 100-kW system is being developed to provide electrical power to a variety of commercial and military projects including SDI. Developments in thermoelectrics that have taken place in the western world during the past 30 years are primarily due to United States interest and involvement in the exploration of space. This paper reviews US applications of nuclear-powered thermoelectric generators in space.