engineering question #1920



Gord Vance, a 34 year old male from Ottawa asks on February 19, 2004,

Q:

Could Uranium 238 be used in conjunction with photovoltaic cells in order to create a viable power source?

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the answer

Jeremy Whitlock answered on February 20, 2004, A:

A photovoltaic cell uses photons of light to generate electrical energy, and in this sense there is nothing that a radioactive source like U-238 could do for it since the energy of nuclear radiation is too high compared to the electron excitation energies required by a photovolatic cell.

However, if you're wondering if we can generate electricity with some kind of solid-state device using a radioactive source (i.e. other than through nuclear fission), then the answer is yes -- through something called a Radioisotope Thermoelectric Generator (RTG).

RTGs use the heat emitted from a radioisotope (usually plutonium 238, which is perhaps the isotope you were thinking of) to heat what is essentially a blanket of thermocouples -- devices that convert heat to electricity. Thermocouples are junctions of two dissimilar metals which raise a voltage dependent upon their temperature -- they're used quite commonly in electronic thermometers.

RTGs have the advantage of being independent of sunlight, long-lasting, and relatively lightweight. These attributes make them perfect for powering deep-space probes that travel too far from the Sun for photovoltaic solar panels to be of use, such as the Cassini probe that is scheduled to rendezvous with Saturn later in 2004, after a seven-year flight from Earth.

RTGs also have also been used here at home on Earth in applications requiring a long-term, continuous, reliable, low wattage power source, independent of sunlight.  Russia has used RTGs to power lighthouses and remote arctic beacons.  The most common application in North America, perhaps, was in cardio "pacemakers", although this is no longer practiced due to increasingly restrictive regulatory issues, and public concerns about safety. 

These concerns, in fact, partly explain why RTGs are not likely to significantly replace conventional electricity supplies on a planet like Earth, where sufficient alternatives exist.  RTGs are also expensive due to both the limited supply of suitable radioisotopes, and the regulatory process that surrounds the acquisition and use of these radioisotopes.  The high cost and relatively low output of RTGs are likely to limit their use to specialized, non-commercial applications.

Note that public concerns about safety are largely a perception issue, since radioisotopes like Pu-238 emit mainly alpha particles, a form of radiation that is immediately converted to heat within the radioisotope material itself (thus its usefulness in an RTG), leading to zero external exposure.

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