The nuclear battery shown in [Figure 13] performs this trick. A central rod is coated with an electron-emitting radioisotope (a beta-emitter; say, strontium-90). The high-velocity electrons emitted by the radioisotope cross the gap between the cylinders and are collected by a simple metallic sleeve and sent to the load. Simple, but why don’t space charge effects prevent the electrons from crossing the gap as they do in the thermionic converter? The answer lies in the fact that the nuclear electrons have a million times more kinetic energy than those boiled off the thermionic converter’s emitter surface. Consequently, they are too powerful to be stopped by any space charge in the narrow gap.
Nuclear batteries are simple and rugged. They generate only microamperes of current at 10,000 to 100,000 volts.
Figure 13 A NUCLEAR BATTERY
The nuclear battery depends upon the emission of charged particles from a surface coated with a radioisotope. The particles are collected on another surface.
ENERGY OUT INSULATOR LAYER OF BETA-EMITTING RADIOISOTOPE VACUUM
Double Conversion
In the earlier description of the energy conversion matrix, we saw that we could go through the energy transformation process repeatedly until we obtained the kind of energy we wanted. This is exemplified in a type of nuclear battery which uses the so-called double conversion approach. First, the high-velocity nuclear particles are absorbed in a phosphor which emits visible light. The photons thus produced are then absorbed in a group of strategically placed solar cells, which deliver electrical power to the load. Although efficiency is lost at each energy transformation, the double conversion technique still ends up with an overall efficiency of from 1 to 5%, an acceptable value for power supplies in the watt and milliwatt ranges.
ADVANCED CONCEPTS
Ferroelectric and thermomagnetic conversion are subtle concepts which depend upon the gross alteration of a material’s physical properties by the application of heat. Devices employing such concepts are true heat engines. Instead of the gaseous and electronic working fluids used in the other direct conversion concepts, the ferroelectric and thermomagnetic concepts employ patterns of atoms and molecules that are actually rearranged periodically by heat.