Figure 1 Direct conversion devices, such as the spokelike lead telluride thermoelectric elements inside the SNAP 3 radioisotope generator shown above (courtesy Martin Company), convert heat into electricity without moving parts. In contrast, the SNAP 2 dynamic converter shown below SNAP 3 (courtesy Thompson Ramo Wooldridge, Inc.) includes a high-speed turbine, an electric generator, and pumps to produce electricity from heat. (NaK is a liquid mixture of sodium and potassium.)
DIRECT VERSUS DYNAMIC CONVERSION SNAP 3 LESS THAN 5 WATTS 5″ SNAP 2 3000 WATTS 24″ ALTERNATOR ROTOR ALTERNATOR STATOR TURBINE ROTORS NaK PUMP DIFFUSER NaK PUMP ROTOR MERCURY JET BOOSTER PUMP MERCURY CENTRIFUGAL PUMP MERCURY THRUST BEARING MERCURY BEARING MERCURY BEARING
Why is Direct Conversion Desirable?
There are places where energy conversion equipment must run for years without maintenance or breakdown. Also, there are situations where the ultimate in reliability is required, such as on scientific satellites and particularly on manned space flights. Direct conversion equipment seems to offer greater reliability than dynamic conversion equipment for these purposes.
We should recognize that our belief in the superiority of direct conversion is based more on intuition than proof. It is true that direct converters will never throw piston rods or run out of lubricant. Yet, some satellite power failures have been caused by the degradation of solar cells under the bombardment of solar protons. The other types of direct conversion devices described in the following pages may also break down in ways as yet unknown. Still, today’s knowledge gives us hope that direct conversion will be more reliable and trustworthy than dynamic conversion. Direct conversion equipment is beginning to be adopted for small power plants, producing less than 500 watts, designed to operate for long periods of time in outer space and under the ocean. Some day, large central-station power plants may use direct conversion to improve their efficiencies and reliabilities.
How is Energy Transformed?
What is energy and how do we change it? Energy is a fundamental concept of science involving the capacity for doing work. Kinetic or mechanical energy is the most obvious form of energy. It is defined as
E = ½ mv²
where
E = energy (expressed in joules)