A revolution is in the making. We know now that we can force the
heat-and-electricity-carrying electrons residing in matter to do our
bidding without the use of shafts and pistons. This is a leading
accomplishment of modern technology: energy transformation without
moving parts. It is called _direct_ conversion.
The thermoelements shown above the turbogenerator in Figure 1 illustrate
the contrast between direct and dynamic conversion. The thermoelements
convert heat into electricity directly, without any of the intervening
machinery seen in the turbogenerator.
[Illustration: 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)
m = mass of the moving object (in kilograms)
v = velocity (in meters per second)
Energy can also be stored in chemical and nuclear substances or in the
water behind a dam. In these quiescent states it is called _potential_
energy. If the potential energy in a substance is abundant and easily
released, the energy-rich substance is called a _fuel_.