Part 3
Historically, the sun’s energy has most often been used by concentrating it with a lens or mirror and then converting it to heat. We could do this and run a heat engine, but a more direct avenue is open.
About a decade ago it was found that the junction between _p_ and _n_ semiconductors would generate electricity if illuminated. This discovery led to the development of the _solar cell_, a thin, lopsided sandwich of silicon semiconductors. As shown in Figure 12, the top semiconductor layer exposed to the sun is extremely thin, only 2.5 microns. Solar photons can readily penetrate this layer and reach the junction separating it from the thick main body of the solar cell.
_p_ SILICON _n_ SILICON ELECTRON-MOLE PAIRS JUNCTION PHOTONS FROM SUN OR RADIOISOTOPE ELECTRONS ENERGY OUT
Whenever _p-_ and _n-_type semiconductors are sandwiched together a voltage difference is created across the junction. The separated holes and electrons in the two semiconductor regions establish this electric field across the junction. Unfortunately, there are usually no current carriers in the immediate vicinity of the junction so that no power is produced.
The absorption of solar photons in the vicinity of the junction will create current carriers, as the photons’ energy is transformed into the potential energy of the hole-electron pairs. These pairs would quickly recombine and give up their newly acquired potential energy if the electric field existing across the junction did not whisk them away to an external load.
The solar cell produces electricity when hole-electron pairs are formed. Any other phenomenon that creates such pairs will also generate electricity. The source of energy is irrelevant so long as the current carriers are formed near the junction. Thus, particles emitted by radioactive atoms can also produce electricity from solar cells, although too much bombardment by such particles can damage the cell’s atomic structure and reduce its output.
The solar cell is not a heat engine. Yet it loses enough energy so that the sun’s energy is converted at less than 15% efficiency. Losses commonly occur because of the recombination of the hole-electron pairs before they can produce current, the absorption of photons too far from the junction, and the reflection of incident photons from the top surface of the cell. Despite these losses solar cells are now the mainstay of nonpropulsive space power.
NUCLEAR BATTERIES
Energy from Nuclear Particles
As we have seen, solar cells are able to convert the kinetic energy of charged nuclear particles directly into electricity, but a simpler and more straightforward way of doing this exists. This involves direct use of the flow of charged particles as current.
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.
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.
Ferroelectric Conversion
Ferroelectric conversion makes use of the peculiar properties of _dielectric_[12] materials. Barium titanate, for example, has good dielectric properties at low temperatures, but, when its temperature is raised to more than 120°C, the properties get worse rapidly. We cannot discuss dielectric behavior thoroughly in this booklet; suffice it to say that in this process heat is absorbed in a realignment of molecules within the barium titanate latticework.
If we now place a slab of barium titanate between the two plates of an electrical condenser and charge the condenser, as shown in Figure 14, we have a unique way of converting heat into electricity directly. When the barium titanate is heated above its _Curie point_[13] of 120°C, the condenser’s capacitance is radically reduced as the dielectric constant falls. The condenser is forced to discharge and move electrons through an external circuit consisting of the load and the original source of charge. Useful electrical energy is delivered during this step. Figure 14 shows the process schematically and mathematically. When the dielectric is cooled, waste heat is given up by the barium titanate, and the cycle is complete.
(a) CIRCUIT HEAT IN BARIUM TITANATE DIELECTRIC WASTE HEAT OUT SWITCH #2 LOAD SWITCH #1 BATTERY (b) CYCLE DIAGRAM charge volts Q₂, Q₁, E₁, E₂, V₀ V₁ V₂ GENERAL INFORMATION: C₂ < C₁ V = Q/C
Thermomagnetic Conversion
The _analog_[14] of ferroelectricity is ferromagnetism. A converter employing similar principles to those in ferroelectricity can be made using an electrical _inductance_ with a ferromagnetic core. When the temperature of the ferromagnetic material is raised above its Curie point, its magnetic _permeability_ drops quickly, causing the magnetic field to collapse partially. Energy may be delivered to an external load during this change. Instead of energy being stored in an electrostatic field, it is stored in a magnetic field.
Ferroelectric and thermomagnetic conversion both represent a class of energy transformations which involve internal molecular or crystalline rearrangements of solids. There is no change of phase as in a steam engine, but the energy changes are there nevertheless. In thermodynamics such internal geometrical changes are called _second-order_ transitions, as opposed to the _first-order_ transitions observed with heat engines using two-phase working fluids like water/steam.
On the Frontier
Other potential energy conversion schemes are being investigated by scientists and engineers. Those listed in the Energy Conversion Matrix (Figure 2) only scratch the surface.
In particular, we are just learning how to manipulate photons. There are photochemical, photoelectric, and even photomechanical transformations. These have hardly been tapped.
Consider the reaction when an electron and its antimatter equivalent, the positron, meet. They mutually annihilate each other in a burst of energy! This energy will be harnessed someday.
What energy conversion device are we going to use to completely convert mass into energy? The energy requirements for interstellar exploration are so great that these voyages will be impossible unless a new device is found that can completely transform mass into energy.
Then again, we haven’t the faintest idea of how to control gravitational energy, but we may learn.
The panorama is endless.
Problem 5
A 1,000,000-kilogram spaceship takes off for Alpha Centauri, our nearest star, 4.3 light years away. If it accelerates to nine-tenths the velocity of light, what is its kinetic energy? How much fuel mass will have to be completely converted to energy to acquire this velocity?
SUGGESTED REFERENCES
Articles
Fuel Cells, Leonard G. Austin, _Scientific American_, 201: 72 (October 1959). A survey of the different types.
Nuclear Power in Outer Space, William R. Corliss, _Nucleonics_, 18: 58 (August 1960). A review of all nuclear space power plants.
Fuel Cells for Space Vehicles, M. G. Del Duca, _Astronautics_, 5: 36 (March 1960).
Fuel Cells, E. Gorin and H. L. Recht, _Chemical Engineering Progress_, 55: 51 (August 1959).
Thermionic Converters, Karl G. Hernqvist, _Nucleonics_, 17: 49 (July 1959).
The Revival of Thermoelectricity, Abram F. Joffe, _Scientific American_, 199: 31 (November 1958). Excellent historical and technical review.
The Photovoltaic Effect and Its Utilization, P. Rappaport, _RCA Review_, 20: 373 (September 1959). Recommended for advanced students.
The Prospects of MHD Power Generation, Leo Steg and George W. Sutton, _Astronautics_, 5: 22 (August 1960).
Conversion of Heat to Electricity by Thermionic Emission, Volney C. Wilson, _Journal of Applied Physics_, 30: 475 (April 1959). Recommended for advanced students.
Improved Solar Cells Planned for IMP-D, R. D. Hibben, _Aviation Week & Space Technology_, 83: 53 (July 26, 1965).
Thin-film Solar Cells Boost Output Ratio, P. J. Klass, _Aviation Week & Space Technology_, 83: 67 (November 29, 1965).
Books
_Direct Conversion of Heat to Electricity_, Joseph Kaye and John A. Welsh, John Wiley & Sons, Inc., New York 10016, 1960, 387 pp., $11.50. Recommended for advanced students.
_Selected Papers on New Techniques for Energy Conversion_, Sumner N. Levine, (Ed.), Dover Publications, Inc., New York 10014, 1961, 444 pp., $3.00. A reprinting of many classical papers on direct conversion.
_Energy Conversion for Space Power_, Nathan W. Snyder, (Ed.), Academic Press, Inc., New York 10003, 1961, 779 pp., $8.50. A collection of American Rocket Society papers.
_Man and Energy_, Alfred Rene Ubbelohde, George Braziller, New York 10016, 1955, 247 pp., $5.00 (hardback); $1.25 (paperback), from Penguin Books, Inc., Baltimore, Maryland 21211. A popular treatment of energy and power.
Motion Pictures
The following films are produced by Educational Services, Inc., and are available from Modern Learning Aids, A Division of Modern Talking Picture Service, Inc., 3 East 54th St., New York 22, New York.
_Energy and Work_, 0311, 29 minutes, $150. _Mechanical Energy and Thermal Energy_, 0312, 27 minutes, $120. _Conservation of Energy_, 0313, 27 minutes, $150. _Photo-Electric Effect_, 0417, 28 minutes, $220.
ANSWERS TO PROBLEMS
First, mechanical energy drives the car’s electric generator. Second, the electrical energy is converted into chemical energy when the battery is recharged.
* * * * * * *
From the kinetic energy equation we get
v = √(2 E/m)
Since the engine is 25% efficient, the energy available to propel the car is 48,000 × 0.25 or 12,000 joules. So
v = √(24,000/1,000) = 2√6 = 4.9 meters per second
* * * * * * *
e = (300 - 20)/300 = 14/15 = 0.93 = 93%
The crossover point, t, in hours is found by equating the nuclear power plant mass and that of the fuel cell with its associated fuel. The equation is
1000 = 50 + 25 + ½t t = 1850 hours = 77 days
* * * * * * *
E = ½ mv² = (10⁶(0.9 × 3 × 10⁸)²)/2 = 3.6 × 10²² joules
The ship will use the same amount of energy to decelerate at its destination. Note that this calculation assumes a perfect efficiency in converting the energy of matter annihilation into the kinetic energy of the space ship. The mass consumed is
m = E/c² = (3.6 × 10²²)/(9 × 10¹⁶) = 4.0 × 10⁵ kg
almost half the spaceship mass.
Footnotes
[1]Systems for Nuclear Auxiliary Power.
[2]Described in this booklet.
[3]Magnetohydrodynamics.
[4]The Kelvin temperature scale starts with zero at absolute zero instead of at the freezing point of water. Therefore, °K = °C + 273; °K = ⁵/₉ (°F + 460).
[5]Termed _valence_ or _conduction_ electrons, these are responsible for chemical properties, bonds with other atoms, and the conduction of electricity.
[6]See the companion Understanding the Atom booklet, _Power from Radioisotopes_.
[7]Discovered by Thomas Edison in 1883.
[8]An electron volt is equal to the kinetic energy acquired by an electron accelerated through a potential difference of 1 volt. It is equal to 1.6 × 10⁻¹⁹ joule.
[9]In outer space, waste heat must be radiated away. The rate at which heat is radiated is proportional to the fourth power of T_c (Stefan-Boltzmann law).
[10]The newton and the weber are mks (meter-kilogram-second) units.
[11]An angstrom unit (A) is a unit of distance measurement equal to 10⁻¹⁰ meter.
[12]Dielectric materials are nonconductors such as are those used between the plates of a condenser to increase its electrical capacity.
[13]The Curie point is the temperature at which a material’s crystalline structure radically changes and becomes less orderly.
[14]Ferroelectricity and ferromagnetism are very similar. The equations describing these phenomena are almost identical except that capacitance is replaced by its magnetic analog, inductance, and so on.
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Transcriber’s Notes
—Silently corrected a few typos.
—Modified some image references to reflect the pageless flowable eBook format.
—Retained publication information from the printed edition: this eBook is public-domain in the country of publication.
—In the text versions only, text in italics is delimited by _underscores_.