THE amphibious DUKW of World War II has come back as a civilian. A West German manufacturer has crossbred an automobile with a motorboat and come up with a pleasure vehicle called Amphicar that performs equally well on land or on the water. The car has conventional fins, tires, and hubcaps, but it also has two screw propellers and a bumper strip that goes around the entire car at the water line. From the back or profile view, the car looks like an ordinary convertible, unless you happen to notice the two large propellers mounted under the rear end.
With a four-cylinder 45-horsepower Austin engine and standard gear shift with four forward speeds and reverse, Amphicar can travel 75 miles an hour on land. Gas consumption is said to be about 32 miles to the gallon. When the car hits the water, it goes into a special gear, with forward and reverse speed, that drives it at speeds up to 12 miles an hour. Landing on the beach is accomplished by using the pushing force of both screws, or the driving force of both rear wheels, or both forces at once.
Amphicar Corporation of New York, importers of the car, assert that its amphibious nature does not lessen its desirability as an economical, comfortable highway vehicle. And they say it is equally desirable as a boat. Introduced to American foreign-car enthusiasts late in April at the International Auto Show in New York, the Amphicar is expected to be available for purchase in June at a price under $3000.
New ways to make electricity
As our energy-hungry world each year demands more and more power, the electric generator has become the most important of man’s machines. Yet despite its vital part in our economy, the conventional process of generating electricity remains inherently wasteful. The heat energy of burning fuel must be first transformed into mechanical energy in a steam turbine or diesel engine, then transformed again to electrical energy in the generator. In each of these processes, energy is lost. What is worse, the unyielding laws of thermodynamics put a low ceiling on the efficiency of heat engines: the best steam-generating system presently conceived must lose 60 per cent of the heat energy fed into it-tons of coal or barrels of oil from which we receive no return.
In recent years, research groups here and abroad have been exploring methods of generating electricity that could end our dependence on the steam-driven generator. If they succeed, fuel consumption could be cut by a third. Investment in plant equipment could be reduced, since no boilers or turbines would be needed. And these compact, silent generators without moving parts could lead to new applications for electrical energy.
Three approaches are under intensive study, all based on principles long known to science. The thermoelectric generator is based on the device, known since 1821, that controls the temperature in an automatic hot-water heater. The thermionic converter is much like a simple radio tube, using an effect first observed by Edison. Magnetohydrodynamics utilizes the same principle as the conventional generator, discovered by Faraday in 1831, but applies it in a new way.
The thermoelectric generator
The thermoelectric generator is based on the thermocouple, a simple device in which two strips of dissimilar metals are joined at both ends to form a ring. When one junction is maintained at a higher temperature than the other, a current flows in the circuit. This capacity of the metals is called their thermoelectric power. If the cold junction is opened and the ends are connected to a load, the current can be used to perform work. High electrical conductivity in the two materials is necessary. Low heat conductivity is equally important, since heat flow from the hot to the cold junction would destroy the temperature difference.
Metals are good electrical conductors but too efficient as heat conductors and low in thermoelectric power. Metal-based thermocouples produce such tiny currents that they have been used only as temperature-measuring devices. The discovery in recent years of a whole new class of materials, the semiconductors (such as silicon and germanium, used in transistors), has changed the outlook for the thermocouple. Semiconductors have superior thermoelectric power and they do not readily conduct heat. They can be made even more useful by the addition of selected impurities that improve their electrical conductivity.
The recent development of mixedvalence compounds promises even greater efficiency by making it possible to operate at higher temperatures than are possible with semiconductors.
Westinghouse, a leader in research into thermoelectric generation, has contracted to build for the Navy a 5000-watt generator using a mixedvalence compound that operates in a range of 850° to 1500° F. Westinghouse has already developed a 100watt generator for the Air Force known as TAP-100 (TAP stands for Terrestrial Auxiliary Power), which weighs forty pounds and is heated by propane gas. When TAP-100 was announced last June, the Air Force said it was the most powerful thermoelectric generator yet built, delivering three times as much power per pound of weight as any other generator of its type.
Westinghouse’s present devices turn 10 to 15 per cent of the available heat energy into electric power the efficiency of a small steamgeneration plant. Westinghouse sees many military applications for thermoelectric generators because of their simplicity, ruggedness, and light weight. The company does not expect the device to be used in largescale power generation, but points out that thermoelectric devices can be combined with conventional power plants to make use of heat now being wasted.
Westinghouse is also utilizing the fact that thermoelectric generation is a reversible process: instead of using a difference in temperature in thermoelcctrical materials to maintain a flow of electricity, a flow of electricity can be used to maintain a difference in temperature. Westinghouse has built several prototype devices, including a mobile serving cart that provides both refrigeration and oven compartments, and a home refrigerator, based on thermoelectric cooling.
Thermionic converters offer higher efficiency than thermocouples, but they are more complicated and require a higher temperature heat source. They are based on the fact that if you heat a metal plate, electrons on its surface boil off. If a relatively cooler plate is nearby, the electrons flow to it. The moving electrons repel each other, setting up a space charge in the gap between the plates which eventually blocks the flow of current.
One way of meeting this problem is to add cesium vapor to the cell. The cesium vapor becomes ionized on contact with the hot cathode and facilitates the flow of electrons to the second electrode.
RCA is working on thermionic converters designed to draw heat either from the sun or from the waste heat of a rocket exhaust. One 270watt converter, produced by RCA this past year in cooperation with Thiokol, is a hollow, double-walled cylinder that slips over the exhaust tube of a solid-fuel rocket motor. The exhaust heats the inner wall, forming a hot electrode, and the electrons flow to the cooler outer wall. Cesium vapor fills the space between the two walls to control the space charge.
The electrical power that is generated by this process is fed to the rocket steering control mechanism or to electronic apparatus by means of cables attached to the cathode and the second electrode. RCA suggests teaming up a thermionic converter with a steam turbine generator. The heat removed from the converter by the collector coolant would be used to generate steam to drive a conventional turbogenerator. Such a combination might have an overall efficiency of 60 per cent.
General Electric has also developed a thermionic converter using cesium vapor, as well as a very simple, compact ceramic converter — it looks rather like a sandwich made of silver quarters — in which the space charge is overcome by putting the two plates extremely close together in a vacuum.
A similar approach is taken by two M.I.T. scientists, Joseph Kaye and George N. Hatsopolous, who have set up a special company, Thermo Electron Engineering Corporation, to develop commercially their tube, which generates electricity with a thermal efficiency of 13 per cent. Here the space charge is handled by machining the electrodes, made of a special material developed in the Netherlands, to very close tolerances. The emitter and collector plates are set only .001 inch apart. The two scientists hope to raise the efficiency of their converter to 30 per cent.
A cesium-vapor converter under development by General Atomic Division of General Dynamics produces both alternating current and direct current at different ranges of operation. The alternating current is in the high range of 100 kilocycles, the frequency used for radio transmission, rather than the frequency of household current, 60 cycles. General Atomic, whose program is supported by nine Western utility companies, asserts that this is the first successful conversion of heat directly to alternating current in significant amounts.
The compactness of the cesium cell and its efficiency at high temperatures suggest its use for conversion of heat produced in nuclear reactors. Such a device has been achieved experimentally in the Los Alamos Scientific Laboratory. Described by its developers as a plasma thermocouple, it uses ionized cesium gas (plasma) in place of one metallic clement and produces direct current at several hundred times the power of conventional thermocouples. The plasma thermocouple is a container about the size and shape of a frozenfruit-juice can, in the center of which a rod of enriched uranium is suspended. When the container is lowered into the reactor core, the neutron flux activates uranium fission, which heats the center of the can, while the flow of reactor coolant around the outside of the can lowers the temperature of the cesium plasma. The essential requirements of a thermocouple are thus met, and electricity is produced.
Gas as a conductor
A third method of producing electricity directly from heat is magnetohydrodynamics, usually referred to as MHD. In principle, MHD is based on Faraday’s discovery that a current is generated within a conductor moved through a magnetic field. In a conventional generator, the moving conductor is a rotating coil of wire. The conductor can be a fluid or a gas. In MHD the conductor is a hot gas.
General Electric has reported that last September its scientists were able to obtain one kilowatt for five seconds in a laboratory MHD device, a white quartz chamber mounted within a three-inch graphite cylinder. The moving gas was an air plasma. Looking to the future, GE proposes two types of MHD converters, both designed for space vehicles. A closed-cycle model would use solar energy for a nuclear reactor as the heat source, and the conductor gas would be cooled and recirculated. Such a converter would be designed to power radio, light, heat, air conditioning, even electrical propulsion during long space flights. A second, open-cycle GE model designed for one-shot tasks in space, such as TV transmission, would operate for only one to three seconds, drawing its heat from a small solid-fuel rocket motor.
In contrast, an MHD system proposed last November by AvcoEverett is designed expressly for a central power station. Avco has already produced an experimental device, using argon gas heated to several thousand degrees, that has generated about 11 kilowatts for five seconds. Avco scientists hope to increase the output to about 100 kilowatts in a new series of experiments through the development of materials able to stand 3000° to 5000° F. for continuous operation.
In the meantime, Avco and the American Electric Power Service Corporation, representing ten U.S. utilities, have revealed a plan for a huge 450,000-kilowatt, coal-fired, open-cycle power plant, in which the MHD generator replaces the gas turbine. In this design, atmospheric air is compressed to 140 pounds per square inch and the temperature is raised to 5300° F. by compression and heating. Seeded with a vaporized metal such as potassium to increase its conductivity, the gas develops about 360,000 kilowatts of direct current when run through a magnetic field. The heat of the exhaust gas is used to generate steam, which both powers the compressor and generates an additional 107,000 kilowatts of alternate current electricity. The predicted capital cost is forty dollars per kilowatt.
Many other organizations are studying direct generation of electricity, either from heat or from chemical energy, as in the fuel cell. They include Pratt & Whitney, International Telephone and Telegraph, Allis Chalmers, Republic Aviation, National Carbon, Lockheed, and others, as well as universities and government departments. It is too early to predict which generators hold commercial promise.
General Electric believes that its open-cycle MHD generator can be developed for practical use in satellites by the latter part of 1961, its closed-cycle continuously operating model in “several years,” and that it will be at least two decades before MHD could produce electricity for general use. General Atomic scientists believe that the cesium converter is the most promising converter under investigation today, but caution that “a great deal of research and development lies ahead before we can begin to speak of large-scale commercial applications.”