1. Field of the Invention
The present invention relates to the conversion of thermal energy to electrical energy, and electrical energy to refrigeration, and more particularly to a solid state thermionic converter using semiconductor diode implementation.
2. Relevant Technology
Thermionic energy conversion is a method of converting heat energy directly into electrical energy by thermionic emission. In this process, electrons are thermionically emitted from the surface of a metal by heating the metal and imparting sufficient energy to a portion of the electrons to overcome retarding forces at the surface of the metal in order to escape. Unlike most other conventional methods of generating electrical energy, thermionic conversion does not require either an intermediate form of energy or a working fluid, other than electrical charges, in order to change heat into electricity.
In its most elementary form, a conventional thermionic energy converter includes one electrode connected to a heat source, a second electrode connected to a heat sink and separated from the first electrode by an intervening space, leads connecting the electrodes to an electrical load, and an enclosure. The space in the enclosure is either highly evacuated or filled with a suitable rarefied vapor, such as cesium.
The essential process in a conventional thermionic converter is as follows. The heat source supplies heat at a sufficiently high temperature to one electrode, the emitter, from which electrons are thermionically evaporated into the evacuated or rarefied vapor-filled interelectrode space. The electrons move through this space toward the other electrode, the collector, which is kept at a low temperature near that of the heat sink. There the electrons condense and return to the hot electrode via external electrical leads and an electrical load connected between the emitter and the collector. The flow of electrons through the electrical load is sustained by the temperature difference between the electrodes. Thus, electrical work is delivered to the load.
Thermionic energy conversion is based on the concept that a low electron work function cathode in contact with a heat source will emit electrons. These electrons are absorbed by a cold, high work function anode, and they can flow back to the cathode through an external load where they perform useful work. Practical thermionic generators are limited by the work function of available metals or other materials that are used for the cathodes. Another important limitation is the space charge effect. The presence of charged electrons in the space between the cathode and anode will create an extra potential barrier which reduces the thermionic current. These limitations detrimentally affect the maximum current density, and thus present a major problem in developing large-scale thermionic converters.
Conventional thermionic converters are typically classified as vacuum converters or gas-filled converters. Vacuum converters have an evacuated medium between the electrodes. These converters have limited practical applications.
Embodiments in a first class of gas-filled converters are provided with a vaporized substance in the interelectrode space that generates positive ions. This vaporized substance is commonly a vaporized alkali metal such as cesium, potassium and rubidium. Because of the presence of these positive ions, liberated electrons can more easily travel from the emitter to the collector. The emitter temperature in this type of conventional device is in part determined by the vaporization temperature of the substance that generates the positive ions. Generally, the emitter temperature should be at least 3.5 times the temperature of the reservoir of the positive ion generating substance if efficient production of ions is to be achieved in these conventional devices.
Embodiments in a second class of gas-filled converters are provided with a third electrode to generate ions, and the gas in the interelectrode space in these conventional devices is an inert gas such as neon, argon, or xenon. Although these converters can operate at lower temperatures, such as about 1500 K, they are more complex.
Typical conventional thermionic emitters are operated at temperatures ranging from about 1400 K to about 2200 K, and thermionic collectors are operated at temperatures ranging from about 500 K to about 1200 K. Under optimum conditions of operation, overall efficiencies of energy conversion range from 5% to 40%, electrical power densities are of the order of 1 to 100 watts/cm2, and current densities are of the order of 5 to 100 A/cm2. In general, the higher the emitter temperature, the higher the efficiency, the power and current densities with designs accounting for radiation losses. The voltage at which the power is delivered from one unit of a typical converter is 0.3 to 1.2 volts, i.e., about the same as that of an ordinary electrolytic cell. Thermionic systems with a high power rating frequently include many thermionic converter units connected electrically in series. Each thermionic converter unit is typically rated at 10 to 500 watts.
The high-temperature attributes of thermionic converters are advantageous for certain applications, but they are restrictive for others because the required emitter temperatures are generally beyond the practical capability of many conventional heat sources. In contrast, typical thermoelectric converters are operable at heat source temperatures ranging from about 500 K to about 1500 K. However, even under optimum conditions, overall efficiencies of thermoelectric energy converters only range from 3% to 10%, electrical power densities are normally less than a few watts/cm2, and current densities are of the order of 1 to 100 A/cm2.
From a physics standpoint, thermoelectric devices are similar to thermionic devices. In both cases a temperature gradient is placed upon a metal or semiconductor, and both cases are based upon the concept that electron motion is electricity. However, the electron motion also carries energy. A forced current transports energy for both thermionic and thermoelectric devices. The main difference between thermoelectric and thermionic devices is in the transport mechanism; ballistic and diffusive transport for thermionics and ohmic transport for thermoelectrics. Ohmic flow is macroscopically diffusive, but not microscopically diffusive. The distinguishing feature is whether excess carriers are present. In thermoelectrics, the carriers normally present are responsible for current. In thermionics, the current is due to putting excess carriers in the gap. A thermionic device has a relatively high efficiency if the electrons ballistically go over and across the gap. For a thermionic device all of the kinetic energy is carried from one electrode to the other. The motion of electrons in a thermoelectric device is quasi-equilibrium and ohmic, and can be described in terms of a Seebeck coefficient, which is an equilibrium parameter.
In structures with narrow barriers, the electrons will not travel far enough to suffer collisions as they cross the barrier. Under these circumstances, the ballistic version of thermionic emission theory is a more accurate representation of the current transport.
Solutions to the foregoing problems have been sought according to the present state of the art by using vacuum converters or gas-filled converters. Attempts to reduce space-charge effects with vacuum converters have involved the reduction of the interelectrode separation to the order of micrometers. Attempts to reduce the same effects with gas-filled converters have led to the introduction of positive ions into the cloud of electrons in front of the emitter. Nevertheless, these conventional devices still present shortcomings such as those related to limited maximum current densities and temperature regimes.
Consequently, there remains a need to provide a more satisfactory solution to converting thermal energy to electrical energy at lower temperature regimes with high efficiencies and high power densities.