thermoelectric power generator

Introduction

      any of a class of solid-state devices (solid-state device) that either convert heat directly into electricity or transform electrical energy into thermal power for heating or cooling. Such devices are based on thermoelectric effects involving interactions between the flow of heat and of electricity through solid bodies.

 All thermoelectric power generators have the same basic configuration, as shown in the figure—>. A heat source provides the high temperature, and the heat flows through a thermoelectric converter to a heat sink, which is maintained at a temperature below that of the source. The temperature differential across the converter produces direct current (DC) to a load (R_L) having a terminal voltage (V) and a terminal current (I). There is no intermediate energy conversion process. For this reason, thermoelectric power generation is classified as direct power conversion. The amount of electrical power generated is given by I²R_L, or VI.

      A unique aspect of thermoelectric energy conversion is that the direction of energy flow is reversible. So, for instance, if the load resistor is removed and a DC power supply is substituted, the thermoelectric device can be used to draw heat from the “heat source” element and lower its temperature. In this configuration, the reversed energy-conversion process of thermoelectric devices is invoked, using electrical power to pump heat and produce refrigeration.

      This reversibility distinguishes thermoelectric energy converters from many other conversion systems, such as thermionic power converters (thermionic power converter). Electrical input power can be directly converted to pumped thermal power for heating or refrigerating, or thermal input power can be converted directly to electrical power for lighting, operating electrical equipment, and other work. Any thermoelectric device can be applied in either mode of operation, though the design of a particular device is usually optimized for its specific purpose.

      Systematic study began on thermoelectricity between about 1885 and 1910. By 1910 Edmund Altenkirch, a German scientist, satisfactorily calculated the potential efficiency of thermoelectric generators and delineated the parameters of the materials needed to build practical devices. Unfortunately, metallic conductors (conductor) were the only materials available at the time, rendering it unfeasible to build thermoelectric generators with an efficiency of more than about 0.5 percent. By 1940 a semiconductor-based generator with a conversion efficiency of 4 percent had been developed. After 1950, in spite of increased research and development, gains in thermoelectric power-generating efficiency were relatively small, with efficiencies of not much more than 10 percent by the late 1980s. Better thermoelectric materials will be required in order to go much beyond this performance level. Nevertheless, some low-power varieties of thermoelectric generators have proven to be of considerable practical importance. Those fueled by radioactive isotopes (radioactive isotope) are the most versatile, reliable, and generally used power source for isolated or remote sites, such as for recording and transmitting data from space.

Major types of thermoelectric generators

      Thermoelectric power generators vary in geometry, depending on the type of heat source and heat sink, the power requirement, and the intended use. During World War II, some thermoelectric generators were used to power portable communications transmitters. Substantial improvements were made in semiconductor materials and in electrical contacts between 1955 and 1965 that expanded the practical range of application. In practice, many units require a power conditioner to convert the generator output to a usable voltage.

Fossil-fuel (fossil fuel) generators

      Generators have been constructed to use natural gas, propane, butane, kerosene, jet fuels, and wood, to name but a few heat sources. Commercial units are usually in the 10- to 100-watt output power range. These are for use in remote areas in applications such as navigational aids, data collection and communications systems, and cathodic protection, which prevents electrolysis from corroding metallic pipelines and marine structures.

Solar-source (solar energy) generators

      Solar thermoelectric generators have been used with some success to power small irrigation pumps in remote areas and underdeveloped regions of the world. An experimental system has been described in which warm surface ocean water is used as the heat source and cooler deep ocean water as the heat sink. Solar thermoelectric generators have been designed to supply electric power in orbiting spacecraft, though they have not been able to compete with silicon solar cells (solar cell), which have better efficiency and lower unit weight. However, consideration has been given to systems featuring both heat pumping and power generation for thermal control of orbiting spacecraft. Utilizing solar heat from the Sun-oriented side of the spacecraft, thermoelectric devices can generate electrical power for use by other thermoelectric devices in dark areas of the spacecraft and to dissipate heat from the vehicle.

Nuclear-fueled generators

      The decay products of radioactive isotopes (radioactive isotope) can be used to provide a high-temperature heat source for thermoelectric generators. Because thermoelectric device materials are relatively immune to nuclear radiation and because the source can be made to last for a long period of time, such generators provide a useful source of power for many unattended and remote applications. For example, radioisotope thermoelectric generators provide electric power for isolated weather monitoring stations, for deep-ocean data collection, for various warning and communications systems, and for spacecraft. In addition, a low-power radioisotope thermoelectric generator was developed as early as 1970 and used to power cardiac pacemakers. The power range of radioisotope thermoelectric generators is generally between 10⁻⁶ and 100 watts.

Principles of operation

      An introduction to the phenomena of thermoelectricity is necessary to understand the operating principles of thermoelectric devices.

Seebeck effect

      In 1821 the German physicist Thomas Johann Seebeck (Seebeck, Thomas Johann) discovered that when two strips of different electrically conducting materials were separated along their length but joined together by two “legs” at their ends, a magnetic field developed around the legs, provided that a temperature difference existed between the two junctions. He published his observations the following year, and the phenomenon came to be known as the Seebeck effect. However, Seebeck did not identify the cause of the magnetic field. This magnetic field results from equal but opposite electric currents in the two metal-strip legs. These currents are caused by an electric potential difference across the junctions induced by thermal differences between the materials. If one junction is open but the temperature differential is maintained, current no longer flows in the legs but a voltage can be measured across the open circuit. This generated voltage (V) is the Seebeck voltage and is related to the difference in temperature (ΔT) between the heated junction and the open junction by a proportionality factor (α) called the Seebeck coefficient, or V = αΔT. The value for α is dependent on the types of material at the junction.

Peltier effect

      In 1834 the French physicist and watchmaker Jean-Charles-Athanase Peltier (Peltier, Jean-Charles-Athanase) observed that if a current is passed through a single junction of the type described above, the amount of measured heat generated is not consistent with what would be predicted solely from ohmic heating caused by electrical resistance. This observation is called the Peltier effect. As in Seebeck's case, Peltier failed to define the cause of the anomaly. He did not identify that heat was absorbed or evolved at the junction depending on the direction of the current. He also did not recognize the reversible nature of this thermoelectric phenomenon, nor did he associate his discovery with that of Seebeck.

Thomson effect

      It was not until 1855 that William Thomson (later Lord Kelvin (Kelvin, William Thomson, Baron)) drew the connection between the Seebeck and Peltier effects, which was the first significant contribution to the understanding of thermoelectric phenomena. He showed that the Peltier heat or power (Q_p) at a junction was proportional to the junction current (I) through the relationship Q_p = πI, where π is the Peltier coefficient. Through thermodynamic analysis, Thomson also showed the direct relation between the Seebeck and Peltier effects, namely that π = αT, where T is the temperature of the junction. Furthermore, on the basis of thermodynamic considerations, he predicted what came to be known as the Thomson effect, that heat power (Q_τ) is absorbed or evolved along the length of a material rod whose ends are at different temperatures. This heat was shown to be proportional to the flow of current and to the temperature gradient along the rod. The proportionality factor τ is known as the Thomson coefficient.

Analysis of a thermoelectric device

      Practically, the thermoelectric property of a device is adequately described using only one thermoelectric parameter, the Seebeck coefficient α. As was shown by Thomson, the Peltier coefficient at a junction is equal to the Seebeck coefficient multiplied by the operating junction temperature. The Thomson effect is comparatively small, and so it is generally neglected.

 While there is a Seebeck effect in junctions between different metals, the effect is small. A much larger Seebeck effect is achieved by use of junctions (p-n junction) between -type and -type semiconductor materials, typically silicon or germanium. The figure—> shows p-type and n-type semiconductor legs between a heat source and a heat sink with an electrical power load of resistance R_L connected across the low-temperature ends. A practical thermoelectric device can be made up of many p-type and n-type semiconductor legs connected electrically in series and thermally in parallel between a common heat source and a heat sink. Its behaviour can be discussed considering only one couple.

      An understanding of the thermal and electric power flows in a thermoelectric device involves two factors in addition to the Seebeck effect. First, there is the heat conduction in the two semiconductor legs between the source and the sink. The thermal flow down these two legs is given by

2κ(a/L)ΔT,

where κ is their average thermal conductivity in watts per metre-kelvin, a (or w²) is the area in square metres of the base of each leg, L is the length of each leg in metres, and ΔT is the temperature differential between source and sink in kelvins. The second factor is the ohmic heating that occurs in both of the legs because of electrical resistance. The heat power produced in each leg is given by

ρI²(L/a),

where ρ is the average electrical resistivity of the semiconductor materials in ohm-metres and I is the electric current in amperes. Approximately half of the resistance-produced heat in each of the two legs flows toward the source and half toward the sink.

      In a thermoelectric power generator, a temperature differential between the upper and lower surfaces of two legs of the device can result in the generation of electric power. If no electrical load is connected to the generator, the applied heat source power results in a temperature differential (ΔT) with a value dictated only by the thermal conductivity of the p- and n-type semiconductor legs and their dimensions. The same amount of heat power will be extracted at the heat sink. However, because of the Seebeck effect, a voltage V_α = αΔT will be present at the output terminals. When an electrical load is attached to these terminals, current will flow through the load. The electrical power generated in the device is equal to the product of the Seebeck coefficient α, the current I, and the temperature differential ΔT. For a given temperature differential, the flow of this current causes an increase in the thermal power into the device equal to the electric power generated. Some of the electric power generated in the device is dissipated by ohmic heating in the resistances of the two legs. The remainder is the electrical power output to the load resistance R_L.

      The leg geometry has a considerable effect on the operation. The thermal conduction power is dependent on the ratio of area to length, while ohmic heating is dependent on the inverse of that ratio. Thus, an increase in this ratio increases the thermal conduction power but reduces the power dissipated in the leg resistances. An optimum design normally results in relatively long and thin legs.

      In choosing or developing semiconductor materials suitable for thermoelectric generators, a useful figure of merit is the square of the Seebeck coefficient (α) divided by the product of the electrical resistivity (ρ) and the thermal conductivity (κ).

Joseph W. Harpster G. Ralph Strohl

Additional Reading

Reiner Decher, Direct Energy Conversion: Fundamentals of Electric Power Production (1997), provides a technical treatment. Stanley W. Angrist, Direct Energy Conversion, 4th ed. (1982), provides a historical introduction and overview.G. Ralph Strohl

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