THERMOELECTRICS AND THERMIONICS BASIC AND TUTORIALS


Thermoelectrics
Thermal energy can be directly converted to electrical energy by using the thermoelectric effects in materials. Semiconductors offer the best option as thermocouples since thermojunctions can be constructed using a p-type and an n-type material to cumulate the effects around a thermoelectric circuit.

Moreover, by using solid solutions of tellurides and selenides doped to result in a low density of charge carriers, relatively moderate thermal conductivities and reasonably good electrical conductivities can be achieved.

In a thermoelectric generator, the Seebeck voltage generated under a temperature difference drives a dc current through the load circuit. Even though there is no mechanical conversion, the process is still Carnot limited since it operates over a temperature difference.

In practice, several couples are assembled in a seriesparallel configuration to provide dc output power at the required voltage. Typical thermoelectric generators employ radioisotope or nuclear reactor or hydrocarbon burner as the heat source.

They are custom-made for space missions as exemplified by the SNAP (systems for nuclear auxiliary power) series and the RTG (radioisotope thermoelectric generator) used by the Apollo astronauts.

Maximum performance over a large temperature range is achieved by cascading stages. Each stage consists of thermocouples electrically in series and thermally in parallel. The stages themselves are thermally in series and electrically in parallel.

Tellurides and selenides are used for power generation up to 600° C. Silicon germanium alloys turn out better performance above this up to 1000° C. With the materials available at present, conversion efficiencies in the 5 to 10% range can be expected.

Whenever small amounts of silent reliable power is needed for long periods of time, thermoelectrics offer a viable option. Space, underwater, biomedical, and remote terrestrial power such as cathodic protection of pipelines fall into this category.

Thermionics
Direct conversion of thermal energy into electrical energy can be achieved by employing the Edison effect the release of electrons from a hot body, also known as thermionic emission. The thermal input imparts sufficient energy (≥ work function) to a few electrons in the emitter (cathode), which helps them escape.

If these electrons are collected using a collector (anode) and a closed path through a load is established for them to complete the circuit back to the cathode, then electrical output is obtained. Thermionic converters are heat engines with electrons as the working fluid and, as such, are subject to Carnot limitations.

Converters filled with ionizable gases such as cesium vapor in the interelectrode space yield higher power densities due to space charge neutralization. Barrier index is a parameter that signifies the closeness to ideal performance with no space charge effects. As this index is reduced, more applications become feasible.

A typical example of developments in thermionics is the TFE (thermionic fuel element) that integrates the converter and nuclear fuel for space nuclear power in the kW to MW level for very long (7 to 10 years) duration missions.

Another niche is the thermionic cogeneration burner module, a high-temperature burner equipped with thermionic converters. Electrical outputs of 50 kW/MW of thermal output have been achieved. High (600 to 650° C) heat rejection temperatures of thermionic converters are ideally suited for producing flue gas in the 500 to 550° C range for industrial processes. A long-range goal is to use thermionic converters as toppers for conventional power plants. Such concepts are not economical at present.

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