A unit thermionic power cell (TPC) concept has been developed that converts natural heat found in high-temperature environments (460 to 700 ºC) into electrical power for in situ instruments and electronics. Thermionic emission of electrons occurs when an emitter filament is heated to “white hot” temperatures (>1,000 ºC) allowing electrons to overcome the potential barrier and emit into the vacuum. These electrons are then collected by an anode, and transported to the external circuit for energy storage.

Schematic representation of the Thermionic Power Cell (TPC) with alkali-metal coated CNT bundles to enhance thermionic emission.
The thermionic emission current density (A/m2) = AT 2e(–φ/kT); where A= a constant, T = temperature (K), φ = work function (eV), and k = Boltzmann constant. The efficiency of emission increases with decreasing work function of the emitter material and increasing temperature. For example, the emission efficiency is much higher for cesium (φ = 2.4 eV) compared to pure carbon nanotube (CNT) (4.9 eV) and tungsten (4.5 eV). Additionally, the total current produced can be increased by enhancing the emitter surface area.

In this proposed approach, the higher emission efficiency of low-work function metal is combined with the enormous surface area achievable using CNT bundles to produce mA to A range current at lower temperatures of 460 to 700 ºC range. This is achievable by conformally coating CNT (see figure)bundle arrays (or simply arrays of CNTs) with alkali metals such as potassium (φ = 2.3 eV) or cesium using an atomic layer deposition process. Projected emission area of such an alkali metal-coated CNT bundle array (2-μm diameter, spaced 2 μm apart) over a 4-in. (≈10-cm) diameter wafer is ≈3.0 × 104 cm2. This leads to an estimated current production of ≈500 μA (> 200 Wh/kg) at 460 ºC to ≈1.3 A at 700 ºC, which is comparable to standard hightemperature batteries (for example, for Na-NiCl2, high-temperature batteries produce ≈90–130 Wh/kg), and sufficient to power communication, computational and control electronics, as well as sensors and miniature motors. Large areas of TPC or multiple TPC plates can be employed to produce much higher electrical energy to power heavier systems.

This highly miniaturized, high-temperature, long-life power source can be supplementary to primary high-temperature battery. The concept applies familiar thermionic emission principle for power generation by harnessing the local heat in the application environments. The approach of power production and design flexibility naturally provides an attractive option to harness in situ heat to produce power enough to operate electronics and miniature instrumentation. TPC can be designed to support geo thermal explorations by harnessing heat energy of the local environment.

This work was done by Harish Manohara, Mohammad Mojarradi, and Harold F. Greer of Caltech for NASA’s Jet Propulsion Laboratory. NPO-46967

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Thermionic Power Cell to Harness Heat Energies for Geothermal Applications

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This article first appeared in the December, 2011 issue of NASA Tech Briefs Magazine.

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