How do thermionic power converters turn heat directly into electricity?

A thermionic power converter functions as a heat engine that converts thermal energy directly into electricity through thermionic emission. The process relies on two electrodes: a "hot plate" (emitter) and a cooler "collector".1

The conversion process works as follows:

• Electron Emission: The emitter is heated to a high temperature, providing electrons with enough energy to overcome the material's electronic work function—the energy barrier holding them within the metal.12
• Electron Flow: These liberated electrons travel across the gap between the electrodes toward the collector.1
• Energy Conversion: By connecting the two electrodes through an external load, the flow of electrons creates an electric current. Part of the thermal energy used to liberate the electrons is thus converted directly into electrical energy.1
• Maintaining Flow: To prevent the space-charge effect—where emitted electrons repel one another and disrupt current—the gap is often filled with a gas like cesium to create a neutral plasma.1

Some thermal energy inevitably heats the collector and must be removed to maintain the necessary temperature differential.1