How efficient can thermionic power converters be at temperatures from 900 to 1,500 kelvins?

At temperatures ranging from 900 to 1,500 kelvins, thermionic power converters can achieve an efficiency in the range of 12 to 15 percent.1

These devices function by converting heat directly into electricity through thermionic emission, a process where electrons are emitted from a heated electrode (the "hot plate") and collected by a cooler electrode. Because they operate most effectively at high temperatures, they are particularly well-suited for specialized applications, such as providing power for spacecraft or serving as potential topping devices in fossil fuel power plants.1