If implemented, generators based on the thermionic process  could considerably enhance the efficiency of focused solar energy conversion or of coal-combustion power plants, yielding a corresponding reduction of CO2 emissions. In thermionic energy conversion the electrodes are separated by vacuum rather than by a solid conductors that may give rise to the thermoelectric effect. Thereby, the parasitic heat conduction from the hot to the cold electrode is radically decreased. Thermionic generators can operate with input temperatures Tin that are sufficiently high to match the temperatures at which concentrating-solar power plants or fossil-fuel power stations generate heat. In principle, electric power may therefore be generated from these energy sources with outstanding efficiency, because the maximum possible efficiency – the Carnot efficiency ηC=1–Tout/Tin, where Tout is the generator's output temperature – increases with Tin. In contrast, a significant amount of energy is wasted today in the conversion of heat to electricity. Coal, from which 40% of the world's electricity is currently generated  is burned in power stations at ≈1500°C, whereas, due to technical limitations, the steam turbines driven by this heat are operated below ≈700°C, to give just one example.
Yet, thermionic generators have never been deployed to harvest solar energy or to convert combustion heat into electricity in power stations, although the conversion process is straightforward: electrons are evaporated from a heated emitter electrode into vacuum, then the electrons drift to the surface of a cooler collector electrode, where they condense. As a result of the electron flow, the electrochemical potentials of the emitter and collector differ by a voltage Vout and an output current can be sourced through a load. Turning this elegant operation principle into commercial devices has not yet been possible, however, because space-charge clouds suppress the emission current.
Principle of Operation
We have shown that the space-charge problem can be solved (Fig. 1) . To remove the static space charges, a positively charged gate electrode (Fig. 2) is inserted into the emitter-collector space to create a potential trough. In a virtually lossless process the gate electric field accelerates the electrons from the emitter surface and decelerates them as they approach the collector. A nominally homogeneous magnetic field applied along the electron trajectories prevents loss of the electrons to a gate current by directing them through holes in the gate on helical paths circling straight axes. This "thermoelectronic" process turns the static space-charge cloud, which previously blocked the electron emission, into a useful output current. The design is analogous to that of ion thrusters used for spacecraft propulsion. Compared to thermoelectrics, the thermoelectronic process benefits from the use of vacuum to separate the emitter from the collector. While the thermal conduction between both electrodes is minimized, the electrical conductance between both electrodes is high, as the electrons travel ballistically.
Based on our experimental and theoretical work  we conclude that by using the thermoelectronic process a highly efficient conversion of heat or focused solar radiation into electric power may be possible, indeed. Optimization of the conversion efficiencies requires the development of metal or semiconductor surfaces with the desired effective work functions and electron affinities, which may also be done by nanostructuring the electrode surfaces. These surfaces need to be stable at high temperatures in vacuum. Such devices may be realized, for example, in a flip-chip arrangement of micromachined emitter and collector wafers separated by 10–100μm using a microfabricated gate with holes of comparable size, and ceramic spacers (Fig. 3). According to our calculations, such devices may produce hundreds of Watts of power from active areas of some 100 cm2. The magnetic fields, typically ≈0.2–0.5T with large tolerances in strength and spatial distribution, can be generated by permanent magnets or, for applications such as power plants, by superconducting coils .