Search Results (4)

In this paper, the influence of thermionic emission on He ionization and plasma enhancement in thermionic energy conversion (TEC) are studied by experiment and numerical simulation. A 1D unsteady plasma TEC model, which includes a He ionization model, plasma conservation equations, and a thermionic emission formula for the wall, is developed. A He plasma thermionic energy conversion device composed of a barium–tungsten cathode and a tungsten anode is established. The volt–ampere curves of the He plasma TEC device are measured at 1050 K, 1150 K, 1250 K, 1300 K, and 1350 K temperatures. Both important cathode parameters, work function and emission area, are estimated. Based on the modelling simulation and the experiment, the He ionization mechanism in plasma TEC is discovered. The effects of cathode temperature on several distributions of plasma reaction rates, particle number density, and potential in He plasma TEC are described. Some important parameters, including electron mobility, resistivity, and plasma equilibrium are analyzed. The relationship of thermionic emission on plasma enhancement to the output power of plasma TEC is presented. The output powers of plasma TEC and vacuum TEC are compared at various cathode temperatures. A dimensionless analyzing method concerning thermionic emission intensity and plasma enhancement power is proposed. A brief dimensionless relationship is deduced regarding thermionic emission intensity and the plasma enhancement contribution of TEC. The principles and methods for quantitative calculations concerning the output power of plasma TEC under the action of thermionic emission are established. It is possible to do quantitative research on the effects of thermionic emission on plasma-enhanced TEC. Full article

Photon-enhanced thermionic emission (PETE) is an efficient solar energy conversion mechanism that combines photovoltaic effects and thermionic emissions. In this study, a diffusion–emission model of electrons for the InGaN cathode was deduced based on one-dimensional continuity equations. The temperature dependence of the excess electron concentration, current density, and conversion efficiency at different cathode electron affinities was simulated, and the performance of the PETE converter under isothermal and nonisothermal state was compared. The results show that the improvement in conversion efficiency under isothermal condition was limited by the increase in anode temperature and reached the maximum of ~22% at an electron affinity of 0.56–0.59 eV and the operating temperature of 710–740 K. When the anode temperature was 500 K, the conversion efficiency increased with the increase in the electron affinity and exceeded the maximum value of the isothermal state at 0.6 eV. We explored the behavior of the converter at bias voltages as well as the determination of the maximum conversion efficiency point. The open-circuit voltage in the isothermal state was lower than that in the nonisothermal state, and the output voltage at the maximum conversion efficiency was eventually greater than the flat-band voltage. Full article

Small-size concentrated solar power (CSP) plants are presently not diffused due to a too-high levelized cost of electricity (LCoE), contrarily to CSP plants with capacity >100 MW, which provide LCoE < 20 cEUR/kWh. The integration of solid-state converters within CSP plants can enhance the scalability and economic competitiveness of the whole technology, especially at smaller scales, since the conversion efficiency of solid-state converters weakly depends on the size. Here a system with a high-temperature thermionic energy converter (TEC), together with an optical concentrator designed to be cheap even providing high concentration ratios, is proposed to improve the cost-effectiveness of CSP plants, thus achieving conditions for economic sustainability and market competitiveness. This is possible since TEC can act as a conversion topping cycle, directly producing electricity with a possible conversion efficiency of 24.8% estimated by applying realistic conditions and providing useful thermal flows to a secondary thermal stage. Under established technical specifications for the development of optical concentrator and TEC and according to reasonable economic assumptions, the overall plant conversion efficiency is estimated to be 35.5%, with LCoE of 6.9 cEUR/kW and considering the possibility of an 8 h storage tank for a 1 MW input solar energy system. The calculated projected value is an extremely competitive value compared with other available renewable energy technologies at small capacity scales and opens the path for accelerating the deployment of technological efforts to demonstrate the proposed solution. Full article

The stability of the electron thermionic emission current is one of the most important requirements for electron sources used, inter alia, in evaporators, production of rare gas excimers, and electron beam objects for high energy physics. In emission current control systems, a negative feedback signal, directly proportional to the emission current is transferred from the high-voltage anode circuit to the low-voltage cathode circuit. This technique, especially for high-voltage sources of electrons, requires the use of galvanic isolation. Alternatively, a method of converting the emission current to voltage in the cathode power supply circuit was proposed. It uses a linear cathode current intensity distribution and multiplicative-additive processing of two voltage signals, directly proportional to the values of cathode current intensity. The simulation results show that a relatively high conversion accuracy can be obtained for low values of the electron work function of the cathode material. The results of experimental tests of the dynamic parameters of the electron source and the steady-state I_e-V characteristic of the converter are presented. The implementation of the proposed I_e-V conversion method facilitates the design of the emission current controller, especially for high-voltage sources of electrons, because a negative feedback loop between the anode and cathode circuits is not required, all controller sub-components are at a common electrostatic potential. Full article