Progress Toward High Power Output in Thermionic Energy Converters

Abstract

Thermionic energy converters are solid‐state heat engines that have the potential to produce electricity with efficiencies of over 30% and area‐specific power densities of 100 Wcm−2. Despite this prospect, no prototypes reported in the literature have achieved true efficiencies close to this target, and many of the most recent investigations report power densities on the order of mWcm−2 or less. These discrepancies stem in part from the low‐temperature (<1300 K) test conditions used to evaluate these devices, the large vacuum gap distances (25–100 µm) employed by these devices, and material challenges related to these devices' electrodes. This review will argue that, for feasible electrode work functions available today, efficient performance requires generating output power densities of >1 Wcm−2 and employing emitter temperatures of 1300 K or higher. With this result in mind, this review provides an overview of historical and current design architectures and comments on their capacity to realize the efficiency and power potential of thermionic energy converters. Also emphasized is the importance of using standardized efficiency metrics to report thermionic energy converter performance data.

A Three‐Terminal Hybrid Thermionic‐Photovoltaic Energy Converter

“…However, power densities around 1 W cm −2 have been obtained by TECs with interelectrode gaps ≤ 10 µm. [3,[22][23][24] In this context, these experimental results indicate that the power generated by the PV sub-device is significant and will therefore contribute to increase output power and conversion efficiency of optimized future TECs. Besides, the PV contribution is expected to rise when the interelectrode distances are reduced to the micro/nanoscale, which is necessary to address the space-charge issues of thermionic energy conversion, and the near-field thermal radiation becomes significant.…”

“…Thermophotovoltaics (TPV) [1,2] and thermionic energy converters (TECs) [3] are highly efficient and high temperature effect and finally extracted from the conduction band to produce useful energy before they are reinjected in the cathode. The proof of concept experiment of a two terminal TIPV device has been recently reported using a high-temperature polycrystalline tungsten cathode and both GaAs [12] and InGaAs PV anodes, [13] demonstrating a higher output voltage and power generation capacity than a baseline thermionic converter that uses the same materials.…”

“…As a matter of fact, TPV cells made of low bandgap energy semiconductors like In 0.53 Ga 0.47 As ( E g ≈ 0.75 eV) have recently demonstrated conversion efficiencies around 30% at hot‐side temperatures of ≈1200 °C. [ 5,6 ] On the other hand, current TECs have reported efficiencies only slightly above 10% at hot‐side temperatures above 1000 °C, [ 3 ] which is still in the range of thermoelectric generators. [ 7 ] Possible solutions to increase the conversion efficiency of TEC include i) the development of ultra‐low work function coatings for the cathode and the anode and ii) the establishment of micro‐scale vacuum gaps that prevent the accumulation of electrons within the gap and thus, avoid the formation of an electric field that opposes the flow of electrons (the so‐called “space charge effect”).…”

“…Thermophotovoltaics (TPV) [ 1,2 ] and thermionic energy converters (TECs) [ 3 ] are highly efficient and high temperature alternatives to thermoelectrics [ 4 ] that rely on the non‐isothermal transport of fundamental particles (photons for TPV and electrons for TEC) through space. Unlike thermoelectrics, the lack of solid continuity between the hot and cold side of the device confers both TPV and TEC the ability to operate at higher temperature gradients with minimal parasitic internal heat loss, theoretically enabling higher conversion efficiencies.…”

GaAs-Based Near-Field Thermophotonic Devices: Approaching The Idealized Case With One-Dimensional PN Junctions

“…Thanks to these benefits, solid-state heat engines can be interesting candidates for both terrestrial and spatial applications. Several of these engines have seen good recent development in the last 20 years thanks to dynamic research [1,2]: we can cite thermoelectric devices [3,4], thermophotovoltaic (TPV) devices [5], thermionic devices [6,7] or hot-carrier cells [8,9]. In the TPV field, if a large fraction of the research focuses on the development of improved emitters [10] or rear-face reflectors [11], advanced concepts are also under development [12], including hybridization with thermionic devices [13,14], near-field (NF) enhancement [15,16,17] and thermophotonics (TPX) [18,19].…”