Thermionic energy converter (TEC) is a heat engine that generates electricity directly using heat as its source of energy and electron as its working fluid. Despite having a huge potential as an efficient direct energy conversion device, the progress in vacuum-based thermionic energy converter development has always been hindered by the space charge problem and the unavailability of materials with low work function. It is only recently that researchers have started to look back into this technology as recent advances in manufacturing technology techniques have made it possible to solve these problems, making TECs a viable option in replacing current energy production systems. The focus of this paper is to review the challenges of producing efficient and practical TECs, along with recent findings and developments in mitigating these challenges. Furthermore, this paper looked into potential applications of TECs, based on recent works and technologies, and found that, with certain improvements, it can be applied in many sectors.

Review on Thermionic Energy Converters

Thermal energy is abundantly available, and especially low-grade heat is often wasted in industrial processes as a by-product. Tapping into this vast energy reservoir with cost-attractive technologies may become a key element for the transition to an energy-sustainable economy and society. We propose a novel heat-to-current converter which is based on the temperature dependence of the cell voltage of charged supercapacitors. Using a commercially available supercapacitor, we observed a thermal cell-voltage rise of around 0.6 mV K−1 over a temperature window of 0 °C to 65 °C. Within our theoretical model, this can be used to operate a Stirling-like charge–voltage cycle whose efficiency is competitive to the most-efficient thermoelectric (Seebeck) engines. Our proposed heat-to-current converter is built from cheap materials, contains no moving parts, and could operate with a plethora of electrolytes which can be chosen for optimal performance at specific working temperatures. Therefore, this heat-to-current converter is interesting for small-scale, domestic, and industrial applications.

/pdf/heat-to-current-conversion-of-low-grade-heat-from-a-1trs34riog.pdf

Heat-to-current conversion of low-grade heat from a thermocapacitive cycle by supercapacitors

In this paper, we propose a model to investigate the electron thermionic emission from a single-layer graphene (ignoring the effects of substrate) and to explore its application as the emitter of thermionic energy convertor (TIC). An analytical formula has been derived, which is a function of temperature, work function and Fermi energy level. The formula is significantly different from the traditional Richardson-Dushman (RD) law for which it is independent of mass to account for the supply function of the electrons in the graphene behaving like massless Fermion quasiparticles. By comparing with a recent experiment [Kaili Jiang et al., Nano Research 7, 553 (2014)] measuring electron thermionic emission from a suspended single layer graphene, our model predicts that the intrinsic work function of a single-layer graphene is about 4.514 eV with a Fermi energy level of 0.083 eV. For a given work function, a new scaling of $T^{3}$ is predicted, which is different from the traditional RD scaling of $T^2$. If the work function of the graphene is lowered to 2.5 to 3 eV, and the Fermi energy level is increased to 0.8 to 0.9 eV, it is possible to design a graphene cathode based TIC operating at around 900 K or lower, as compared with the metal-based cathode TIC (operating at about 1500 K). With a graphene based cathode (work function = 4.514 eV) at 900 K, and a metallic based anode (work function = 2.5 eV) like LaB$_6$ at 425 K, the efficiency of our proposed-TIC is about 45$\%$

Electron Thermionic Emission from Graphene and a Thermionic Energy Converter

The recent developments in nanoscale solid-state cooling are reviewed. This includes both theoretical and experimental studies of different physical concepts, as well as nanostructured material design and device configurations. We primarily focus on thermoelectric, thermionic and thermo-magnetic coolers. Particular emphasis is given to the concepts based on metal-semiconductor superlattices, graded materials, non-equilibrium thermoelectric devices, Thomson coolers, and photon assisted Peltier coolers as promising methods for efficient solid-state cooling. Thermomagnetic effects such as magneto-Peltier and Nernst-Ettingshausen cooling are briefly described and recent advances and future trends in these areas are reviewed. The ongoing progress in solid-state cooling concepts such as spin-calorimetrics, electrocalorics, non-equilibrium/nonlinear Peltier devices, superconducting junctions and two-dimensional materials are also elucidated and practical achievements are reviewed. We explain the thermoreflectance thermal imaging microscopy and the transient Harman method as two unique techniques developed for characterization of thermoelectric microrefrigerators. The future prospects for solid-state cooling are briefly summarized.

https://iopscience.iop.org/article/10.1088/0034-4885/79/9/095901/pdf

Nanoscale solid-state cooling: a review.

Thermionic energy conversion for concentrating solar power

Electric power may, in principle, be generated in a highly efficient manner from heat created by focused solar irradiation, chemical combustion, or nuclear decay by means of thermionic energy conversion. As the conversion efficiency of the thermionic process tends to be degraded by electron space charges, the efficiencies of thermionic generators have amounted to only a fraction of those fundamentally possible. We show that this space-charge problem can be resolved by shaping the electric potential distribution of the converter such that the static electron space-charge clouds are transformed into an output current. Although the technical development of such thermoelectronic generators will require further substantial efforts, we conclude that a highly efficient transformation of heat to electric power may well be achieved.

/pdf/highly-efficient-thermoelectronic-conversion-of-solar-energy-5ei4vu3h31.pdf

Highly-efficient thermoelectronic conversion of solar energy and heat into electric power

Electric power may, in principle, be generated in a highly efficient manner from heat created by focused solar irradiation, chemical combustion, or nuclear decay by means of thermionic energy conversion. As the conversion efficiency of the thermionic process tends to be degraded by electron space charges, the efficiencies of thermionic generators have amounted to only a fraction of those fundamentally possible. We show that this space-charge problem can be resolved by shaping the electric potential distribution of the converter such that the static electron space-charge clouds are transformed into an output current. Although the technical development of practical generators will require further substantial efforts, we conclude that a highly efficient transformation of heat to electric power may well be achieved.

Highly-Efficient Thermoelectronic Conversion of Solar Energy and Heat into Electric Power

Graphene, being an atomically thin conducting sheet, is a candidate material for gate electrodes in vacuum electronic devices, as it may be traversed by low-energy electrons. The transparency of graphene to electrons with energies between 2 and 40 eV has been measured by using an optimized vacuum-triode setup. The measured graphene transparency equals ∼60% in most of this energy range. Based on these results, nano-patterned sheets of graphene or of related two-dimensional materials are proposed as gate electrodes for ambipolar vacuum devices.

/pdf/transparency-of-graphene-for-low-energy-electrons-measured-5edn8ue7wn.pdf

Transparency of graphene for low-energy electrons measured in a vacuum-triode setup

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Highly-efficient thermoelectronic conversion of solar energy and heat into electric power

Highly-Efficient Thermoelectronic Conversion of Solar Energy and Heat into Electric Power

Transparency of graphene for low-energy electrons measured in a vacuum-triode setup