Thermionic-enhanced near-field thermophotovoltaics

Abstract

Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential.Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps.We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ~ 30% and ~ 70 W/cm 2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (~10% and ~10 W/cm 2 ). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.

Review of Carnot Battery Technology Commercial Development

“…The combination of a thermionic and photovoltaic cell as a new hybrid thermionic photovoltaic converter with an efficiency of approximately 40% is considered for discharging. The system should also have a high variable power in the range of kW to MW [141][142][143]. Similar is the "Sun in a box" project at MIT [144].…”

Evaluation of Applicability of Thermophotovoltaic System in Combi Boiler

“…In TPV systems, any heat source with the operating temperature above 1000 °C can be used. In the literature, TPV systems operating at low temperatures are also available [5,[8][9][10]. For example, Xu has prepared a dissertation on the thermal change of a system consisting of an emitter with an emitter temperature ranging from 76.85 °C to 276.85 °C and a photovoltaic (PV) material [11].…”

Dielectric Micro‐ and Sub‐Micrometric Spacers for High‐Temperature Energy Converters

“…Similarly, the performance of thermophotovoltaic (TPV) modules can be significantly boosted exploiting near‐field thermal radiation across vacuum gaps of few hundred nanometers or less, [ 4 ] as well as the novel concept of near‐field thermionic‐TPV device predicts. [ 5 ] Anyway, obtaining such small gaps is quite a difficult technological challenge, because the spacer must minimize the thermal conductance and maximize the electrical insulation over all the active area and, at the same time, prevent thermal or electrical shorting due to spacer cracking or collapsing. For addressing these requirements, the research has been focused on two main objectives: materials and geometrical constraints.…”