Solar energy conversion with photon-enhanced thermionic emission

Abstract

Photon-enhanced thermionic emission (PETE) converts sunlight to electricity with the combined photonic and thermal excitation of charge carriers in a semiconductor, leading to electron emission over a vacuum gap. Theoretical analyses predict conversion efficiency that can match, or even exceed, the efficiency of traditional solar thermal and photovoltaic converters. Several materials have been examined as candidates for radiation absorbers and electron emitters, with no conclusion yet on the best set of materials to achieve high efficiency. Analyses have shown the complexity of the energy conversion and transport processes, and the significance of several loss mechanisms, requiring careful control of material properties and optimization of the device structure. Here we survey current research on PETE modeling, materials, and device configurations, outline the advances made, and stress the open issues and future research needed. Based on the substantial progress already made in this young topic, and the potential of high conversion efficiency based on theoretical performance limits, continued research in this direction is very promising and may yield a competitive technology for solar electricity generation.

Near-field enhanced thermionic energy conversion for renewable energy recycling

“…In the present work, we assumed C PR to be 0.1 by following previous works [16,[37][38][39]. Another electron recycling process may occur at the cathode interface when a portion of the emitted electrons are reflected back from the anode and reach the cathode conduction band, contributing to the increase of the thermionic power output and the efficiency [40]. However, the present study assumes that all the reflected electrons are lost due to surface recombination at the surface of the cathode.…”

Theoretical studies of thermionic conversion of solar energy with graphene as emitter and collector

“…4,9,10,[15][16][17] The SLAC/ Stanford University research team is creating a new solid-state energy conversion technology based on microfabricated heterostructure semiconductor cathodes with appropriate band engineering and photon-enhanced thermionic energy converters (PETECs). The microfabrication allows very small gaps (a few microns) between the emitter and the collector and thus reduces the space-charge effect drastically.…”

“…As a result, no practical device has yet been built based on PETEC with high efficiency. 17 Schwede et al 45 have carried out interesting theoretical work on photo-enhanced thermionic emission from semiconducting surfaces in which incident photons photo excite the electrons in a semiconductor, which are then thermally emitted. The idea of the photo excitation of electrons and thermal ejection from the excited state seems to bear a resemblance to the idea proposed a long time ago on the creation of a high brightness-electron beam.…”

A PVTC system integrating photon-enhanced thermionic emission and methane reforming for efficient solar power generation

“…In the latter case, net thermal radiation energy flux onto the anode vanishes, and thermal energy extracted from the anode is contributed mainly by heating (J Á / A in Fig. 1 [16,19]) associated with the capture of electrons at the anode.…”

“…carrier separation) is achieved by the vacuum, which does not lose efficacy at high temperature (the built-in electric field of p-n junction in PV cells lose efficacy at high temperatures [18]). High-temperature thermal energy can be extracted from the anode to drive a heat engine [16] and efficiency promotion has been proved in PETE and heat engine hybrid systems [19]. PETE devices can be operated either with a temperature difference (between the two electrodes) or isothermally [18,20].…”